WO2001085342A1

WO2001085342A1 - Metering dispenser with capillary stop

Info

Publication number: WO2001085342A1
Application number: PCT/GB2001/002106
Authority: WO
Inventors: John Robert Dodgson; John Edward Andrew Shaw
Original assignee: Central Research Laboratories Limited
Priority date: 2000-05-12
Filing date: 2001-05-14
Publication date: 2001-11-15
Also published as: GB0011574D0; AU2001254987A1

Abstract

A dispenser (10) for dispensing a known volume of liquid (22), the dispenser comprising a pathway (12) for receiving the liquid, at least one capillary arrester (14) in said pathway and located to define a limit of said volume, pressure means (18) for establishing a pressure differential across said arrester so that, in use, liquid is drawn into the pathway, and release means to dispense said volume of liquid. The dispenser enables accurate dispensing of very small known volumes of liquid.

Description

METERING DISPENSER WITH CAPILLARY STOP

Technical Field

The present invention relates to a dispenser. It relates particularly, but not exclusively, to a dispenser for dispensing small volumes of liquid.

Background Art

In fluid handling robotics used in chemical and biological research, small volumes of liquid are typically dispensed by filling into a capillary tube held at a controlled pressure, moving to another location and then forcing the liquid out of the tube at a higher pressure. This is followed by a wash cycle. There is a trend towards lower volumes of liquid to be handled as the number of wells on a micro-titre plate, for example, increases. This increases the demands made by the need for accurate and reproducible volume control on the accuracy of pressure control and uniformity of fill rate of the capillary tubes. A system where the volume that is drawn up into the tube is made less critically dependent on the filling force and timing will be advantageous. There is also a need for the dispensing of accurately known volumes of liquid from a larger volume of liquid, either in a single volume or repeatedly, such dispensing being as independent as possible of the force driving the liquid.

In another scenario, there is a need in sampling, for example in medical sampling, to retain a given volume of sample while disposing of the rest to waste, this metered volume of sample then being moved on to further parts of the device. Various means have been developed to meter a volume in a sample space while retaining remaining liquid in a separate space - see for example US 5,975,153 which shows a sample space which fills from a larger sample collection space. However, the prior art does not disclose a system whereby once metered, the metered volume can be readily separated from the remaining sample and then acted on separately.

An aim of the invention is to provide a dispenser which is suitable for dispensing small volumes of a liquid. Another aim of the invention is to provide a dispenser for dispensing accurately known volumes of a liquid from a larger volume of a liquid. Disclosure of Invention

A dispenser for dispensing a known volume of liquid, typically less than a microlitre, and preferably less than a nanolitre, comprises a pathway for receiving the liquid, at least one capillary arrester in said pathway and located to define a limit of said volume, pressure means for establishing a pressure differential across said arrester so that, in use, liquid is drawn into the pathway and release means to dispense said volume of liquid.

The pathway is preferably a capillary tube. The capillary tube has at least one capillary stop within it at a position within the capillary tube that defines the volume of liquid to be dispensed. A capillary stop is a sudden change of the capillarity of the capillary tube which acts to hinder the motion of liquid in the tube past that point.

The pressure means is preferably a pump, either a suction pump or a pump for applying pressure to the liquid. The pressure means is preferably coupled to the capillary tube via a manifold. A pressure gauge may also be provided for measuring the pressure applied to the liquid. The pressure gauge is preferably located between the manifold and the pressure means.

In general, the dispensing tubes are made from a material, for example polypropylene, that is not wetted by the solution to be dispensed (the solvent is usually polar: water or DMSO). The capillary force between the liquid and the capillary tube then acts to oppose formation of a meniscus inside the capillary tube, and so a lowered pressure (i.e., suction) is needed in the manifold to draw liquid into the capillary tube.

For a liquid that does not wet the capillary tube, a capillary stop will take the form of a narrowing of the minimum dimension of the capillary tube. The minimum dimension of the capillary tube is defined herein as the narrowest part of the cross- section of the tube. Increased driving force, i.e., a lower pressure inside the manifold, is then needed to draw the meniscus past the narrowing. A change of the wetting properties of the inside surface of the capillary tube to increase the contact angle (i.e., the angle the meniscus forms with the capillary tube) of the liquid further would also act as a capillary stop. This change may be, for example, from a hydrophobic to a hydrophilic material. This principle may be used to provide a more precise control of the volume of liquid filled into the capillary than would control of the pressure alone. If the value of the manifold pressure is sufficient to draw liquid into the capillary tube, but insufficient to draw it past the capillary stop, then filling will cease at that capillary stop. This can be made very precise - it depends on the dimensions of the capillary tube, not on the value of the pressure or the viscosity of the liquid. The contact angle of the liquid to the tube is not critical (provided it is known to be either greater or less than 90 degrees) as the difference between the capillarity of the tube and that of the stop region can be made large.

In the case that the liquid does wet the tube, it will tend to fill by capillary action alone, and the manifold pressure is positive in order to control and/or halt the filling.

In this case, the capillary stop is created by an increase in the minimum dimension of the capillary tube. A change in surface properties of the capillary tube to increase the contact angle may also be used to improve the control of the volumes of liquid dispensed, as previously described.

Preferably, the release means is a pump, so that the liquid can be dispensed from the capillary tube by application of pressure to the liquid.

The capillary tube may need to be flushed with a wash liquid. During washing of the capillary tube, the wash liquid needs to be moved further within the tube than the original fill liquid, in order to ensure complete flushing. This is done by filling of the capillary with wash liquid using a manifold pressure greater than that needed to overcome the capillary stop. The metering effect is obviously not needed during the wash step.

The invention is not limited to tubes of any particular cross-sectional shape or means of fabrication. The capillarity of a tube is determined by the minimum dimension of the cross-section of the tube. For example, for a liquid that does not wet the surface of the tube, a capillary stop can be formed in a tube of circular cross-section by a decrease of the diameter of the tube, or by a change in cross-section from circular (of diameter d) to a rectangular cross-section of minimum side dimension less than d. By tube is meant any enclosure for liquid which has two open ends, at least one of which is passable by liquid, the other by liquid, or by gas only. Therefore tubes as envisaged in the invention may be formed in the surface of a planar substrate which is covered with a second cover component. The cross-section can be of any shape convenient in the fabrication process. Capillary stops may also be fabricated by treatment or modification of a part of the surface of the tube to change the contact angle of the liquid with the surface. This might be done by modification of a part of the surface of the tube formed by a continuous piece of the same material, or by introducing a different bulk material into part of the tube.

In the invention, forces acting on the liquid other than capillarity are usually neglected. Obviously in the case of filling a vertical tube, gravity will act in addition to pressure difference across a meniscus. However, in most cases in micro-fluidic devices the effect will be negligible. In larger fluidic devices these effects can be taken into account in the design. Thermal effects, causing expansion of the liquid or tube(s) or changes in the contact angle at the liquid/solid interface are also neglected in the following discussion, but will be significant in a micro-fluidic environment. They can also be accommodated in detailed design of a micro-fluidic device.

In a further aspect of the invention, the manifold pressure can be monitored by the pressure gauge. The fact that in the system of the invention the volume filled into the tubes is independent of the manifold pressure allows the manifold pressure itself to vary during the filling procedure, provided it remains within the limits required to fill the tube and to allow the capillary stop to operate. When the liquid reaches the capillary stop, the manifold pressure will rise slightly and this can be used to control the manifold pressure - for example to reduce a suction force to just that needed to hold the liquid at the stop position. This allows for a greater driving force to be used during filling and hence faster operation. A further embodiment using the aforedescribed principle uses a tube or channel with a tapering minimum dimension, such that the width at any position of the meniscus determines the pressure needed to allow the meniscus to move further, and hence gives a measure of the volume in the channel. The taper can be in a series of steps, giving a discrete set of volume measurements which are easily identifiable by the pressure measuring apparatus.

In a further aspect of the invention, more than one capillary stop position can be provided to allow the possibility of repeated metering or dispensing operations in a single tube. For example, if liquid is aspirated into a tube comprising a number of capillary stops, then each time the meniscus reaches a stop position the differential pressure across the meniscus needs to be increased in order for it to pass the stop. Therefore if a transient pressure pulse is applied of duration sufficient to allow the meniscus to pass the stop but less than the time the liquid takes to fill the tube to the next stop position, then the liquid will travel only to the next stop. A subsequent pulse will allow movement to a further stop, and so on. The invention is likely to be useful for repeated dispensing of a metered volume of liquid following a single charging step, where the tube is filled using a pressure such that the liquid flows past a number of stop positions, and is then dispensed by a transient pressure pulse as described above, allowing liquid to flow out from the tube until the meniscus reaches a stop position. A subsequent pressure pulse will then dispense a second volume. In this way a number of aliquots of liquid can be dispensed repeatedly from a filled tube using a simple actuation pressure pulse whose magnitude does not affect the amount dispensed at each step. The volumes in the tube between the capillary stops can be the same or different. The pressure in the tube above the meniscus can be monitored and the process actively controlled. Alternatively, as the pressures needed to overcome the capillary stop behaviour will be known, an actuation pressure in excess of the known value will cause the liquid to move in a predictable way.

In a further aspect of the invention, as the differential pressure across the meniscus needed to move the meniscus in a tube depends on the minimum dimension of the tube, then capillary stops can be designed to require different differential pressures to allow the meniscus to move past them. In a situation where the liquid wets the tube surface (contact angle < 90 degrees), the larger the minimum dimension in the capillary stop region, the greater the pressure differential needs to be. Where the liquid does not wet the tube (contact angle > 90 degrees), the smaller the minimum dimension the greater the pressure differential needs to be. Similarly, capillary stops can be designed to resist different differential pressures by controlling the contact angle in the capillary stop, for example by choice of material at, or coating, the surface in the stop region. Therefore a series of capillary stops can be provided in a tube and the differential pressure that they will resist can be chosen as appropriate. For example, a series of increasing stop pressures measured by a pressure monitoring means can be used as a measure of how much liquid will be filled into, or dispensed out from the tube. Alternatively, these amounts can be set by choice of the pressure applied to the tube to effect filling or dispensing. The liquid will move until the meniscus encounters a capillary stop with a pressure required to overcome it that is greater than that applied.

A further aspect of the invention includes an overflow channel to allow for the possibility of a positive diversion of part of the liquid into this channel, leaving a metered volume of liquid in the first channel. This then acts more positively than the simple version described above in cases where a sharp capillary stop can not be achieved. For example, with liquids that are highly viscous, have wetting properties which may vary greatly between samples, from a source at high pressure, where a larger amount of liquid is desired to be taken into a device and a smaller portion then used for a purpose (e.g., a blood sampling device), or where smaller portions are to be metered from a larger amount of liquid.

This embodiment of the invention may includes an inlet port communicating with a volume which has a capillarity Cl and a pressure port which is either a vent, if the device is intended to fill by capillary action alone, or a port for application of pressure, if the device is to be filled actively. The first volume contains a capillary stop area with capillarity Cs where liquid which fills the first volume through the port by capillary action will tend to stop. Communicating with the first volume is a second volume with a second capillarity C2 which acts to receive excess liquid once the first volume has filled. At the far end of C2 from the inlet port is a second port, which again is either a vent or can be pressurised externally. The device is designed such that Cs < C2 < Cl, to maintain the first volume full and the second volume partially full. The two volumes can then be emptied by application of pressure to the ports. Applying pressure to the port in the first volume will dispense the metered volume into and outlet channel (which may be the same as the inlet channel). Applying pressure to the port in the second volume will flush the device ready for subsequent use. The device can be washed out completely if so desired (as in the previous aspect of the invention) by increasing the suction pressure to draw wash liquid past the capillary stop in the first volume. Optionally a second capillary stop region can be provided in the second volume in order to signal to the system, using the idea that the suction pressure will rise when the liquid encounters a capillary stop, that the second volume also has been completely washed out in the wash process.

Brief Description of Drawings

A number of embodiments of the invention will now be described with reference to the Figures, where:

Figure la shows a cross-sectional view of a capillary stop dispensing apparatus; Figure lb shows a cross-sectional view of part of the capillary stop dispensing apparatus of Figure la;

Figure 2 shows a cross-sectional view of a capillary stop dispensing apparatus having a plurality of capillary stops;

Figure 3 shows a cross-sectional view of a capillary stop dispensing apparatus having an overflow channel;

Figure 4 shows a cross-sectional view of a capillary stop dispensing apparatus having another type of overflow channel;

Figure 5 shows a cross-sectional view of a another capillary stop dispensing apparatus; Figure 6a shows a plan view of a further capillary stop dispensing apparatus; and

Figure 6b shows a cross-sectional view of the apparatus shown in Figure 6a. Detailed Description of Preferred Embodiments

Figure la shows a capillary stop dispensing apparatus (10). The apparatus includes a capillary tube (12) which is connected to a manifold (16), which in turn is connected to a suction pump (18). The apparatus also includes an optional pressure gauge (20) which is placed in the pathway between the manifold (16) and the suction pump (18). Sampling liquid (22) is aspirated into the capillary tube (12) by low pressure created in the manifold (16) by the suction pump (18).

The capillary tube (12) has a constriction formed therein, the constriction acting as a capillary stop region (14). The apparatus (10) is designed for a liquid which does not wet the capillary tube material, hence an inverted meniscus (24) is formed by the liquid. The pressure in the manifold (16) is set to be that at which the meniscus (24) stops at the constriction (14). To wash the capillary tube (12), wash liquid is drawn past the capillary stop (14) using a lower pressure in the manifold than for filling the capillary with the sample liquid (make sure sample liquid is defined) in the manifold (16). If the suction established by the pump (18) is chosen correctly (i.e., it is not too great), the pressure gauge (20) (if used) will indicate when the capillary stop (14) has been reached. A second stop position might be used to give a further indication of the extent of travel of the wash liquid.

Figure lb shows the capillary tube (12) for the case where the liquid wets the tube, the capillary stop (14) in this case being a widening of the bore, rather than a constriction as previously described. The manifold (16) and pressure control devices (18,20) are not shown, but would act slightly differently from those in Figure la. A positive pressure rather than suction might be necessary to control the rate of capillary filling of the capillary tube (12).

The embodiments in Figures la and lb can be made from freestanding capillary tubing, or might be made from a channel in a substrate, for instance in a micro-fluidic device. The capillary stops might be achieved by a change of the material in the tube or channel, to give a change in contact angle of the liquid with the capillary tube. Figure 2 shows a further embodiment of the invention including a capillary tube (12) having a series of capillary stops (14a,b,c,d). It is assumed here that the liquid (22) wets the capillary tube material, so that the capillary stops (14a,b,c,d) consists of a widening of the capillary tube (12). A pressure gauge (20) is positioned above the highest capillary stop (14d) so that a plurality of metered volumes of liquid can be selected by monitoring the number of capillary stop positions the liquid meniscus has passed. In this case, the capillary stop positions are defined by widened portions of the capillary tube (12). The capillary tube (12) leads to a further tube (26) which may have a larger cross-section than the capillary tube.

The capillary tube (12) is filled with liquid (22) via an inlet (38) by capillary action and a controlled pressure in tube (26). This process is monitored by a pressure gauge (20). As the meniscus (24) of the liquid passes the capillary stop positions (14a) to (14d), the pressure gauge (20) will register a transient change in pressure. The pressure required to move the liquid (22) in the capillary stop will be different from that required in the narrow part of the tube, and this will be an indication of where the meniscus (24) is, and therefore of the volume of liquid (22) in the tube (12). The broader the capillary stop (14), the bigger the change in pressure to overcome the stop (14) will be. Therefore, assuming a series of capillary stops having increasing widths, the width of the capillary stop can be used as a further indication of which stop the meniscus (24) has reached.

The apparatus shown in Figure 2 may also be used in the following way. Again, the liquid wets the material of the tube, so the capillary stops are in the form of an increase in the minimum dimension of the tube. The capillary tube (12) is filled from the inlet (38), as before, using a pressure P in pressure supply tube (26) of value PI relative to the pressure in a reservoir communicating with the inlet (38) (not shown). PI is sufficiently low such that the liquid flows past the capillary stops (14a) to (14c). The liquid may be flowed to an arbitrary point in the capillary tube (12) beyond the last capillary stop (14d), or it may be halted at a capillary stop by observation of the pressure rise in the tube (26) when the capillary stop is reached. The last capillary stop in the tube, shown as (14d) in fig. 2, might require a greater pressure to move the meniscus past it than do the others, so by setting the pressure P such that stops (14a) to (14c) are passed but (14d) is not, the process will be self-regulating and continuous monitoring of the pressure is not necessary.

Once the device has been filled, pressure P is raised to P2 > PI, sufficient to expel liquid from the tube (i.e., it will cause liquid to flow in the main part of the tube but not past the capillary stop). This will dispense the volume of liquid held between the stops (14c) and (14d). In order to overcome the next capillary stop (14c), the pressure is raised transiently to P3 > P2, sufficient to overcome the stop action, and then returned to P2. Liquid between stops (14b) and (14c) will then be dispensed. The process can be repeated for as many capillary stops are provided in the capillary tube (12).

If the liquid does not wet the material of the tube, then the same principles as above can be applied, except that the capillary stops are now formed by a reduction in the maximum dimension of the tube. Increased pressure is required to force the meniscus past the capillary stop in the same way as before.

Figure 3 shows a further embodiment of the invention. This embodiment includes a capillary tube (12) having a constriction formed therein which acts as a capillary stop (14). The capillary tube (12) also has an orifice (36) formed therein. The orifice (36) is formed just below the capillary stop (14) and leads to an overflow tube (30) so as to make a provision for accommodation of overflow liquid (22) once the capillary stop (14) has been reached. The initial portion of the overflow tube (30) (where the overflow tube (30) meets the capillary tube (12)) is perpendicular to the capillary tube (12). An overflow valve (32) is also placed in the overflow tube (30). A capillary tube valve (28) is placed in the capillary tube (12) above the capillary stop (14). The capillary tube then feeds into a further tube (26).

The capillary tube (12) is filled with liquid by low pressure in the tube (26) and in the overflow tube (30), with valves (28) and (32) open. The capillarity of the tubes is arranged such that capillary tube (12) fills more readily than the overflow tube (30), which again fills more readily than the liquid (22) can move past the capillary stop (14). The apparatus is designed for liquid (22) which does not wet the tube material, so the minimum dimension of the capillary tube (12) is greater than that of the overflow tube (30), which in turn is greater than the width of the capillary stop region (14). This being the case, the capillary tube (12) fills with liquid until the meniscus (24) reaches the capillary stop (14). Liquid (22) then flows through the orifice (36) at which point previously a meniscus formed but, the capillarity of the overflow tube (30) being lower than that of the capillary tube (12), liquid did not flow further into the overflow tube (30). At an arbitrary point in the overflow tube (30), before the meniscus (34) reaches the overflow valve (32), the valves (28) and (32) are closed. This leaves a measured volume of liquid (22) in the capillary tube (12).

To dispense liquid, the capillary tube valve (28) is opened, and pressure applied to the liquid so that only the liquid in tube (12) is dispensed, the liquid in the overflow tube (30) being held in place, and a new meniscus formed at the orifice (36). The remaining liquid in the overflow tube (30) is then cleared to waste, and a washing operation performed by sucking wash liquid past the capillary stop (14), and past the point where liquid had previously reached in the overflow tube (30). This latter point might be determined by using a second capillary stop in the overflow tube (30), and detecting a pressure change in the overflow tube when liquid reaches this.

Figure 4 shows a further embodiment of the principle of providing an overflow channel for any excess liquid (22) to flow along. In this case, the capillary tube (12) branches into two separate but parallel tubes (26,30). The point at which the tubes (26,30) meet the capillary tube (12) acts as a capillary stop (14). The capillary tube (12) is filled with liquid (22) by applying suction to the tubes (26) and (30), until the meniscus reaches the capillary stop (14) at position (24). Any further suction leads the meniscus to contact orifice (36) at the entrance of the tube (30). This has a greater minimum dimension than tube (26), and therefore any excess liquid above the maximal position in the capillary tube (12), set by the design of the capillary stop (14) and the precise position of orifice (36), will fill preferentially into the overflow tube (30). Suction can then be stopped at an arbitrary point. Dispensing of liquid is then done by closing a valve (not shown) to isolate the overflow tube (30), and applying positive pressure to the liquid in tube (26). Only the liquid in the capillary tube (12) will be dispensed. Washing can be accomplished by a sequence of suction operations with valves to tubes (26) and (30) opened and closed as appropriate. Alternatively, the contact of the meniscus with the orifice of the overflow tube (30) will cause a rise in pressure in the overflow tube, which can be used to control the filling process.

Figure 5 shows a further embodiment of the invention which is aimed at metering part of a sample. This might be used in, for example, a blood test device. A metering tube or channel (12) has a tapered filling inlet (38) at one end and a capillary stop (14) at the other. A reservoir (40) connects with the tube (12) above the capillary stop (14) and has a closeable vent (42) at the opposite end of the reservoir from the tube (12). A tube (26) is situated below the capillary stop (14), and connects to the rest of the device. The capillarity of the various sections of the device is as follows: the tube (12) > reservoir (40) > inlet (38) > capillary stop (14).

When liquid (22) is introduced to inlet (38) it fills into the tube (12) until it reaches the capillary stop (14). Any excess liquid remaimng in the inlet (38) is drawn into the reservoir (40) by its capillarity being greater than that of the inlet. The vent (42) is open to allow escape of air from the reservoir (40). When the meniscus reaches position (44) at the junction of the tube (12) and the inlet (38), capillary forces at position (44) oppose further flow into the reservoir (40) and flow stops. This results in a metered amount of liquid in the tube (12). The metered amount of liquid is then dispensed into the rest of the device via tube (26), by closing vent (42) to trap liquid in the reservoir (40), and then by applying a pressure difference (for example by suction in tube (26)) to the column of liquid in tube (12) to allow it to overcome the capillary stop.

Figures 6a and 6b show a further embodiment of the invention (50) designed to achieve filling of a capillary fill port (60) with a measured amount of liquid. The metering apparatus (50) is formed in the surface of a substrate (52). An application recess (54) communicates with a metering channel (56) and one or more overflow channels (58). The channel (56) has at its other end a capillary stop (14), past which the channel leads to a capillary fill port (60). Capillarities in the design are such that the capillarity of the capillary stop (14) < application recess (54) < that of the overflow channel (58) < the metering channel (56) < the capillary fill port (60). This means that when liquid is applied to recess (54), it flows preferentially into channel (56) until it reaches the capillary stop, with meniscus position (24), then into overflow channel(s) (58), leaving a meniscus that may lie within the region of the application recess as shown in Figure 6, or may preferably be at the ends of the channels where they open into the recess. This leaves a metered amount of liquid in channel (56) and the remainder in channel(s) 58. Application of positive pressure to the application recess (54), for example by pressing a semi gas-tight cover placed over the recess, moves the meniscus (24) past the capillary stop (14), allowing channel (12) to empty into the port (60), and moving the meniscus(es) (62) further into the overflow channels. Thereby a metered amount of material is introduced to port (60).

An advantage of the apparatus is that it is simple, can handle variations in fill rate for a number of capillaries in parallel, and requires only minimal changes to presently used fluid handling apparatus. The viscosity of the liquid will also affect the capillary stop behaviour, in that with a high viscosity liquid the pressure difference between that needed to aspirate the liquid and that needed to overcome the stop will be reduced. In general, although robotic liquid handling uses liquids with viscosities close to that of water, problems with viscosity are not likely to be encountered.

The advantage of the invention over the prior art is that the manifold pressure and time it is applied do not control the volume of liquid filled into the tubes, and neither does the time taken for the tubes to fill, which depends also on the viscosity of the liquid. Therefore the degree of control of these variables, and possible errors from variation in them, are reduced. This is particularly valuable when many tubes are being filled simultaneously from a common manifold.

Claims

1. A dispenser (10) for dispensing a known volume of liquid (22), the dispenser comprising a pathway (12) for receiving the liquid, at least one capillary arrester (14) in said pathway and located to define a limit of said volume, pressure means

(18) for establishing a pressure differential across said arrester so that, in use, liquid is drawn into the pathway, and release means to dispense said volume of liquid.

2. A dispenser (10) according to claim 1 wherein the volume of liquid (22) dispensed is less than a microlitre.

3. A dispenser (10) according to claim 1 wherein the volume of liquid (22) dispensed is less than a nanolitre.

4. A dispenser (10) according to claim 3 wherein the pressure means (18) is a pump.

5. A dispenser (10) according to any previous claim wherein the release means is a pump.

6. A dispenser (10) according to any previous claim further including a manifold (16) disposed between the pressure means (18) and the pathway (12).

7. A dispenser (10) according to any previous claim further including a pressure monitoring means (20).

8. A dispenser (10) according to claim 7 wherein the pressure monitoring means (20) is disposed in a pathway between the manifold (16) and the pressure means (18).

9. A dispenser (10) according to any previous claim wherein the at least one capillary arrester (14) comprises an increase in the minimum dimension of the pathway

(12).

10. A dispenser (10) according to claims 1 to 8 wherein the at least one capillary arrester (14) comprises a decrease in the minimum dimension of the pathway (12).

11. A dispenser (10) according to claims 1 to 8 wherein the at least one capillary arrester (14) comprises a change in the wetting properties of the pathway (12).

12. A dispenser (10) according to claim 11 wherein the pathway (12) includes a region of substantially hydrophobic material.

13. A dispenser (10) according to claim 12 wherein the pathway (12) includes a region of substantially hydrophilic material.

14. A dispenser (10) further including an overflow channel (30) for receiving excess liquid (22).

15. A dispenser (10) according to claim 14 further including a valve (28) disposed in the pathway (12).

16. A dispenser (10) according to claim 14 or claim 15 further including a valve (32) disposed in the overflow channel (30).

17. A dispenser (10) according to any previous claim wherein the pathway is formed in a planar substrate.

18. A method of dispensing a known volume of liquid using the apparatus claimed in claims 1 to 17, the method comprising the steps of: drawing liquid (22) into the pathway (12) until the liquid reaches a capillary arrester (14); and applying sufficient pressure to the liquid to eject a known volume of liquid from the pathway (12).

19. A method of dispensing a plurality of known volumes of a liquid using the apparatus claimed in claims 1 to 17, the method comprising the steps of: drawing liquid (22) into the pathway (12) until the liquid reaches a capillary arrester (14); increasing the suction so that the liquid flows past said capillary arrester (14); detecting the change in pressure to indicate that the said known volumes of liquid have been drawn into the pathway; and applying pressure pulses to eject said known volumes of liquid from the pathway (12).

20. A dispenser (10) substantially as described herein with reference to Figures 1 to 6 of the accompanying drawing.