Abstract:
A method of forming a fluid tight seal between a first fluid pathway and a second fluid pathway a volume is defined between an outer surface of the first fluid pathway and an inner surface of the second fluid pathway. The surfaces are maintained in a given orientation and distance with respect, one to another, so as to achieve a desired capillaric property therebetween. A quantity of sealant is delivered to a junction region of said surfaces. The sealant is caused or permitted to flow into the defined volume, so as to achieve capillary balance. Only substantially sufficient sealant is delivered to fill the volume. The sealant is then caused or permitted to cure or set.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a method of forming a fluid tight seal and a device so formed.  
         BACKGROUND ART  
         [0002]    A rapid means of forming a reliable and permanent fluid tight seal between a micro-fluidic chip and external fluidic circuitry is sought. FIG. 1 illustrates how this may be achieved. A micro-fluidic device ( 10 ) has an inlet channel ( 12 ) into which a capillary ( 14 ) is inserted in order to make fluidic connection to the channel ( 12 ). The inlet channel ( 12 ) communicates with a channel ( 20 ) which leads into the device. In the prior art, sealant ( 16 ) is applied to the exterior of the device ( 10 ) at the opening of the inlet channel ( 12 ), and flows by capillary action into the seal space ( 18 ) between the capillary and the channel walls. Ideally the flow ceases when the meniscus of the sealant reaches the end of the seal space, forming a surface ( 17 ). However, in practice, as the dimensions involved are so small, a relatively large volume of sealant tends to be deposited at the opening of the inlet channel ( 12 ). This forms a surface ( 19 ), whose precise shape depends on the wetting properties of the sealant to the outside of device ( 10 ) and the capillary ( 14 ), but will be generally gently curved owing to the volume of material it contains. Capillary action tends to draw the sealant past the end of the capillary ( 14 ) into channel ( 20 ) and/or the interior of the capillary, forming menisci at positions ( 21 ) and ( 23 ). This problem arises from lack of control of capillary action at the area of application of the sealant ( 16 ). Thus the flow of sealant needs to be controlled.  
           [0003]    A method of connecting a capillary to a micro-fluidic chip is described in U.S. Pat. No. 5,890,745 (Kovacs). A capillary is inserted into a channel whose diameter is only very slightly larger than the capillary. Sealing of the capillary in the channel is achieved either by close fit alone, by a compressive plastic component mounted on the chip, or by contraction of the capillary at low temperature before insertion into the channel. In a preferred embodiment, adhesive is used to hold the capillary in place. However, no means are provided to control the flow of this sealant, and so the success of this method is likely to depend critically on maintaining close tolerance between the capillary and the channel. Also, the method will be awkward to implement for multi-way connections.  
           [0004]    U.S. Pat. No. 5,985,086 (Peall) discloses a method of sealing an optical fibre into a v-groove etched in a silicon wafer by means of adhesive wicked from reservoirs remote from the fibre, to a region where it contacts the fibre and effects a seal. The specification is clear in that a measured amount of adhesive is applied, and that the reservoir and the wicking channels are designed to convey the sealant to the sealing site, rather than to control the amount that is applied. As such, the operation still needs careful control and therefore is open to error.  
           [0005]    Our earlier UK patent application (GB9625491.7) describes a method of using capillary connections inserted into inlet channels formed in the end surface of a chip. Various means were described of sealing the capillaries into the chip, including flowing sealant into the space between the capillary and the inlet channel (referred to herein as the “seal space”). In order to prevent excess sealant entering and flowing past the end of the capillary, control over the flow or location of the sealant is necessary. Our earlier patent application described methods involving active control by an operator, for instance application of closely measured amounts of sealant, or observation to determine when sufficient amounts of sealant had been applied. Also described was the use of blocking methods, such as a removable insert inside the capillary or application of ultra-violet light to stop the flow of a UV-curable adhesive at the end of the capillary. These methods were suitable for handling of one capillary at a time, and are potentially overly time consuming for multi-way connections. The present invention overcomes these drawbacks and provides a method of applying the correct amount of sealant without accurate pre-measurement of the amount or monitoring by the operator.  
           [0006]    An aim of the invention is to provide a method of forming a fluid tight seal which allows the amount of sealant to be carefully controlled. Another aim of the invention is to provide a method of forming a fluid tight seal that is suitable for forming multi-way connections between components.  
         DISCLOSURE OF INVENTION  
         [0007]    According to a first aspect of the invention there is provided a method of forming a fluid tight seal between a first fluid pathway and a second fluid pathway comprising the steps of: defining a volume between an outer surface of the first pathway and an inner surface of the second pathway; maintaining said surfaces in a given orientation and distance with respect, one to another, so as to achieve a desired capillaric property therebetween; delivering a quantity of sealant to a junction region of said surfaces; causing or permitting the sealant to flow into said volume, so as to achieve capillary balance whereby only substantially sufficient sealant is delivered to fill the volume; and causing or permitting the sealant to cure or set.  
           [0008]    Preferably the first fluid pathway is a capillary tube. The second fluid pathway is preferably disposed within a micro-fluidic device, and is known as the inlet channel. The inlet channel may be of any cross-section, but is advantageously close in size and shape to the capillary tube.  
           [0009]    The device may have a rectangular cross-section having first and second major (i.e., upper and lower) surfaces. However, the device may be of any other suitable shape. The inlet channel preferably extends from a side surface of the device into the device, parallel to the upper and lower surfaces. However, the inlet channel could equally well be formed so that it extends from the upper (or lower) surface of the chip so that it is perpendicular to this surface.  
           [0010]    The sealant may be delivered to the junction by way of a further capillary tube, or applicator. The applicator may be used to meter a volume of sealant as well as its delivery. To this end, the applicator might have variable capillarity along its length, for example a capillary break at a certain point, which means that a controlled volume of sealant can be filled into it by capillary action and the allowed to flow out into the seal space. Such a capillary break might be formed by a sudden widening of the applicator tube (the sealant is taken to wet the applicator), or by a change in the inner surface of the applicator so as to change the angle of contact. Alternatively, more than one capillary break might be used to control flow of the adhesive under pressure: a narrowing of the applicator might increase its capillarity to the point where the seal space will no longer fill spontaneously. A pressure increase applied to the applicator might then urge the sealant past the capillary break and fill the seal space. Such an arrangement might be used for sequential filling of a number of seal spaces from a series of slugs of sealant and air in the applicator tube. As a further alternative, a sponge applicator might be used to deliver an accurately metered amount of sealant.  
           [0011]    Preferably the capillarity of the applicator is greater than the capillarity of the second fluid pathway, and the capillarity of the volume (or seal space) is greater than the capillarity of the applicator.  
           [0012]    In an alternative aspect of the invention, the sealant may be delivered to the junction region by way of an application reservoir. The application reservoir may be formed by the second fluid pathway having a stepped profile so that the volume of the second fluid pathway is larger towards the edge of the device. The reservoir and inlet channel again might be of any appropriate cross section, but advantageously are close in size and shape to the first fluid pathway. Preferably the volume of the application reservoir (whatever its shape) is greater than the volume of the seal space.  
           [0013]    Alternatively, the application reservoir may be formed by a further fluid pathway defined in the device. This further fluid pathway is preferably defined in one of the major surfaces of the device so that it is substantially perpendicular to the second fluid pathway. The reservoir might have parallel sides, a stepped profile, or a tapering profile. The stepped or tapering reservoir has the advantages that a larger opening (or port) into the device is provided so that it is more easily filled, but that the smaller dimensions needed to achieve the capillary control of filling are provided further within the structure. Initially also the capillary force opposing wicking is small, and so the seal space fills quickly. As the level of sealant in the port lowers, the minimum dimension in the reservoir profile decreases and so the opposing force increases, slowing the wicking of the sealant in to the inlet channel.  
           [0014]    Preferably the capillarity of the application reservoir is less than the capillarity of the volume. The capillarity of the application reservoir is preferably greater than the capillarity of the second pathway.  
           [0015]    Obviously the concept of a stepped or tapering reservoir might also be applied to the capillary inlet arrangement where the reservoir is formed by the shape of the second fluid pathway.  
           [0016]    A variation of the previous embodiment of the invention uses the principle of a plurality of narrow pathways through which, or past which, the sealant has to flow in order to reach the seal space. The region of the reservoir communicating with the seal space can be made to have a number of parallel narrow flow channels that together can sustain a significant flow rate, but individually have a high degree of capillarity. This might be achieved, for example, by using a porous filter in the lower portion of the reservoir.  
           [0017]    Once flow has been established through the porous filter from the reservoir (the flow being driven by capillarity in the seal space), it will continue until the meniscus of the sealant reaches the porous filter, where a number of individual much smaller menisci will form, each with large capillarity, so stopping the flow. This is particularly advantageous when there is a tendency for the sealant to wick on beyond the end of the capillary into the channel, despite the overall capillarity of the channel being lower than that of the reservoir. This is likely to happen if the channel cross-sections have sharp angle corners, for example, square, semicircular or triangular channels. The porous filter might be fabricated from a filter membrane or a micro fabricated stature, for example micro-fabricated mesh.  
           [0018]    The function will be the same if the sealant does not actually flow through the structure, but rather that the meniscus of the sealant encounters a series of narrow channels as is retreats towards the end of the reservoir. The overall effect is to introduce a region of higher capillarity than the seal space in between the reservoir and the seal space which is more effective than that of the outer end of the seal space in stopping the flow.  
           [0019]    The reservoir may also be used as a metering device for the amount of sealant applied. The amount of sealant applied relies on the rate of exit of sealant from the reservoir being lower than the rate at which it is applied to the reservoir. The use of balance of capillarity to stop the process means that the precise amount of sealant applied is immaterial. However, a further embodiment of the invention may be provided to control the rate at which the sealant leaves the reservoir, or to halt it altogether until the reservoir has been filled, and then to allow it to start. Such control can be provided by means of a capillary stop or impedance at the end of the reservoir between the reservoir and seal space. An increase in the minimum dimension of the seal space near the end of the reservoir would provide a stop, and a constriction in the seal space would provide a limitation through viscosity of the rate of outflow.  
           [0020]    Activation past the capillary stop night be by for example a positive pressure pulse applied to the reservoir inlet, a negative pressure pulse applied to a port to reduce the pressure inside the seal space, or a temperature change to change the degree of wetting of the reservoir or the surface of the seal space, to expand the sealant past the stop or to expand a gas bubble included in the sealant in the reservoir for this purpose.  
           [0021]    The methods of the invention may also include a further pathway (known as an overflow or run-off channel) for receiving excess sealant.  
           [0022]    According to another aspect of the invention there is provided a method of forming a fluid tight seal between a first substrate (or component) and a second substrate (or component) comprising the steps of maintaining said substrates in a given orientation and distance with respect, one to another, so as to define a volume therebetween which has a desired capillaric property; delivering a quantity of sealant to a junction region of said substrates; causing or permitting the sealant to flow into said volume, so as to achieve capillary balance whereby only substantially sufficient sealant is delivered to fill the volume; and causing or permitting the sealant to cure or set.  
           [0023]    A micro-fluidic chip may be formed from a first component and a second component. The first and/or second components may have a first recess formed therein which, when the first component is joined to the second component, forms a channel. The first and second components are joined by adhesive which is introduced into the seal space (i.e., volume) between the components from a reservoir at the edge of the device. The reservoir is formed simply by a recess in the first and/or second components. The reservoir may be formed by the first and/or second component having a stepped cross-section. The reservoir may be at edge of the device, or formed in one of the major surfaces of the device. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0024]    A number of embodiments of the invention will now be described with reference to the Figures, where:  
         [0025]    [0025]FIG. 1 shows a method of forming a fluid tight seal, according to the prior art;  
         [0026]    [0026]FIG. 2 shows a cross-sectional view of a first embodiment of the invention;  
         [0027]    [0027]FIG. 3 shows a cross-sectional view of a second embodiment of the invention;  
         [0028]    [0028]FIG. 4 shows a cross-sectional view of a third embodiment of the invention;  
         [0029]    [0029]FIGS. 5 a  to  5   d  show the connection of a capillary to a micro-fluidic device which is formed using an embodiment of the invention;  
         [0030]    [0030]FIGS. 6 a  to  6   f  shows various views of another embodiment of the invention;  
         [0031]    [0031]FIG. 7 shows a cross-sectional view of a further embodiment of the invention; and  
         [0032]    [0032]FIG. 8 shows a plan view of a micro-fluidic chip having a metering device.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]    A first embodiment of the invention is shown in FIG. 2, and is a system which comprises an applicator with appropriately designed capillary properties. The Figure shows a micro-fluidic chip ( 10 ) which has a rectangular cross-section, the upper and lower surfaces of the chip being longer than the side surfaces. The chip ( 10 ) has an inlet channel ( 12 ) formed therein which extends from a side surface of the chip into the chip, parallel to the upper and lower surfaces. The inlet channel ( 12 ) communicates with a channel ( 20 ) which leads further into the chip ( 10 ). The inlet channel ( 12 ) is shaped and dimensioned to receive a capillary tube ( 14 ). The diameter of the inlet channel ( 12 ) is larger than that of the capillary tube ( 14 ) so that a space ( 18 ) (the seal space) is left between the inlet channel and the capillary tube. The diameter of the channel ( 20 ) is smaller than the diameter of the capillary tube ( 14 ) so that the capillary tube is prevented from passing into the channel ( 20 ).  
         [0034]    To form a fluid tight seal between the capillary ( 14 ) and the inlet channel ( 12 ), an applicator ( 22 ) filed with sealant ( 16 ) is brought into contact with the end of the inlet channel. Sealant ( 16 ) is allowed to flow out of the applicator ( 22 ) and into the seal space ( 18 ). The applicator ( 22 ) is a capillary tube in its simplest form, and has a capillary action which tends to retain sealant ( 16 ) within it. However, the seal space ( 18 ) is designed to have a greater capillarity than the applicator ( 22 ), so that sealant ( 16 ) will be drawn out of the applicator and into the seal space.  
         [0035]    The capillarity of the applicator is also chosen to be greater than that of the inlet channel ( 12 ) (or channel ( 20 ), if the end of the capillary ( 14 ) is in contact with channel ( 20 ) and this is of narrower minimum dimension than inlet channel ( 12 )), so that when the sealant ( 16 ) reaches the end of the seal space ( 18 ), there is a net force opposing its continuing progress. In other words, the meniscus inside the applicator ( 22 ) controls the force which is opposing the filling force, rather than an uncontrolled capillary force from the ill-defined meniscus ( 19 ) which is shown in FIG. 1. In FIG. 2, residual menisci ( 11 ) are shown around the interface between the end of the applicator ( 22 ) and the side surface of the chip device ( 10 ). Capillary action tends to retain sealant ( 16 ) inside the applicator ( 22 ), and thus will ensure that the residual menisci ( 11 ) have lower capillarity than other parts of the system.  
         [0036]    A second embodiment of the invention is shown in FIG. 3. As in the first embodiment of the invention, there is shown a micro-fluidic chip ( 10 ) which has an inlet channel ( 12 ) formed therein. The inlet channel is designed to accept a fluid connection capillary ( 14 ), and communicates with a channel ( 20 ) which leads into the chip ( 10 ). In this embodiment of the invention, the inlet channel ( 12 ) has a stepped profile so that its radius, r R , in the region where the inlet channel leads out of the chip ( 10 ) is larger than its radius, r C , towards the inner region of the chip ( 10 ), thereby forming an application reservoir ( 24 ). The radius of the channel ( 20 ) is the same as the radius r C  of the inner region of the inlet channel ( 12 ).  
         [0037]    To form a fluid tight seal between the capillary tube ( 14 ) and the micro-fluidic chip ( 10 ), sealant ( 16 ) is introduced into the application reservoir ( 24 ). The capillarity of the application reservoir ( 24 ) is designed to be less than that of the seal space ( 18 ), but greater than that of the channel ( 20 ). In this way, the capillarity of the application reservoir ( 24 ), the seal space ( 18 ), and the channel ( 20 ) control the application of the sealant. For circularly symmetric channels ( 12 , 20 ), with an application reservoir ( 24 ) and capillary ( 14 ) made of identical materials and hence contact angles, the condition for the movement of sealant ( 16 ) to be controlled via differences in capillarity is as follows: (r c −r t )&lt;(r R −r t )&lt;r j &lt;r c , where r c  is the diameter of the channel ( 20 ) and the inner region of the inlet channel ( 12 ), r t  and r j  are the outer radius and inner radius of the capillary ( 14 ), and r R  is the diameter of the application reservoir ( 24 ).  
         [0038]    The condition for channels of other cross sections and for differing contact angles will (all other parameters being equal) depend on the order of the minimum dimension in the system. In other words, the diameter of the seal space ( 18 ) is less than the diameter of the application reservoir ( 24 ), which is less than the diameter of the channels ( 12 , 20 ) beyond the capillary ( 14 ) and the internal dimension of the capillary ( 14 ). The condition for the size of the reservoir ( 24 ) is that its volume must be greater than the volume of the seal space ( 18 ), and that the reservoir ( 24 ) can be filled by an operator before the seal space ( 18 ) has filled substantially. The preferred condition of operation is that the operator can fill the reservoir ( 24 ), and the opposing force of the menisci ( 17 ) and ( 25 ) will draw the sealant to the ends of the capillary ( 14 ), where it will stop.  
         [0039]    A single reservoir ( 24 ) and a single inlet channel ( 12 ) are shown in FIG. 3. However, the same principle can be applied to a number of channels and capillaries with a common reservoir ( 24 ), provided that the volume of the reservoir is adequate for them all, and that the minimum dimension of the reservoir maintains capillarity greater than that of the channel ( 20 ). A preferable arrangement for the reservoir is a rectangular shape with a height close to that of the capillaries, with the capillaries and inlet channels arranged side-by-side.  
         [0040]    A third embodiment of the invention is shown in FIG. 4, where the application reservoir ( 24 ) is formed in the upper surface of the micro-fluidic chip ( 10 ). As in the aforedescribed embodiments of the invention the chip ( 10 ) has an inlet channel ( 12 ) formed therein which receives a capillary tube ( 14 ). The application reservoir ( 24 ) leads to a parallel sided channel ( 34 ) which in turn leads to the inlet channel ( 12 ). In order to form a fluid tight seal between the chip ( 10 ) and the capillary ( 14 ), sealant ( 16 ) is introduced into the application reservoir ( 24 ). The sealant passes into die parallel sided channel ( 34 ) to the seal space ( 18 ) between the inlet channel ( 12 ) and the capillary ( 14 ).  
         [0041]    As shown in FIG. 4, the application reservoir has a tapered profile. This is advantageous as a larger opening into the chip ( 10 ) is provided which is more easily filled with sealant ( 16 ), and the smaller dimensions needed to achieve capillary control of the sealant are provided at the base of the reservoir ( 24 ), leading into the inlet channel ( 12 ). In addition, the capillary force opposing wicking of the sealant into the seal space ( 18 ) is small and so the seal space fills quickly. The parallel sided channel ( 34 ) provides a region of constant high capillarity near the end of the filling process, and so gives better control over the stopping point of the sealant  
         [0042]    As the level of sealant in the application reservoir ( 24 ) decreases, the force opposing the movement of the sealant increases, thereby slowing the wicking of the sealant into the inlet channel ( 12 ). At its base, the application reservoir ( 24 ) has a diameter such that its capillarity lies between that of the seal space ( 18 ) and that of the channel ( 20 ) beyond the seal space, thereby stopping the wicking of the sealant into the channel.  
         [0043]    The aforedescribed embodiments of the invention describe the concept of sealing a capillary tube ( 14 ) to a fluidic inlet channel ( 12 ) of a micro-fluidic chip ( 10 ). This concept can also be applied to the introduction of sealant into any other space, for example the region between two components which are to be joined to form a device. This gives an advantageous way of controlling the ingress of adhesive between two flat components of a micro-fabricated device.  
         [0044]    Frequently, micro-fluidic devices are fabricated in more than one part, with details on one fare which define channels or other features, this face then being joined to another face to close the features. The faces are held together by various means, one of which is the ingress of liquid adhesive to the space between the faces, this ingress driven by the capillarity of the space. In the prior art devices, it is difficult to control the ingress of adhesive closely—the amount which is taken into the space depends on the stopping of the flow when the meniscus reaches a defined boundary between the faces where the capillarity suddenly drops. This may occur by increase of the minimum dimension of the space, as happens at the boundary of an open channel feature, or by a change in the contact angle at one or both of the faces, as might be achieved by a change in the surface nature or the bulk nature of the materials making up the faces. This may not be sufficient in some cases, and there is a danger that the channels will fill and blocks.  
         [0045]    A fourth embodiment of the invention is shown in FIG. 5 a . This Figure shows a cross-section through part of a micro-fluidic chip ( 10 ) formed from a first component ( 40 ) and a second component ( 42 ). The first component ( 40 ) has a first recess ( 44 ) formed therein which, when the first component ( 40 ) is joined to the second component ( 42 ), forms a channel. The first ( 40 ) and second ( 42 ) components are joined by adhesive ( 16 ) which is introduced into the seal space ( 46 ) between the components ( 40 , 42 ) from a reservoir ( 48 ) at the edge of the device. The reservoir ( 48 ) is formed simply by a recess ( 50 ) in the first component ( 40 ).  
         [0046]    The position of the adhesive ( 16 ) after capillary action has reached equilibrium is determined by the minimum dimensions of the pathways at the positions of the adhesive menisci ( 52 ) and ( 54 ), and the amount of adhesive applied. The amount of adhesive applied is determined by the volume of the reservoir ( 48 ), which is larger than the volume of adhesive ( 16 ) intended to be wicked into the seal space ( 46 ). The reservoir ( 48 ) is designed to fill quickly relative to the movement of adhesive ( 16 ) out of the reservoir into the seal space ( 46 ). To this end, the reservoir ( 48 ) may have features formed within it to speed the ingress of adhesive and to render wall effects less important, giving a more uniform front to the adhesive as it enters the seal space. Grooves in one or more walls of the reservoir ( 48 ) are an example of such a feature.  
         [0047]    [0047]FIG. 5 b  shows an alternative arrangement which is advantageous when the adhesive ( 16 ) has a significant viscosity. The seal space ( 46 ) is provided with features which act to ease the flow of adhesive ( 16 ) through it, optionally directing the flow preferentially into certain regions, while ensuring that it comes to a capillary stop where it is required to. The closer the spacing between the components ( 40 , 42 ), i.e., the narrower is seal space ( 46 ), the more marked will be the capillary stop when the adhesive reaches the channel ( 44 ). However, if the adhesive is viscous, a narrow space will fill only slowly from a remote reservoir. Seal space ( 46 ) is therefore provided with regions ( 56 ) of lower capillarity and concomitant lower viscous impedance, and regions ( 58 ) of higher capillarity next to the channels ( 44 ) where a good seal and a defined capillary stop are required. Regions ( 56 ) might be channels which act to direct flow of adhesive ( 16 ) from the reservoir ( 48 ) to areas where it is required. Alternatively, regions ( 56 ) might be larger so as to form an open sealing area rather than a narrow channel; the sealing area might be subdivided by ribs or similar structures to form adjoining areas which fill in a pre-ordered manner from one or more reservoirs. The capillary stop is still provided by the balance of capillarity at the interior meniscus ( 52 ) and the exterior meniscus ( 54 ).  
         [0048]    An example of an application of this embodiment of the invention is shown in FIG. 5 c  and FIG. 5 d . FIG. 5 c  shows a capillary ( 80 ) connected to a plug component ( 82 ). The capillary is inserted into a channel ( 84 ) formed in the plug component ( 82 ) leaving a short length ( 88 ) of capillary protruding from the end of the plug. Sealant ( 16 ) is then wicked into the seal space ( 86 ) between the plug ( 82 ) and the capillary ( 80 ), and subsequently hardened. The end length ( 88 ) of the capillary is then removed to form a plug assembly ( 90 ). Typically the sealant ( 16 ) will form a reproducible meniscus around the capillary ( 80 ) and so the profile of the surface ( 89 ) which is formed when the end length ( 88 ), of the capillary ( 80 ) is removed will be predictable, and can be accommodated into the design of the opening in the device that receives it. Alternatively the surface ( 89 ) of the plug component might easily be ground or polished to be largely flat.  
         [0049]    The plug assembly ( 90 ) so formed is then connected to a micro-fluidic device ( 10 ), as shown in FIG. 5 d . The device ( 10 ) has an opening (or socket) formed to be a close fit to the plug assembly ( 90 ). A seal space is formed between the surfaces ( 94 ) and ( 96 ) of the socket and plug respectively. This seal space is filled with sealant in the controlled manner of the invention. The surfaces form a reservoir ( 98 ) with a known capillarity which is designed to be intermediate between the capillarity of the space ( 100 ) between the end of the plug and the recess near the channel ( 93 ) in the device. In this way the correct amount of sealant is drawn into the seal space. Optional projections ( 104 ) might be provided on the base of the plug to control the capillarity of the space ( 100 ). Also projections or surface profiling might be provided on one or both of the surfaces ( 94 ,  96 ) to control the movement of sealant ( 16 ) in the space between them. In this way, a reliable permanent attachment is made between the capillary ( 80 ) and the device ( 10 ).  
         [0050]    While the connection of a single capillary to a device has been described, a similar connection might use multiple capillaries in any practical geometric arrangement. The capillaries would be attached first to the plug, and then the whole multi-way plug sealed in one process into the device port. Devices might be connected together by ready-formed plug-to-plug multi-way capillary ‘cables’. The means of sealing a capillary into a ready formed component as in FIG. 5 c  might be applied to a socket component also, allowing the seal structure and method in FIG. 5 d  to join two capillary ‘cables’ end-to-end.  
         [0051]    In situations where overfilling of a reservoir might occur, it is advantageous to provide run-off channels to accommodate excess sealant. FIG. 6 a  shows a cross sectional view, and FIGS. 6 b ,  6   c  and  6   d  plan views, of another embodiment of the invention which incorporates this idea In FIGS. 6 a  to  6   d , there is shown a device ( 10 ) formed of components ( 40 ) and ( 42 ), which define a reservoir ( 48 ) for sealant ( 16 ). The device also has a second channel ( 60 ) which communicates with the reservoir ( 48 ), and has a vent ( 62 ) at its other end. The reservoir ( 48 ) leads to a channel or seal space ( 46 ) into which sealant is to be moved by capillary action. Sealant ( 16 ) is applied to the inlet of the reservoir ( 48 ) and wicks into the reservoir, and then into channel ( 60 ).  
         [0052]    In the case that the reservoir ( 48 ) opens to the edge of the device ( 10 ), the sealant will tend to form a meniscus ( 66 ) which protrudes from the reservoir and from the edge of the device, as shown in FIG. 6 b . This is shown by way of example only, and it is likely in preferable embodiments that sealant will be dispensed into a specifically provided receiving area into which the reservoir ( 48 ) opens. As the sealant moves into the reservoir ( 48 ) under capillary action, it encounters the start of the channel or seal space ( 46 ) which has a greater capillarity than the reservoir, and moves quickly into this, the driving force being the difference in capillarity at the seal space meniscus ( 68 ) and that at the reservoir meniscus ( 66 ). The minimum dimension of channel ( 60 ) is smaller than that of the reservoir ( 48 ), so when the sealant in the reservoir is emptied into the seal space, the greater capillarity in channel ( 60 ) means that that a portion of sealant is left in situ. The system provides a competition between tide rate of flow into the seal space and that into the side channel, so acting to meter the amount that flows into the seal space.  
         [0053]    Preferably the system includes a capillary break, which is shown in FIG. 6 a  as a narrowing of the channel ( 72 ). Once the slug of sealant ( 16 ) in the reservoir ( 48 ) has been separated into two by detachment of the main flow from the excess in the channel ( 60 ), the capillary break will act to halt the main flow of sealant into the seal space at the break as shown in FIG. 6 d . This allows an accurate amount of sealant to enter the seal space.  
         [0054]    An embodiment preferred for some applications is shown in cross-section in FIG. 6 e , and in plan view in FIG. 6 f . Here, the overflow channel ( 60 ) is provided at the start of the seal space. The greater capillarity of the seal space means that that will fill first. Excess sealant is then drawn into the overflow channel ( 60 ). The vent channel ( 62 ) might be designed to end up filled with sealant, or a capillary stop might be provided to halt the flow at the end of the channel ( 60 ). In this and previous embodiments, the vent channel ( 62 ) might be as shown, or might exit through another face of the device ( 10 ).  
         [0055]    [0055]FIG. 7 shows an alternative embodiment of the invention, suitable for use in sealing capillary connections in place, in which a runoff channel ( 30 ) is provided As before, when sealant is placed in the port ( 24 ) it wicks into both the seal space ( 18 ) and into channel ( 30 ). The processes will compete, and the dimensions of channel ( 30 ), port ( 24 ) and the seal space ( 18 ) are chosen so that the effect of channel ( 30 ) is to lower the level of sealant in the port to below the junction of the port and channel ( 30 ) in a time less than that taken for sealant to reach the end of the seal space, preferably much less. In this manner, the amount of sealant which will fill the seal space is set approximately by the dimensions of the lower part of the port, the excess sealant being taken up by the ran-off channel ( 30 ). Correct choice of the dimensions of the channels will make this a close approximation.  
         [0056]    A metering device can be incorporated in the surface of the micro-fluidic chip ( 10 ) as in FIG. 8, allowing an accurate amount sealant ( 16 ) to be metered from an unmeasured dispensing process, and then injected by transient pressure past a capillary stop into the seal space ( 18 ). The metering apparatus ( 150 ) is formed in the surface of a substrate ( 152 ). An application recess ( 154 ) communicates with a metering channel ( 156 ) and one or more overflow channels ( 158 ). The channel ( 156 ) has at its other end a capillary stop ( 114 ), past which a channel leads to a capillary fill port ( 160 ). Capillarities in the design are such that the capillarity of the capillary stop ( 114 )&lt;application recess ( 154 )&lt;that of the overflow channel ( 158 )&lt;the metering channel ( 156 )&lt;the capillary fill port ( 160 ). This means that when liquid is applied to recess ( 154 ), it flows preferentially into channel ( 156 ) until it reaches the capillary stop, with meniscus position ( 124 ), then into overflow channel(s) ( 158 ), leaving a meniscus that may lie within the region of the application recess as shown in FIG. 8, or may be at the ends of the channels where they open into the recess. This leaves a metered amount of liquid in channel ( 156 ) and the remainder in channel(s) ( 158 ). Application of positive pressure to the application recess, for example by pressing a semi gas-tight cover placed over the recess, then moves the meniscus ( 124 ) past the capillary stop; allowing channel ( 12 ) to empty into the port ( 160 ), and moving the meniscus(es) ( 162 ) further into the overflow channels. Thereby a metered amount of material is introduced to port ( 160 ).  
         [0057]    In summary, the present invention controls capillary forces at or near the point of application of the sealant to reduce or prevent the tendency of a meniscus to flow beyond a desired stopping point This can be achieved in three ways, all within the scope of the invention. The sealing system can be designed to control the movement using opposing capillary forces, filling from a reservoir of lesser capillarity than the space between the capillary and the channel wall, but greater capillarity than the channel beyond the end of the capillary; the seal space might fill in competition with a second space, the capillarity of the second space being less than that of the seal space but greater than that of the channel beyond the capillary, or the amount of sealant needed might be metered by a capillary fill structure before it is introduced to the seal space. By each of these methods the sealing process is made either less dependent, or completely independent of the amount of sealant initially applied, and is self-terminating, i.e., no observation or control of the process by an operator is needed. This makes the seal process much easier and more reliable and no longer is the quality of the seal dependent upon the volume of sealant supplied because this volume is self-regulating.  
         [0058]    An advantage of the invention is that it provides a means of accurately controlling the movement of and the amount of adhesive introduced into a micro-fluidic device, formed from more than one component fixed together, under the influence of capillary action.