Directional flow ion-junction bridge

Abstract: An ion-junction fibrous bridge and a potentiometric test element incorporating the same, wherein the fibers extending generally along the center-line between two liquid access apertures in the bridge, are readily capable of conducting aqueous liquid through the bridge, and fibers extending at an angle of at least 20° from said center-line are not readily capable of conducting aqueous liquid.

Description: This invention relates to an ion-junction bridge, and a potentiometric test element incorporating the same, for clinical diagnosis of an ionic analyte. In the field of clinical diagnosis, it is common to test certain ionic analytes using a potentiometric slide test element providing a differential potential between a reference liquid and the patient sample liquid, both of which are aqueous. Such an element features two substantially identical ion-selective electrodes held spaced apart in a frame, and an ion-junction bridge connecting them. The bridge is apertured so as to provide two liquid access apertures each aligned and in fluid contact with one of the electrodes. To ensure the two liquids will flow together within the bridge, the bridge is constructed to induce liquid flow away from the entrance aperture toward the other aperture. For example, the bridge can be a sheet of fibers, for example, paper; on or embedded within the frame. An example of the first is shown in US-A-4,053,381 and an example of the second is shown in US-A-4,273,639. Although such examples have worked admirably, they do have a minor problem - liquid flow within the bridge tends to occur in all directions, rather than just from one aperture to the other. At best, this non-directional flow requires more than the minimum amount of liquid. At worst, liquid flowing in non-desired directions can lead to shorting. As a result, mechanical means are preferably added to keep liquid flow from extending in the non-desired direction, namely, directions angled away from the straight-line direction between the apertures. An example of dams placed parallel to the straight-line direction to cut down on flow away from that direction, is shown in US-A-4,556,474. Although such dams can reduce the amount of liquid required, they do further complicate the manufacturing process. Thus, it has been a problem, prior to the invention, to provide for an ion-junction bridge that inherently favors only flow directions that are the desired flow directions, minimizing the volume of liquid needed. We have constructed an ion-junction bridge, and a test element using the same, which solve the aforesaid need. More specifically, the above-noted problems are solved, in accord with one aspect of the invention, by an ion-junction bridge for bringing two liquids together to make an ion exchange junction, the bridge comprising a sheet of fibers having two apertures therethrough; whereby the fibers comprise a material which favors aqueous flow in no more than two generally orthogonal directions and suppresses aqueous flow in directions angled away from either of the orthogonal directions, one of the orthogonal directions being generally aligned with the two apertures, so that aqueous liquids deposited in said apertures will flow primarily in the orthogonal direction being aligned with the apertures. In accord with another aspect of the invention, the above-noted problems are solved by a potentiometric test element comprising a frame, two substantially identical ion-selective electrodes mounted spaced apart on the frame, and an ion-junction bridge extending between the electrodes, the bridge comprising a sheet of fibers having two liquid access apertures through the sheet, each aperture being disposed above and in fluid contact with one of the electrodes, whereby the fibers comprise a material which favors aqueous flow in no more than two generally orthogonal directions and suppresses aqueous flow in directions angled away from either of the orthogonal directions, one of the orthogonal directions being generally aligned with the two apertures, so that aqueous liquids deposited in said apertures will flow primarily in the orthogonal direction being aligned with the apertures. Accordingly, it is an advantageous feature of the invention that an ion bridge for a test element inherently provides no more than orthogonally-directed flow, one of which is generally aligned with the two apertures, thereby reducing the amount of liquid needed to form the ion-junction needed to do the potentiometric test. It is a related advantageous feature that such a bridge reduces the likelihood of liquid flow to areas of the bridge that can cause problems. Other advantageous features will become apparent upon reference to the following Detailed Description, when read in light of the attached drawings. Fig. 1 is an isometric view of a slide test element, such element featuring an ion-junction bridge that is either a prior art bridge or is constructed as per this invention; Fig. 2 is an exploded isometric view of another test element, again featuring an ion-junction bridge that is either a prior-art bridge or constructed in accordance with the invention; Fig. 3 is a fragmentary, partially schematic, isometric view of just an ion-junction bridge constructed in accordance with the invention, for use in the test element of either Fig. 1 or Fig. 2; Fig. 4 is an exploded isometric view of an alternate embodiment of the invention; Fig. 5 is a partially schematic plan view of still another embodiment of the invention; and Fig. 6 is a section view taken generally along line VII-VII of Fig. 5, wherein the number of fibers shown is schematic for simplicity. The invention is described hereinafter in connection with preferred slide test element constructions having a particular arrangement of electrodes, bridge, and a slide frame, and woven fibers of particular materials in the bridge. In addition, the invention is applicable regardless of the specific arrangement of parts of the slide test element, so long as the ion-junction bridge has two access apertures and at least one predominant aqueous flow direction that is generally aligned with the two apertures. As used herein, "generally aligned" means, along the center line drawn between the centers of the two apertures, plus or minus 20°. The invention is also useful whether or not the sheet of fibers of the bridge is woven with only two directions (warp and weft) and regardless of the materials selected to produce a predominant flow in a direction. Figs. 1 and 2 illustrate some typical preferred constructions for slide test elements. In test element 10, Fig. 1, two solid electrodes 12 and 14 are mounted on a frame 16, and a capillary bridge 18 is provided for promoting ionic migration between two fluid access holes 20 and 22 at the electrodes. The capillary bridge includes a nonporous support layer, a porous layer with ionic access to both electrodes, and a top nonporous cover layer which is preferably non-aqueous-conducting. When a drop of reference solution of known ion activity is applied to one fluid access hole and a drop of test solution is applied to the other fluid access hole, the drops spread into the porous layer until contact is made at a thin junction interface between the apertures, permitting ionic migration between the drops. An electrometer 24 is provided to measure the electrical potentials at the interfaces between each solution drop and its associated electrode to provide an indication of ion activity in the test solution. In a similar manner, an alternative useful construction, Fig. 2, features a test element 30 which has an electrically insulative mounting frame 32 formed of a base web 34 and a spacer web 36. Two solid electrodes 38 and 40 are mounted in the frame and electrically isolated from each other. A cover sheet 42, with an internal capillary ion bridge 44 embedded therein, promotes ionic migration between solution drops deposited in fluid access holes 46 and 48. The fluid access holes extend through the cover sheet in the region of, and to provide fluid contact with, electrodes 38 and 40. Two electrical access holes 50 and 52 are also formed in the cover sheet. In both embodiments (of Fig. 1 and Fig. 2), the electrodes and their use are conventional and require no further discussion. It will be readily appreciated that if the embodiment of Fig.. 1 or Fig. 2 is constructed in accord with the prior art, the ion-junction bridge comprises a porous sheet of material, such as fibers, which allows flow generally in all directions. For example, bridge 44, Fig. 2, is formed from paper such as the Whatman #2 chroma sheet described in the aforesaid US-A-4,273,639. It is this kind of bridge that is improved upon by this invention. In accordance with the invention, the ion-junction bridge in either the embodiment of Fig. 1 or Fig. 2 comprises bridge 100 shown in Fig. 3. More specifically, such a bridge comprises a sheet 102 of fibers, preferably a woven sheet in which the fibers extend in two orthogonal directions. The machine direction, or warp fibers, are those fibers 110 and 110', and the weft direction fibers are fibers 112 extending generally perpendicularly to fibers 110. As in most ion-junction bridges, liquid access apertures 46 and 48 extend all the way through sheet 102, to provide fluid contact with an ion-selective electrode located underneath (not shown). Optionally, but not necessarily, sheet 102 is embedded in a layer 120 of plastic, such as polystyrene or polyethylene. Aperture 46 and 48 each has a side wall 140 which intersects fibers 110' and some of fibers 112. Stated in other words, apertures 46 and 48, when punched out of the bridge, have exposed at wall 140, ends or side portions of fibers 110' and 112. See for example, end 144 in the sidewall of aperture 46 of a fiber 110'. However, to provide mono-directional flow, only the thus-exposed portions of fibers 110' are capable of extensive absorption and carrying of the liquid deposited in the aperture, as will become apparent. (The illustration in Fig. 3 is schematic, because preferably there are more fibers per inch than shown - 20 to 40 per inch, for example. Also, there may be more than one layer.) Unlike fibers 110', fibers 110 are not active carriers of the liquid as they do not intersect wall 140. Importantly, fibers 110 extend in a direction that is generally aligned with center line 130 extending between center-points C of apertures 46 and 48. That is, the lines along which the warp fibers 110 extend deviate from center-line 130 by an angle of no greater than 20°, and most preferably, no greater than 17° to ensure that liquid flow along fibers 110 intersects both apertures. Also importantly, fibers 110,110' comprise an aqueous-conducting material, whereas fibers 112 comprise a non-aqueous-conducting material. As used herein, "aqueous-conducting" means, conducting aqueous liquid readily along the length of the fiber. Any suitable materials having these properties can be used. Preferred aqueous-conducting materials are selected from the group consisting of cotton, acetate, rayon, and polyester multifilament or hollow core yarn, as well as hollow core yarn made from water-repellant materials such as polyester and polypropylene. The latter hollow core yarn are useful because the hollow core will conduct aqueous liquid through the core, notwithstanding that the shell around the core is water-repellant. Preferred non-aqueous-conducting materials are mono- or multifilament yarn selected from the group consisting of polyester, polypropylene, and any aqueous-conducting material coated with a water-repellent composition. Preferred water-repellent compositions are selected from the group consisting of wax, poly(tetrafluoroethylene), silicone, and fluourinated vinyl polymers such as "Scotchgard" ® polymer available from 3M Company. Thus, a highly preferred non-aqueous-conducting material is cotton yarn coated with "Scotchgard" ®, obtainable from 3M Company. Thus it is that an aqueous liquid, for example, a reference liquid or patient sample, when placed in either aperture 46 or 48, will traverse along primarily fibers 110', and not fibers 112, not even those fibers 112 that intersect a sidewall 140. An ion junction then forms somewhere between the two apertures, for example along wavefront 150, created when the two liquids meet along fibers 110'. It is not necessary that the ion bridge be the only item in the potentiometric test element that has a mono-directional flow property. Fig. 4 illustrates the manner in which a mono-directional flow can be expanded to include other, but separate, parts. Parts similar to those previously described bear the same reference numeral to which the suffix "B" is appended. Thus, a cover sheet 42B has two liquid access apertures 46B, 48B, and an ion bridge 100B comprising a sheet 102B of fibers as described above. However, instead of the ISEs being disposed directly underneath sheet 102B (which may be embedded in plastic), there is interposed a new sheet of plastic 200 apertured at 202, 204. Below that is a second sheet 210 of fibers which comprises aqueous-conducting fibers 110B and non-aqueous-conducting fibers 112B, just as with bridge sheet 100B, except that fibers 110B of sheet 210 run substantially perpendicular to the direction of fibers 110B in sheet 100B. Also, unlike sheet 100B, sheet 210 should not be embedded in plastic since it needs to adequately wet the apertures in the plastic sheet below it, discussed below. The orientation of fibers 110B causes flow of liquid reaching sheet 210 to proceed away from contact points 220 of sheet 210 towards end 222 of sheet 210, that is, orthogonal to the direction of predominant flow in sheet 100B. End 222 overlies apertures 310 in the sheet of plastic 300 disposed below. It is apertures 310 which in turn conduct liquid flow down to the ISEs 38B, 40B that are mounted below sheet 300. To ensure liquid will "turn the corner" and flow from sheet 210 to the ISEs, apertures 310 are constructed with the properties explained in commonly-owned US-A-4,271,119. For example, the length L can be much greater than the width W of these apertures, as described in the '119 patent. Additionally, a filter paper material, such as Whatman filter paper, not shown, can be inserted into apertures 310 to aid in wetting the ISEs below. As with sheet 100B, fibers 110B of sheet 210 can also deviate from the direction exactly orthogonal to the center line 130B between centers of contact 220, but by no more than 20°. The advantage of this construction is that the portion of the ISEs that is contacted with liquid (shown shaded) is no longer exposed to air. Such air exposure has been found to be detrimental in certain circumstances, and this embodiment avoids that. It is also not necessary that each sheet of fibers conducting the liquid, be only mono-directional. That is, sheets 210 and 102B can be combined into a single sheet, Figs. 5 and 6. Parts similar to those previously described bear the same reference numeral to which the distinguishing suffix "C" is applied. Thus, a test element can be constructed as in the case of the embodiment of Fig. 4, except that sheets 200 and 210 are omitted, and sheet 100B is modified as follows: Fig. 5 illustrates an ion bridge 100C that also acts to provide aqueous liquid flow in a direction generally orthogonal to the centerline direction of line 130C, but suppresses the flow in any direction that is angled more than 20° from said orthogonal directions. Thus, as used herein "generally orthogonal" means, orthogonal within plus or minus 20°. Specifically, sheet 102C of bridge 100C, shown embedded in plastic, has only fibers 110C and 110C' (in actuality, more numerous than those shown), going in both orthogonal directions as both the warp (110C) and the weft (110C') of the sheet. (In Fig. 6, the embedded warp fiber 110C is shown in dotted line.) To ensure these both are aqueous-conducting, they both comprise hollow-core fibers with water-repellant (non-aqueous-conducting) shells. They still conduct aqueous liquid, but only if they intersect vertical sidewall 140C of either aperture 46C or 48C, such as at openings 300, Fig. 6. However, there is a suppression of any flow that departs by more than 20° from the direction of the orthogonally-oriented fibers 110C and 110C'. That is, fibers 110C' can be off the centerline 130C, Fig. 5, by an angle ∝ 1 which can be up to 20°, and the fibers 110C can extend in a direction that departs from the direction 500 that is orthogonal to fibers 110C', whatever direction that is, by an angle ∝ 2 , which can be up to 20°. To ensure that liquid conducted along fibers 110C from apertures 46C, 48C is in fact delivered to apertures 310 of sheet 300 below, apertures 400, 404 are formed in sheet 100C, producing sidewalls 410 that intersect the hollow cores of fibers 110C. Apertures 400, 404 are preferably substantially the same size and shape as the apertures 310 below them, and aligned with them. It will be readily apparent that only the fibers 110C and 110C' that are intersected by aperture 46C or 48C will conduct the liquid. By this construction, the bridge will conduct fluid flow in substantially only the two directions that are needed, thus reducing the volume of liquid that would be needed compared to a bridge that flows in all directions.

Claims: An ion-junction bridge (100) for bringing two liquids together to make an ion-exchange junction, said bridge comprising a sheet (102) of fibers having two apertures (46, 48) therethrough, characterized in that the sheet of fibers comprises fibers extending in two generally orthogonal directions, one of which is the warp direction and the other the weft direction, said warp direction fibers (110, 110') comprising an aqueous-conducting material and said weft direction fibers (112) comprising a non-aqueous-conducting material, such that the sheet of fibers favours aqueous flow in said warp direction and suppresses aqueous flow in directions angled away from said warp direction, said warp direction being generally aligned with said two apertures, so that aqueous liquids deposited in said apertures will flow primarily in said warp direction between said apertures. A bridge as defined in claim 1, wherein said warp direction fibers comprise hollow core fibers having a shell that is water-repellant, so that the fibers conduct liquid only if the core thereof is in fluid contact with the apertures. An ion-junction bridge as defined in claim 1, wherein said warp direction fibers are cotton, acetate, rayon or polyester fibers. An ion-junction bridge as defined in claim 3, wherein said weft direction fibers are polyester 1 or polypropylene fibers or fibers of any aqueous-conducting material coated with a water-repellent composition. An ion-junction bridge as defined in claim 4, wherein the water-repellent composition is wax, poly(tetrafluoroethylene), silicone or a fluorinated vinyl polymer. An ion-junction bridge as defined in claim 5, wherein the weft direction fibers comprise cotton coated with a fluorinated vinyl polymer. An ion-junction bridge (100c) for bringing two liquids together to make an ion-exchange junction, said bridge comprising a sheet (102c) of fibers having two apertures (46c, 48c) therethrough; characterized in that the sheet of fibers comprises a weave of hollow core fibers (110c, 110'c) having a shell that is water-repellant, so that the fibers conduct liquid only if the core thereof is in fluid contact with the apertures, wherein the fibers extend in two generally orthogonal directions, one of which is the warp direction and the other the weft direction, such that the sheet of fibres favours aqueous flow in said two generally orthogonal directions and suppresses aqueous flow in directions angled away from said two orthogonal directions, one of said orthogonal directions being generally aligned with said two apertures, so that aqueous liquids deposited in said apertures will flow primarily in said orthogonal direction being aligned with said apertures. A potentiometric test element comprising: a frame; two substantially identical ion-selective electrodes mounted spaced apart on the frame; and an ion-junction bridge as claimed in any one of claims 1 to 7 extending between the electrodes.

Inventors: DEACON MARK ELLIOTT, GOTTERMEIER WILLIAM FRANKLIN, DEACON, MARK ELLIOTT, GOTTERMEIER, WILLIAM FRANKLIN
Applicants: JOHNSON & JOHNSON CLIN DIAG, JOHNSON & JOHNSON CLINICAL DIAGNOSTICS, INC.
Application year: 1996
Main classification: G01N27
IPC codes: G01N27