WO2008119945A2

WO2008119945A2 - Microbubble producing device, microbubble dividing device, process for producing microbubbles and process for dividing microbubbles

Info

Publication number: WO2008119945A2
Application number: PCT/GB2008/001050
Authority: WO
Inventors: Mohan Edirisinghe; Eleanor Stride; Reza Moaleji; Ketan Pancholi; Uthumankandu Farook
Original assignee: Ucl Business Plc
Priority date: 2007-03-29
Filing date: 2008-03-28
Publication date: 2008-10-09
Also published as: GB0706175D0; WO2008119945A3

Abstract

The invention provides a process for producing microbubbles using a T-junction device which includes a junction; a first capillary, connected to a gas supply; a second capillary, connected to a liquid supply, and arranged perpendicular to said first capillary, and a third capillary, wherein said first, second and third capillaries are connected to said junction, and in use, gas and liquid enter the junction, through said first and second capillaries respectively, producing microbubbles which leave the junction via said third capillary.

Description

Microbubble Producing Device, Microbubble Dividing Device, Process For Producing Microbubbles And Process For Dividing Microbubbles

Introduction

The invention relates to a device and process for producing microbubbles, which achieve improved control of the size distribution of the microbubbles produced. The preparation of stable microbubble suspensions is fundamental to a wide range of technological applications across the scientific, engineering, medical and industrial sectors; from the production of basic foodstuffs to the self-assembly of smart materials. A key requirement in such preparation is to control the size distribution (i.e. mean and standard deviation) of the resulting microbubbles, with ideally a monodispersed solution being achieved. This is particularly the case when using microbubbles as an ultrasound contrast agent, because using bubbles of the wrong size can have serious implications with respect to patient safety.

Such quality control in respect of microbubble size distribution is important because when the microbubbles are used in, for example, ultrasound imaging, the response of such a microbubble to ultrasound excitation is strongly influenced by the radius and the thickness of the microbubble and also the mechanical properties of any coating material. This response, in turn, determines the amplitude and frequency content of the ultrasound signal scattered by the microbubbles and the threshold for microbubble destruction in response to ultrasound excitation. As more advanced imaging techniques are developed, which exploits the complex, non-linear features of the microbubble signal, the need for a high degree of control over these parameters is becoming increasingly strong. Similarly, for therapeutic applications, a well-defined microbubble size distribution, drug content and destruction threshold are critical in order to ensure correct dosage. However the required level of control is not provided by existing microbubble preparation technologies.

Existing methods for the manufacture of coated microbubbles for medical applications involve the production of gas bubbles in a solution containing a suitable surfactant such as serum albumin or a mixture of phospholipids. The surfactant forms a coating on the surface of the bubble, which counteracts surface tension and stabilises the bubble against dissolution. The bubbles are produced by entraining gas from the surroundings by agitation of the vessel containing the solution or by direct sonication. The bubbles may either be used immediately or else lyophilised and then resuspended when required. Further microbubble stability may be attained by using high molecular weight gases such as perfluorobutane or sulphur hexafluoride. Alternative techniques have also been developed. The contrast agent EchoGen® (Sonus Pharmaceutical, Bothell, WA₅ USA) forms microbubbles after injection, via the vaporisation of perfluoropentane, whose boiling point coincides with normal body temperature (37⁰C). Echovist® and its successor Levovist® (Schering AG, Berlin, Germany) are formed when an injected suspension of galactose microcrystals dissolves in the blood, releasing air microbubbles from defects on the crystal surfaces. In the case of Levovist®, additional stability is provided by the presence of a layer of palmitic acid on the bubble surface.

Producing microbubble suspensions via the methods described above, results in a broad size distribution and it is necessary to remove microbubbles (or microbubble precursors, in the case of agents forming microbubbles post-injection) having diameters greater than lOμm that would pose a risk of causing an embolism in vivo. However, the remaining size distribution is still relatively broad, as shown in Figure Ia, resulting in a wide range of microbubble resonance frequencies within a microbubble suspension. Moreover, there is a wide variation in the coating properties of individual microbubbles, which has a significant effect upon their dynamic and acoustic response. For example, Figure Ib shows how the microbubble scattering cross section varies with the stiffness of the coating. This makes predicting and controlling the microbubble response, in particular the point at which the coating will rupture, extremely difficult.

An alternative method for microbubble production is via electrohydrodynamic atomization. As shown in Figure 2, electrohydrodynamic research on model systems has already generated microbubbles in the range 2-8 μm. In this way, practical systems have also realised similar size, that is less than lOμm, lipid-coated microbubbles. However, it is not easy to achieve perfect monodispersity and the 1- lOμm size range using electrohydrodynamics alone.

Therefore, there is a problem in providing an improved apparatus and process for producing microbubbles that overcome the aforementioned disadvantages. It is an object of the present invention to provide a method and apparatus for producing microbubbles to at least partially alleviate some of the problems identified above. It is a further object to provide a method and apparatus for producing microbubbles with a diameter less than 1 Oμm.

Meeting these objects will enable the development of advanced diagnostic and therapeutic agents that are currently beyond the limits of existing technology. For medical applications, e.g. ultrasound contrast agents, microbubbles with diameters smaller than lOμm are required but with concentrations of 2 x 10⁸ microbubbles per ml. Therefore, it is a further object of the present invention to produce microbubbles with diameters smaller than lOμm at high bubble concentrations.

Summary of Invention

According to a first aspect of the invention, there is provided a microbubble producing device, comprising: a first capillary, connectable to a gas supply; and a second capillary, connectable to a liquid supply; wherein said first and second capillaries meet at a junction and, in use, gas and liquid enter the junction, through said first and second capillaries respectively, producing microbubbles.

Preferably, the microbubble producing device further comprises a third capillary which meets said first and second capillaries at said junction, and, in use, said produced microbubbles leave said junction via said third capillary.

Preferably, the microbubble producing device is arranged to produce bubbles which are substantially monodispersed. Preferably, said a first capillary is connected to a constant pressure or constant flow gas supply.

Preferably, said a second capillary is connected to a syringe, and further comprising a syringe pump for controlling the flow rate of liquid from said syringe.

Preferably, said second capillary is arranged at an angle between 0 and 90° to said first capillary, and preferably perpendicular to said first capillary.

Preferably, said third capillary is arranged such that it has a longitudinal axis that is coincident with a longitudinal axis of the first capillary. In preferably embodiments the spacing between an end of said first capillary and an end of said third capillary is 30μm -150μm, preferably 50μm -lOOμm, and more preferably 70μm.

Preferably the capillaries have a diameter of 50μm - lOOOμm, preferably 80μm - 500μm, more preferably lOOμm - 200μm, and even more preferably 150μm.

Preferably, the microbubble producing device is arranged to produce microbubbles with diameters in the range 1-1 Oμm.

According to a second aspect of the invention, there is provided a microbubble dividing device, comprising a first bubble dividing stage, said first bubble dividing stage comprising: a supply capillary connectable at a first end to a microbubble supply, wherein said supply capillary is divided at a second end into two capillary side arms; and such that, in use, microbubbles passing from the supply capillary into the two capillary side arms are divided into two smaller microbubbles, one of said two smaller bubbles passing into the first of said two capillary side arms, the second of said two smaller microbubbles passing into the second of said two capillary side arms.

Preferably, the microbubble dividing device further comprises at least one further microbubble dividing stage, said further stage(s) comprising: a supply capillary connected at a first end to a capillary side arm of a previous microbubble dividing stage, and wherein said supply capillary is divided at a second end into two capillary side arms; such that, in use, a microbubble passing through any bubble dividing stage is divided into two smaller microbubbles, one of said two smaller microbubbles passing into the first of said two capillary side arms of said bubble dividing stage, and the second of said two smaller microbubbles passing into the second of said two capillary side arms of said bubble dividing stage.

Preferably, said microbubble dividing device is arranged to divide microbubbles into two smaller microbubbles of substantially equal size at each bubble dividing stage.

Preferably, said supply capillaries at each of said microbubble dividing stages are constructed such that said supply capillaries are divided into two capillary side arms which are narrower than the supply capillary of that microbubble dividing stage.

Preferably said capillary sides arms at the same bubble dividing stage are constructed to have the same diameter as each other.

Preferably, the microbubble dividing device further comprises means for applying an electric field to said microbubble dividing device to facilitate co-axial electrohydrodynamic flow.

According to one embodiment there is provided a device for producing and dividing microbubbles, comprising the microbubble producing device according to any embodiment of the first aspect of the invention, and the microbubble dividing device according to any embodiment of the second aspect of the invention.

According to a third aspect of the invention, there is provided a process for producing microbubbles using a microbubble producing device, said process comprising the steps of: supplying a gas to a junction along a first capillary; supplying a liquid to a junction along a second capillary; and producing microbubbles at said junction from said supplied gas and supplied liquid. Preferably, said process further comprises removing said microbubbles from said junction via a third capillary.

Preferably, said step of producing microbubbles produces substantially monodispersed microbubbles.

Preferably, said step of supplying a gas to a junction further comprises supplying said gas at a constant pressure or constant flow.

Preferably, said microbubble producing device is the microbubble producing device according to any embodiment of the first aspect of the invention.

Preferably, said step of producing microbubbles further comprises producing microbubbles with diameters in the range 1-1 Oμm.

According to a fourth aspect of the invention, there is provided a process for dividing microbubbles using a microbubble dividing device, said process comprising a first bubble dividing stage of: supplying microbubbles to a supply capillary which divides into two capillary side arms; dividing each of said microbubbles into two smaller microbubbles by passing said microbubbles from said supply capillary into said two capillary side arms; such that each microbubble divides into two smaller microbubbles, one of said two smaller microbubbles is passed into the first of said two capillary side arms, and the second of said two smaller microbubbles is passed into the second of said two capillary side arms.

Preferably, the process for dividing microbubbles using a microbubble dividing device further comprises at least one further stage, wherein said steps of supplying and dividing are repeated at each stage.

Preferably, each step of dividing further comprises dividing said microbubbles into two smaller bubbles of substantially equal size. Preferably, said microbubble dividing device is the microbubble dividing device according to any embodiment of the second aspect of the invention.

Preferably, the process further comprises the step of applying an electric field to said microbubble dividing device to facilitate co-axial electrohydrodynamic flow.

According to one embodiment, there is provided a process for producing and dividing microbubbles, comprising the process for producing microbubbles using a microbubble producing device according to any embodiment of the third aspect of the invention, and the process for dividing microbubbles using a microbubble dividing device according to any embodiment of the fourth aspect of the invention.

According to one embodiment, the process achieves enhanced control of microbubble size distribution through the use of a T-junction assembly. Preferably the T-junction assembly comprises two axially aligned capillaries with a small spacing between, with a third capillary inserted perpendicular to the other two to form the T-junction. Preferably, the capillary tubes are of diameter 50μm - lOOOμm. More preferably, the capillary tubes are of diameter 80μm - 500μm. Still more preferably, the capillary tubes are of diameter lOOμm - 200μm. Even more preferably, the capillary tubes are of diameter 150μm. Preferably, the spacing between the axially aligned capillary tubes is 30μm -150μm. More preferably, the spacing between the axially aligned capillary tubes is 50μm -lOOμm. Even more preferably, the spacing between the axially aligned capillary tubes is 70μm.

The present invention has already demonstrated the ability to produce microbubbles with the size range 1-1 Oμm that tend towards perfect monodispersity. References to 'monodispersity' and 'monodispersed' in this document refer to size distributions in which all the microbubbles are within 2% of the desired or stated bubble diameter.

Description of Figures The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows graphs describing the properties of microbubbles produced by an existing method for producing gas bubbles in a solution. Figure Ia shows the size distribution for two commercial microbubble agents, and Figure Ib shows the effect of a variation in coating stiffness upon the scattering cross-section of a coated microbubble at different insonation frequencies.

Figure 2 is a graph showing an example of a microbubble size distributions achieved by existing electrohydrodynamic methods.

Figure 3 is a schematic diagram showing the arrangement of a single T-junction device for generating microbubbles in accordance with an embodiment of the invention. Figure 3 a shows a plan view of the device, whilst Figures 3 b and 3 c show side views of the device. Figure 3d shows the detail of the junction of the device.

Figure 4 is a micrograph, showing monodisperse microbubbles, with a diameter of lOOμm, of a lipid suspension prepared from the single T-junction device shown in Figure 3.

Figure 5 is a schematic diagram showing the arrangement of a multi-T junction apparatus in accordance with an embodiment of the invention. The Figure shows successive branching capillary T-junctions connected in stages.

Figure 6 shows schematic representations of possible drug encapsulation strategies in accordance with embodiments of the invention. Figure 6a shows one final T- j unction in a branching multi-T junction apparatus, as shown in Figure 5, in which drug is introduced to each capillary leaving the T-junction. Figure 6b depicts directing a steam exiting a first T-junction device, as shown in Figure 3, through a subsequent T-junction, in which a drug is introduced to the microbubble suspension. Description of the Invention

Figure 3 shows a T-junction device 10 for producing microbubbles, in accordance with an embodiment of the invention, for achieving improved control of the microbubble size distribution. One arm 7, of the T-junction device 10 for producing microbubbles, carries a flow of gas 20 that is to be encapsulated, whilst another arm 6 may carry material with which the bubbles 40 are to be coated in a suitable suspending liquid 30. Any liquid or suspension may be used as the liquid 30, including liquids with high viscosities. Microbubbles 40 are generated at the junction 11 of the T-junction device 10. The microbubbles 40 are produced within a defined size distribution, according to the parameters of the system. An example of the microbubble distribution is further illustrated in Figure 4, which shows a micrograph of a microbubble suspension produced by such a device. A third arm 9 of the device 10 carries the microbubbles 40 away from the junction 11. The T-junction device 10 is further sealed within a body, so that the gas 20 and liquid 30 can only flow through the capillaries and the junction 11.

The T-junction device 10 relies on a simple pressure balance of the gas 20 and liquid 30 phases at the junction 11 to generate bubbles 40 at a constant rate without any disruption or change in bubble size over a long period of time. The gas 20 is supplied from a capillary tube 7 positioned upstream from the end of the capillary tube 6, through which the suspending liquid 30 is supplied. These two capillaries 6, 7 are arranged such that the gas 20 and liquid 30 exiting the capillaries 6,7 meet near one end of a focusing capillary 9 at the junction 11. This ensures that the liquid 30 and gas 20 flow together through the focusing capillary 9 and, when the gas column 20 is surrounded by slower moving liquid 30, that the gas column 20 starts breaking-off at constant time intervals to produce a stream of substantially monodisperse, coated bubbles 40 exiting the focusing capillary 9. These bubbles 40 can be collected at the outlet of the device 10. The volume of bubbles 40 collected depends only on the ratio of gas 20 flow rate to liquid 30 flow rate: the higher the ratio, the larger the gas 20 volume fraction, and therefore the larger the volume of bubbles collected. The ends 1,2 of the gas supply and focusing capillaries 7, 9 have internal diameters 3. These diameters are preferably 50-1000μm, more preferably 80-500μm, still more preferably 100-200μm and even more preferably 150μm. In preferred embodiments, the gas supply capillary 7 has an internal diameter 3 between lOμm and 500μm, and the focussing capillary 9 has an internal diameter 3 between lμm and 500μm. The space 4 between the coupled tubes 7, 9 may be varied, according to various factors, such as flow rates and diameter of microbubbles 40 required. Typically, for example, when the capillary tubes 7, 9 have internal diameters 3 in the range 120-180μm (e.g. 150μm), the tubes 7, 9 may be spaced at a distance of 30-150μm, for example 50- lOOμm, preferably 70μm. For internal diameters 3 of the gas supply and focussing capillaries 7,9 in the range 10-50μm, a space 4 of 20-3 Oμm is preferred. For internal diameters 3 of the gas supply and focussing capillaries 7,9 in the range 50-500μm, a space 4 of 50-70μm is preferred.

As shown in Figure 3 a the gas supply and focusing capillaries 7, 9 are housed within a rigid body block 5, preferably made of polymer and preferably with 10 mm thick walls. These two tubes 7, 9 are fixed in the block 5 so they can be aligned at an angle to each other. In a preferably embodiment, the gas supply capillary 7 and the focusing capillary 9 are aligned such that the longitudinal axes of the capillaries are coincident. A third, liquid supply, tube 6 is inserted in to the block 5, preferably so that it is substantially perpendicular to the axis of gas supply tube 7. Therefore, in the preferred embodiment mentioned, in which the axes of the gas supply and focusing capillaries 7, 9 are coincident, the liquid supply tube 6 will be substantially perpendicular to the axes of both the other two tubes 7, 9 to produce the junction 11 of the T-junction device 10.

All the tubes 6, 7, 9 are secured mechanically to the block 5 so that they can withstand high pressures without slipping away from the junction 11. The gas supply tube7 of the T-junction device 10 is preferably connected to a tank supplying gas 20 at constant pressure or flow rate and also preferably connected to a digital manometer so that the gas pressure inside the tube 7 can be measured. The tube 6, preferably perpendicular to the gas supply tube 7, is preferably connected securely to a syringe supplying liquid. In a preferred embodiment, the syringe is a 20 ml stainless steel syringe. The syringe is preferably coupled via a leak proof mechanical coupling, and the liquid 30 flow rate into the T-junction device 10 is preferably controlled using a syringe pump. In a preferred embodiment the device 10 is robust and rigid, withstanding high pressures at the junction 11 , preferably up to 6 MPa.

The above-described T-junction device 10 for producing microbubbles only produces bubbles 40 over a narrow range of pressure values for a constant flow rate. When the gas 20 supply pressure is too high for microbubble formation, atomization of the liquid 30 is observed. When the gas 20 supply pressure is too low for microbubble formation, liquid 30 chokes the gas supply capillary 7. Increasing gas 20 pressure increases the number of bubbles 40 but also increases the size of bubbles 40 produced. Consequently, the smaller the size of the bubbles 40 produced, the smaller the number of bubbles 40 per unit volume of bubble-liquid suspension. As previously mentioned, it is desirable to have high concentrations of microbubbles 40 for medical applications.

According to the present invention, it is possible to produce microbubbles 40 at a high concentration which have a small diameter and narrow size distribution. A simple analogue for this aspect of the present invention is the observation that when liquid is passed through a sufficiently small-sized capillary at reasonably high pressure it breaks into smaller droplets. Similarly, in the present invention, bubbles 40 passing along a capillary that splits into two new, narrower, capillaries are divided into a pair of smaller bubbles 40. One of each of these bubbles 40 then passes into each of the two new capillaries. In one embodiment, it is contemplated that the focusing capillary 9, of the T-junction device 10 for generating microbubbles 40, divides, at an end remote from the junction 11 of the T-junction device 10, into two further capillaries. The two further capillaries are preferably narrower than the focusing capillary 9.

In the present aspect of the invention, bubbles 40 which have already been created are resized by passing them through an apparatus comprising a branching series of T- junctions 52 with increasingly small-sized capillaries at successive stages, in a so- called multi-T junction apparatus 50 as shown in Figure 5. This apparatus 50 not only changes the size of the bubbles 40, but also increases the total number of bubbles 40. Table 1 shows an example of the lengths of the side arm capillaries 51 in each stage of a multi-T junction apparatus 50, with reference to Figure 5.

The multi-T junction apparatus 50 is preferably constructed such that each stage results in the symmetric division of each bubble 40. This can be achieved by ensuring that all the side arms 51 at a given stage have the same length. Asymmetric microbubble division is observed when the side arms 51 have different lengths. The division depends upon the Capillary Number and the ratio of bubble 40 slug length and bubble 40 diameter flowing in the capillary leading to a T-junction 52. Preferably the Capillary Number is in the range 0-10 and the ratio of the bubble 40 length to bubble 40 diameter is in the range 1-10. To reduce the diameter of bubbles 40 further, this stream of bubbles 40 can be passed through a further series of branching T-junctions 52 to obtain the desired size and volume of bubbles 40. The result is the controlled production of substantially monodispersed microbubbles at concentrations sufficient to be used as, for example, ultrasound contrast agents.

Table 1. Example of dimensions of multi-T junction device.

As mentioned, Table 1 shows an example of the lengths of the side arms 51 in each stage of a multi-T junction apparatus (50). These dimensions can be varied and further stages could be added in alternative embodiments to get the required diameter and volume of bubbles 40 desired. Bubble 40 division to achieve the desired bubble 40 size can also be accomplished by having a contra-flow obstruction pellet, which provides a physical obstacle that divides the flow of bubbles coming into a T- junction 52. An electric field may also be applied to facilitate co-axial electrohydrodynamic flow in order to further regulate microbubble 40 size.

In accordance with embodiments of the invention, in-situ drug dosing of the microbubbles 40 may be carried out in a single-step production operation, according to one of the following alternative scenarios:

1. When the bubbles 40 are flowing through the last stage of the multi-T junction apparatus (50), co-axial flow of the drug will be facilitated as shown in Figure 6a. Figure 6a shows one final division of a multi-T junction apparatus (50). The drug is introduced to each capillary leaving the T- junction 52, which contain the microbubble 40 suspension. This will enable mixing of the drug with the pre-formed microbubbles 40, resulting in the microbubbles 40 becoming coated with the drug.

2. Alternatively, the bubble-liquid suspension exiting the T-junction device 10 for generating microbubbles, as shown in Figure 3, will be made to focus through another T-junction, as shown in Figure 6b, at which the drug is supplied, as a liquid or in solution, perpendicular to the bubble-liquid suspension flow. This will have the effect of coating the microbubbles 40 with the drug before they are subjected to any subsequent resizing operations.

3. The air or gas 20 used with the T-junction device 10 for generating microbubbles, as shown in Figure 3, may be loaded with particulate or aerosol drug plumes before being supplied to the T-junction device 10 for generating microbubbles. Alternatively the dosage may be supplied via the liquid phase supplied to the T-junction device 10 for generating microbubbles.

Industrial Application

In medicine, microbubbles have become well established as the most effective form of contrast agent for diagnostic ultrasound imaging, owing to their high compressibility and their ability to scatter ultrasound non-linearly. More recently, the use of coated microbubbles in therapeutic applications such as targeted drug delivery has also become an active area of research. By incorporating drugs into the microbubble coating, it may be used as a carrier particle that can be traced through the body using low intensity ultrasound and then destroyed with a high intensity burst in order to release the drug in a specific region. By localising the treatment in this way, harmful side-effects from chemotherapy can be reduced.

Monodisperse bubbles will improve ultrasound imaging beyond what can currently be achieved with commercial bubbles, and this will benefit the user as follows. The resonance frequency and destruction threshold of coated microbubbles is determined primarily by their size and the thickness and material properties of the coating. By maximising bubble uniformity therefore, the number of bubbles which will be acoustically active at a given ultrasound frequency also be maximised, thus reducing the need for large doses of contrast agent which can produce imaging artefacts (and pose a potential risk for harmful bio-effects). For drug delivery procedures, having a uniform population will enable a much higher percentage of bubbles to be destroyed simultaneously than in a polydisperse suspension, since their dynamic response to ultrasound excitation will also be more uniform. This will reduce the dosage required and also the risk of harmful chemotherapeutic drugs being released away from the target site. It will also improve dosing accuracy at the encapsulation stage.

In non-medical applications the drug dosing stage may not be required. Suspensions of gas microbubbles stabilised by a surfactant or polymer coating play a vital role in processing applications across a wide range of sectors including materials science, chemical engineering, medicine, biochemistry and the food, cosmetic and pharmaceutical industries. In environmental technology, for example, they are used in activated sludge treatment to increase the rate of degradation and particulate removal. In polymer processing, coated microbubbles are used as fillers for improving the factional, electrical and thermal properties of products such as non- woven textiles, cable sheathing and paint; and for reducing the density of construction materials such as synthetic wood. In the food and cosmetic industries, the manufacture of a wide range of products such as bread, aerated dairy products, foams and lathers relies upon the production of stable microbubbles whilst, in molecular biology, microbubbles are central to the mesoscale self assembly of smart materials, microfabrication and DNA-driven assembly techniques.

Claims

1. A microbubble producing device, comprising: a first capillary, connectable to a gas supply; and a second capillary, connectable to a liquid supply; wherein said first and second capillaries meet at a junction and, in use, gas and liquid enter the junction, through said first and second capillaries respectively, producing microbubbles.

2. The microbubble producing device according to claim 1, further comprising a third capillary which meets said first and second capillaries at said junction, and wherein, in use, said produced microbubbles leave said junction via said third capillary.

3. The microbubble producing device according to claim 1 or claim 2, wherein said microbubble producing device is arranged to produce bubbles which are substantially monodispersed.

4. The microbubble producing device according to any one of the previous claims, wherein said a first capillary is connected to a constant pressure or constant flow gas supply.

5. The microbubble producing device according to any one of the previous claims, wherein said a second capillary is connected to a syringe, and further comprising a syringe pump for controlling the flow rate of liquid from said syringe.

6. The microbubble producing device according to any one of the previous claims, wherein said second capillary is arranged at an angle in the range of from 0 to 90° to said first capillary, and preferably perpendicular to said first capillary.

7. The microbubble producing device according to 2, or any one of claims 3 to 6 when appendant to claim 2,, wherein said third capillary is arranged such that it has a longitudinal axis that is coincident with a longitudinal axis of the first capillary.

8. The microbubble producing device according to claim 2, any one of claims 3 to 7 when appendant to claim 2, or claim 7, wherein the spacing between an end of said first capillary and an end of said third capillary is 30μm -I5θμm, preferably 50μm -1 OOμm, and more preferably 70μm.

9. The microbubble producing device according to any one of the previous claims, wherein the capillaries have a diameter of 50μm - lOOOμm, preferably 80μm - 500μm, more preferably lOOμm - 200μm, and even more preferably 150μm.

10. The microbubble producing device according to any previous claim, wherein the microbubble producing device is arranged to produce microbubbles with diameters in the range 1-1 Oμm.

11. A microbubble dividing device, comprising a first bubble dividing stage, said first bubble dividing stage comprising: a supply capillary connectable at a first end to a microbubble supply, wherein said supply capillary is divided at a second end into two capillary side arms; and such that, in use, microbubbles passing from the supply capillary into the two capillary side arms are divided into two smaller microbubbles, one of said two smaller bubbles passing into the first of said two capillary side arms, the second of said two smaller microbubbles passing into the second of said two capillary side arms.

12. The microbubble dividing device according to claim 11, further comprising at least one further microbubble dividing stage, said further stage(s) comprising: a supply capillary connected at a first end to a capillary side arm of a previous microbubble dividing stage, and wherein said supply capillary is divided at a second end into two capillary side arms; such that, in use, a microbubble passing through any bubble dividing stage is divided into two smaller microbubbles, one of said two smaller microbubbles passing into the first of said two capillary side arms of said bubble dividing stage, and the second of said two smaller microbubbles passing into the second of said two capillary side arms of said bubble dividing stage.

13. The microbubble dividing device according to claim 11 or claim 12, wherein said microbubble dividing device is arranged to divide microbubbles into two smaller microbubbles of substantially equal size at each bubble dividing stage.

14. The microbubble dividing device according to any one of claims 11 to 13, wherein said supply capillaries at each of said microbubble dividing stages are constructed such that said supply capillaries are divided into two capillary side arms which are narrower than the supply capillary of that microbubble dividing stage.

15. The microbubble dividing device according to any one of claims 11 to 14, wherein said capillary sides arms at the same bubble dividing stage are constructed to have the same diameter as each other.

16. The microbubble dividing device according to any one of claims 11 to 15, further comprising means for applying an electric field to said microbubble dividing device to facilitate co-axial electrohydrodynamic flow.

17. A device for producing and dividing microbubbles, comprising the microbubble producing device according to any one of claims 1-10, and the microbubble dividing device according to any one of claims 11 to 16.

18. A process for producing microbubbles using a microbubble producing device, said process comprising the steps of : supplying a gas to a junction along a first capillary; supplying a liquid to a junction along a second capillary; and producing microbubbles at said junction from said supplied gas and supplied liquid.

19. The process according to claim 18, further comprising removing said microbubbles from said junction via a third capillary.

20. The process according to claim 18 or claim 19, wherein said step of producing microbubbles produces substantially monodispersed microbubbles.

21. The process according to any one of claims 18 -20, wherein said step of supplying a gas to a junction further comprises supplying said gas at a constant pressure or constant flow.

22. The process according to any one of claims 18 - 21, wherein said microbubble producing device is the microbubble producing device according to any of claims 5 — 8.

23. The process according to any one of claims 18 - 22, wherein said step of producing microbubbles further compήses producing microbubbles with diameters in the range 1-1 Oμm.

24. A process for dividing microbubbles using a microbubble dividing device, said process comprising a first bubble dividing stage of: supplying microbubbles to a supply capillary which divides into two capillary side arms; dividing each of said microbubbles into two smaller microbubbles by passing said microbubbles from said supply capillary into said two capillary side arms; such that each microbubble divides into two smaller microbubbles, one of said two smaller microbubbles is passed into the first of said two capillary side arms, and the second of said two smaller microbubbles is passed into the second of said two capillary side arms.

25. The process according to claim 24, further comprising at least one further stage, wherein said steps of supplying and dividing are repeated at each stage.

26. The process according to claim 24 or claim 25, wherein each step of dividing further comprises dividing said microbubbles into two smaller bubbles of substantially equal size.

27. The process according to any one of claims 242 - 26, wherein said microbubble dividing device is the microbubble dividing device according to claim 13 or claim 14.

28. The process according to any one of claims 24 - 27, further comprising the step of applying an electric field to said microbubble dividing device to facilitate coaxial electrohydrodynamic flow.

29. A process for producing and dividing microbubbles, comprising the process for producing microbubbles using a microbubble producing device according to any one of claims 18-23, and the process for dividing microbubbles using a microbubble dividing device according to any one of claims 24 to 28.