EP1099483A1

EP1099483A1 - Liquid droplet dispensing

Info

Publication number: EP1099483A1
Application number: EP00650123A
Authority: EP
Inventors: Igor Shvets; Sergei Makarov; Juergen Osing
Original assignee: Allegro Technologies Ltd
Current assignee: Allegro Technologies Ltd
Priority date: 1999-11-11
Filing date: 2000-09-04
Publication date: 2001-05-16
Anticipated expiration: 2020-09-04
Also published as: EP1099483B1

Abstract

A dispensing assembly for liquid droplets which comprises a dispenser (60) connected to a liquid carrying pipe which in turn is connected to a source of pressurised liquid. The dispenser (60) has an elongate body member (41) having a main bore (42) connected to the liquid carrying pipe. At the other end, the main bore (42) has a valve seat (43) proud of the base (49) of the body member (41) terminating in a dispensing tip (46). The valve seat 43 is the upper part of a nozzle formed by a capillary tube 64. A valve boss (61) of a ferromagnetic material covered with one or more layers of soft polymer or polymers (62) is mounted in the main bore (42) and has a cross-sectional area considerably less than that of the main bore (42). A separate valve boss actuating coil assembly comprising upper and lower coils (50) and (51) is provided which coils (50, 51) are separate from the main body (41) which can be unplugged from both the coils (50) and (51) and from the liquid carrying pipe . It is thus a disposable body member (41). In operation the valve boss is accelerated away from the initial position with the valve closed, by the coils (50) and (51) typical for a time of the order of 0.2 to 0.5 ms. The valve is then maintained open and the current in the coils is considerably reduced. The duration of this second phase determines the volume of the droplet dispensed. The second phase can be of duration of 0.1 to 5 ms, which will result in a volume of droplet dispensed in the range of 100 nl to some microlitres. Then in the third phase the valve is closed with a short pulse of a high current, typically in the range of 0.2 to 0.4 ms. The fourth phase is maintaining the valve in the closed position until the next droplet has to be dispensed. There is further disclosed various electrodes to provide drop-off of any droplet on the tip (46) of the capillary tube (64) . Deflection electrodes are also described in conjunction with various substrates and various means for measuring the volume of droplet is also described. A floating valve boss is not essential for the operation of the invention and other types of valve may be used.

Description

Introduction

The present invention relates to a dispensing assembly for liquid droplets of the type comprising a dispenser, having a main bore communicating with the nozzle having a nozzle bore terminating in a dispensing tip and delivery means for moving liquid to the dispenser and from there through the bore to form a droplet on the exterior of the tip and then to cause a droplet to fall off therefrom. The invention is further concerned with a method of dispensing a droplet from such a dispenser.
The present invention is generally related to liquid handling systems and in particular to systems for dispensing and aspirating small volumes of reagents. It is particularly directed to a high throughput screening, polymerase chain reaction (PCR), combinatorial chemistry, microarraying, medical diagnostics and other similar tasks. In the area of high throughput screening, PCR and combinatorial chemistry, the typical application for such a fluid handling system is in dispensing small volumes of reagents, e.g. 1 ml and smaller and in particular volumes around 1 microliter and smaller. It is also directed to the aspiration of volumes from sample wells so that the reagents can be transported between the wells. The invention relates also to microarray technology, a recent advance in the field of high throughput screening. Microarray technology is being used for applications such as DNA arrays: in this technology the arrays are created on glass or polymer slides. The fluid handling system for this technology is directed to dispensing consistent droplets of reagents of submicrolitre volume.
Development of instrumentation for dispensing of minute volumes of liquids has been an important area of technological progress for some time. Numerous devices for controlled dispensing of small volumes of liquids (in the range of 1µl and smaller) for ink jet printing application have been developed over the past twenty five years. More recently, a wide range of new areas of applications has emerged for devices handling liquids in the low microlitre range. These are discussed for example in "Analytical Chemistry" [A.J. Bard, Integrated chemical systems, Wiley-lnterscience Pbl, 1994], and "Biomedical Applications [A.G. Graig, J.D. Hoheeisel, Automation, Series Methods in Microbiology, vol 28, Academic Press, 1999].
The present invention is also directed to medical diagnostics e.g. for printing reagents on a substrate covered with bodily fluids for subsequent analysis or alternatively for printing bodily fluids on substrates.
The requirements of a dispensing system vary significantly depending on the application. For example, the main requirement of a dispensing system for ink jet applications is to deliver droplets of a fixed volume with a high repetition rate. The separation between individual nozzles should be as small as possible so that many nozzles can be accommodated on a single printing cartridge. On the other hand in this application the task is simplified by the fact that the mechanical properties of the liquid dispensed namely ink are well defined and consistent. Also in most cases the device used in the ink jet applications does not need to aspire the liquid through the nozzle for the cartridge refill.
For biomedical applications such as High Throughput Screening (HTS) the requirements imposed on a dispensing system are completely different. The system should be capable of handling a variety of reagents with different mechanical properties e.g. viscosity. Usually these systems should also be capable of aspiring the reagents through the nozzle from a well. On the other hand there is not such a demanding requirement for the high repetition rate of drops as in ink jet applications. Another requirement in the HTS applications is that cross contamination, between different wells served by the same dispensing device, be avoided as much as possible.
The most common method of liquid handling for the HTS applications is based on a positive displacement pump such as described in US Patent Specification No. US 5,744,099 (Chase et al). The pump consists of a syringe with a plunger driven by a motor , usually a stepper or servo-motor. The syringe is usually connected to the nozzle of the liquid handling system by means of a flexible polymer tubing. The nozzle is typically attached to an arm of a robotic system which carries it between different wells for aspiring and dispensing the liquids. The syringe is filled with a system liquid such as water. The system liquid continuously extends through the flexible tubing into the nozzle down towards the tip. The liquid reagent which needs to be dispensed, fills up into the nozzle from the tip. In order to avoid mixing of the system liquid and the reagent and therefore cross-contamination, an air bubble or bubble of another gas is usually left between them. In order to dispense the reagent from the nozzle, the plunger of the syringe is displaced. Suppose this displacement expels the volume ΔV of the system liquid water from the syringe. The front end of the system liquid filling the nozzle is displaced along with it. The system liquid is virtually incompressible. If the inner volume within the flexible tubing remains unchanged, then the volume ΔV displaced from the syringe equals the volume displaced by the moving front of the system liquid in the nozzle. If the volume of the air bubble is small it is possible to ignore the variations of the bubble's volume as the plunger of the syringe moves. Thus the back end of the reagent is displaced by the same volume ΔV in the nozzle, and therefore the volume ejected from the tip is the same ΔV. This is the principle of operation of such a pump. The pump works sufficiently accurately if the volume ΔV is much greater than the volume of the air bubble. In practice, the volume of the air bubble changes as the plunger of the syringe moves. Indeed in order to eject a drop from the tip, the pressure in the tubing should exceed the atmospheric pressure by an amount determined by the surface tension acting on the drop before it detaches from the nozzle. This is discussed in more detail below. Therefore at the moment of ejection the pressure in the tubing increases and after the ejection, it decreases. As common gasses are compressible, the volume of the air or gas bubble changes during the ejection of the droplet and this adds to the error of the accuracy of the system. The smaller the volume of the air bubble, the smaller is the expected error. In other words the accuracy is determined significantly by the ratio of the volumes of the air bubble and the liquid droplet. The smaller this ratio is the better the accuracy. For practical reasons it is difficult to reduce the volume of the air or gas bubble to below some one or two microlitres and usually it is considerably greater than this. Therefore, this method with two liquids separated by an air or gas bubble and based on a positive displacement pump is not well suited for dispensing a small volume of the order of 1 microlitre or lower. There are also additional limitations on accuracy when sub-microlitre volumes need to be dispensed. For example, as the arm of the robotic system moves over the target wells, the flexible tubing filled with the system liquid bends and consequently its inner volume changes. Therefore, as the arm moves, the front end of the system liquid in the nozzle moves to some extent even if the plunger of the syringe does not. This adds to the error of the volume dispensed. Other limitations are discussed in Graig et al referred to above. Examples of such positive displacement pumps are shown in US Patent Specification No. 5744099 (Chase et al). Similarly the problems of dispensing drops of small volume are also described and discussed in U.S. Patent Specification Nos. 4574850 (Davis) and 5035150 (Tomkins).
Dispensing of drops of liquids using a conventional solenoid valve is well known. It has been used in ink printing applications for more than a decade (Technical Handbook, The Lee Company, 6^th edition, 2 Pettipaug Road, P.O. Box 424, Westbrook, Connecticut, 06498-0424, 1994).
U.S. Patent Specification No. 5741554 (Tisone) describes another method of dispensing submicrolitre volumes of fluids for biomedical application and in particular for depositing bodily fluids and reagents on diagnostic test strips. This method combines a positive displacement pump and a conventional solenoid valve. The positive displacement pump is a syringe pump filled with a fluid to be dispensed. The pump is connected to a tubing. At the other end of the tubing there is a solenoid valve located close to the ejection nozzle. The tubing is also filled with the fluid to be dispensed. In this method the piston of the pump is driven by a motor with a well-defined constant speed. The speed determines the flow rate of the fluid from the nozzle provided the solenoid valve is opened frequently enough and the duty cycle between opening and closing of the valve is long enough. The solenoid valve is actuated with a defined repetition rate. The repetition rate of the valve and the flow rate of the pump determine the size of each drop. For example, if the pump operates at a flow rate of 1 µl per second and the repetition rate is 100 open-close cycles per second, then the size of each drop is 10 nl. This method is suitable for dispensing of large number of identical droplets. However, for dispensing of liquids for HTS applications, this method is often inappropriate since it is commonly required to aspire a liquid through the nozzle in small quantities (say 1 µl) and then dispense it in fractions of this quantity, say in a series of only five drops or even a single drop on demand. To avoid mixing of the fluid aspired with the one in the syringe pump, it is probably necessary to place a bubble of gas in the tube with the attendant problems described above.
Without such a bubble, if the solenoid valve open time and/or operating frequency are too small for a given pump flow rate, the pressure in the dispenser will become too great, causing possible rupture or malfunctioning of the system. Another disadvantage of this solution is that the heat from the coil actuating the plunger of the valve may cause the heating effect of the liquid in the valve that can be a serious problem for some applications.
As the solenoid valve is normally not used as a disposable element due to its high cost, the used portion or potentially contaminated chamber of the valve needs to be washed frequently to avoid cross contamination. This is a major issue for HTS applications and microarraying as the dispenser typically switches from one liquid to another up to several times a minute. The fluid path in the valve is torturous, the valve contains a number of parts and pockets where the contamination can build up complicating the cleaning routine.
Various attempts in the past have been made to address the problem of such conventional solenoid valves. A typical example of these is the invention described in PCT Patent Specification No. WO 99/42752 (Labudde). This patent specification discusses the problems associated with using conventional solenoid valves for many HTS applications. Various solutions to the problem are proposed. None of these unfortunately overcome the major problems of the use of conventional solenoid valves for these applications. Relatively complex constructions of actuator and plungers with diaphragms are described.
This patent specification, for example, attempts to tackle the problem of bubble formation in the valve and aspiration. WO 99/42752 (Labudde) discusses in some detail the problems relating to the structure and geometry of the valve. The solution proposed is to design a "non torturous" flow path for the liquid. In this patent specification, the effect of the use of a blunt or rounded valve seat is discussed as in the effect of the area of the valve seat orifice opening. This specification discloses a valve seat with an internal diameter of the order of 7.5 mm. Further, in this latter patent specification, the plunger of the valve is attached to a diaphragm limiting its movement. The displacement of the plunger between the open and closed position is of the order of 50 µm. There is also a discussion in this patent specification of the heating effects and a solution is proposed by separating the actuating coil from the valve. US Patent Specification No. 5,741,554 (Tisone) again describes substantially the same construction.
Patent Specification No. WO 98/52640 (Shalom) describes a flow control device for medical infusion systems. These systems are used for the slow injection of relatively large volumes, namely millilitres up to a litre, into a patient over a relatively long period of minutes, if not hours. Essentially, there is described a system for the slow injection of fluids into a patient with real time control of the process. The system uses valves to mix or select fluids coming from a number of inlets and to route their flow via selected outlets. Nothing in the description would lead one to believe that such a valve would be suitable for the dispensing of droplets of liquid with volumes of the order of 5 nl at a high frequency. In this patent specification, there is illustrated an actuation coil embedded in the body of the valve and the use of spherical magnetic bosses or a multiple of bosses, as described, is to increase the resistance of fluid flow through the valve.
US Patent No 5,758,666 (Carl O. Larson, Jr. et al) describes a surgically implantable reciprocating pump having a floating piston made of a permanent magnetic material and incorporating a check valve. The piston can be moved by means of energising the coils in a suitable timing sequence. The piston allows the flow of liquid through it when it moves in one direction as the check valve is open, and when it moves in the opposite direction, the check valve is closed and the liquid is pumped by the piston.
US Patent No 4,541,787 (Sanford D. DeLong) describes an electromagnetic reciprocating pump with a "magnetically responsive" piston so called as it contains some ferromagnetic material. The piston is actuated by at least two coils located outside the cylinder containing the piston. The coils are energised by a current with a required timing.
Drops of microlitre volume and smaller can be also generated by the method of electrospray which is mainly used for injection of a fluid into a chemical analysis system such as a mass spectrometer. In most cases the desired output of an electrospray system is not a stream of small drops but rather of ionised molecules. The method is based on supplying a liquid under pressure through a capillary tube towards its end or tip and then a strong electrostatic field is generated at the tip by applying a high voltage, typically over 400V, between the tip of the capillary and a conductor placed close to it. A charged volume of fluid at the tip of the capillary is repelled from the rest of the capillary by Coulomb interaction as they are charged with the like charge. This forms a flow of charged particles and ions in the shape of a cone with the apex at the tip of the capillary. A typical electrospray application is described in US Patent Specification No. 5115131 (Jorgenson et al). There are inventions where the droplets emitted from a capillary are charged in order to prevent them from coming together with coagulation. This approach is described in US Pat No 5,891,212 (Tang et al) for the fabrication of uniform charged spheres. US Pat. No 4,302,166 (Fulwyler et al) teaches how to handle uniform particles each containing a core of one liquid and a solidified sheath. In this latter invention, the electric field is applied in a similar way to keep the particles away from each other until the sheath of the particles has solidified. In this invention the particles are formed from a jet by applying a periodic disturbance to the jet. US Pat. No 4,956,128 (Martin Hommel et al) teaches how to dispense uniform droplets and convert these into microcapsules. A syringe pump supplies the fluid into a capillary. A series of high voltage pulses is applied to the capillary. The size of the droplets is determined by the supply of fluid through the capillary and the repetition rate of the high voltage pulses. The specification does not discuss generation of a single drop on demand. US Pat. No 5,639,467 (Dorian et al) teaches a method of coating of substrates with a uniform layer of biological material. A droplet generator is employed which consists of a pressurised container connected to a capillary. A high constant voltage is applied between the capillary and a receiving gelling solution.
There are numerous methods for ink jet dispensing . The ink jet printing industry is the main driving force in the continuing progress in this field. Some of the well known methods are listed below:
a) One of the oldest methods of creating separated and uniform droplets is based on breaking a jet of liquid emerging from the nozzle. To control the breaking up of the jet into separated droplets periodical vibrations are applied to the jet of liquid. The optimal frequency F of such vibrations was estimated by Lord Rayleigh over a hundred years ago: F = V4.51d where
V - emerging jet velocity
d - jet diameter.
b) In numerous implementations of ink jet printing, pressure waves inside a liquid-holding chamber are created by a piezoelectric actuator. Accelerated by pressure waves, the liquid in the chamber achieves sufficient speed to move through the nozzle and to overcome capillary forces at the tip. In such a case a small droplet will be formed.
c) According to one method, the piezoelectric transducer changes the volume of a container and creates pressure waves in the liquid in the container. The action of compression wave causes some amount of the liquid (ink) to go through the nozzle and to form droplets which are separated from the bulk liquid in the container, see for example U.S. Patent No. 5,508,726 (Sugahara).
d) In U.S. Patent No. 5,491,500 (Inui) an ink jet head is described where liquid in the printing head is "pushed" by progressive waves created by a synchronized row of piezoelectric devices. Eventually, liquid in the printing head obtains enough speed to spray sequences of droplets through the nozzle.
In the methods b) to d) listed above it is necessary to have liquid without vapor and bubbles. Droplet viscosity and surface tension are very important. In the b) and c) cases droplets can be only of a fixed size.
In summary, the most common method of handling reagents used in HTS applications is based on a positive displacement pump and a gas bubble. The problem is that when dispensing volumes of reagents around 1 microlitre or smaller the variation in the volume of the bubble during the dispensation compromises the accuracy. It has been found difficult to eject small droplets of precisely required volume using this method.
A solenoid valve has two main disadvantages when used for HTS applications. The first one is the relatively high cost of a solenoid valve such that it cannot be a disposable element and thus cross contamination can be a major problem. Further difficulties have been experienced in achieving dead volumes smaller than 1 to 2 microlitres in a conventional solenoid valve.
Piezo dispensers while used are often not well suited for dispensing reagents for medical applications. The reason is that the piezo dispenser commonly requires that fluid to be dispensed has well-defined and consistent properties. Unfortunately, reagents and bodily fluids used in medical and biomedical applications have broadly varying properties and often contain particles and inhomogenities which can block the nozzle of the piezo dispenser. At the same time, what must be appreciated is that everybody in this field is aware that if solenoid valves could be used, they would have enormous commercial and technical advantages. They would have considerable advantages over positive displacement pumps, piezo dispensers and various contact mode techniques.
As the size of wells becomes smaller and smaller, the problem of missing the correct well or dropping the liquid reagent at the wrong place of the substrate on which the reagent is being deposited becomes more and more significant. Measurement of the volume of the drops dispensed in the submicrolitre range is a formidable task. It would be a highly desired and valuable feature of a liquid handling instrument to be capable of measurement of volume of individual droplets especially in the submicrolitre range, and also measurement of the dispensation event which will allow excluding missing a drop.
US Patent No. 5,559,339 (Domanik) teaches a method for verifying a dispensing of a fluid from a dispenser nozzle. The method is based on coupling of electromagnetic radiation which is usually light from a source to a receiver. As a droplet of fluid travels from the nozzle it obstructs the coupling and therefore the intensity of the signal detected by the receiver is reduced. The mechanism of such an obstruction is absorption of electromagnetic radiation by the droplet. The disadvantage of this method is that the smaller the size of the droplet, the smaller is the absorption in it. Almost certainly the method will not work for fluids which do not absorb the radiation. For a range of applications such as high throughput screening where minute droplets of fluids with a broad range of optical properties need to be dispensed the methods disclosed in this specification are inappropriate. Further the specification acknowledges that it will only operate satisfactorily with major droplets.
Also, in biomedical applications such as HTS, as well as being able to handle a variety of reagents with different mechanical properties, for example, viscosity, it can be important to be able to measure the viscosity of such reagents.
In summary, there is a major problem in finding a suitable way of dispensing submicrolitre volumes for applications as described above such as HTS applications. This problem can be said to be currently the bottleneck in changing to assay formats of higher density. Numerous publications in the specialised literature indicate that a technical solution to this problem has not been found so far. For example, according to surveys carried out by the journal Genetic Engineering News (Vol. 20, No. 2, Jan 2000), absence of an adequate technology for low volume liquid dispensing is named as the number one reason preventing researchers from moving to denser microplates.
The present invention is directed towards providing an improved method and apparatus for dispensing of volumes of liquids as small as 10 nl =10^-8l or even smaller, while at the same time it should be possible to dispense larger droplets such as those as large at 10 microlitres or even greater.
Another objective is to provide a method where the quantity of the fluid dispensed can be freely selected by the operator and accurately controlled by the dispensing system. The system should be capable of dispensing a drop of one size followed by a drop of a widely differing size, for example, a 10 nl drop followed by a 500 nl one. This is in contrast to for example ink jet printing where the volume of one dispensation is fixed, and dispensations are only possible in multiples of this quantity.
The invention is also directed towards providing a method where the fluid can be dispensed on demand, i.e. one quantity can be dispensed at a required time as opposed to a series of dispensations with set periodic time intervals between them. Yet, the method should also allow for dispensation of doses with regular intervals between subsequent dispensations, for example, printing with reagents.
Another objective of the present invention is to provide a method and a device suitable for dispensing a fluid from a supply line to a sample well and also for aspiring a fluid from the sample well into the supply line. The device should be able to control accurately the amount of the fluid aspired into the nozzle of the dispenser from a supply well.
Another objective is to provide a low cost front end of the dispensing device called herein the dispenser which could be disposed of when it becomes contaminated namely the part which comes in direct contact with the reagents dispensed. It is an important objective of the invention to provide a dispenser such that the disconnection and replacement is achieved simply such as by an arm of a robot.
Another objective is to provide a method for handling fluids in a robotic system for high throughput screening or microarraying which would be suitable for accurate dispensing and aspiring volumes smaller than the ones obtainable with current positive displacement pumps.
Yet another objective is to provide means of more accurate delivery of a drop of liquid reagent to a correct target well on a substrate and also to improve the accuracy of delivery of the drop to a correct location in a well forming part of a receiving substrate.
Yet another objective is to provide means for directing the doses of fluids into different wells of a sample well plate and means of controlling the delivery address of the dose on the sample well plate to speed up the liquid handling procedure.
Yet another objective of the invention is to reduce "splashing" as the drop arrives at the well.
An additional objective is to improve the operation of the conventional solenoid valve for the dispensing and aspiring of liquids and in particular the invention is directed towards providing specific constructions of such solenoid valve type dispensers.
Another objective of the invention is to provide information if the drop was dispensed or not. It is additionally an objective to measure the volume of the drop which was dispensed. Similarly, the measurement of other properties such as the viscosity of the droplet is desirable

Statements of Invention

According to the invention, there is provided a dispensing assembly for liquid droplets of the type comprising a dispenser having a main bore communicating with a nozzle having a nozzle bore terminating in a dispensing tip, and delivery means for moving liquid to the dispenser and from there through the bore to form a droplet on the exterior of the tip and then to cause the droplet to fall off therefrom, characterised in that:
the delivery means comprises a separate pressurised liquid delivery source for moving pressurized liquid to the dispenser; and
the dispenser is a metering valve dispenser and comprises:
an elongate body member having a base including a valve seat forming an entrance to the nozzle which valve seat projects proud of the base;
a valve boss in the bore, the cross-sectional area of which is sufficiently less than that of the main bore to permit the free passage of liquid therebetween bypassing the valve boss and means for altering the relative positions of the valve boss and the valve seat between an open position with the valve boss spaced-apart from the valve seat and a closed contact position sealing the valve seat and spaced-apart from the base.
Ideally, the valve seat is in the form of a capillary tube projecting proud of the base.
Preferably, the valve boss is covered with a layer of soft polymer.
Ideally, the valve boss is a floating valve boss of a magnetic material and the means for altering the relative position of the valve boss and valve seat comprises a separate valve boss actuating assembly adjacent the body member which may be of a hard magnetic material or manufactured from a flexible polymer bonding magnetic material.
Ideally, the valve boss actuating assembly is an electrical coil surrounding the body member and may be biased into a closed position into engagement with the valve seat by an external magnetic field generated by the actuating coil assembly. Indeed, the actuator coil assembly may comprise two separate sets of coils for moving the valve boss in opposite directions within the body member and may comprise a source of electrical power and a controller for varying the current over time as each droplet is being dispensed.
A valve boss actuating assembly may comprise a permanent magnet and means for moving the magnet along the elongate body member towards and away from the valve seat which magnet may be substantially U-shaped to embrace the body member.
In another embodiment of the invention, the valve actuating assembly comprises a pair of spaced apart magnetizing assemblies each comprising a coil wrapped around a core of soft magnetic material which core may be substantially U-shaped to embrace the body member.
Ideally, the valve boss, the body member and nozzle form the one separate sub assembly releasably detachable from the remainder of the dispenser. Preferably, the valve boss is constructed for limited movement out of line with the main bore longitudinal axis.
The valve boss may be a cylindrical plug which in some instances may have a convex valve seat engaging surface and in some instances may have both upper and lower convex surfaces.
In one embodiment of the invention, an annular rim is formed on the exterior wall of the plug intermediate its ends.
There may be mounted in the main bore, remote from the valve seat, a stop to limit movement of the floating valve boss. Further, the invention comprises means for measuring the velocity of the valve boss and in which when the valve boss actuating assembly comprises an electrical actuating coil surrounding the body member, the means for measuring the velocity of the valve boss comprises a secondary coil adjacent the valve boss and means for measuring the induced voltage in the secondary coil.
In another embodiment of the invention, when the valve boss actuating assembly is an electrical actuating coil surrounding the body member, means are provided to measure the total voltage induced in the actuating coil to provide a measure of the velocity of the valve boss.
In one particular dispensing assembly according to the invention, the body member is a two part body member having an upper portion and a lower portion interconnected in liquid tight manner, the upper portion housing the valve boss which is rigidly mounted therein and the lower portion housing the base and valve seat and in which actuation means are provided for moving the upper and lower portions relative to each other to cause the valve boss to move between the open and closed positions.
In this latter embodiment, the upper and lower portions are connected by a flexible concertina type wall. In another embodiment, when the dispenser is a two part body member having an upper portion and a lower portion, they are telescopically connected in liquid tight manner, the upper portion housing the valve boss which is rigidly mounted therein and the lower portion housing the base and the valve seat and in which means are provided for causing the movement between the open and closed positions.
In another embodiment of the invention, the dispenser comprises a solenoid valve. In one embodiment, the solenoid valve comprises a solenoid and core external of the body member, the core mounting the valve boss on a lower valve boss carrying rod forming an extension of the core. In another embodiment, the rod is a two part rod releasably connected together external of the body member. It has been found preferably that the cross sectional area of the main bore is between 50 to 1500 times greater than that of the nozzle bore, but generally is of the order of 100 times that of the nozzle bore. The nozzle bore diameter is usually between 300 and 75 µm and preferably between 200 and 100 µm. The body member and the nozzle ideally form the one integral moulding of plastics material or are made from stainless steel.
In another embodiment of the invention, the dispensing assembly comprises:
an electrode incorporated in the dispensing tip;
a separate receiving electrode remote from the tip; and
a high voltage source connected to one of the electrodes to provide an electrostatic field therebetween.
The receiving assembly may be below the dispensing tip and a droplet receiving substrate is mounted between the receiving electrode and the dispensing tip.
In another embodiment, when a droplet receiving substrate is mounted below the receiving electrode, the receiving electrode having at least one hole for the droplet to pass through to the receiving substrate
Ideally, there is a plurality of receiving electrodes at least one of which is activated at any time.
In one embodiment of the invention, synchronous indexing means are provided for the dispenser and the receiving electrode for accurate deployment of droplets on the substrate.
In another embodiment, there is more than one receiving electrode forming droplet deflection electrodes which are mounted below the dispensing tip and above the droplets receiving substrate and in which the high voltage source has control means to vary the voltage applied to the deflection electrodes.
Ideally, a detector is provided for sensing the separation of the droplet from the dispening tip which detector may comprise:
a source of electromagnetic radiation;
means for focussing the radiation on the end of the dispensing tip; and
means for collecting the radiation transmitted by a droplet on the dispensing tip.
In this latter embodiment, the source of radiation is mounted within the dispenser nozzle.
Further, the invention provides means for measuring the charge of the droplet which may comprise a standard Faraday Pail or a bottomless Faraday Pail.
Further, the invention provides a method of dispensing a droplet having a volume less than ten micro litres (10µl) from a pressurised liquid delivery source through a metering valve dispenser comprising an elongate body member having a base including a valve seat forming an entrance to the nozzle which valve seat projects proud of the base, a valve boss in the bore, the cross-sectional area of which is sufficiently less than that of the main bore to permit the free passage of liquid therebetween by passing the valve boss and means for altering the relative positions of the valve boss and the valve seat between an open position with the valve boss spaced-apart from the valve seat and to a closed contact position sealing the valve seat and spaced-apart from the base comprising the steps of:
delivering the pressurised liquid to the dispenser;
opening the valve for a preset time to deliver liquid around the valve boss into the nozzle bore; and
closing the valve as the droplet falls off.
In the method, when the valve boss is a floating valve boss of a magnetic material and there is means for moving the valve boss comprising a separate valve boss actuating assembly including an actuating coil adjacent the body member and in which the method includes actuating the assembly by energising the actuating coil..
In this latter embodiment, the speed of the floating boss is obtained by masuring the voltage induced in an actuating coil due to its velocity and magnetisation or may be obtained by measuring the voltage indicated in a coild adjacent the floating boss. The speed of the valve boss may be used to determine the viscosity of the liquid. When there is no electrostatic drop off and there has been dispensed droplets of the order of 100 nl and less, the liquid is pressurised in excess of 5 bar.
However, in many instances, on the valve being shut off, there is performed the step of generating a pulse of voltage at a receiving electrode remote from the dispensing tip to generate an electrostatic field to cause an electrostatic potential between the droplet and the receiving electrode to detach it from the dispensing tip.
In this latter embodiment, the liquid is not as highly pressurised and may be pressurised at less than 4 bar or even as low as 2 bar or less.
In accordance with the method, the receiving electrode is mounted beneath a droplet receiving substrate or between a droplet receiving substrate and the nozzle. Preferably, there are usually two receiving electrodes which receiving electrodes may be moved after each droplet is dispensed to direct the next droplet to another position on the substrate.
In another method according to the invention, spaced-apart deflection electrodes are placed between the dispensing tip and a droplet receiving substrate and the electrodes are differentially charged to cause the droplet to move laterally as it drops from the dispensing tip.
Further, the invention provides a method comprising the steps of:
measuring the volume of a droplet of a particular liquid for different drop off voltages;
storing a database of the measurements;
recording the drop off voltage when a droplet detaches from the dispensing tip; and
retrieving the volume from the database.
The drop off voltage may be measured by a Faraday Pail. To record the drop off of voltage, there is performed the steps of:
directing an electromagnetic beam from a source of electromagnetic
radiation at the droplet as it forms at the tip; and
monitoring the electromagnetic radiation coupled by the droplet at a collector remote from the droplet.
In this method, a light beam may be used as a source of electromagnetic radiation and the amount of light reflected and/or refracted by the droplet is monitored.
Further, the invention provides a method of performing the steps of:
measuring the charge of droplets of a particular liquid for different volumes of droplets;
storing a database of the measurements;
recording the charge on each droplet; and
retrieving the volumes of the drops from the database.
Additionally, in this method, there is provided the steps of:
measuring the width of the voltage pulse in the Faraday pail;
determining the time taken for the droplet to pass through the pail;
deriving the speed of the droplet from the time taken to pass through the pail; and
calculating the mass of the droplet from the charge to mass ratio.

Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which:
Fig. 1 (a) and (b) are diagrammatic views of a positive displacement pump arrangement of the prior art;
Figs. 2 and 3 are diagrammatic views of a dispensing assembly according to the invention;
Fig. 4 illustrates an alternative construction of dispenser;
Figs. 5 and 6 illustrate diagrammatically another alternative construction of dispensing assembly,
Fig. 7 illustrates an alternative construction of dispenser;
Fig. 8 illustrates another construction of dispenser;
Fig. 9 illustrates, in more detail, operation of the dispenser of Fig. 8;
Fig. 10 illustrates another construction of dispenser;
Fig. 11 illustrates portion of another dispenser;
Fig. 12 illustrates portion of a still further dispenser;
Figs. 13 (a) and (b) illustrate a further construction of dispenser in closed and open modes;
Fig. 14 is an illustration of portion of a still further dispenser;
Fig. 15 illustrates another dispensing assembly according to the invention;
Fig. 16 illustrates a still further dispensing assembly;
Fig. 17 illustrates another dispensing assembly;
Fig. 18 is a graph of low pressure droplet formation;
Fig. 19 is a graph of high pressure droplet formation;
Fig. 20 is a graph showing the effect of a droplet volume on the drop-off voltage;
Fig. 21 is a graph of drop-off voltage against distances from tip to an electrode;
Fig. 22 illustrates diagrammatically a test assembly;
Fig. 23 is a graph of the effect of deflection electrode voltage on a droplet deflection;
Fig. 24 illustrates diagrammatically an electromagnetic balance;
Fig. 25 gives the circuit diagram of the electromagnetic balance of Fig. 24;
Figs. 26 to 30 show various droplet drop-off detectors according to the invention,
Fig. 31 records a test to ascertain that the volume of a droplet is related to the electrostatic charge it holds;
Fig. 32 records a similar test to that of Fig. 31 under different conditions;
Fig. 33 shows the effect in a Faraday Pail of a droplet;
Fig. 34 illustrates graphically the noise and sensitivity of one dispensing assembly;
Fig. 35 illustrates an electronic circuit used with a Faraday Pail according to the invention;
Fig. 36 is a diagrammatic view of one form of application of Faraday Pail;
Fig. 37 is a diagrammatic view of another alternative form of application of Faraday Pail;
Fig. 38 is a side view of an alternative construction of dispenser;
Fig. 39 is a plan view of the dispenser of Fig. 38;
Fig. 40 is a sectional view of the dispenser of Fig. 38;
Fig. 41 is a side view of a still further dispenser;
Fig. 42 is a plan view of the dispenser of Fig. 41;
Fig. 43 is a sectional view of the dispenser;
Fig. 44 is a view of another dispenser according to the invention;
Fig. 45 is a view of a still further construction of dispenser;
Fig. 46 is an illustration of a different dispenser;
Fig. 47 is an illustration of a still further dispenser;
Fig. 48 is an illustration of another dispenser;
Fig. 49 is an illustration of a still further dispenser; and
Fig. 50 is an illustration of a different dispenser.
Referring to the drawings and initially to Figs. 1 (a) and (b) there is illustrated the prior art showing a conventional method of liquid droplet production using a positive displacement pump. There is illustrated a motor 1 driving a piston 2 of a positive displacement pump 3 containing a system liquid, namely, water 4 connected by flexible tubing 5 to a robotic arm 6 carrying a nozzle 7 having a tip 8 into which the tubing 5 projects. A reagent 9 is contained in the nozzle 7 adjacent to the tip 8 and separated from the water 4 by a gas bubble 10 see Fig. 1 (b). The motor 1 which is usually a stepper or servo motor will each time move the piston 2 to dispense reagent.
Referring now to Figs 2 and 3 there is illustrated a dispensing assembly for liquid droplets according to the invention, indicated generally by the reference numeral 20. The dispensing assembly 20 comprises a delivery means indicated generally by the reference numeral 21 which, in turn, comprises a pressure source 22 feeding a pressure regulator 23 and a pressure readout device 24 all connected to an electronic controller 25. The pressure readout device 24 in turn feeds through a high pressure airline 26, a switch 27 which is also fed by a vacuum pump 28 and vacuum line 29. The switch 27 is also connected to the electronic controller 25. The switch 27 connects by a further airline 30 to a reagent reservoir 31 which in turn feeds by a liquid carrying pipe 32, a dispenser, indicated generally by the reference numeral 40. Such a reagent reservoir 31 will normally only be used where there is a relatively large amount of reagent been used.
The dispenser 40 is illustrated in more detail in Fig. 3 and comprises of an elongated body member 41 having a main bore 42 connected at one end to the liquid carrying pipe 32. At the other end the main bore has a valve seat 43 in a base 49, connecting to a nozzle 44 having a nozzle bore 45 terminating in a dispensing tip 46. The valve boss 47 of a ferromagnetic material covered with a soft polymer material 48 is mounted in the main bore 42 and has a cross sectional area less than that of the main bore 42. The top or exposed portion of the valve seat 43 is essentially sharp to engage firmly in the soft polymer material 48. The valve seat 43 projects proud of the base 49 such that when in the closed position with the valve boss 47 seated on the valve seat 43, there is a gap between the base 49 and the boss coating 48.
A separate valve boss actuating coil assembly comprising upper and lower coils 50 and 51 respectively are provided separate from the body member 41 and are also connected to the electronic controller 25. As can be seen in Fig. 2 the power source for the coils 50 and 51 is not illustrated.
Again referring to Fig. 2 a droplet receiving substrate 55 usually in the form of a series of wells is mounted below the dispensing tip 46 and above a conducting plate 56. The conducting plate 56 is connected to the electronic controller 25 through a high voltage source 57. Reagent when in the form of droplets is identified by the reference numeral 58 in Fig. 2.
It will be noted that the dispenser 40 is grounded to earth through an earthline 59, in effect making the dispensing tip 46 an electrode. In some cases, the tip may have to be made out of a conducting material such as metal or an electrically conducting polymer.
In operation the reagent is stored in the main bore 42 of the body member 41 and the controller 25 is operated to cause the coils 50 and 51 to be activated to raise the valve boss 47 off the valve seat 43 and to allow the reagent to pass between the valve boss 47 and the walls of the main bore 42 down into the nozzle bore 45 until the coils are activated again to shut off the valve by lowering the valve boss 47. As the valve opens the reagent is supplied to the dispensing tip 46 and the droplet 58 grows. The volume of the droplet 58 is obviously determined by the length of time the valve is open and, the viscosity of the liquid, the cross-sectional area of the nozzle bore, its length and also the pressure exerted on the liquid through the valve from the switch 27. It will be appreciated that if the pressure exerted on the liquid is sufficiently above ambient which is normally atmospheric (1 bar) the droplet will be ejected from the tip 46. However, in many instances, when the pressure is too low or in any case for accuracy, applying a relatively high voltage to the conducting plate 56 will cause an electrostatic field to be exerted between the dispensing tip 46 and the substrate 55 thus causing the droplet 58 to be pulled downwards onto the substrate 55 by a force considerably in excess of gravity.
To aspire reagent from a substrate or indeed from any reagent reservoir or container the vacuum pump 28 is operated and the switch 27 suitably arranged to ensure that the vacuum pump 28 and vacuum line 29 is connected to the dispensing assembly 20. The valve is opened and the liquid sucked up into the dispenser 40.
Referring now to Fig. 4, there is illustrated an alternative construction of dispenser, indicated generally by the reference numeral 60 in which parts similar to those described with reference to Fig. 3 are identified by the same reference numerals. In this embodiment, there is provided a cylindrical valve boss 61 of a ferromagnetic material surround by a polymer coating 62. There is also provided a floating valve boss stop 63 which is mounted in the main bore 42 remote from the base 49. In this embodiment, the nozzle is formed from a capillary tube 64 again forming the valve seat 43 which, it will be noted, projects a distance I above the base 49.
Referring now to Figs 5 and 6 there is illustrated an alternative construction of dispensing assembly. In this embodiment the dispenser is indicated generally by the reference numeral 70 and parts similar to those described in the previous Fig. 3 are identified by the same reference numerals. The only difference between the dispenser 70 and the dispenser 40 is that there is a boss stopper 71 provided in the main bore 42. In this embodiment referring specifically to Fig. 5, the delivery means indicated generally by the reference numeral 72 comprises a positive displacement liquid handling system. There is provided a stepper motor 73 incorporating suitable controls operating a piston 74 of a pump 75 containing water 76 delivered by flexible tubing 77 to the dispenser, air 78 separates the water 76 from reagent 80. The tubing 77 is connected by a suitable seal 79 to the dispenser 70. A pressure sensor 81 is connected to the tubing 77 and to a pump controller 82.
Referring to Fig 7, there is illustrated an alternative construction of a dispenser, indicated generally by the reference numeral 90 in which parts similar to those described in the previous drawings are identified by the same reference numerals. In this embodiment the dispenser 90 includes a cylindrical valve boss 91 of permanent magnetic material surrounded by a polymer coating 92. The polymer coating 92 is thicker adjacent the lower portion to form a convex valve seat engaging surface 93. In this embodiment, the valve seat 43 does not project above the base 49. Again, it will be noted that the cross sectional area of the valve boss 91 with the coating is less than that of the main bore 42. It is advantageous to have the cylinder 91 magnetised along its axis as indicated by the arrow.
Fig. 8 illustrates a dispenser indicated generally by the reference numeral 94 substantially similar to the dispenser 90 of Fig. 7, except that the nozzle is provided by the capillary tube 64 and the valve seat 43 projects above the base 49. There is an upper convex surface 95 as well as the lower convex surface 93 on the valve boss 90 for improved hydrodynamic performance and faster movement.
Fig. 9 illustrates the bottom portion 94 of Fig. 8 and shows the contour of the convex valve seat engaging surface 93 by the subscript (a) when it is still open, by the subscript (b) when it is just about to open and by the subscript (c) when it is fully closed. It will thus be noted that the contour of the convex valve seat engaging surface changes as it closes but it still remains spaced-apart from the base 49. Similarly the shape will change as it opens. On opening there is some movement of the valve boss 90 before the seal on the valve seat 43 is broken. The significance of this is discussed later.
Fig 10 shows another construction of dispenser, identified generally by reference numeral 100, again parts similar to those described in the previous drawings are identified by the same reference numerals. In this embodiment there is provided a valve seat 101 with a sharpened peripheral tip 102 which will engage the polymer coating of 92 of the cylindrical valve boss 91. In this embodiment there is only one coil 50 as the cylindrical valve boss 91 is of a permanent magnetic material. It is advantageous to have the cylindrical valve boss 91 magnetised along its axis as indicated by the arrow.
While all the bosses described above have been either spherical or cylindrical and thus circular in cross-section and similarly all the bores have been circular in cross-section, this is not necessarily essential. For example a boss could be made of any suitable shape. It could be square, rectangular or polygonal in cross-section. Similarly the bore does not have to be circular in cross-section. Nor indeed do both the bore and the boss have to be of the same shape in cross-section.
Essentially the boss can form an elongate plug-like member which effectively is constructed for limited movement at a line with the main bore.
Fig. 11 illustrates portion of an alternative construction of dispenser, indicated generally by the reference numeral 105, in which parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, the body member 41 and nozzle 44 are manufactured from the one piece, for example, of a plastics material.
Fig. 12 illustrates a still further construction of dispenser, indicated generally by the reference numeral 107, in which the body member 41 and the nozzle, which in this case is formed from a capillary tube 64, are again of the one piece moulding. In this embodiment, the base 49 forms a conical inner surface rising towards the valve seat 43 from the sides of the body member 41.
Referring now to Figs 13(a) and 13(b) there is illustrated another dispenser indicated generally by the reference numeral 110 in which parts similar to those described with reference to Fig. 7 are identified by the same reference numerals. This shows clearly the opening and closing of the dispenser 110 together with the direction of the liquid flow around the cylindrical valve boss 91. Two sets of coils 50 and 51 are used though the valve boss 91 is of a permanent magnetic material.
Before discussing in more detail the various dispensing assemblies according to the present invention, it is advantageous to discuss briefly the features of the invention already disclosed and their advantages. It is also advantageous to discuss constructional details before continuing further with the description.
In the embodiments described already, reference has been made to capillary tubes and to the use thereof, and in particular to the size of the main bore and the nozzle bore. Ideally, the cross sectional area of the nozzle bore A_N is significantly smaller than that of the main bore A_M. Accordingly 50 <A_M/A_N < 1500. Indeed, it could be greater than 1500. Ideally, the valve seat should project above the base of the body member of the dispenser such that as, for example, illustrated in Fig. 9 where even with the floating valve boss fully home and pressed against the valve seat 43, there is still a gap between the bottom of the floating boss and the base of the body member. Thus, the length I has to be carefully chosen and depends on various parameters such as the thickness of the coating and its elastic properties. Essentially, the thickness of the coating is determined by the requirement of forming a reliable seal on the valve seat which in turn will depend on the geometry of the valve seat and also the axial alignment of the floating boss on the valve seat which will obviously vary depending on the gap between the valve boss and the inner surface of the main bore and also on the actual construction of the valve boss, namely, where there is a cylindrical valve boss or a spherical valve boss.
While, as described below, in certain circumstances, a conventional solenoid valve can be used in accordance with the invention, there are certain drawbacks in the use of the conventional solenoid valve that has already been discussed which disadvantages are overcome by the use of a floating boss solenoid valve. Ideally, after each aspirate and dispense cycle, the "contaminated" part of the dispenser needs to be washed. In technical terms, this means that the inner area of the dispenser exposed to the liquid being handled is as small as possible and that it does not contain numerous pockets where liquid can accumulate. This, it will be appreciated, can be a major drawback in certain circumstances with a conventional solenoid valve. In a conventional solenoid valve, there are several functional elements exposed to the liquid. The actuator area of the valve is separated from the plunger area by a seal. There is a shaft connecting the plunger to the actuator passing through the seal. The seal, the shaft and the spring pressing against the plunger are the areas where contamination can build up. The situation with the cross contamination in the solenoid valve gets worse with the reduction in the overall size of the valve. The area S exposed to the liquid is proportional to the second power of the characteristic dimension d of the valve: S=Ad². The inner volume V of the valve is proportional to the third power of the dimension: V=Bd³. The ratio of the two, S/V is the essential indicator of the cross-contamination. By dividing the two formulas, one can see that as the size of the solenoid valve gets smaller, the cross-contamination due to the valve will become more significant. It is inversely proportional to the dimension of the valve: S/V=A/(B*d). The floating valve boss addresses this problem. It reduces the number of the functional elements in the dispenser to just two elements by removing the seal, the shaft, the spring and other optional elements. Mathematically, we reduce the coefficient A for the same value of the coefficient B. This leads to a reduction in cross contamination.
A further advantage of the use of a floating boss solenoid valve is that the dead volume inside the dispenser is reduced as is the wastage of reagents. It is important to consider the constraints which are imposed on the diameter of the outlet of the dispenser for it to dispense nanolitre range volumes. Suppose there are two different dispensers having outlets of inner diameters of 2r and 4r respectively. Further presume a straight length of tube with a constant bore is connected to each outlet. Suppose both dispensers are designed for the same flow rate meaning that they can dispense comparable volumes of liquids. The formula for the laminar flow rate Q through the tube (Hagen-Poiseuille law): Q = π*δP*r4/(8*η*L), where

π: is the constant equal to approximately 3.1415926 ...
δP = (P₂-P₁): is pressure difference at the two ends of the tube.
r: is the tube inner radius.
η: is the liquid viscosity.
L: is the tube length.

If the two dispensers are designed for the same flow rate, the length of the tube on the second dispenser must be 2⁴=16 times greater than that of the first one. The volume of the capillary tube which effectively forms the nozzle bore is: Vdead = π*r2*L,
This volume indicates the amount of liquid stored in the nozzle bore which effectively forms an outlet tube. This volume will stay in the outlet tube during the last dispensation to maintain the accuracy of the dispensation and is effectively wasted. Therefore, as the diameter of the second dispenser's outlet is twice that of the first one, and the length is 16 times as large, the dead volume of the second dispenser is 64 times greater. The important conclusion here is that the diameter of the outlet should be as small possible to reduce the dead volume and wastage of the liquids. Diameters considerably smaller than 100 micron are unpractical as the capillaries begin to be blocked by the inhomogenities in the liquids which can often be of the size of some 20-50 microns for some applications. In certain circumstances, this may not be a problem.
There is a critical pressure necessary for the drop off without the use of an electrostatic field as described below with reference to other dispensing assemblies according to the invention. This pressure must be exceeded for reliable dispensing. The value of the critical pressure essentially depends on viscosity and density of liquid, radius of the tube, its length and air-liquid surface tension. We can estimate the critical pressure using a simple model whereby the liquid moves in a capillary which is what the nozzle bore effectively is with a constant velocity V_M due to the pressure difference δP at the two ends: VM = δP*r2/48*η*L where

r: is the tube's inner radius.
η: is the liquid viscosity.
L: is the tube length.

The kinetic energy of the drop must exceed the energy of surface tension for the drop off to occur: VM 2ρ*v/2= σ*πr2 where

v: is the volume of the drop.
σ: is air-liquid surface tension coefficient.
ρ: is the density of liquid.

We assume the drop is a perfect sphere and we can estimate the volume of the drop as πr³. By substituting this into the formulae (3) and (4), the critical pressure needed to separate the droplet from the tip of capillary is: δPcrit ≈ Const*η*(L/r5/2)*(σ/ρ)½,
It may appear at first sight that decreasing the radius r leads to an increase in the critical pressure and therefore is undesirable. However, what must be taken into consideration is that the flow rate is also a function of radius: Q=const*r⁴/L. By substituting this into (5) we get the answer: δP_crit=const*r^3/2. Therefore, for the same flow, it is beneficial to reduce the radius of the tube. By inserting the capillary into the body of the dispenser, and by changing from a large diameter above the seal or valve seat to a lot smaller diameter below the seal in one step, we reduce the value of the critical pressure. These formulas may somewhat change depending on the model used. Still the qualitative conclusion will remain.
Another important reason making it necessary to reduce the diameter of the valve seat outlet into the nozzle bore or capillary is that when particulate reagents are being dispensed, the particles suspended in the liquid can get attached to the inner walls of the dispenser thus blocking the flow path. The problem of blockage can be particularly severe for microvalves, i.e. when dispensing small volumes and also for dispensing of biological liquids and bodily fluids which can normally contain cells, particles etc. Surprisingly, a valve seat of smaller diameter (in the region of some 100-200 µm) will actually make the blockage less likely to occur. This may appear to be contrary to the common logic which would suggest that the smaller the size of the outlet, the greater is the chance of the blockage. The following explanation is advanced for this. The flow rate through a tube is Q=u*π*r2, where
U is the velocity of the flow in the tube.
π*r² is the cross sectional area of the tube.
Suppose, a particle with diameter d is about to be attached to the wall of the tube or the valve seat. The drag force acting of the particles is (Mechanics of Fluids by A.C. Walshaw, D.A. Jobson, Longman Ltd): Fdrag=3d*π*η*u
The smaller the diameter of the valve seat and of the tubing protruding inside the dispenser main bore, the greater is the velocity in the nozzle bore for the same flow rate. Therefore, the smaller the size of the nozzle bore and its outlet, the greater is the drag force on the particles in the flow preventing them from being attached to the wall of the valve seat and the tube.
It is accepted that by reducing the diameter of the valve seat and keeping all the other dimensions unchanged, blockage is more likely to occur. This is because the flow velocity will be decreased as the diameter is reduced. Thus, it is essential when reducing the diameter of the valve seat and hence the nozzle bore, that the length of the nozzle bore decreases to keep the flow rate constant.
One of the major problems which is encountered in accurate metering of nanolitres within a dispenser is the effect of "piston action" by any valve boss against a valve seat. This is illustrated by reference to Fig. 14 which illustrates a dispenser indicated generally by the reference numeral 115 substantially similar to the dispenser 90 disclosed in Fig. 7 where the valve seat 43 does not project above the base 49 of the main body member 41. In this embodiment, the cylindrical boss is illustrated with a polymer coating 92 having a valve seat engaging portion 116 having a dished configuration identified by the reference numeral 117 with subscript (a) and interrupted lines showing the shape of the surface of the bottom of the floating valve boss 91 when the surface 117 first contacts the base 49 and then by a full line and the subscript (b) when it finally is at rest sealing the valve seat 43. When the valve boss 91 first contacts the base 49, liquid is trapped between it and the valve seat 43. As the valve boss 91 continues its movement, the contour of the valve seat engaging portion 116 changes with the surface changing from the configuration identified by the reference numeral 117(a) to the configuration identified by the reference numeral 117(b). Essentially, there is a pumping action on the trapped liquid through the valve seat 43 until the movement is finally stopped. The effect is shown schematically by the arrows. It is important to appreciate, that the liquid does not have to be hermetically trapped for the onset of the piston action. The piston action will occur even if there is a small gap between the surface of the boss 117 and the base 49. This is because the liquid will rush out of this area and the viscosity of the fluid expelled through a small gap will result in an additional pressure appearing at the valve seat 43 and thus in piston action of the boss 91. It is important to appreciate that the piston action will become more significant for dispensing of small volumes. The reason is that in this case, the boss must be actuated as fast as possible. This will result in a greater additional piston action pressure. It should be appreciated that this phenomenon will be present also in many other configurations of the boss thus compromising the accuracy of dispensations. The correct geometry of the valve seat will reduce the piston action significantly some of which are described above. The gap of dimension I in Fig. 9 allows for an easier flow of the liquid and thus reduces the piston action. The solution with the conical inner surface of the base of the dispenser as illustrated in Fig. 12 may also help to reduce the piston action of the boss.
It has been found that with the present invention, the dispensing assembly can operate at a higher pressure than in conventional assemblies. It is suggested that this is due to the fact that the force acting on the boss due to the pressure difference is proportional to area of the outlet, i.e. of the valve seat. As the area of the outlet is reduced to some 10⁴-10⁵ µm², the pressure range at which the boss can still be reliably actuated, increases to over 10 Bar. For comparison, a typical state-of-the art miniature solenoid valve from the Lee company used for the ink jet printing applications, can be used up to the pressure of only approximately 0.7 Bar (Technical Handbook, The Lee Company, 6^th edition, 2 Pettipaug Road, P.O. Box 424, Westbrook, Connecticut, 06498-0424, 1994, page 21). Although as explained above, the critical pressure is decreased as the radius of the tube decreases, still relatively high pressure is required for dispensing of submicrolitre volumes. High pressure is particularly important for dispensing of small volumes of high viscosity liquids. We have established that for accurate dispensation in the volume range of 100 nl and smaller, a pressure in excess of 5 Bar is desirable, if electrostatic drop off, as described below, is not used.
As an aside such information is well known to those skilled in the art and would direct them away from using a state of the art solenoid valve for the dispensing of nanolitre sized drops since they would be aware of the need for such pressures.
As there is a gap between the lower surface of the boss and the base of the body of the dispenser (Fig. 9), the liquid does not rush to fill in this space as the boss is actuated for opening. This movement of liquid otherwise creates negative pressure which sucks the liquid from the tube through the valve seat. This suction action in a conventional solenoid valve reduces the accuracy of the dispensation. Further, this suction action of the valve boss, which is in effect a dynamic negative pressure appearing at the moment of opening slows down the boss. As explained above, the movement of the boss to open the valve does not result in the displacement of the liquid to fill in the space between the boss and the base of the body. As the movement of the liquid is dampened by the presence of the gap, the boss can be actuated faster, making dispensing of smaller volumes more accurate.
A further advantage in the reduction in the cross sectional area of the valve seat and hence the nozzle bore is that it reduces the flow rate of the liquid. The flow rate is proportional to the fourth power of the inner radius r of the tube: Q = π*δP*r4/(8*η*L)
Therefore, by reducing the radius of the valve seat, the flow rare is reduced. This means that the dispenser open time can be made longer, resulting in a more accurate control of the volume dispensed.
Yet a further advantage comes from the fact that the seal is open and closed at a higher velocity of the boss.
The position z of the boss immediately after the moment it is energized, is given by the formula: z = t2*a/2 where
a is the acceleration of the boss.
t is the time after the moment the coil is energized.
By differentiating this formula, we get δz/δt=t*a or δt=δz/t*a= δz/2vb where v_b=t*a is the velocity of the boss. Suppose the position tolerance defining the difference between the dispensers' open and closed states, is Δz. This value is determined by the tolerance of the seal, the mechanical properties of the elastomer on the boss, the smoothness of the elastomeric coating, the hydrodynamics of the viscous flow around the seal etc. It is therefore a characteristics of any given dispenser. We can now see that if v_b increases, the error of the open/close time decreases. As illustrated in Fig. 9, as the valve boss is energised to open, the seal remains closed for certain time after the coil is being actuated and the boss starts moving. Only after a certain duration of time, when the boss passes through the length by which the elastomeric coating on the boss is compressed, the valve seat becomes open. As the boss moves with a constant acceleration during this time, its velocity at the moment of the opening of the seal is a lot higher than the velocity at the moment of the actuation of the coil. The same applies for the closure of the boss.
For the HTS and microarraying applications the essential requirement is to reduce the cross contamination and carry over of the reagents. The dispenser typically aspirates a few microlitres of a reagent, dispenses it in other locations and then moves to a new reagent. Commonly the contaminated part of the dispenser needs to be disposed of after a few cycles of aspiration and dispensation, and in some cases after each cycle. The conventional solenoid valve is a rather complex device designed for hundreds of thousand of open-close cycles. Solenoid valves are not used as disposable elements due to the complexity of design and cost considerations. The simplicity of the present invention addresses and solves this problem making the contaminated part of the dispenser essentially a disposable component. At the same time, in certain situations, conventional solenoid valves with nozzles and valve seats according to the invention are advantageous.
It will be appreciated that with a construction of dispenser according to the present invention, the only moving part is the valve boss with the soft polymer material coating. All the mass of the valve boss can be effected and actuated by the electromagnetic action of the actuating coil assembly. This minimises very much the inertia of the dispenser. This minimisation of inertia leads to a reduction in the open-close cycle thus minimizing the amount of liquid that can be dispensed. This also reduces the heating of the liquid as the boss can be operated at a lower current to achieve the same open-close cycle. The present dispenser can also operate at a significantly higher pressure than a conventional solenoid microvalve, by a factor of over two or three (the typical specifications for the solenoid microvalves used in ink printing can be seen e.g. in the Technical Handbook, The Lee Company, 6^th edition, 2 Pettipaug Road, P.O. Box 424, Westbrook, Connecticut, 06498-0424, 1994 pages 22-23). This increase in the pressure range is also due to the fact that almost the entire boss can be affected by the electromagnet. Operating at a higher pressure may be beneficial for the dispensation of high viscosity liquids and also volumes in the low nanolitre range.
Referring now to Fig. 15 there is illustrated a dispensing assembly indicated generally by the reference numeral 120 incorporating a dispenser 40 as described above with reference to Figs. 2 and 3. In this embodiment the droplets are identified by the numeral 58 and successive subscripts thus 58(a) to 58 (c). The dispensing tip 46 effectively forms or incorporates an electrode by virtue of being grounded by the earth line 59. There is mounted below the dispenser 40 a receiving substrate 121 incorporating reagent wells 122. For three of the wells 122 a, b and c there are, for simplicity identified by the same subscript letters, droplets 58 a, b and c both approaching the wells 122 and in them. Positioned below the receiving substrate 121 is a receiving electrode 123 in turn mounted on an indexing table 124. The receiving electrode 123 is connected to a high voltage source 125.
The indexing table 124 is used to position the receiving electrode 123 below the appropriate reagent well 122 as shown by the interrupted lines in the drawing.
Referring now to Fig. 16 there is illustrated an alternative construction of dispensing assembly, indicated generally by the reference numeral 130 in which parts similar to those described in Fig. 15 are identified by the same reference numerals. In this embodiment there is provided a plurality of receiving electrodes 131 on the indexing table 124, which are individually connected to the high voltage source 125.
Referring now to Fig. 17 there is illustrated still further construction of dispensing assembly indicated generally by the reference numeral 140 in which parts similar to those described with reference to Fig. 15 are identified by the same reference numerals. In this embodiment there are provided additional deflecting electrodes 141 and 142. It will be appreciated that depending on the voltage on the deflecting electrodes 141 and 142, the droplets 58 will in conjunction with the receiving electrodes 123 navigate into the appropriate reagent well 122. This is illustrated clearly in Fig. 17 by the interrupted lines. In Fig. 17 there is also shown a receiving electrode 123 but it will be appreciated that such a receiving electrode 123 will not always be necessary. It is also possible to use a conducting plate such as illustrated in Fig. 2 or it is possible to use only deflecting electrodes. However, what will be appreciated by consideration of the dispensing assemblies as illustrated in Figs. 15 to 17 inclusive is that electrostatic navigation of the drops by means of both the receiving electrodes and the deflecting electrodes can be relatively easily achieved. For example, the receiving electrode could be in the form of a plate having at least one hole to allow a droplet pass therethrough.

Before discussing in any more detail certain other aspects of the present invention it is necessary to discuss in some detail the nature of droplet formation, the effect of the electrostatic field on its drop-off from a dispensing tip and the various other factors that govern the volume of the droplet and its formation.

Test No. 1.
Liquid	Water
Temperature	20°C
Delivery pressure	1 Bar (15 psi)
Valve boss	Samarium Cobalt permanent magnet, magnetised along its axis
	Length	5.5mm
	Diameter	1.8mm
	Lower valve seat contacting side -nitrile rubber 1 mm thick
Actuating coil resistance	30Ohm
Nozzle	Length	35mm
Internal diameter	100 micron
Outside diameter	170 micron

In this experiment the pressure was not sufficiently high to eject the droplet from the nozzle and a grown drop remained on the dispensing nozzle. Tolerance for the drop volume was ± 1nl. The drop volume was measured by transferring the drop grown to a calibrated capillary.

Activation phases:

Phase 1 (strong force to open the valve quickly)
Voltage 22 V
Duration 0.2 to 0.5ms
Phase 2 (no applied force).
Voltage 0V
Duration 0.1 to 1ms
Phase 3 (strong force to close the valve quickly)
Voltage 22 V
Duration 0.2 to 0.4ms
Phase 4 (small force to keep the valve closed to prevent leakage and dump oscillations)
Voltage 4V
Phase 4 is the interval between cycles.
Fig. 18 shows the dependence of the volume of the droplet grown at the dispensing tip as a function of the duration of phase 2.

Test No. 2.

All the conditions remained the same as in Test No. 1 except that the pressure in the line connected to the dispenser was increased to 10 bar (150 psi). In this experiment drops were ejected from the nozzle by the pressure gradient which was sufficient to eject the drops and the tolerance of the measuring volume of the drops was ± 3nl. Fig. 19 illustrates the results obtained.
In both of the above two tests it is important to appreciate that the shape and construction of the nozzle will vary the test results and thus different test results will be achieved for different constructions of nozzle.

Test No. 3

The conditions of the dispensing assembly were identical as for Tests No. 1 and No. 2 with the addition of a conducting plate. This was spaced from the dispensing tip by 10mm and had dimensions 100mm X 100mm.
A high voltage was applied to the conducting plate which was arranged in substantially the same manner as the dispensing assembly of Fig. 2.
The test was carried out by growing a droplet on the dispensing tip of the nozzle by opening the valve. Then the valve was closed and the voltage was gradually increased until drop off occurred, when it was recorded. The volume of the droplet was measured by repeating this with the electromagnetic balance, details of which are described later.
Fig. 20 shows clearly the dependence of the drop off voltage as a function of the volume of the drop grown at the end of the dispensing tip.

Test No. 4

A volume of droplet 40 nanolitre was chosen with the remainder of the conditions the same as Test No. 3. In this test the dependence of the drop off voltage as a function of the distance between the end of the nozzle and a conducting plate was tested and the results are given in Fig. 21.

Test No. 5

With the same construction of dispensing assembly as for Test No. 4 and with referring specifically to Fig. 22 there is illustrated a test assembly indicated generally by the reference numeral 150 incorporating a dispensing assembly as illustrated in Fig. 5 and 13. There is provided a substrate 151 below which is mounted a pair of receiving electrodes in the form of plates 152 and 153 which in turn are connected to an electrical circuit indicated generally by the reference numeral 154 incorporating a high voltage supply 155 of approximately 5 KV. The separation between the dispensing tip and the substrate 151 was 15 mm. Tests were carried out, the results of which are shown in Fig. 23.
Fig. 23 shows the deviation of a droplet as a function of the potential difference applied to the plates 152 and 153. The potential difference between the plates 153 and 152 is measured in percentage of the potential difference between the average of the potentials of 152 and 153 and the nozzle 46.
Referring now specifically to Figs. 24 and 25 there is illustrated an electromagnetic balance for the measurement of the mass of droplets dispensed in accordance with the invention.
The electromagnetic balance 160 comprises a receiving coil 161 across which a magnetic field may be applied suspended on a fine spring provided by a twisted spring coil 162 and powered by a controlled current source 163. Lines of the magnetic field are schematically indicated with the numeral 169. The receiving coil 161 supports a balance arm 164 carrying a droplet receiving plate 165. A position sensor 166 is provided adjacent the balance arm 164 and is connected to a feed back controller 167 which in turn is connected to the controlled current source 163. The position sensor 166 in one embodiment is a light emitting diode and a photo diode coupled optically. It will be appreciated that the torque acting on the receiving coil 161 is proportional to the current carried by the receiving coil 161.
To measure the gravity force of a droplet identified by the reference numeral 168 on the receiving plate 165 when the position sensor 166 senses a deviation of the balance arm 164, the feedback controller 167 signals the controlled current source 163 to change the current into the receiving coil 161 until the previous unloaded position is attained. Thus the gravity force exerted by the droplet 168 is proportional to the change in current in the coil 161, then using simple calibration the mass of droplets can be measured directly and accurately.
Fig. 25 shows in some more detail the electronic circuit of the electromagnetic balance 160. D1 is the light-emitting diode, Q1 is the photodiode. Output J1 supplies the voltage which is dependent on the position of the arm. This output is connected to the analogue-to-digital converter and processor controlled feedback circuit for continuous comparison of the actual position of the arm with the preset value. The feedback circuit produces signal proportional to the current needed to be supplied to the coil to control the position of the arm. This signal in the form of a voltage is applied to the input J2 and the current is taken from the output as marked "Moving Coil" normally the coil 161.
As has been shown already that the drop off voltage depends on the volume of the droplet on the dispensing tip. It becomes important to ascertain exactly when the droplet is released from the dispensing tip. Accordingly the invention provides various methods of detection of the separation of a droplet from the dispensing tip. Once the electrostatic field causing the drop off to be achieved is known, then the volume of the droplet can be calculated within relatively fine limits.
Referring to Fig. 26, there is illustrated a detector indicated generally by the reference numeral 170, for sensing the separation of a droplet from the dispensing tip. Again for illustrative purposes the dispenser 40 of Fig. 2 is illustrated. The detector 170 comprises source 171 of electromagnetic radiation, an electromagnetic collector 172 and a controller 173 connected to the electromagnetic radiation source 171 and collector 172.
In this embodiment the electromagnetic radiation source 171 is a laser. There is illustrated a laser beam 174 emanating from the electromagnetic radiation source 171 and then either being reflected as a further laser beam 175 to the electromagnetic collector 172 or as a beam 176 passing straight beyond the dispensing tip 46 when a droplet 58 is not in position.
The term "radiation transmitted" when used in this specification in respect of a droplet covers both reflection and refraction.
It will be appreciated that only a fraction of the laser beam 174 returns as the beam 175 to the electromagnetic radiation collector 172.
Referring to Fig. 27, there is illustrated another construction of detector arrangement indicated generally by the reference numeral 180 in which parts similar to those described with reference to Fig. 26 are identified by the same reference numerals. In this embodiment, the laser beam 174 is either refracted by the droplet 58 if it is in position as shown by the numeral 181 or simply bypasses undeflected when the droplet 58 is not in position as shown by the numeral 176.
Referring now to Fig. 28 there is illustrated a slightly different arrangement of the detector illustrated in Fig. 27 and thus parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment additional scattered light beams 185 are illustrated as is a modulator 186 and a lock-in amplifier 187. A signal input to the lock-in amplifier 187 is identified by the reference numeral 188 and a reference input signal is identified by the reference numeral 189.
Referring now to Fig. 29 there is illustrated a further construction of detector indicated generally by the reference numeral 190 again used with the dispenser of Fig. 2 and in which parts similar to those described with reference to Figs. 26 and 27 are identified by the same reference numerals.
In this embodiment the electromagnetic radiation source 171 delivers radiation through a fibre-optic cable 191 down the nozzle 44. Reference numerals 192 and 193 show the meniscus of a droplet being formed on the dispensing tip 46, namely one forming a flat meniscus 192 and the other a curved meniscus 193. The beam 174 when there is flat meniscus 192 on the dispensing tip 46 will be delivered through it as the beam 194 to the detector 172. However when the meniscus is a curved meniscus 193, the beam 174 will be delivered as a beam 195 and a further beam 196 away from the detector 172.
Referring now to Fig. 30 there is illustrated a further construction of detector indicated generally by the reference numeral 200 in which the parts similar to those described with reference to the previous drawings are identified by the same reference numerals. It will be appreciated that in this embodiment the beam 174 will always form a reflected beam 201 once a droplet whether formed or not is present. The reflected beam will vary in intensity. Thus there will be a variation detected at the detector 172. It will be appreciated that an optical coupler will need to be installed between the electromagnetic radiation source 171 and the collector 172 on one side and the fibre-optic guide 191 on the other.
It will be appreciated that in certain embodiments of the invention it will be necessary to calibrate the dispensing assembly for each new liquid or reagent handled since as explained above the volume dispensed depends on the properties of the liquid and especially on the viscosity thereof. Therefore each time a new liquid of unknown properties is to be dispensed, the dispenser should be calibrated. As explained above the use of an electromagnetic balance as described herein would be particularly suitable. Further as has been explained already, the drop off voltage is a function of the volume of the droplet, and over a substantial range of volumes it is effectively a monotonous function. That is to say the smaller the volume of the drop, the greater it is the drop off voltage for a given diameter of the nozzle and a given fluid. As was shown already with reference to Fig. 20 this is monotonous for a range of some 40 nl to well over one microlitre for water. Further, the range of volumes in which the function is monotonous can be adjusted by changing the bore of the nozzle. Therefore, by varying the voltage and monitoring the moment when the droplet is detached from the dispensing tip, one can ascertain clearly the volume of the droplet. The voltage at the moment of drop off needs to be recorded for this. Monitoring the moment of the drop off and measuring the drop off voltage is a much simpler task than the one of complex measurement of the drop volume in flight. However, as will be explained later this can also be done.
As explained already one method for the direct measurement of the volume of the drop which is not based on the detection of the separation of the droplet from the dispensing tip would be to measure the charge of the droplet as will be described thereinafter. It is proposed in the present invention to use a Faraday Pail for this purpose.
Faraday Pails are well known and are described in many published documents (see for example Industrial Electrostatics by D.M. Taylor and P.E. Secker, Research Studies Press, 1994 ISBN 0-471-0523333-8) and Electrostatics: Principles, Problems and Applications by J. Cross, Adam Hilger ISBN 0-85274-589-3). Essentially, the Faraday Pail consists of an outer shield and an inner conductive box or chamber. The shield and chamber are well insulated from each other and indeed it is advantageous to keep the outside shield and the chamber at the same potential. In this situation, a charged droplet arriving at the chamber induces the same charge with opposite sign at the surface of the chamber. This charge is created by the current flowing from inside to outside which can be easily measured by a charge measurement circuit. Generally, the dispenser and hence the nozzle will be maintained at a relatively high voltage, and the shield and chamber connected to ground potential, as will be described hereinafter, the charge can be measured without catching the droplet in the pail. Thus charged droplets will progress through the induced charge detector which is effectively the function of the Faraday Pail.

Test No. 6

Faraday Pail is at ground potential
Dispensing tip is at the potential 2KV to 4KV.
Distance to Faraday Pail is 17mm
Rest of dispensing assembly as Test No. 1.

Activation Phases

Phase 1	0.2ms
Phase 2	0.3ms
Phase 3	0.3ms
Phase 4	105ms

Each actuation of the boss resulted in the same subnanolitre volume added to the drop grown at the end of the tip. Therefore, each drop was grown as a result of a number of actuations.
Fig. 31 illustrates that the charge is directly related to the volume of the droplet.

Test No. 7

A further test was carried out without the use of the pail at ground potential All the conditions remain the same as in Test No.6.
Fig. 32 shows the results obtained from this test again the charge is directly related to the volume of the droplet.
Referring now to Figs. 33 and 34 there is shown typical signal detection traces from the Faraday Pail. In Fig. 33 there is shown a change in the output voltage of a charge amplified as a result of the charge of approximately 3*10^-11C on a droplet and it is easy to calculate the volume of the drop from the calibration curves of Figs. 31 and 32.
Fig. 34 shows the zoom in to indicate the extent of the noise and sensitivity of the system.
Referring now the Fig. 35 there is illustrated the electronic circuit of the amplifier measuring the charge in the Faraday Pail. The two inputs of the amplifier are connected to the chamber and the shield of the Faraday Pail, respectively. The relay is added to the circuit to prevent damage to the amplifier by electrostatic charge when the circuit is idle. By deactivating relay the two inputs are connected together and they are also connected to the output voltage of OPA111 to bypass the storage capacitor C1. It is advantageous to have the storage capacitor C1 having a value of capacitance much greater than the capacitance between the chamber and the shield of the Faraday Pail.
Referring now to Fig. 36 there is illustrated the use of a Faraday Pail indicated generally by the reference numeral 210 for use in a dispensing assembly similar to that described with reference to the Figure 10 above. In this embodiment a high voltage source 211 is connected to the nozzle 44. The Faraday Pail 210 comprises of an inner chamber 212 and an outer shield 213 connected to a controller 214 in the form of a charge amplifier. In use samples of droplets are taken and an average for droplet volume and mass is calculated.
To measure some parameters of a dispensed droplet (charge, mass) a contactless method is implemented. This method is based on the Faraday Pail principle.
In a conventional Faraday Pail as described in the disclosure a droplet reaches the bottom of the inner chamber and sticks to it. An output signal of the charge amplifier will be a step-like function. The height of the step indicates the value of the arrived charge.
It is important to emphasise that it is not necessary for the droplet to contact the inner chamber at all. The charge measured can be created by induction. Putting the charge inside the Faraday Pail induces charge on the inner chamber, and removing the charge from it cancels the induced charge.
When the droplet passes the bottomless Faraday Pail, the charge amplifier will create only a short pulse at its output. The rising edge of this pulse will correspond to the arrival of the charge in the chamber while a falling edge corresponds to the charge leaving.
The width of this pulse is proportional to the time of the droplet flight through the pail and therefore inversely proportional to the speed of droplet.
The height of the pulse peak is proportional to the charge of droplet.
From these parameters we can obtain value of the droplet's charge on the flight as well as the speed of the droplet accelerated by electric field after it left the tip.
Information about the voltage between the tip and the Pail, charge and speed of droplet provides an estimate of the charge-to-mass ratio for the flying droplet. Droplets with different charge to mass ratios will have different acceleration and final speed in viscose air, which can be detected by the pail. This means that charge-to-mass ratio can be estimated if the applied voltage and the final speed of droplet are both known. Dividing the droplet charge by its charge-to-mass ratio gives mass of droplet. The speed of the droplet and the calculation of its mass from the calculated charge to mass ratio can be achieved.
Referring now to Fig. 37 there is illustrated a further construction of Faraday Pail indicated generally by the reference numeral 220 having an inner chamber 221 an outer shield 222 and a charge amplifier circuit forming a controller 223.
In this embodiment the drop off voltage is determined by the potential difference between the shield 222 and the dispensing tip 46 of the nozzle 44. 224 is the high voltage source connected to the tip.
Referring to Figs. 38 to 40 inclusive there is illustrated an alternative construction dispenser indicated generally by the reference numeral 240 substantially similar to the dispenser 70 illustrated in Fig. 5 and thus the same reference numerals are used to identify the same or similar parts. In this embodiment there is provided a spherical valve boss 241 of a soft magnetic material. The dispenser 41 is mounted between an upper coil 242 and a lower coil 243, each wrapped around a core of soft magnetic material 244 and 245 respectively. This construction is particularly advantageous in that it allows removing the dispenser 41 while keeping the source of the gradient magnetic field in place. This is particularly advantageous for replacing contaminated dispensers.
Referring now to Figs. 41 to 43 inclusive there is illustrated an alternative construction of dispenser indicated generally by the reference numeral 250 in which parts similar to those described with reference to Fig. 38 to 40 inclusive are identified by the same reference numerals.
In this embodiment there is provided a separate valve boss actuating assembly indicated generally by the reference numeral 251. In this embodiment the dispenser 250 incorporates a spherical valve boss 252 of a soft magnetic material. The actuating assembly 251 comprises a permanent magnet 253 mounted in a nozzle embracing U shaped sleeve 254 movable up and down relative to the body member 41 by a pneumatic ram of which only a plunger 255 is shown connected to the sleeve 254.
Referring now to Fig. 44, there is illustrated a still further construction of dispenser indicated generally by the reference numeral 260 in which parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, there is provided a two-part elongated body member comprising an upper portion 261 and a lower portion 262 interconnected in liquid tight manner, in this case by a flexible type concertina type wall 263 of a suitable elastomeric material. Mounted in the upper portion 261 by a cantilever arm 264 is a valve boss 265 including a lower valve seat engaging portion 266 of a suitable resilient polymeric material having a valve seat engaging surface 267. The lower portion 262 forms the base 49 of the body member and carries the nozzle formed from the capillary tube 64 such as illustrated in Fig. 4 which projects above the base 49. Actuation means 268 is provided to move the upper portion 261 towards and away from the lower portion 262 in the direction of the arrows A. It will be noted that there is a considerable gap between the valve boss 265 and the bore of the upper portion 261 and the lower portion 262. The actuation means 268 is simply illustrated by two blocks as it could be of any suitable construction such as an electromagnetic coil arrangement, a mechanical or fluid power actuator and so on. Indeed any suitable means can be used to push them towards and away from each other and indeed the upper portion 261 could form effectively the boss of a solenoid valve similar to the floating bosses previously described and actuating coils could be used to raise and lower the upper portion 261. Alternatively, the actuator could be attached to the lower portion 262.
Referring to Fig. 45, there is illustrated an alternative construction of dispenser, indicated generally by the reference numeral 270 in which parts similar to those described with reference to the Fig. 44 are identified by the same reference numerals. In this embodiment, there is provided an upper portion 271 telescopic and slidable within a lower portion 272. Again, actuating means 273 are provided for raising and lowering the lower portion 272 relative to the upper portion 271 in the direction of the arrows A.
Referring now to Fig. 46, there is provided a still further construction of dispenser indicated generally by the reference numeral 280 in which parts similar to those described with reference to the previous drawings are identified by the same reference numerals. In this embodiment, there is provided a conventional solenoid valve 281 carrying a rod 282 projecting into the bore 42 of the main body member 41 carrying on its extremity a piston forming a valve boss 283 mounting a polymeric valve seat engaging seal 284 having a convex surface 285. Again, there is a considerable gap between the inner wall of the bore 42 and the piston or valve boss 283. The liquid carrying pipe 32 feeds the body member 41. Portion of the rod 282 forms the core of the solenoid 281.
Referring to Fig. 47, there is illustrated an alternative construction of the dispenser identified by the reference numeral 290 which dispenser 290 is substantially similar to the dispenser 280 except that the rod 282 engages a further rod 291 which is connected thereto by a suitable quick release connector. A suitable membrane seal 292 is provided. Thus, the body member 41 and the piston 283 are totally disposable.
Referring now to Fig. 48, there is illustrated an alternative construction of dispenser, indicated generally by the reference numeral 300 having a main body 301 in which is mounted a fixed valve boss 302 having a soft polymer coating 303. The nozzle is again formed by the capillary tube 64, in this case, projecting through the main body 301 through a seal 304. The capillary tube 64 is mounted in an actuator 305 which can move the capillary tube 64 up and down in the direction of the arrow A and thus now the valve seat 43 moves towards and away from contact with the fixed valve boss 302.
Fig. 49 illustrates a still further construction of dispenser, indicated generally by the reference numeral 310, in which parts similar to those described with reference to Fig. 48 are identified by the same reference numerals. In this embodiment, the capillary tube is mounted in a core 311 of a solenoid 312.
Fig. 50 illustrates an alternative construction of dispenser, indicated generally by the reference numeral 320 in which parts similar to those described with reference to the Fig. 48 are identified by the same reference numerals. In this embodiment, the polymer coating 62 incorporates an annular rim 321 in an exterior wall of the valve boss 61 which is in the form of a cylindrical plug 61. This annular rim 321, in use, gives improved high dynamic performance. The valve boss 61 can move faster and therefore accuracy of dispensation is improved. As mentioned above the valve boss does not have to be circular in cross-section.
It is also envisaged that, for example, in some of the embodiments where the example disclosed in Figs. 2 and 3, the velocity of the valve boss can be measured. For example, if the voltage is measured in one of the coils 50 or 51, that voltage will have essentially three components. Firstly, there will be the voltage due to the inherent resistance R of the coil and the current i being iR; then self inductance Ldi/dt where L is the self-inductance of the coil, and finally the induced voltage which is proportional to the velocity of the valve boss and its magnetisation. As the current i in the coil can be continuously monitored, the first two components can be easily calculated and then eliminated and therefore relatively easily the voltage induced by the motion can be determined which will give the velocity of the valve boss. It will be appreciated that two coils could be used whereby a secondary coil simply placed beside the energising coil 50 or 51. In this case, the voltage induced in the secondary coil would comprise two components. The first one is proportional to the mutual inductance of the two coils L₁₂ and the rate of current change in the coil actuating the valve boss, namely L₁₂ * di₁/dt. The second voltage would be proportionate to the velocity of the boss and its magnetic moment. The first component can be independently measured as the rate of change of current di₁/dt is known and therefore the velocity of the boss can be calculated. This could have considerable importance where you wish to measure the viscosity of a liquid. For example, if the viscosity of a liquid were to change during a test, this might indicate some form of malfunction. It will be possible to calibrate any dispenser according to the present invention to so measure the viscosity.
Preferably the dispenser in so far as it comprises the elongate body member the valve seat and nozzle can be manufactured from a suitable polymer material by micro machining or indeed any standard polymer mass production technique such as injection moulding. The purpose of this is to provide a disposable dispenser. The body of the dispenser could be also manufactured of other materials such as steel.
The valve boss as will be appreciated from the description above can be cylindrical, spherical or indeed a body of any geometric shape made from magnetic material for example iron, ferrite or NdFeB. It is preferably coated with a polymer or inert layer of another material to prevent chemical reaction between the boss and the liquid dispensed. In order to obtain a good seal with the valve seat, the valve boss may need to be coated with a specially selected soft polymer such as chemically inert rubber. The choice of the materials for the coating on the boss depends on the requirements of the liquids which must be handled by the dispenser. The most likely materials include fluoroelastomers such as VITON, perfluoroelastomers such as KALREZ and ZALAK and for less demanding applications, materials with lower cost could be considered such as NITRILE. TEFLON (PTFE) could be used in conjunction with chemically aggressive liquids. VITON, KALREZ, TEFLON and ZALAK are Du Pont registered trademarks.
The valve boss may be made of magnetic material bonded in a flexible polymer. These materials can have either hard or soft magnetic properties as required. The specific choice of material will be determined by the cost-performance considerations. Materials of families FX, FXSC, FXND manufactured by Kane Magnetics are suitable for certain applications. Other materials such as magnetic rubbers can be also used for certain designs. Making the boss of a mechanically soft material can improve the performance of the seal.
It is important to appreciate that the polymer coating on the boss (such as 62, Fig. 4) can consist of two or more layers of different polymers. For example, the first layer could be deposited on the boss 61 to improve the adhesion of the second outer layer with the first one. For certain applications it may be advantageous to have the second layer deposited only on that part of the boss that comes in contact with the seat 43 and in the immediate vicinity.
It is envisaged that the dispenser may be operational in either active or passive mode. In the active mode the valve is actuated to make an open-close circuit for each dispensation and aspiration. In this mode the dispenser is connected to a vacuum/pressure alignment as for example illustrated in Fig. 2 above. In the passive mode the dispenser is connected to a syringe pump as illustrated in Fig. 5.
It is important to note that in a preferred embodiment according to the invention, the valve boss is made of hard magnetic material, i.e. a material having a well-defined direction of magnetisation even in the absence of any external magnetic field. In a conventional solenoid valve, the plunger is usually made of soft magnetic material such as iron or iron-nickel alloy. This material has no significant magnetisation in the absence of an external magnetic field. In a preferred configuration the valve boss is a cylinder with the axisymmetrical magnetisation for instance in direction along its axis. The dispenser could also operate with a boss of soft magnetic material. However, its performance has been found to be not as good for dispensing the minute volumes such as 100 nl and smaller, because the force which can be exerted on the valve boss by a current coil is much smaller. A smaller force means that the valve boss moves slower and the accuracy of the dispensing is reduced. Also, by using a boss of hard magnetic material it is possible to avoid the use of two coils and to use only one. In order to close the valve all that is required is to reverse the direction of the current in the coil. If the boss is made of a soft magnetic material then two coils need to be used; one to open the valve and the other to close it.
In practice, with the present invention the dispenser can dispense volumes as small as 50 nl without any electrostatic field if the pressure in the line is as high as 10 Bar. It is often advantageous to decrease the pressure in the line connected to the dispenser. The dispensing assembly operating at a low pressure has considerable advantages. The connection requirements for the pneumatic components are less stringent. Normally it is desirable to use a basic push fit connector in robotic dispensers for these applications. The invention when used at reduced pressures allows using a simple push-fit connection between the dispenser and the pressure line, which is a desirable feature of the dispenser.
Further at lower pressures the drops are ejected with a lower speed which reduces the chances of splashing as the drop touches the substrate or the well plate. High pressure in the line may result in gases dissolved in the liquids dispensed. This is not acceptable for many biological applications. The gas dissolved in the liquid dispensed can also result in small air bubbles at the nozzle, which make its operation unreliable.
However, reducing pressure in the line compromises the ability of the dispenser to dispense small drops. The drops grow on the nozzle tip but do not get detached from it and electrostatic drop off is required.
Essentially, the technique comprises firstly opening the valve of the dispenser to allow a droplet of the desired size to grow on the dispensing tip. The valve is then closed and subsequently a strong electrostatic field is generated between the dispensing tip and the substrate on which the droplet is to be deposited. As the value of the field increases from the initial zero to a final preset value at some stage it will exceed a critical value which will cause the drop off of the droplet.
The dispenser can also be used with the valve continuously open. In this case the fluid from the dispensing tip is ejected as a jet. The flow of the jet is determined by the pressure in the line connected to the dispenser and where present the value of the electrostatic field at the nozzle. The jet may split into droplets partly due to the electrostatic repulsion between the charged parts of the jet.
With a further miniaturisation of the substrate targets, it becomes increasingly difficult to ensure that the drop reaches the correct destination as it is ejected from a liquid handling system. For applications such as high-density arrays, the size between the subsequent drops covering the substrate, herein called pitch, could be as small as 0.1 mm. In this invention there are two different means of controlling the destination of the drop, both are based on the electrostatic forces acting on the drop as it travels between the nozzle and the well.
The first way is to generate the electrostatic field with a small charged receiving electrode positioned underneath the well instead of a large conducting plate. The size of the electrode is smaller than the size of the well for accurate navigation. It may be advantageous as described above to have the receiving electrode in the shape of a tip to produce the strongest electric field at the centre of a destination well. The electrode produces a strong electric field underneath the well attracting the drop to the required destination position (usually the centre of the well). The receiving electrode may be attached to an arm of a positioner capable of moving it underneath the well plate and pointing to the correct destination well. Alternatively, the sample well plate may be repositioned above the receiving electrode in order to target a different well. It may be necessary to move the dispensing tip and receiving electrode synchronously. It may be advantageous to have a module with a number of receiving electrodes which could be connected to the high voltage supply independently. The distance between the electrodes could be the same as the distance between the centres of the wells in a well plate. In this case the drops could be navigated to different wells without actually moving the dispenser or the receiving electrode.
In an arrangement described above deflection electrodes are positioned along the path between the nozzle and the destination well. The electrodes are charged by means of a high voltage applied to them. As the drops leaving the dispensing tip are charged by the voltage between the dispensing tip and the receiving electrode, they will be deflected by the deflection electrodes.
It is important to realise that during the electrostatic drop off, the electrostatic force acting on the drop could much greater than the gravity force. In this case as the drop flies between the nozzle and the substrate, the direction of the path is determined by the direction of the electrostatic field.
While it is explained above in many instances necessary to calibrate the dispenser for each new liquid because the volume dispensed depends on the properties of the liquid and of the nozzle, in certain instances this is not required as has been explained above.
In the present invention we also envisage, as described above, the monitoring of the droplet in flight. It is important in many instances to be absolutely certain that the droplet was actually dispensed and ideally also to ascertain the volume of the droplet and this has been described in considerable detail above. Also it must be noted that the present invention proposes a method for the direct measurements of volume of the droplet which is not based on the detection or the timing of the drop-off but on direct measurement of the charge on the droplet.
It has been found particularly advantageous to separate the actuation of the dispenser into distinct phases. The first phase is accelerating the valve boss fast from the initial position when the valve is closed by sending a short pulse of a large current through the coil or coils. In the case of one dispenser manufactured in accordance with the invention, the duration of the first phase is typically in the range of 0.2 to 0.5ms. The second phase is maintaining the valve in the open position and during this phase, the current in the coil is considerably reduced. The duration of the second phase mainly determines the volume of the droplet dispensed as demonstrated above. In dispensing assemblies manufactured in accordance with the present invention the duration of the second phase of some 0.1 to 5ms would result in the volume of the droplets dispensed being in the range of 100 nl to some few microlitres. The third phase is closing the valve with a short pulse of a high current. In the case of a specific dispenser constructed the duration of the third phase was typically in the range of some 0.2 to 0.4ms. The fourth phase is maintaining the valve in the closed position, i.e. holding the boss against the seal for the duration between cycles. The value of the current during the fourth phase was typically in the range of some 20% of the peak current supplied through the coil/coils during the first and third phases. Such a separation is advantageous as it allows getting the highest value of the actuating force from the coil or coils. Driving a large current through a coil or coils over an extended length of time may cause overheating with a detrimental effect. However, during a short pulse, a much higher current value is acceptable. A much higher current resulting in much higher actuating force is particularly suitable for dispensing of droplets of submicrolitre volumes.
A similar separation into separate phases can be advantageous during the aspiration of the liquids.
It will also be appreciated in accordance with the present invention that it does not rely on a positive displacement pump nor indeed does it rely on the conventional normal construction of solenoid valve. At the same time the present invention can, as shown above, be applied with advantage to positive displacement pump assemblies. The essential point then is that the positive displacement pump operates as a source of pressure difference, not as a metering device. Unlike in the conventional solenoid valve, there is no mechanical connection between the valve boss and other parts of the dispenser, similarly there is no mechanically actuated means involved or a spring for closing a valve boss. There is virtually zero dead volume in the apparatus according to the present invention which increases the accuracy particularly where smaller volumes are required. By having the dispenser separate from the actuating coils etc., it is possible to produce a very low cost dispenser which can be easily and rapidly removed thus avoiding cost and cross contamination problems. There is thus great disposability with the present invention. It is also advantageous that the present invention can work at both high and low pressures.
In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms "include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.
The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail within the scope of the claims.

Claims

A dispensing assembly for liquid droplets of the type comprising a dispenser having a main bore communicating with a nozzle having a nozzle bore terminating in a dispensing tip, and delivery means for moving liquid to the dispenser and from there through the bore to form a droplet on the exterior of the tip and then to cause the droplet to fall off therefrom, characterised in that:

the delivery means (21) comprises a separate pressurised liquid delivery source (22) for moving pressurized liquid to the dispenser (40); and

the dispenser (40) is a metering valve dispenser and comprises:

an elongate body member (41) having a base (49) including a valve seat (43) forming an entrance to the nozzle (44) which valve seat (43) projects proud of the base (49);

a valve boss (47) in the bore (42), the cross-sectional area of which is sufficiently less than that of the main bore (42) to permit the free passage of liquid therebetween bypassing the valve boss (47) and means for altering the relative positions of the valve boss (47) and the valve seat (43) between an open position with the valve boss (47) spaced-apart from the valve seat (43) and a closed contact position sealing the valve seat (43) and spaced-apart from the base (49).
A dispensing assembly as claimed in claim 1, in which the valve seat (43) is in the form of a capillary tube (64) projecting proud of the base (49).
A dispensing assembly as claimed in claim 1 in which the valve boss (47) is covered with a layer of soft polymer (48).
A dispensing assembly as claimed in claim 2 or 3, in which the valve boss (47) is a floating valve boss of a magnetic material and the means for altering the relative position of the valve boss (47) and valve seat (43) comprises a separate valve boss actuating assembly (50, 51) adjacent the body member (41).
A dispensing assembly as claimed in claim 4 in which the valve boss (47) is of a hard magnetic material.
A dispensing assembly as claimed in claim 4 or 5 in which the valve boss (47) is manufactured from a flexible polymer bonded magnetic material.
A dispensing assembly as claimed in any of claims 4 to 6, in which the valve boss actuating assembly is an electrical coil (50,51) surrounding the body member.
A dispensing assembly as claimed in claim 7 in which the valve boss (47) is biased to a closed position into engagement with the valve seat (43) by an external magnetic field generated by the actuating coil assembly (50,51).
A dispensing assembly as claimed in claim 7 or 8 in which the actuating coil assembly comprises two separate sets of coils (50,51) for moving the valve boss (47) in opposite directions within the body member (41).
A dispensing assembly as claimed in claim 9 in which the actuating coil assembly comprises a source of electrical power and a controller (25) for varying the current over time as each droplet is being dispensed.
A dispensing assembly as claimed in any of claims 4 to 6 in which the valve boss actuating assembly (250) comprises a permanent magnet (253) and means (255) for moving the magnet along the elongate body member (41) towards and away from the valve seat (43).
A dispensing assembly as claimed in claim 11 in which the magnet (253) is substantially U shaped to embrace the body member (41).
A dispensing assembly as claimed in any of claims 4 to 6 in which the valve actuating assembly (240) comprises a pair of spaced apart magnetizing assemblies each comprising a coil (242 and 243) wrapped around a core (244 and 245) of soft magnetic material.
A dispensing assembly as claimed in claim 13 in which the core (244, 245) is substantially U shaped to embrace the body member (41).
A dispensing assembly as claimed in any of claims 4 to 14 in which the valve boss (47), the body member (41) and nozzle (45) form the one separate sub assembly releasably detachable from the remainder of the dispenser (40).
A dispensing assembly as claimed in any of claims 4 to 15 in which the valve boss (47) is constructed for limited movement out of line with the main bore (42) longitudinal axis.
A dispensing assembly as claimed in any preceding claim in which the valve boss (47) is in the form of an elongate plug.
A dispensing assembly as claimed in any preceding claim in which the valve boss (47) is a cylindrical plug (91).
A dispensing assembly as claimed in claim 17 or 18, in which the plug has a convex valve seat engaging surface.
A dispensing assembly as claimed in claim 17 or 18, in which the plug has upper and lower convex surfaces.
A dispensing assembly as claimed in any of claims 17 to 1920, in which an annular rim (321) is formed on the exterior wall of the plug (61) intermediate its ends.
A dispensing assembly as claimed in any of claims 4 to 21 in which there is mounted in the main bore, remote from the valve seat, a stop to limit movement of the floating valve boss.
A dispensing assembly as claimed in any of claims 4 to 22 comprising means for measuring the velocity of the valve boss.
A dispensing assembly as claimed in claim 23 in which, when the valve boss actuating assembly comprises an electrical actuating coil surrounding the body member, the means for measuring the velocity of the valve boss comprises a secondary coil adjacent the valve boss and means for measuring the induced voltage in the secondary coil.
A dispensing assembly as claimed in claim 23 in which when the valve boss actuating assembly is an electrical actuating coil surrounding the body member, means are provided to measure the total voltage induced in the actuating coil to provide a measure of the velocity of the valve boss.
A dispensing assembly as claimed in any of claims 1 to 3, in which the body member is a two part body member having an upper portion (261) and a lower portion (262) interconnected in liquid tight manner, the upper portion (261) housing the valve boss (265) which is rigidly mounted therein and the lower portion (262) housing the base (49) and valve seat (43) and in which actuation means (268) are provided for moving the upper and lower portions (261, 262) relative to each other to cause the valve boss (265) to move between the open and closed positions.
A dispensing assembly as claimed in claim 26, in which the lower portions are connected by a flexible concertina type wall (263).
A dispensing assembly as claimed in any of claims 1 to 3, in which a body member is a two part body member having an upper portion (271) and a lower portion (272) telescopically connected in liquid tight manner, the upper portion (271) housing the valve boss (265) which is rigidly mounted therein and the lower portion (272) housing the base (49) and the valve seat (43) and in which means (273) are provided for causing the movement between the open and closed positions.
A dispensing assembly as claimed in any of claims 1 to 3, in which the dispenser comprises a solenoid valve (280).
A dispensing assembly as claimed in claim 29, in which the solenoid valve (280) comprises a solenoid and core external of the body member, the core mounting the valve boss (283) on a lower valve boss carrying rod (282) forming an extension of the core.
A dispensing assembly as claimed in claim 30, in which the rod is a two part rod (282, 291) releasably connected together external of the body member.
A dispensing assembly as claimed in any preceding claim, in which the cross-sectional area of the main bore is between 50 to 1500 times greater than that of the nozzle bore.
A dispensing assembly as claimed in any preceding claim, in which the cross-sectional area of the main bore is of the order of 100 times that of the nozzle bore.
A dispensing assembly as claimed in any preceding claim, in which the nozzle bore diameter is between 300 and 75 µm.
A dispensing assembly as claimed in any preceding claim, in which the nozzle bore diameter is between 200 and 100 µm.
A dispensing assembly as claimed in any preceding claim in which the body member (41) and the nozzle (45) form the one integral moulding of plastics material.
A dispensing assembly as claimed in any of claims 1 to 35 in which the body member (41) and nozzle (44) are made from stainless steel.
A dispensing assembly as claimed in any preceding claim comprising;

an electrode incorporated in the dispensing tip (46);

a separate receiving electrode (123) remote from the tip (46); and

a high voltage source (125) connected to one of the electrodes to provide an electrostatic field therebetween.
A dispensing assembly as claimed in claim 38 in which the receiving electrode (123) is below the dispensing tip (8).
A dispensing assembly as claimed in claim 38 or 39 in which a droplet receiving substrate (121) is mounted between the receiving electrode (123) and the dispensing tip (8).
A dispensing assembly as claimed in claim 38 or 39 in which a droplet receiving substrate is mounted below the receiving electrode, the receiving electrode having at least one hole for the droplet to pass through to the receiving substrate (121).
A dispensing assembly as claimed in claim 40 or 41 in which there is a plurality of receiving electrodes (131) at least one of which is activated at any time.
A dispensing assembly as claimed in any of claims 38 to 42 in which synchronous indexing means (124) are provided for the dispenser (40) and the receiving electrode (131) for accurate deployment of droplets on the substrate (121).
A dispensing assembly as claimed in any of claims 38 to 43 in which there is more than one receiving electrode forming droplet deflection electrodes (141,142) which are mounted below the dispensing tip (8) and above the droplets receiving substrate (121) and in which the high voltage source (125) has control means to vary the voltage applied to the deflection electrodes.
A dispensing assembly as claimed in any preceding claim comprising a detector (170) for sensing the separation of the droplet from the dispensing tip.
A dispensing assembly as claimed in claim 45 in which the detector (170) comprises:

a source (171) of electromagnetic radiation;

means for focussing the radiation on the end of the dispensing tip; and

means (172) for collecting the radiation transmitted by a droplet on the dispensing tip.
A dispensing assembly as claimed in claim 46 in which the source of radiation (171) is mounted within the dispenser nozzle.
A dispensing assembly as claimed in any of claims 38 to 47 in which means are provided for measuring the charge of the droplet.
A dispensing assembly as claimed in claim 48 comprising a Faraday Pail (210).
A dispensing assembly as claimed in claim 48 comprising a bottomless Faraday Pail (220).
A method of dispensing a droplet having a volume less than ten micro litres (10_µl) from a pressurised liquid delivery source through a metering valve dispenser comprising an elongate body member having a base including a valve seat forming an entrance to the nozzle which valve seat projects proud of the base, a valve boss in the bore, the cross-sectional area of which is sufficiently less than that of the main bore to permit the free passage of liquid therebetween by passing the valve boss and means for altering the relative positions of the valve boss and the valve seat between an open position with the valve boss spaced-apart from the valve seat and to a closed contact position sealing the valve seat and spaced-apart from the base comprising the steps of:

delivering the pressurised liquid to the dispenser;

opening the valve for a preset time to deliver liquid around the valve boss into the nozzle bore; and

closing the valve as the droplet falls off.
A method as claimed in claim 51, in which the valve boss is a floating valve boss of a magnetic material and there is means for moving the valve boss comprising a separate valve boss actuating assembly including an actuating coil adjacent the body member and in which the method includes actuating the assembly by energising the actuating coil.
A method as claimed in claim 52 in which the speed of the floating boss is obtained by measuring the voltage induced in an actuating coil due to its velocity and magnetisation.
A method as claimed in claim 52, in which the speed of the floating boss is obtained by measuring the voltage induced in a coil adjacent the floating boss.
A method as claimed in claim 53 or 54 in which the speed of the floating boss is used to determine the viscosity of the liquid.
A method as claimed in any of claims 49 to 55, in which when dispensing droplets of the order of 100 nl and less, the liquid is pressurised in excess of 5 bar.
A method as claimed in any of claims 51 to 55 in which the step is performed on the valve being shut off of generating a pulse of voltage at a receiving electrode remote from the dispensing tip to generate an electrostatic field to cause an electrostatic potential between the droplet and the receiving electrode to detach it from the dispensing tip.
A method as claimed in claim 57, in which the liquid is pressurised at less than four bar.
A method as claimed in claim 57 or 58, in which the liquid is pressurised to less than two bar.
A method as claimed in any of claims 51 to 59 in which the receiving electrode is mounted beneath a droplet receiving substrate.
A method as claimed in any of claims 51 to 59 in which the receiving electrode is mounted between a droplet receiving substrate and the nozzle.
A method as claimed in claim 60 or 61, in which there are at least two receiving electrodes.
A method as claimed in any of claims 51 to 61, in which the receiving electrode is moved after each droplet is dispensed to direct the next droplet to another position on the substrate.
A method as claimed in any of claims 51 to 59 in which spaced apart deflection electrodes are placed between the dispensing tip and a droplet receiving substrate and the electrodes are differentially charged to cause the droplet to move laterally as it drops from the dispensing tip.
A method as claimed in any of claims 57 to 64 comprising the steps of:

measuring the volume of a droplet of a particular liquid for different drop off voltages;

storing a database of the measurements;

recording the drop off voltage when a droplet detaches from the dispensing tip; and

retrieving the volume from the database.
A method as claimed in claim 65 in which the drop off voltage is measured by a Faraday Pail.
A method as claimed in claim 66 in which recording of the drop-off of a droplet includes the steps of:

directing an electromagnetic beam from a source of electromagnetic radiation at the droplet as it forms at the tip; and

monitoring the electromagnetic radiation coupled by the droplet at a collector remote from the droplet.
A method as claimed in claim 67 in which a light beam is the source of electromagnetic radiation and the amount of light reflected and/or refracted by the droplet is monitored.
A method a claimed in any of claims 57 to 64 in which the steps are performed of:

measuring the charge of droplets of a particular liquid for different volumes of droplets;

storing a database of the measurements;

recording the charge on each droplet; and

retrieving the volumes of the drops from the database.
A method as claimed in claim 69 of:

measuring the width of the voltage pulse in the Faraday pail;

determining the time taken for the droplet to pass through the pail;

deriving the speed of the droplet from the time taken to pass through the pail; and

calculating the mass of the droplet from the charge to mass ratio.