Field of the invention

The present invention concerns a device for controlling the flow rate of a fluid, in particular a device for use with microfluidic or nanofluidic devices.

Background of the invention

Microfluidic and nanofluidic devices are well known in the art, and are designed to manipulate fluids that are constrained in the microscale or nanoscale respectively. Microfluidic and nanofluidic devices have been used in many different fields which require the use of very small volumes of fluids, including engineering and biotechnology. For example, microfluidic systems have been used in the development of inkjet printheads and DNA chips.

Microfluidic or nanofluidic devices typically comprise one or more channels for passage of a fluid. Methods by which fluid actuation through the channels can be achieved are known in the art. For example, the fluid may be pumped through the device using a positive displacement pump such as a syringe pump. Alternatively, pressure driven flow may be used. For example a pneumatic pressure source may be used, wherein a gas line provides a flow of gas which displaces the fluid.

Each method of fluid actuation has particular advantages and disadvantages.

At the micro or nanoscale, methods relying on the use of a syringe pump, peristaltic pump or piston pump have a disadvantage in that the time to achieve the required flow rate (steady state flow rate) of the fluid in the device (the equilibrium time) is longer than desired when using a gas phase fluid to move the working fluid in the chip. The use of such pumps may also cause pulsation which is problematic for many applications. Furthermore, at such a small scale it is difficult to integrate a syringe pump with a microfluidic or nanofluidic device. The use of pneumatically driven flow allows the fluid (reagent or similar) to be moved via an external gas, thus removing the risk of cross contamination between instrument and microfluidic device as there is no liquid contact with any part of the instrument. Disadvantages are also associated with the use of pressure driven flow, in particular in systems which use devices with channels having internal diameters of hundreds of micrometers. For channels having such dimensions, the hydrodynamic resistance due to the channel itself is not large, and even a tiny pressure can lead to a flow rate which is too high for most microfiuidic and nanofiuidic applications.

It is known within the art that a major concern with the manufacture of microfiuidic chips is the requirement to accurately control the dimensions of the microfiuidic channels in order to repeatedly control the flow rate of the working fluid as the dimensions of the channel can control the flow rate due to restrictions. Additionally this variation is increased when channels having internal diameters of hundreds of micrometers are used as the other restrictions begin to govern the flow rate, such as the interconnection variation, or the supply tubing diameter and length.

US 2006/275179 discloses a microfiuidic device wherein actuation of the fluid is achieved using pressure driven flow. The microfiuidic device comprises at least one microchannel connected at one end to a chamber. The flow rate within the microchannel is controlled by means of an inlet circuit and an outlet circuit. The inlet circuit and outlet circuit are connected to the chamber, and a flow of fluid is established between the two circuits without contact with the microchannel. Preferably, the fluid flowing in the inlet and outlet circuits is a gas whilst the fluid flowing in the microfiuidic device is a liquid.

In this device at least one of the inlet and outlet circuits are controllable so as to modify the pressure at one end of the microchannel independently of the pressure at another end of the microchannel. This arrangement allows control over the pressure at one end of the microchannel and therefore over the flow rate in the microchannel. This system avoids some of the drawbacks of a system relying on pump-driven flow, for example unwanted pulses in the flow can be minimised. However, the requirement for separate inlet and outlet circuits increases the complexity, and consequently the cost of such microfiuidic systems. A further disadvantage is that the flow control system according to US 2006/275179 is only suitable for use with microfiuidic devices having channels with dimensions typically in the range of 10 to 100 μm and is not suitable for use with devices having channels of larger internal diameter, for example 100 to 1000 μm due to the flow rate of the working fluid being too high even when the pressures are very low. It is also known in the art that controlling very low pressures is very difficult and therefore not ideal.

Accordingly, it is an aim of the present invention to solve one or more of the problems with the prior art described above. Specifically, it is an aim of the present invention to provide an improved apparatus for controlling the flow of a fluid in microfluidic or nanofluidic devices. In particular, it is an aim of the present invention to provide an apparatus which is capable of lowering the flow rate of a fluid to a desired level. It is desired that the apparatus achieves the desired flow rate quickly and maintains the flow rate with minimal variability.

Summary of Invention

Accordingly, the present invention provides a flow control element for a microfluidic or nanofluidic device, comprising: a) a fluid source; and b) a flow restriction component; wherein the flow restriction component is situated downstream from the fluid source, and the flow restriction component has a hydrodynamic resistance at least 5 times higher than the hydrodynamic resistance of the device.

Preferably, the flow restriction component has a hydrodynamic resistance at least 10 times higher than the hydrodynamic resistance of the device. The method of measurement of hydrodynamic resistance is not especially limited, and any known method may be employed. In determining the ratio of hydrodynamic resistance between the restriction component and the device, the same method is preferably employed.

The hydrodynamic resistance of a conduit can be determined using the formula:

\28μL

R = πd ^A wherein L is the length of the element being measured, d is its internal (cross-sectional) diameter, and μ is the dynamic viscosity of the solution. Typically, the flow restriction component is comprised of a conduit having a cross-sectional diameter of less than 300 μm and a length of from 5 to 100 mm. The conduit preferably has a cross-sectional diameter of from 50 to 150 μm. The length of the conduit is preferably from 40 to 60 mm. In a most preferred embodiment the conduit has a cross-sectional diameter of about 100 μm and a length of about 50 mm.

The fluid source is typically a container containing a fluid, preferably with an air space above the fluid to be moved.

The conduit of the flow restriction component may be directly connected to the fluid source or may be situated within the fluid of the fluid source.

The use of a flow control element comprising a flow restriction component provides several advantages in microfluidic or nanofluidic applications. In particular, the flow restriction component reduces the flow rate of the fluid exiting the component to a rate which is suitable for fluids passing through microfluidic or nanofluidic devices. The flow control element is preferably configured to maintain the flow rate of the liquid exiting the flow control element in the range of from 10 μL/min to 200 μL/min. The inventors have also found that microfluidic systems comprising the flow control element have reduced flow rate variability relative to microfluidic systems known in the art.

If the restriction in the restriction component is greater than the restriction inherent in the device then the variation in the restriction component becomes more significant than the variation in the microfluidic device. This is important as the restriction component can be made using a more robust production method, and hence have a much smaller variation in restriction than the microfluidic device.

In a preferred embodiment, the ratio of the cross-sectional diameter to the length of the narrow portion controls the restriction. It has been found that the variation of the cross- sectional diameter is the more difficult to control dimension in manufacture, thus a longer length is preferred rather than a smaller diameter. The flow control element may further comprise a pressure source for controlling fluid flow. Preferably, the pressure source is a pneumatic pressure source with air as a driving fluid, however any gaseous fluid may be used. In a yet further preferred embodiment the pressure source is adapted to apply pressure directly to a fluid in the fluid source. Typically, the pressure source is adapted to apply a pressure of up to 1 bar to the fluid. Preferably, the pressure source is adapted to apply a pressure of from 0.08 bar to 0.5 bar to the fluid.

Provision of the flow control element according to the invention enables the use of standard pressure sources, particularly pneumatic pressure sources, with microfluidic and nanofluidic devices. If no flow control element is used, even the smallest pressure available from a high performance pressure regulator (approximately 0.08 bar) leads to a flow rate of a fluid which is too large for use with microfluidic or nanofluidic devices, necessitating the use of other methods of fluid actuation, for example the use of syringe pumps or other types of pump. A microfluidic system using a combination of pressure driven flow and the flow control element of the invention has particular advantages over microfluidic systems relying on syringe pump driven flow. One such advantage is that pressure driven flow devices do not cause pulsation effects which can be problematic when using syringe pumps. Additionally, the time required to achieve the desired flow rate using the apparatus according to the invention is significantly lower than when using a known syringe pump driven apparatus.

In a preferred embodiment the flow control element is configured to achieve a steady state flow rate within a period of time of between 200 milliseconds to 2 seconds from application of a pressure to the fluid. The term steady state flow rate is meant to refer to the flow rate which is achieved at equilibrium. The application of pressure to the fluid source causes an initial increase in flow rate which then levels out to an equilibrium (steady state) value. The time taken for the flow rate to reach steady state is referred to as the equilibrium time, i.e. the equilibrium time is preferably from 200 milliseconds to 2 seconds. Preferably, the equilibrium time is from 400 milliseconds to 1 second from application of a pressure to the fluid. As well as the improved dynamic performance of an apparatus comprising pressure driven flow and the flow control component, the apparatus according to the invention also provides improved performance once the steady state is reached. Specifically, there is reduced fluctuation of the steady state flow rate when using the apparatus according to the invention as opposed to known devices with pneumatic pressure sources (without a flow control component) or syringe pumps. The apparatus according to the invention typically allows a substantially constant value of the flow rate to be achieved over timescales appropriate for microfluidic or nanofluidic applications.

In a further aspect, the present invention provides an apparatus comprising at least one flow control element according to any preceding claim, and further comprising a microfluidic or nanofluidic device. The microfluidic or nanofluidic device comprises at least one channel. Preferably, at least one channel has a cross-sectional diameter of from 50 to 1000 μM, more preferably at least one channel has a cross-sectional diameter of from 100 to 1000 μM, and in a most preferred embodiment at least one channel has a cross-sectional diameter of from 100 to 300 μM.

In one embodiment the flow control element and the microfluidic or nanofluidic device are separate components of the apparatus and the flow control element is connected upstream from the microfluidic or nanofluidic device. Preferably, the flow control element is connected to one or more channels of the microfluidic or nanofluidic device.

In an alternative embodiment the flow control element is integral with the microfluidic or nanofluidic device.

In a further embodiment the conduit is integral with the microfluidic or nanofluidic device and the fluid source is a separate element.

The conduit of the flow control element may be formed from one or more channels of the microfluidic or nanofluidic device. The material from which the flow restriction component is formed is not especially limited. Preferably the flow restriction component is formed from a metal or a polymer, for example, stainless steel or PEEK (Polyethylethylketone). In a most preferred embodiment the flow restriction component is formed of stainless steel.

In a further aspect, the present invention relates to the use of the flow control element or apparatus described above for regulating the flow rate of a fluid entering, within, or exiting a microfluidic or nanofluidic device.

In a still further aspect, the present invention provides a method of regulating the flow rate of a fluid entering, within or exiting a microfluidic or nanofluidic device, which method comprises altering the pressure applied to a fluid within a fluid source of a flow control element or apparatus as defined in any preceding claim.

Detailed Description of the Invention

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

Figure 1 shows a schematic of a typical pressure driven flow setup. A pneumatic line from a pressure regulator delivers a gas to a to a reagent reservoir so as to apply a pressure to a fluid provided within the reservoir. An outlet tube provided within the fluid in the reservoir is connected to a microfluidic device. The pressure applied by the pneumatic system forces the fluid from the reservoir, through the outlet tube and into the microfluidic chip.

Figure 2 shows a plot of the flow rate achieved over time using the typical pressure driven flow apparatus shown in Figure 1 at three different pressures: 0.08 bar, 0.12 bar and 0.16 bar from left to right in the plot. It can be seen that even when the minimum possible pressure is applied, a flow rate of nearly 300 μL/min results.

Figure 3 shows a schematic of an apparatus according to the present invention. To reduce the flow rate a restriction is situated between the reagent reservoir and the microfluidic or nanofluidic device. Figure 4 shows a schematic of a reservoir in a typical pressure driven flow apparatus.

Figure 5 shows a schematic of a modified reservoir according to an embodiment of the present invention. In this embodiment a large gauge needle (with a small internal diameter) is used as the flow restriction component. The needle is placed into the reservoir for better integration.

Figure 6 shows a plot of flow rate against time for devices according to the invention using needles of internal diameters of 254, 203, 178, 152, 127 or 102 μm as the flow restriction component or a known device using a standard needle (internal diameter 330 μm) at pressures of 0.08 bar, 0.12 bar and 0.16 bar (from left, centre and right in the plot respectively).

Figure 7 shows the results of an experiment to measure the variability of the flow rate achieved using the apparatus according to the invention. The plot shows flow rate against time for a device using a 152 μM needle at pressures of 0.08 bar, 0.12 bar and 0.16 bar, with 10 repeats at each pressure.

Figure 8 shows a plot of flow rate against time for the pressure driven flow set up according to the embodiment of the invention shown in Figure 5 and for an apparatus using a syringe pump.

Figure 9 shows a plot of flow rate against time to demonstrate the steady state behaviour of (a) an apparatus using a syringe pump (b) a standard pressure driven flow apparatus without a restriction using a needle of internal diameter 330 μm and (c) a pressure driven flow apparatus according to the invention using a needle of internal diameter 102 μm as the narrower portion of the flow restriction component.

The flow control element according to the present invention comprises a fluid source. The term fluid source is not especially limiting, and includes any source of fluid. The fluid is preferably a liquid. The fluid may be any fluid to be used with a microfluidic or nanofluidic device. Examples include samples comprising one or more proteins, polypeptides, peptides, oligonucleotides, reagents, buffers, wash solutions, bead solutions or small molecules. The fluid source may be a container containing the fluid. In this case the container may be any kind of container of any suitable size or shape. Examples of typical containers include a test tube, an Eppendorf™ tube, a well, such as a well of a 96-well plate, a custom designed reagent container or a flask. The fluid source may contain a finite volume of fluid. The fluid source may contain volumes from a few nanolitres up to several litres depending on the application. Typically, the container contains from 20 to 1000 μl.

The flow control element further comprises a flow restriction component. The flow restriction component is situated downstream from the fluid source. This means that a fluid flows from the fluid source to the restriction component, and flows through the restriction component.

The flow restriction component has a hydrodynamic resistance at least 5 times higher than the hydrodynamic resistance of the microfluidic or nanofluidic device. Preferably, the flow restriction component has a hydrodynamic resistance at least 10 times higher than the hydrodynamic resistance of the device.

Methods for determining hydrodynamic resistance are well known in the art, and any such methods may be employed here.

The hydrodynamic resistance of a conduit can be determined using the formula:

\28μL

R = πd ^A

With L is the length of the element being measured, d is its internal (cross-sectional) diameter, and μ is the dynamic viscosity of the solution.

Typically, the flow restriction component is comprised of a conduit having a cross-sectional diameter of less than 300 μm and a length of from 5 to 100 mm. Where it is possible to measure more than one cross-sectional diameter of a conduit, the smallest cross-sectional diameter is measured. The term cross-sectional diameter refers to the internal diameter of the conduit. The conduit preferably has a cross-sectional diameter of from 30 to 200 μm, more preferably 50 to 150 μm, more preferably still from 80 to 120 μm. In a particularly preferred embodiment, the cross-sectional diameter of the conduit is about 100 μm. The length of the conduit is preferably from 20 to 80 mm, more preferably from 40 to 60 mm. In a particularly preferred embodiment the length of the conduit is about 50 mm. In a most preferred embodiment the conduit has a cross-sectional diameter of about 100 μm and a length of about 50 mm.

In a preferred embodiment the flow restriction component comprises a flow restriction conduit having at least one narrower portion and at least one wider portion, the narrower portion being upstream of the wider portion. Multiple different conduits can be envisioned with different numbers and configurations of narrower and wider portions. In a most preferred embodiment the conduit comprises a single narrower portion upstream of a single wider portion. The narrower and wider portions may be integral with one another, or may be distinct elements connected together. Typically, the narrower portion is directly connected to the wider portion.

It will be understood that the term narrower portion refers to a portion having a smaller cross- sectional diameter than a wider portion. Where it is possible to measure more than one cross- sectional diameter of a portion of a conduit, the smallest cross-sectional diameter is measured in order to determine whether a portion is wider or narrower than another portion.

The narrower portion preferably has a cross-sectional diameter of less than 300 μm and a length of from 5 to 100 mm. The narrower portion preferably has a cross-sectional diameter of from 30 to 200 μm, more preferably 50 to 150 μm, more preferably still from 80 to 120 μm. In a particularly preferred embodiment, the cross-sectional diameter of the narrower portion is about 100 μm. The length of the narrower portion is preferably from 20 to 80 mm, more preferably from 40 to 60 mm. In a particularly preferred embodiment the length of the narrower portion is about 50 mm. In a most preferred embodiment the narrower portion has a cross-sectional diameter of about 100 μm and a length of about 50 mm. The cross-sectional diameter of the wider portion is determined by the manufacturing capability of the device fabricator, and in the inventors preferred embodiment is 500 μm.

Typically, the conduit (or narrower and wider portions of the conduit) is substantially cylindrical and has a circular cross section. However, the shape of the conduit is not especially limited provided fluid can flow through the conduit. For example, the conduit may have a square, rectangular or oval cross section.

The conduit of the flow restriction component may be directly connected to the fluid source or may be situated within the fluid of the fluid source. When the conduit comprises a narrower portion and a wider portion, preferably the narrower portion of the conduit is directly connected to the fluid source or is situated within the fluid source. In a particular embodiment the flow control element further comprises a connecting portion between the conduit and the fluid source. The connecting portion is not especially limited and may in some embodiments comprise a further narrower portion or a further wider portion.

The flow control element may further comprise a pressure source for controlling fluid flow. Preferably, the pressure source is a pneumatic pressure source utilising a gas phase fluid. Such pressure sources generally comprise a pressure regulator for accurately regulating the pressure applied. In a preferred embodiment the pressure source is adapted to apply pressure directly to a fluid in the fluid source. For example, a pneumatic line may be placed within a fluid provided in a container, and a flow of gas through the pneumatic line into the fluid provides the necessary increase in pressure. Typically, the pressure source is adapted to apply a pressure of up to 1 bar to the fluid in the fluid source. Preferably, the pressure source is adapted to apply a pressure of from 0.08 bar to 0.5 bar to the fluid. In a most preferred embodiment the pressure is from 0.15 bar to 0.3 bar.

A flow control element is preferably configured to maintain the flow rate of the liquid exiting the flow control element in the range of from 10 μL/min to 200 μL/min. In a further preferred embodiment the flow control element is configured to achieve a steady state flow rate within a period of time of between 200 milliseconds to 2 seconds from application of a pressure to the fluid. The term steady state flow rate is meant to refer to the flow rate which is achieved at equilibrium. The application of pressure to the fluid source causes an initial increase in flow rate which then levels out to an equilibrium (steady state) value. The time taken for the flow rate to reach steady state is referred to as the equilibrium time, i.e. the equilibrium time is preferably from 200 milliseconds to 2 seconds. The equilibrium time may be 2 seconds, 1 second, 800 milliseconds, 600 milliseconds, 400 milliseconds or 200 milliseconds. Preferably, the equilibrium time is from 400 milliseconds to 1 second from application of a pressure to the fluid.

In a further aspect, the present invention provides an apparatus comprising at least one flow control element as described above, and further comprising a microfluidic or nanofluidic device. Microfluidic and nanofluidic devices are commonly described as devices for handling small volumes of liquid which comprise at least one channel having at least one dimension of less than 1 mm or 1 μm respectively. Accordingly, the microfluidic or nanofluidic device according to the invention comprises at least one channel. Preferably, at least one channel has a cross-sectional diameter of from 50 to 1000 μM, more preferably at least one channel has a cross-sectional diameter of from 100 to 1000 μM, and in a most preferred embodiment at least one channel has a cross-sectional diameter of from 100 to 300 μM. Otherwise, the architecture of the microfluidic or nanofluidic device is not especially limited.

In addition to comprising one or more channels the microfluidic or nanofluidic device may further comprise one or more valves to control the flow of fluid through the channels. The device also typically comprises one or more chambers. One or more chambers may be provided for storing reagents or samples. One or more chambers may also be provided for mixing or reaction of samples or reagents.

In one embodiment the flow control element and the microfluidic or nanofluidic device are separate components of the apparatus and the flow control element is connected upstream from the microfluidic or nanofluidic device. Preferably, the flow control element is connected to one or more channels of the microfluidic or nanofluidic device. More preferably the flow control element is connected to the one or more channels of the microfluidic or nanofluidic device by means of a wider portion of the conduit. In an alternative embodiment the flow control element is integral with the microfluidic or nanofluidic device.

In a further embodiment the conduit is integral with the microfluidic device and the fluid source is a separate element.

The conduit of the flow control element may be formed from one or more channels of the microfluidic or nanofluidic device.

The material from which the flow restriction component is formed is not especially limited. Preferably the flow restriction component is formed from a metal or a polymer. Ideally the flow restriction component is formed from a material with a high Young's modulus and low thermal expansion to limit the amount of variation due to external stress or temperature. Preferred materials are stainless steel or PEEK (Poly Ethyl Ethyl Ketone). In a most preferred embodiment the metal is stainless steel. Typically, the narrower portion of the flow restriction component is a metal needle. In one embodiment the wider portion of the flow restriction component is formed of polymer tubing, preferably Tygon® tubing.

In a further aspect, the present invention relates to the use of the flow control element or apparatus described above for regulating the flow rate of a fluid entering, within or exiting a microfluidic or nanofluidic device.

The present invention will be described further by way of example only.

EXAMPLES

Example 1

Examining flow rates achieved with standard pressure drive flow arrangements

A standard apparatus as shown in Figure 1 was provided. Fluid was placed in the reservoir, which was connected to a microfluidic chip by means of a conduit. A pneumatic line connected to a pressure regulator was also provided within the reservoir to apply a pressure to the fluid within the reservoir. The flow rate of the fluid entering the microfluidic chip was then measured with time using a thermometric flow meter (Sensirion) under 3 different pressure regimes. Figure 2 shows a plot of the flow rate achieved over time using this typical pressure driven flow apparatus at pressures of 0.08 bar, 0.12 bar and 0.16 bar from left to right in the plot. It can be seen that even when the minimum possible pressure is applied, a flow rate of nearly 300 μL/min results. This flow rate is too high for the majority of applications of microfluidic and nanofluidic devices.

Example 2

Comparison of a standard pressure-driven flow microfluidic apparatus with the apparatus according to the invention

The flow rate achieved using a standard pressure-driven flow apparatus comprising a needle with an internal diameter of 330 μm was compared to the flow rate achieved using an apparatus according to embodiments of the invention using needles of internal diameters of 254, 203, 178, 152, 127 or 102 μm as the narrower region of the flow restriction component. Figure 6 shows a plot of flow rate against time for each device at pressures of 0.08 bar, 0.12 bar and 0.16 bar (left hand, centre and right hand sections of plot respectively). It can be seen that the smaller the internal diameter, the smaller flow rate that is achieved. With a device comprising a needle of internal diameter 102 μm the flow rate is reduced by nearly a factor of 10 compared to a device comprising a standard needle. This Figure demonstrates that providing a flow restriction component (in this case a narrow gauge needle) between the reservoir and the microfluidic device reduces the flow rate significantly for a given pressure.

Example 3 Variability of flow

A significant problem with microfluidic and nanofluidic devices is that the variability of the flow rate achieved between different repeats of an experiment is often too high. This can be due to the interconnect or inherent chip variability. This variation comes from the inability to control the dimensions of the microfluidic chip or from slight variations of the interconnect alignment or pressure. An experiment was carried out wherein a flow control element was connected and disconnected to a microfluidic device 10 successive times, hence varying the interconnect pressure, alignment and therefore restriction. Figure 7 shows a plot of flow rate against time for a device according to the invention, using a needle with an internal diameter of 152 μm as the narrower portion of the flow control component, at pressures of 0.08 bar, 0.12 bar and 0.16 bar, with the 10 repeats at each pressure. The same experiment was carried out using further devices according to the invention with needles of internal diameter of 127 μm and 102 μm, and also with a standard device comprising a needle of internal diameter 330 μm. Table 1 shows the mean flow rate, the standard deviation and the error calculated per pressure and per needle. The flow restriction component clearly reduces the variability due to the interconnect from more than 70% to less than 20%.

Table 1

Example 4

Dynamic behaviour: comparison of pressure driven flow and syringe pump driven flow It is preferable for the steady state flow rate to be achieved as soon as possible after application of pressure to the fluid. The flow rate dynamics of a pressure driven flow apparatus using the flow restriction component according to the present invention were compared to the flow rate dynamics of an apparatus with a syringe pump to drive the flow. The syringe pump is connected with air as its working fluid rather than a liquid phase fluid. Figure 8 shows a plot of flow rate against time for both the apparatus according to the invention and the syringe pump apparatus. It can be seen that the syringe pump apparatus struggles to set the flow rate in less than 10 seconds whereas the apparatus according to the invention allows the flow rate to be achieved in less than 1 second.

Example 5

Steady state behaviour: comparison of pressure driven flow and syringe pump driven flow As well as achieving the steady state flow rate within a small period of time, it is also preferable that the apparatus achieves a steady flow rate which does not fluctuate significantly with time. Figure 9 shows a plot of flow rate against time to demonstrate the steady state behaviour of (a) an apparatus using a syringe pump (b) a standard pressure driven flow apparatus without a restriction using a needle of internal diameter 330 μm and (c) a pressure driven flow apparatus according to the invention using a needle of internal diameter 102 μm as the narrower portion of the flow restriction component. It can be seen that the syringe pump struggles to deliver a steady flow rate and the flow rate fluctuates between 90 and 120 μL/min. Furthermore, the standard reservoir comprising a needle with an internal diameter of 330 μm shows a decrease in flow rate due to the reduction in hydrostatic pressure which occurs while the reservoir empties. By contrast the apparatus according to the invention, using a needle of internal diameter 102 μm as the narrower portion of the flow restriction component, provides a steady state that does not fluctuate and the flow rate remains at the approximately the same value throughout the experiment.