WO2006008519A1

WO2006008519A1 - Electrode manufacture

Info

Publication number: WO2006008519A1
Application number: PCT/GB2005/002840
Authority: WO
Inventors: Robert Andrew Porter
Original assignee: Inverness Medical Switzerland Gmbh
Priority date: 2004-07-20
Filing date: 2005-07-20
Publication date: 2006-01-26
Also published as: GB0416209D0

Abstract

A method of making an electrode structure. In the method, a substrate is provided having at least a formation. The formation corresponds to a desired electrode form. A curable ink is applied to the formation and the curable ink is cured. The curate ink may be electro-conductive. An electrode structure is also described.

Description

ELECTRODE MANUFACTURE

Field of the invention The present invention relates to a method of making a structure, to a microelectrode structure having a substrate and to a substrate having at least an electrode structure.

Background to the invention Macroelectrodes find uses in many fields of electrochemistry. They are conductors of electricity and may be fabricated in a variety of ways. Examples are cutting metal foil or wires, deposition of metal-containing inks (subsequently dried) by printing methods. In the main these are mature technologies, which can result in batches of electrodes of repeatable, well- characterised surface areas.

Microelectrodes are used for a wide variety of analytical purposes due to their properties. Their size provides certain advantages over conventional macroelectrodes. Microelectrodes can have various geometries, e.g., hemispheres, disks, bands, tubes, rings, and cylinders and generally have one or more dimension on the order of 0.1 to 20 micrometers. The efficiency of diffusion of species to the electrode surface rises with reduction in electrode area. The ideal hemispherical diffusion profile of a microelectrode induces an improved mass transfer compared to that of macro-electrodes which results in a high current density and an improved electrochemistry.

Further advantages provided by the use of the microelectrodes for electrochemical measurements are a high signal to noise ratio, true steady state amperometric behaviour as well as the ability to work in very high resistive media without loss of sensitivity.

Due to their small size, microelectrodes are difficult to make reliably and reproducibly. They also yield very small currents. In order to overcome the latter disadvantage, microelectrode arrays have been developed such that the advantages of microelectrodes may be combined with the advantages conferred by macroelectrodes. Microelectrode arrays typically consist of an array of identical microelectrodes that may be used in a similar fashion to one or more macroelectrodes.

One such method for producing a microelectrode array is disclosed by US5512489. A photo-ablation technique is used to create apertures in a layer of electrically insulating material and allow electrically conducting material exposed through the apertures to create the microelectrodes.

Photolithographic techniques may also be employed to create microelectrodes. US6110354 discloses such a technique to produce a microband electrode array.

Known ways of making macroelectrodes include printing methods such as screen- printing, using an ink or like material forced through a screen, as disclosed by US5437999. However such methods yield microelectrodes having insufficiently good edge definition due to the inherent inaccuracy of the screen-printing process which results in variation in the electrode areas. Furthermore, such screen printing techniques only allow for the reliable formation of structures and patterns having a length or width dimension of approximately 70μm or greater. Since the current obtained at the electrode surface is proportional to the electrode area it is essential that the electrode is well defined. Similarly, ink-jet printing methods are not capable of producing microelectrodes of the required accuracy.

Embodiments of the invention provide methods of making an electrode provided within a substrate that is reproducible, lends itself easily to mass manufacture and is economic. In particular the method provides a simple and economic way of making microelectrodes with high degree of reproducibility.

Embodiments of the invention further provide methods of making an array of electrodes provided within a substrate, in particular, a microelectrode array. Embodiments of the invention also allow structures other than electrodes to be made.

Summary of the invention

According to one aspect of the present invention there is provided a method of making a structure comprising: providing a substrate having a formation, said formation corresponding to a desired electrode form; applying a curable ink to the said formation; and curing the curable ink.

The curable ink may be electroconductive, whereby the structure is an electrode.

According to a further aspect, the invention provides a substrate having an electrode structure comprising a formation containing cured ink.

The ink may be electroconductive. The electroconductive ink may comprise a dispersion of a suitably conductive powder such as carbon, gold, silver, silver/silver chloride, platinum or palladium in a suitable resin. The ink may contain other components such as enzymes or antibodies. The resin may be a photocurable resin. The properties of the electroconductive ink, such as its viscosity, degree of hydrophilicity, rate of expansion or contraction upon curing, flow properties, shear properties and so on may be optimised as appropriate for example by the choice of resin, conductive matter, optional solvents and/or the substrate itself.

The term curing includes drying and/or photopolymerising.

According to one embodiment, the conductive material is an ink which is able to flow into and/or along the formation. The electrode may be a macroelectrode or a microelectrode or a combination of the two.

The substrate may have any irregular or regular shape such as a sheet, a tube, or a rod and so on. The substrate may comprise one or more sub-structures such as one or more laminates. The material of the substrate may be chosen from any suitable material such as plastic, glass, a curable resin and so on. A convenient plastic of choice is polycarbonate or polyester. The substrate may comprise more than one material.

The term "formation" for the purposes of this invention is intended to refer to a structure provided at an outer surface of a substrate which defines an interior volume in at least a portion of the substrate. The formation may be recessed into the substrate or protruded from the substrate. The formation may be provided as fluidic element such as a well, a channel, a depression, a chamber, a groove, a fluidic splitting element such as a bifurcation and so on. The formation may be of any suitable shape, both regular and irregular. More than one formation may be provided in the substrate.

One or more formations may be arranged to define a fluidic network of elements.

The formation may penetrate though a further outer surface of the substrate such as a hole through the substrate. This further exposed surface might provide the basis for the active electrode surface.

Formations acting as microelectrode structure may be chosen from a variety of shapes and designs. Examples of such are microband arrays, interdigitated combs, arrays of concentric circles, micro disks, micro ring-disk electrodes, micro-dot electrodes, and spiral shaped electrode configurations. Where a plurality of electrodes are provided, adjacent electrodes may be configured so as to be capable of being driven with the same or of a different polarity. Individual microdot electrodes may be round in shape, triangular, square, rectangular and so on.

There may be any desired number of microelectrodes making up an array, from a small number, for example four, to many thousands.

The process of the invention can be such as to provide dimensions of individual electrodes in the width dimension down to lOOnm or less, for example in the case of a microband array. There is no upper limit on the width of the gap or spacing separating each individual microelectrode and may be chosen as desired. A lower limit of width may be typically lOOnm. hi the case of a microdot array consisting of round circles, the diameter could be chosen to be from between lOOnm to 200μm , although smaller sizes are also envisaged.

The dimensions of the desired electrode and therefore of the corresponding formations determine how the electrode is prepared and the type of ink used. For example a formation having a very low (sub μm) dimension might be too small to enable capillary action (due to the drag forces at the surface), hi this particular case, the ink could be provided in the formations by printing into them. The dimensions of the channel would also determine the type of ink that could be used. For example it would be necessary to choose an ink having particulates that were significantly smaller than the formation. For example, metal colloids having a size of 30nm and smaller are available and may be used to prepare a suitable ink.

In the case where a plurality of electrode structures are provided within the substrate as defined by the respective formations, the electrodes may be of the same shape and size or of differing shapes and/or sizes.

According to an embodiment, an array of similarly shaped electrodes is provided of substantially the same size. According to an embodiment, one or more microfluidic channels are provided within a substrate, the microfluidic channel having at least an opening to an outer surface of the substrate. Curable, e.g. electroconductive, ink is provided at the opening of the channel and which is then able to flow along the capillary channel by capillary action. The channel may be subsequently be cured to provide an electrode structure. The channel may be conveniently connected to a further microfluidic element that can act as a liquid reservoir. This provides an easy and simple way of applying the ink to the microfluidic channel as the reservoir may be chosen to be of a much larger dimension than the channel.

In an embodiment, the method comprises applying conductive material to at least a portion of said sheet and cleaning the sheet to remove conductive material outside said formations.

In one embodiment, the electrode structure has a surface that is substantially flush with an outer surface of the substrate.

Alternatively the conductive ink may be provided such that it partially fills the volume defined by the formation. The outer surface of the electrode (after curing) may be flat or have a degree of convexion or concavity.

One advantage of the invention is that embodiments are simple to effect and can provide electrodes having good edge definition and reproducible electrode areas. Another advantage is that electrodes may be formed flush with a substrate to provide virtually no effect on liquid movement across the surface.

According to yet a further embodiment, a microelectrode array is produced by preparing a laser ablated structure in a plastic sheet whose depth defines the depth of the electrode layers. The plastic sheet, for example polyester or polypropylene may then be laminated to a lower metallic surface which acts as an electrode connector. An ink such as a carbon ink may then be pushed or forced into the substrate formation by printing or by other means such as with a squeegee blade. Residual ink is then removed from the surface to provide a microelectrode array. The thickness of the sheet could vary and may be typically between lOOμm and lmm. The shape the individual electrodes, the number of them per unit area and the resulting shape of the array would depend upon the laser pattern.

Electrodes and electrode arrays embodying the invention are suitable for a variety of analytical techniques. These include, for example only, anodic stripping voltammetry (or polarography) with a potential scan which may be linear, cyclic, square-wave, normal pulse or differential pulse, or with a superimposed sinusoidal voltage, may be used. Alternatively, anodic stripping chronopotentiometry may be used. Other techniques may be used, such as ion exchange voltammetry, adsorptive cathodic stripping voltammetry (or polarography) with a scan which may be linear, cyclic, square-wave, normal pulse or differential pulse, or with a superimposed sinusoidal voltage, or clironoamperometry, chronocoulometry or linear, cyclic, square-wave, normal pulse or differential pulse voltammetry (or polarography) or voltammetry (or polarography) with a superimposed sinusoidal voltage.

The electrode arrays of the invention may be used to detect a wide variety of analytes of a biological, environmental or industrial nature.

Brief description of the drawings

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

Figure 1 shows a side elevation of a part of a device for performing a method embodying the invention;

Figure 2 shows a perspective view of the drum of Figure 1;

Figure 3 shows a further part of a device for performing a method embodying the invention;

Figure 4 shows a plan view of a microelectrode structure embodying the invention; Figure 5 shows a lateral cross section through the structure of Figure 4, taken along the line V-V;

Figure 6 shows a lateral cross section through the structure of Figure 4, taken along the line VI-VI'; Figure 7 shows the structure of Figure 4 with dielectric screening material applied;

Figure 8 shows an exemplary electrode structure using carbon ED 5000 printed into micro wells and cleaned with a cloth;

Figure 9 shows a close-up of a portion of the electrode structure of Figure 8;

Figure 10 shows a close-up of the terminal of the electrode structure of Figure 8; Figure 11 shows the formation of gold 10 micron track and gap microelectrodes;

Figure 12 shows the formation of gold 10 micron track and gap microelectrodes with a connector pad;

Figure 13 shows the attachment of gold tracks to the microelectrodes;

Figure 14 shows dielectric insulating the gold tracks; Figure 15 shows a hand made finished gold microelectrode;

Figure 16 shows an end cross-section and partial perspective view of a first channel in a substrate; and

Figure 17 shows an end cross-section and partial perspective view of a second channel in a substrate.

Description of the illustrated embodiments

Referring to Figures 1 and 2, a rotatable drum 10 having an outer nickel layer 11 with positively projecting features 12 is provided. The features 12 are the inverse of the final microfluidic design. A suitable technique for forming the drum is milling.

A UV curable ink 20 from a reservoir 21 is applied using a duct 22 to the underpart of the drum onto a polished glass surface 23 so that the ink is caused to flow under the drum 10 while the drum is being rotated. The UV curable ink 20 is conductive in some embodiments; in others it is semiconductive. In yet others it is insulating, and in those embodiments, the ink may later be treated to make it conductive or semiconductive.

Where the ink is conductive or semiconductive, or is caused to become conductive or semiconductive, the structure formed may be an electrode, or a microelectrode. Other structures may alternatively be formed.

A UV source 30 irradiates the ink 20 with UV light 31 as it flows under the drum to cure it. Thus a cured web 24 emerges from the drum having formations 25 corresponding to the desired microfiuidic features impressed into it.

In other embodiments chemical etching, laser. ablation, mechanical etching (cutting with diamonds), lamination and ink-jet printing several layers on top of one another are used to create 3-D formations in a base substrate, such as a plastics sheet.

In general, the formations include microfiuidic features, such as recesses, wells or channels, which in turn enable electrodes to also be made to a corresponding counterpart or "negative" design.

In some embodiments the ink 20 is applied to a surface that is conductive — e.g. a coated glass surface - and the insulating ink forms insulating lands between conductive portions.

Once the microfiuidic formation is formed in the sheet or web a paste (e.g. a carbon paste) or low temperature gold ink is applied to the microfiuidic channel to effectively fill it.

An advantage of allowing the ink to fill a formation under the influence of capillary action is that the ink may be applied to a nonplanar surface, said surface having outward projections. These may include fluidic elements such as formations, steps, pillars, channels and so on; such formations would make a screen printing method difficult. A further advantage is that the ink may be provided at a remote location and may access a location provided within the substrate that would be difficult to access otherwise. For example the ink could flow into a conduit within the body of the substrate.

In one embodiment, this is done by screen-printing; in another a release layer system is used. In yet a further embodiment, an ink is used that enters the formations using capillary action.

In yet another embodiment, the sheet 24 was placed on a relatively inflexible substrate 50, and ink or paste 40 was placed over the sheet 24 in the vicinity of the formations. A flexible blade 51, e.g. of rubber, was moved across the surface of the plastic sheet to force the ink, or respectively paste, into the formations 25. Any excess ink was then polished off the surface to leave only ink or paste in the formations 25.

In a still further embodiment liquid ink is caused to flow into the channel. The ink is then cured so that it solidifies.

In an embodiment, the formations include a common "terminal" which serves to provide an electrode connection. The terminal is provided on the sheet or web typically by forming a well 52 corresponding in shape to the terminal.

Further electrode connections or terminals may be provided if necessary within or on the electrode substrate. These connections or terminals may be provided by one of many methods including deposition of a suitable conductive ink onto the surface of the substrate by screen-printing, ink-jet printing, spraying, vacuum deposition, painting, laminating and so on.

Referring to Figure 4, the result is an array of microelectrodes 53, and a terminal electrode 152. Referring to Figure 5 the electrodes in this embodiment are generally flush with the surface 26 of the plastics sheet 24 or web. Referring to Figure 6A₅ the terminal 152A is likewise flush with the surface. Alternatively, the or each "terminal" 152 is formed by one or more of painting, printing, chemical or vapour deposition. Some of these techniques give a presence of the terminal on the surface, as shown in Figure 6B. This projection may be disadvantageous by its effect upon fluid flow in some applications. This problem can be avoided by disposing the terminals in zones not subject to fluid flow. One technique for achieving this is to screen part of the microelectrode structure by depositing a dielectric medium 54 over the undesired part. This can be effective for example where there is a part of the structure such as the terminals that stand proud of the surface, and can be achieved by making the electrodes overlong for the application so that the terminals are outside the area of use. Then the dielectric material is deposited over the to-be unused portions of the electrode structure, including the portions that stand proud of the surface of the substrate.

Alternatively terminals could connect to individual electrodes or groups of electrodes where selective addressing is needed. Other forms and shapes of electrodes are possible, including interdigitated electrodes, and curved electrodes. To achieve other shapes, it may be necessary to dispense with the drum and use one of the other techniques described above for creating the formations in the web or sheet surface.

Experimental results are shown in Figures 8 to 15. Figure 8 shows an exemplary electrode structure using Carbon ED 5000 printed into micro wells and cleaned with a cloth. Carbon ED5000 is a proprietary ink. Figure 9 shows a close-up of a portion of the electrode structure of Figure 8, with the two electrodes running parallel Figure 10 shows a close-up of the terminal of the electrode structure of Figure 8, showing a circular terminal extending into an electrode and the end of the adjacent electrode.

Figure 11 shows the formation of a gold 10 micron track and gap microelectrode structure. Figure 12 shows the formation of gold 10 micron track and gap microelectrodes with a connector pad. Figure 13 shows the attachment of gold tracks to the microelectrodes. Figure 14 shows dielectric insulating the gold tracks. Figure 15 shows a hand-made finished gold microelectrode.

Figure 16 shows an end cross-section and partial perspective view of a channel in a substrate 100, the channel 101 having a narrow upper section 102 and a wider lower section 103. The dimensions of the channel are such that ink when applied to the channel would flow along the lower channel section 102 (by capillary action) but not fill the upper channel section 101.

Figure 17 shows an end cross-section and partial perspective view of a channel in a substrate 200, the channel 201 having a narrow lower section 203 and a wider upper section 202. Again, ink would flow along the lower channel section (by capillary action) but not the upper due to the respective dimensions of each.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Any features as claimed may be used in combination or separately.

Claims

CLAIMS:

1. A method of making a structure comprising: providing a substrate having at least a formation, said formation corresponding to a desired structure form; applying a curable ink to the said formation; and curing the curable ink.

2. The method according to Claim 1, wherein the ink is electroconductive, whereby the structure forms an electrode.

3. The method according to Claim 1 , wherein the ink is applied by a printing method.

4. The method according to Claim 2, wherein the printing method is blade- printing.

5. The method according to Claim 2, wherein the electrode form is prepared by causing the ink to flow into and or along the said formation.

6. The method according to Claim 4, wherein the formation has at least a capillary dimension.

7. The method according to Claim 2, wherein the electrode form has the dimensions of a microelectrode.

8. The method according to Claim 2, wherein the structure includes at least one electrode connector.

9. A method of making an electrode structure according to any of Claims 2 to 8, wherein the desired electrode form is an electrode array.

10. A method of making a structure according to of Claims 2 to 9, having one or more of micro disk and micro ring-disk electrodes.

11. A microelectrode structure having a substrate with at least one surface and a plurality of microelectrodes disposed in the substrate, each microelectrode having a respective surface, each surface lying substantially flush with said surface of the substrate.

12. A substrate having at least an electrode structure comprising at least a formation containing a cured ink.

13. A substrate according to Claim 12, wherein the ink is electroconductive.

14. A substrate according to Claim 13, wherein the electroconductive ink comprises a dispersion of a suitably conductive powder such as carbon, gold, silver, silver/silver chloride, platinum or palladium in a suitable resin.

15. A substrate according to Claim 14, wherein the resin is a photocurable resin.

16. A substrate according to Claim 12, wherein the ink is cured by one or more of drying and photopolymerising.

17. A substrate according to any of Claims 12 to 16, wherein the electrode is a macroelectrode.

18. A substrate according to any of Claims 12 to 16, wherein the electrode is a microelectrode.

19. A substrate according to any of Claims 12 to 18, wherein the substrate has one of an irregular and regular shape.

20. A substrate according to any of Claims 12 to 19, wherein the substrate comprises one or more sub-structures such as one or more laminates.

21. A substrate according to any of Claims 12 to 20, wherein the material of the substrate is one or more of the group comprising plastics such as polycarbonate or polyester, glass, and a curable resin.

22. A substrate according to any of Claims 12 to 21, wherein the formation is one of recessed into the substrate and protruded from the substrate.

23. A substrate according to Claim 22, wherein the formation is provided as fluidic element such as a well, a channel, a depression, a chamber, a groove, a fluidic splitting element such as a bifurcation.

24. A substrate according to any of Claims 12 to 23, having one or more formations arranged to define a fluidic network of elements.

25. A substrate according to any of Claims 12 to 24, wherein the formation penetrates through a further outer surface of the substrate such as a hole through the substrate.

26. A substrate according to any of Claims 12 to 25, wherein the electrode is an interdigitated array.

27. A substrate according to any of Claims 12 to 26, wherein the electrode structure defines an array of similarly shaped electrodes having substantially the same size.

28. A substrate according to any of Claims 12 to 27, wherein one or more microfluidic channels are provided within a substrate, the microfluidic channel having at least an opening to an outer surface of the substrate.