Patent Description:
Most diagnostic assays are dependent to some degree on the temperature at which the analysis takes place. Temperature measurement and correction is one approach that is employed to mitigate the effects of temperature on an assay response.

In many portable diagnostic devices using disposable test elements e.g. test sensors such as test strips, the temperature of the assay can be difficult to measure and/or control. Temperature measurement is often done within a measurement instrument and the temperature measured can be quite different to that of the strip itself on which the assay is located.

It would be preferable to be able to control the temperature at which the assay reaction takes place but this requires both temperature measurement at the location where the assay reaction occurs and a means to change the temperature at which the assay takes place in that location.

A typical solution to heating the test area is the provision of a heating block in contact with the test strip. There are a number of drawbacks to this solution in that the thermal mass of the heater block limits the response time to any desired temperature change. Further, it typically requires the test strip to be inserted deep inside an instrument that powers a heater block so that it lies next to the heater block almost in its entirety, adding to the strip size and contamination risk. In addition, the temperature of the heating block is known but assumptions about the thermal transfer to the strip and the assay area need to be made and hold true for all strip insertions. In practice, these assumptions cannot be said to hold true for all strip insertions. Therefore, there may well be a disparity between the assumed and actual temperature of the assay and a disparity in temperature measurement from one strip to the next.

Heating and temperature measurement methods are required that are located as close as possible to where the measurement is taken; have rapid response times; are easily manufacturable using materials and processes commonly used in diagnostic device manufacture and are cheap enough to be applied to affordable single-use disposable test devices.

Nucleic acid tests (NATs) in particular also need either precise temperature cycling (for Polymerase Chain Reaction (PCR) based amplification) or well controlled elevated temperatures (for isothermal amplification methods). A very local, fast responding and accurate means to control the temperature exactly where it is required would use less energy and minimise the effects of raised temperatures beyond where they were needed.

In the case of a printed heater on a diagnostic test device such as a test strip, the voltage applied to the heater element is typically <NUM> - 20V (<NUM> - <NUM>,<NUM> mV), whereas the voltage output from a thermocouple in the temperature range relevant to diagnostic devices (e.g. from ambient to <NUM>) is typically less than <NUM> mV and can be less than <NUM> mV i.e. around <NUM> orders of magnitude smaller than the voltage across the heater element. Thus the heater element and thermocouple must be physically and electrically isolated from each other, leading to at least five different material lay down steps in order to place the thermocouple and heater element in the same location (or very nearly in the same location). The five different material lay down steps are typically as follows.

Any heater element is preferably provided with a corresponding measurement of the temperature to achieve more reliable control, as the lot to lot variation in materials and changes within a heater element due to the heating mean that the temperature cannot reliably be predicted for a particular applied voltage.

<CIT> SMITH describes painting lines of two dissimilar thermal element materials on a non-conducting substrate intersecting at a location where the temperature is to be measured.

<CIT> EICHELBERGER describes a low-cost thermocouple using a first conductor, an insulating layer and a second conductor.

<CIT> SMITH describes a silk screen printed thermocouple.

<CIT> and <CIT> both to GRANDE describe a thermocouple device comprising a flexible, non-planar substrate, first and second printed thermocouple elements, and medical devices comprising the thermocouple. The thermocouple can be made to function as both a heater and a temperature sensor by use of a switching circuit. <CIT> (also published as <CIT>) FUJII describes a method for using temperature correction in biosensors.

<CIT>describes a flexible heater device.

<CIT> discloses a thermoelectric sensor for monitoring biological samples.

Printed flexible heaters and warming elements are available from GSI Technologies at http://www. com/heater-and-warming-element-printing-ptc/.

The present invention seeks to alleviate one or more of the above problems or problems in the art.

in a first aspect of the invention there is provided a test sensor comprising a heater and thermocouple device <NUM>, the heater and thermocouple device <NUM> comprising: a substrate <NUM>;.

Typically the test sensor comprises a test area, e.g. a generally <NUM> dimensional area or a sample chamber or flowpath, for receiving sample and conducting an assay. A flowpath allows sample to flow through it, whereas a sample chamber may simply receive sample (e.g. from a flowpath or otherwise).

Preferably, the second conductivity of the second material is greater, preferably significantly greater, than the first conductivity of the first material. In the case of thin films, sheet resistivity is a useful measure of conductance. Where the first material is a screen printed carbon paste and the second material is a screen printed silver paste then the sheet resistivity of the first material is about <NUM> ohms (Ω) per square @ <NUM> sheet thickness and the second about <NUM> milliohms (mΩ) per square @ <NUM> i.e. the second material is at least between <NUM> to <NUM> orders of magnitude more conductive than the first material.

When powered, field lines extend between heater connector tracks 36A, 36B creating current flow and Joule resistive heating in heater element <NUM>. The layout of the field lines will depend on the separation, orientation, and layout (e.g. peripheral shape of opposing edges) of each of the heater connector tracks 36A, 36B and the second thermocouple element <NUM>.

Preferably, the resistive heater element <NUM> and heater connector tracks 36A, 36B are configured to provide a uniform electric field, for example a generally or substantially uniform electric field across at least part of the resistive heater element <NUM>. Preferably a uniform electric field <NUM> is provided across at least <NUM>%, or at least <NUM>%, or at least <NUM>%, or at least <NUM>%, of the area of resistive heater element <NUM> When a resistive heater element <NUM> of uniform geometry is provided comprised of material of substantially homogenous resistivity in a uniform electric field, substantially even resistive heating is more easily provided across the resistive heater element <NUM>. This is especially useful for swift, accurate heating of small sample volumes in a test sensor e.g. of a test area in, or adjacent to, resistive heater element <NUM>. This is also useful for accurate control (feedback) of the temperature of the resistive heater element <NUM> via the thermocouple <NUM>.

Preferably the second thermocouple element <NUM> is configured to enable a uniform electric field to be provided, for example a generally or substantially uniform electric field across at least part of the resistive heater element <NUM>.

The second thermocouple element <NUM> may have a free distal end on resistive heater element <NUM>, which may be a rounded free distal end. The area of overlap with resistive heater element <NUM> may be rounded e.g. semi-circular.

Preferably, the area of the second thermocouple element <NUM> on resistive heater element <NUM> is ≤ <NUM>%, or ≤ <NUM>%, or ≤ <NUM>% of the area of resistive heater element <NUM> between conductive tracks 36A, 36B.

Preferably the second thermocouple element <NUM> is configured to enable a uniform electric field, for example a generally or substantially uniform electric field across at least part of the resistive heater element <NUM> in co-operation with one or both heater connector tracks 36A, 36B.

Preferably, the second thermocouple element <NUM> has at least one edge parallel to a facing edge of one of the heater connector tracks 36A, 36B.

Preferably, the second thermocouple element <NUM> has two edges parallel to respective facing edges of heater each connector track 36A, 36B.

Preferably, the heater connecter tracks 36A, 36B have facing edges which are parallel to one another (e.g. their central longitudinal axes are parallel to each other).

Preferably, the heater connecter tracks 36A, 36B are parallel to one another. Preferably these heater connector tracks 36A, 36B are each of constant width along their length.

Preferably, the first and/or second thermocouple elements are parallel to one or both the heater connector tracks 36A, 36B (e.g. their central longitudinal axes are parallel to each other).

Preferably, at least a portion of one or both of the first and second thermocouple element(s) <NUM> lie(s) along a region of constant temperature within resistive heater element <NUM>.

Preferably, one or both of the first and second thermocouple element(s) <NUM> lie(s) along a region of equipotential within resistive heater element <NUM>.

Preferably, the first thermocouple element <NUM> and/or first connector track <NUM> intersect(s) a periphery <NUM> of the resistive heater element <NUM> at a first location and the second thermocouple element <NUM>, and/or second connector track <NUM>, intersect the periphery <NUM> at a second location, and the first and second locations are at the same temperature and/or at the same potential. The first and second locations may be the same location, or these may be different locations.

Preferably, the first thermocouple element <NUM>, and/or first connector track <NUM>, intersect (meet) the periphery <NUM> of the resistive heater element <NUM> at a different location to the location where the second thermocouple element <NUM>, and/or the second connector track <NUM>, intersects the periphery <NUM>.

Preferably, the first thermocouple element <NUM>, and/or first connector track <NUM>, intersect (meet) the periphery <NUM> of the resistive heater element <NUM> at a location at the same temperature as that where the second thermocouple element <NUM>, and/or the second connector track <NUM>, intersects the periphery <NUM>. Thus the temperature drop along each respective combined thermocouple element <NUM>, <NUM> to a 'cold' end of a respective connector track <NUM>, <NUM> are the same.

Preferably, the first thermocouple element <NUM>, and/or first connector track <NUM>, intersect (meet) the periphery <NUM> of the resistive heater element <NUM> at a location at the same potential as that where the second thermocouple element <NUM>, and/or the second connector track <NUM>, intersects the periphery <NUM>. Thus, the potential gradient to each respective connector track 36A, 36B from the thermocouple junction <NUM> is the same for both thermocouple elements. Indeed, preferably the entire thermocouple junction <NUM> lies along a region of the same potential so the portions of the resistive heater <NUM> each side of it are exposed to the same voltage gradient.

Preferably, the thermocouple elements <NUM>, <NUM> lie substantially in between the heater connector tracks 36A, 36B.

Preferably the thermocouple elements lie entirely in between the heater connector tracks 36A, 36B e.g. so that no portion of one, or preferably both, of the thermocouple elements <NUM>, <NUM> extend(s) beyond a direct straight line between opposing corresponding ends of heater connector tracks 36A to 36B. Thus, it is preferred that the thermocouple elements <NUM>, <NUM> lie directly between opposing portions of connector tracks (as in <FIG>) but these may be slightly to one side i.e. not lying entirely between connector tracks 36A, 36B, see <FIG> and portion A2 in <FIG>.

Preferably, the first and second thermocouple elements <NUM>, <NUM> are equidistant from both heater connector tracks 36A, 36B.

Preferably, one or both first and second thermocouple elements <NUM>, <NUM> and/or one or both heater connector tracks 36A, 36B are elongate, optionally linear, preferably of rectangular shape.

Preferably, at least one, optionally all, of the first thermocouple element <NUM>, the first connector track <NUM>, the second thermocouple element <NUM>, the second connector track <NUM> and the heater connector tracks 36A, 36B are elongate, preferably substantially rectangular, having a narrow lateral width; in other words these are, preferably, relatively slender structures e.g. compared to resistive heater element <NUM>.

Preferably, the heater connector tracks 36A, 36B are of comparable (preferably the same) size and/or shape as one another.

Preferably, the length of the first and second thermocouple elements <NUM>, <NUM> are the same or less than that of one, and preferably both, heater connector tracks 36A, 36B.

Preferably, the first and second thermocouple elements <NUM>, <NUM> are of substantially the same size and shape. Typically the first thermocouple element <NUM> in the first layer is delimited by the size and shape of the second thermocouple element <NUM> of the second layer in contact with it. Particularly within the region of the heater resistive element <NUM>, the first and second thermocouple elements will be at essentially the same average temperature since the second thermocouple element <NUM> conducts heat relatively well.

The first and second connector tracks <NUM>, <NUM> are preferably contiguous and preferably integrally formed with, respectively, first and second thermocouple elements <NUM>, <NUM>. In at least one embodiment, a portion of thermocouple junction <NUM> (overlapping first and second thermocouple elements <NUM>, <NUM>) is formed from a portion of the first layer lying outside the periphery <NUM> of the heater resistive element forming part of the first thermocouple element <NUM> and overlapping with a portion of the second layer forming part of the second thermocouple element <NUM> also lying outside the periphery <NUM> of the heater resistive element. Preferably, the area of overlap beyond the periphery <NUM> of heater <NUM> of the first and second layers to form part of first and second thermocouple elements <NUM>, <NUM> is small, preferably less than <NUM>%, more preferably less than <NUM>%, more preferably less than <NUM>%, of the total area of the thermocouple junction <NUM> (defined by the overlap between first and second thermocouple element <NUM>, <NUM>). Preferably, the overlap beyond periphery <NUM> is zero. In this way, preferably, the thermocouple junction <NUM> lie(s) entirely within a periphery <NUM> of the heater element <NUM>, although it may intersect (meet) with the periphery <NUM>.

Beyond the periphery <NUM> of the resistive heater element <NUM>, the temperature will drop off, but the area of overlap of the first and second thermocouple elements <NUM>, <NUM> beyond the periphery <NUM> is preferably small. In any case, the temperature of each of the first and second thermocouple elements <NUM>, <NUM> beyond the periphery <NUM> will be closely related to each other and will likely be substantially the same because of the conductivity of the second layer.

Preferably, the resistive heater element <NUM> has a periphery <NUM>, and the periphery <NUM> has at least four edges (and preferably only four edges) and the heater connector tracks 36A, 36B extend along two opposing edges of the periphery <NUM> (preferably along the entirety of one or both opposing edges) so that most, preferably substantially all, of the resistive heater element <NUM> is heated between the heater connector tracks 36A, 36B.

It will be understood that the second layer <NUM> may have a continuous portion but is generally discontinuous having second thermocouple element <NUM> separate and distinct from the two spaced apart heater connector tracks 36A, 36B.

The resistive heater element may be any suitable shape e.g. generally or substantially circular, elliptical, rectangular, rhomboid, square, diamond, polygonal. It may be elongate but preferably its longest dimension is no more than double its widest dimension. Preferably it is four sided, with preferably two pairs of parallel edges.

Preferably, opposing edges of resistive element <NUM> are of the same length and/or the corners of the resistive heater element <NUM> are each <NUM>° (substantially <NUM>°).

Preferably, one or more of the resistive heater element <NUM>, the first thermocouple element <NUM>, and the second thermocouple element <NUM> are configured so that the voltage indicative of temperature remains substantially unaffected by the voltage applied to the resistive heater element. There are various geometries and/or arrangements that can assist with this, as is apparent from the geometries and arrangements described in this disclosure.

Preferably, the geometry of the layers is arranged so that the field developed across the resistive heater element <NUM> is parallel to (e.g. generally or substantially parallel) to the uppermost surface of the first thermocouple element <NUM> (and so to the interface between the first and second thermocouple elements <NUM>, <NUM> when these are in direct contact with one another). Preferably, the geometry is configured to provide a 'vertical' thermocouple junction <NUM> across 'horizontal' lowermost and uppermost surfaces of the thermocouple elements <NUM>, <NUM> respectively. There will be other configurations (physical arrangements) that could be envisaged from the teaching in this application that could be used to provide electric field parallel to the uppermost surface of the first thermocouple element <NUM> in further embodiments of the invention as would be understood by those skilled in the art.

Preferably, the thermocouple junction <NUM> (the area of contact between the two thermocouple elements <NUM>, <NUM>) lies in a substantially horizontal plane (with respect to substrate <NUM>). The electric field within any horizontal plane is preferably substantially uniform. The presence of a second thermocouple element of greater conductivity than the first (and in contact with it) provides a region of equipotential along the thermocouple elements <NUM>, <NUM> in a lateral (horizontal) direction. Preferably, in addition, the thermocouple element <NUM> is provided along a region of equipotential between heater connector tracks 36A, 36B. The expected thermocouple voltage is a few mV. Any voltage indicative of temperature at the 'cold' ends of contact tracks <NUM>, <NUM> e.g. at contact pads <NUM>, <NUM> due to the voltage applied to the resistive heater element (in a lateral or horizontal direction) resulting in Joule heating is preferably negligible i.e. <<mV.

To explain further, preferably the first and second thermocouple elements (e.g. forming thermocouple junction <NUM>) lie along a line of equal temperature and/or equipotential between connector tracks 36A, 36B. As the second thermocouple element <NUM> is (relatively) highly conductive, it will also provide, in itself, a region of equal temperature and equipotential. By arranging for this relatively conductive second thermocouple element <NUM> to lie along a region of equipotential in the resistive heater element <NUM> between connector tracks 36A, 36B, more even heating can be achieved. Thus, preferably, each respective thermocouple element <NUM>, <NUM> are at the same 'hot' temperature and their respective connector tracks <NUM>, <NUM> will experience the same temperature gradient to their respective 'cold' or 'reference' end(s) at contact pads <NUM>, <NUM>.

Second thermocouple element <NUM> may be located wholly, or at least partly, anywhere on (or under) resistive heater element <NUM> but is preferably centrally located, preferably equidistant from each substantially parallel of heater connector tracks 36A, 36B. In this way, in the presence of a uniform electric field between connector tracks 36A, 36B and a uniform resistive heater element, regions of the resistive heater element <NUM> lying between each respective track 36A, 36B and the second thermocouple element <NUM> will be of similar resistance (indeed of similar resistance between corresponding points on the heater connector tracks 36A, 36B and thermocouple element <NUM>).

Preferably, the second layer overlays the first layer.

Preferably at least one, optionally all, of the first thermocouple element <NUM>, the first connective track <NUM>, the resistive heater element <NUM>, the second thermocouple element <NUM>, the second connector track <NUM>, the heater connector tracks 36A, 36B are (e.g. generally or substantially) planar e.g. two-dimensional having two lateral dimensions substantially greater than a respective thickness (or depth) on substrate <NUM>.

Preferably, the first layer <NUM> comprises a first contact pad <NUM> connected to the first connector track <NUM>.

Preferably, the second layer <NUM> comprises a second contact pad <NUM>, 128A connected to the second connector track <NUM>.

Preferably, the first layer <NUM> comprises a second contact pad <NUM> connected to the second connector track <NUM>.

It will be apparent to those skilled in the art that any additional junctions between the first and second materials, or indeed other materials remote from the heater element <NUM>, will not affect the voltage developed indicative of temperature due to the thermal gradients from the 'hot' to the 'cold' ends of thermocouple elements <NUM>, <NUM> and connector tracks <NUM>, <NUM> because, in this in vitro design, these will all be at the same temperature as the 'cold' or 'reference' ends.

Preferably, the first thermocouple element <NUM>, resistive heater element <NUM>, first connector track <NUM>, and optionally the first contact pad <NUM> connected to the first connector track <NUM>, form a continuous portion of the first layer <NUM>.

Preferably, the second thermocouple element <NUM>, second connector track <NUM>, and optionally the second contact pad <NUM>, 128A connected to the second connector track <NUM>, form a continuous portion of the second layer <NUM>.

Preferably the thermocouple elements <NUM>, <NUM> are spaced from the peripheral edges of the resistive heater element <NUM> (preferably equispaced from two respective opposing portions of the periphery e.g. two opposing edges on which the heater connector tracks 36A, 36B are overlaid).

Preferably, one of both thermocouple elements <NUM>, <NUM> extend across the resistive heater element <NUM> from one portion of the periphery <NUM> of resistive heater element <NUM> to an opposing portion of the periphery <NUM>.

Preferably, the distal or terminal ends of one or both of the thermocouple elements <NUM>, <NUM> meet the periphery <NUM>, but one or both of these may stop short by a small amount, or may extend beyond the periphery by a small amount. Any such non-overlapping extension beyond the periphery is immaterial.

Preferably the first and second thermocouple elements <NUM>, <NUM> do not overlap one another or overlap to a small, preferably a minimal, extent beyond the periphery <NUM> of the resistive heater element <NUM>.

Preferably, the first connector track <NUM> intersects (meets) the periphery <NUM> of the resistive heater element <NUM> on an opposing portion, preferably a directly opposing portion of the periphery <NUM>, to that where the second connector track <NUM> intersects the periphery <NUM>.

Preferably, the overlap of the first and second thermocouple elements <NUM>, <NUM> lies, substantially, preferably entirely, within the periphery <NUM> of the resistive heater element <NUM>.

Preferably the overlap of the first and second thermocouple elements <NUM>, <NUM>, i.e. area of the thermocouple junction <NUM>, that lies outside the periphery <NUM> of the resistive heater element <NUM> is small, and preferably is negligible.

Preferably, the first material is selected from one or more of a semi-conductive material, carbon, bismuth, constantan, silicon, germanium, antimony, iron, nichrome (e.g. nickel and chromium (optionally iron) alloys), and molybdenum; and/or the second material is selected from any one or more of a metal, silver, copper, gold, aluminium and nickel.

Preferably, the first and second materials have a relative Seebeck coefficient of <NUM>-65µV/K, or <NUM>-50µV/K, or <NUM>-25µV/K, or <NUM> to 20µV/K or <NUM>µV/K.

Preferably, the resistive heater element <NUM> defines a heated test area.

Preferably, the test sensor comprises a sample chamber, and/or flowpath, for receiving sample.

Preferably, the first and second thermocouple elements <NUM>, <NUM> and the resistive heater element <NUM> are located adjacent to or within the sample chamber, and/or adjacent to or within the flowpath.

The sample chamber may be provided by a flow path e.g. a capillary flow path, or may be filled by capillary flow into it. Alternatively, in addition, the sample chamber may have an open side wall or roof portion to facilitate introduction of a sample e.g. under gravity.

In a second aspect of the invention there is provided a heater and thermocouple device <NUM>, the heater and thermocouple device <NUM> comprising: a substrate <NUM>;.

In a third aspect of the invention there is provided a method of manufacturing a test sensor or a thermocouple or a device as described herein comprising:.

Preferably, the first layer is laid down on the substrate before the second layer.

In a fourth aspect of the invention there is provided a method of conducting an assay comprising: providing a test sensor, or heater and thermocouple device, as described herein with a sample chamber and/or flow path; introducing a sample into a sample chamber and/or flowpath; heating the sample using the heater and thermocouple device; making a measurement on the sample; optionally, allowing the sample to cool and repeating the measurement; optionally, holding the sample at a predetermined temperature and repeating the measurement.

In one embodiment the invention concerns a thermocouple controlled heating element within a disposable test strip using (preferably only using) materials commonly used in biosensor manufacture and with a reduced, preferably a minimum, number of material deposition steps. Preferably, the heating element and thermocouple junction are both constructed from the same materials.

Several embodiments of the invention are described and any one or more features of any one or more embodiments may be used in any one or more aspects of the invention as described above.

The present invention will now be described, by way of example only, with reference to the following figures. In this document, like reference numerals refer to like features and reference numerals are used for the purpose of illustration and are not considered to be limiting.

It will be understood by those skilled in the art that any material properties, temperatures, potentials, electric fields, dimensions, shapes etc. and directions (such as height, depth, width, lateral, planar, horizontal etc) are to be understood as lying within the usages, tolerances and limits for devices used in assays and diagnostics, such as test sensors and test strips, and these terms should be interpreted with this in mind. Further, the term test strip is used as an example of a test sensor on which a fluid, typically liquid, sample is tested. This term is not intended to be limiting and there are various forms, sizes and shapes of test sensors with which the invention may be used and the test strip (a relatively rigid, generally planar test device of any size or shape) is one particularly preferable example.

The heater and thermocouple device <NUM> of the invention is an integrated device providing two functions of heating and control. It is particularly useful when formed including a substrate, and in which the substrate is typically generally or substantially planar and may form the main substrate of a test sensor such as a test strip.

In <FIG>, a first layer <NUM> of a first conductive material is laid down on a substrate <NUM> (not shown). The first layer <NUM> comprises a first thermocouple element <NUM>, a first connector track <NUM>, a first contact pad <NUM> and a resistive heating element <NUM>. The four components (here in this example) form a continuous portion of first layer <NUM>. Layer <NUM> may have additional portions e.g. additional continuous (and/or indeed discontinuous) portions similar, or the same, as those shown in <FIG> (not shown) or indeed different separate portions (not shown). Thus, multiple heater and thermocouple devices of the invention may be formed in one pair of first and second layers <NUM>, <NUM>.

The first layer <NUM> (and/or the second layer <NUM> described below) may be provided in the form of a dry film, having, prior to construction, two opposed, generally parallel planar surfaces. Alternatively, one or both may be formed by wet deposition techniques (e.g. screen printing) forming, when dry, two generally parallel planar surfaces one of which is exposed, the other of which is concealed in contact with the surface on which it is deposited.

Resistive heater element <NUM> is shown here as a four-sided shape (here a square) having two pairs of opposing parallel sides of the same length forming its periphery <NUM>. Resistive heater element <NUM> may be of any suitable size and shape but is preferably generally or substantially planar as determined by the uppermost exposed surface of substrate <NUM> on which it is deposited, and its deposition method (e.g. wet or dry deposition). The resistivity of the resistive heater element will be determined both by the conductivity of the first material and its shape and size (height, width, depth etc.). Here, preferably the first material is of relatively high resistivity (compared to the second material - see below) and preferably it is of (e.g. generally or substantially) homogenous resistivity to facilitate more uniform heating. The first thermocouple element <NUM> forms part of resistive heater element <NUM>, here a rectangular portion of heater element <NUM> spanning from one side of square-shaped periphery <NUM> to an opposite side but not as yet defined as such at this stage of manufacture (although it may be).

In <FIG>, a second layer <NUM> comprising three separate portions of a second conductive material of a second conductivity have been laid down. Preferably, the second layer <NUM> is laid over the first layer <NUM> but the first layer <NUM> may be laid over the second layer <NUM> to similar effect and to provide the same function(s). Preferably, these contact each other directly in the region of overlap forming a direct interface. The second layer <NUM> comprises a second thermocouple element <NUM> of narrow, rectangular shape (seen in plan view) of constant width (in this example) connected to a first connector track <NUM> which in turn connects to and terminates in a second contact pad <NUM>. The second layer <NUM> also comprises spaced apart heater connector tracks 36A and 36B spaced, sized and shaped to overlap the opposite edges of the periphery <NUM> of resistor heater element <NUM> providing one connector track 36A, 36B to each side of the first and second thermocouple elements <NUM>, <NUM>. Heater connector tracks 36A and 36B are also here of narrow, rectangular shape, being elongate and of constant width (in this example). These connect to and terminate in connector pads 38A and 38B typically of wider width. Second layer <NUM> comprises a second material of a second conductivity which is, compared to the conductivity of the first material of first layer <NUM>, relatively high. The heater connector tracks 36A, 36B may be in direct or indirect contact with resistive heater element <NUM>.

Thus, both materials have charge mobility carriers and may be of conductive or semi-conductive materials. Nevertheless, the first and second materials are selected so that the two materials have differing Seebeck coefficients. It is easier to talk about relative Seebeck coefficients between two materials than absolute values for a particular material. The relative Seebeck coefficients of known materials include those for Chromel-Constantan, Chromel-Alumel. For standard thermocouple types the relative Seebeck coefficients range from <NUM>µV/K for Type E thermocouple (Chromel-Constantan) to <NUM>µV/K for a Type B (Platinum(<NUM>% Rhodium)-Platinum(<NUM>% Rhodium)). The most common thermocouple is probably Type K (Chromel-Alumel) with a relative Seebeck coefficient of <NUM>µV/K. In our example the relative Seebeck coefficient can be determined from <FIG> to be about <NUM>µV/K.

Materials that could be used include any one or more materials such as carbon, bismuth, constantan, silicon, germanium, antimony, iron, nichrome, molybdenum etc. for the first material including the heater element, while any one or more materials such as metals including silver, copper, gold, aluminium and nickel etc. may be used for the second material for the highly conductive tracks 36A, 36B, 136A, 136B. Other materials and material combinations will be apparent from this disclosure.

In <FIG>, second layer <NUM> is shown overlaying first layer <NUM>. It can now be seen that second thermocouple element <NUM> overlaps a central portion of resistive heater element <NUM> between the connector tracks 36A, 36B and is here of rectangular shape. Thus, the second thermocouple element <NUM> defines that portion of the first layer <NUM> and typically the resistive heater element <NUM> that will form and function as first thermocouple element <NUM>. First and second thermocouple elements <NUM>, <NUM> overlap and are, preferably directly, in contact with one another and are of the same size and shape, here rectangular and further elongate, parallel to and centrally located in between heater connector tracks 36A, 36B. Respective opposing edges of each of heater connector tracks 36A, 36B and of second thermocouple element <NUM> are preferably parallel to one another so that field lines extend perpendicularly and evenly between these to provide a uniform electric field. Preferably the field lines on both sides of the second thermocouple element <NUM> are parallel to one another.

A thermocouple junction <NUM> (best seen in <FIG>) is formed at the interface where the lowermost face of second thermocouple element <NUM> contacts the uppermost surface of a portion (here the first thermocouple element <NUM>) of resistive heater element <NUM>. Thus, first thermocouple element <NUM> consists of a portion of resistive heater element <NUM> in contact with second thermocouple element <NUM> forming thermocouple junction <NUM>.

The first and second thermocouple elements are preferably in direct contact with one another to form a first, e.g. a 'hot', junction of a thermocouple so that these are at the same temperature as each other. In any thermocouple arrangement there is a reliance on knowing the temperature of the 'cold' or 'reference' junction to determine the difference with, and hence the temperature of, the first 'measuring', or 'hot', junction. The second e.g. 'cold' or 'reference' junction is typically located in the control instrument (e.g. a metering device) remote from any heat source. A calibrating temperature gauge in the control instrument may be provided. Preferably, the temperature of the control instrument, and so of the 'cold' or 'reference' junction, is substantially constant over the time period of the measurement. Indeed, preferably the temperature of the 'hot' or 'measuring' junction is substantially constant over the period of the measurement. Nevertheless it will be understood that the temperature may change from one measurement to the next. Indeed, this 'cold' junction may, in an example embodiment be located on the same substrate but remote from the 'hot' junction.

Heater connector tracks 36A and 36B connect opposing edges of the periphery <NUM> of resistive heater element <NUM> to contact pads 38A and 38B so that power can be delivered across resistive heater element <NUM> in the region between connector tracks 36A and 36B. Thus resistive heater element <NUM> and connector tracks 36A and 36B form a heater <NUM> which can be heated by resistive Joule heating, as is well understood.

As the conductivity of the second material is relatively high, the voltage along each respective connector track 36A, 36B will be a respective constant (but different) value along its length. Thus, upon powering, field lines extend between parallel connector tracks 36A and 36B and a uniform electric field is formed across resistive heater element <NUM> which heats by Joule heating. However, the second thermocouple element <NUM> extending across resistive heater element <NUM> is also formed from the relatively highly conductive second material. Therefore, second thermocouple element <NUM> also represents a region of equipotential. Furthermore, preferably second thermocouple element <NUM> lies along a region of expected equal temperature, and preferably also equipotential (due to geometries and layout arrangements selected) between heater connector tracks 36A, 36B.

In the region of the second thermocouple element <NUM>, there will be low or minimal Joule heating within the portion of resistive heater element <NUM> forming first thermocouple element <NUM>, as current will preferentially pass via the higher conductive material of second thermocouple element <NUM>. It is for this reason that it is preferred that second thermocouple element <NUM> is relatively narrow (e.g. <NUM> or <NUM>, <NUM> or less in width) to limit the reduction in heating within resistive heater element <NUM> in the region of first thermocouple element <NUM> and ensure this 'unheated' region is small and heated quickly by conduction and radiation from neighbouring regions on heater element <NUM>. It is also for this region that second thermocouple element <NUM> is also preferably located along a centreline of the resistive heater element <NUM> between connector tracks 36A, 36B so that the current path across the resistive heater element <NUM> is not foreshortened via the second thermocouple element <NUM> (e.g. if it were at an angle to the centreline). If it were at such an angle this would result in a region of the resistive heater element being bypassed by the current and so not heated.

Furthermore, by arranging second thermocouple element <NUM> between and parallel to both first and second conductor tracks 36A, 36B, the field lines are (relatively) unperturbed and extend evenly (and typically perpendicularly) from first conductor track 36A to second thermocouple element <NUM>, and from second thermocouple element <NUM> to second heater connector track 36B. Indeed, the first and second thermocouple elements <NUM>, <NUM>, and in particular the second thermocouple element <NUM>, has two long edges that are opposite to and substantially parallel to facing edges of connector tracks 36A, 36B facilitating the formation of uniform electric fields between these opposing parallel edges. Where the second thermocouple element <NUM> has two parallel edges (as it does here being a rectangle) and the connector tracks 36A, 36B are mirror images of one another, and with preferably facing edges parallel to the edges of the second thermocouple element <NUM>, then the field lines from one connector track 36A to the second thermocouple element <NUM> will be parallel to the field lines from second thermocouple element <NUM> to the other connector track 36B (except near any free distal end of second thermocouple element <NUM> e.g. that terminates short of the periphery <NUM>, e.g. see <FIG>).

Cross-sections AA and BB respectively are shown in <FIG> illustrating the placement of first layer <NUM> on a substrate <NUM> and subsequently second layer <NUM> on substrate <NUM> and first layer <NUM>. The thermocouple junction <NUM> formed at the interface where first and second thermocouple elements <NUM> and <NUM> meet is shown. It will be understood that sample to be heated may be applied, optionally after the application of an additional layer e.g. a waterproof layer, onto the area of resistive heater element <NUM> which is heated in between connector tracks 36A and 36B. It can be seen, in this example embodiment, that connector tracks 36A and 36B are of similar size, shape and orientation to second thermocouple element <NUM>. It can also be seen that second thermocouple element <NUM> is positioned centrally in between tracks 36A and 36B and is here equidistant from each along its length. Furthermore it is preferably of the same length (as seen in <FIG>). These arrangement(s) assist in providing that the electric field is perturbed as little as possible by the presence of an intervening conductive member in the form of second thermocouple element <NUM> and (e.g. generally or substantially) uniform heating across (e.g. generally or substantially) the entire resistive heater element <NUM> is provided. Further, the temperature of the first and second thermocouple elements <NUM>, <NUM> will be (e.g. generally or substantially) the same along the length of the thermocouple junction <NUM>.

It is of note, particularly in <FIG>, that the relative width of first and second thermocouple elements <NUM> is relatively narrow, e.g. <NUM> to <NUM> times narrower, compared to the overall separation of connector tracks 36A and 36B.

It can be seen from <FIG> that, if a specific geometry of components is used, combined with appropriate material selection, the number of different materials and lay down steps can be reduced from five to two, namely step <NUM> - laying down the resistive heater element <NUM> including a first thermocouple element <NUM> in a first layer, and step <NUM> - laying down heater connector tracks 36A and 36B and a second thermocouple element <NUM>. Typically step <NUM> will take place before step <NUM>, but this is not essential, as the second layer may be formed first and over laid with the first layer.

In <FIG>, in one practical embodiment, the connector tracks 36A, 36B are elongate and parallel to one another and the first (and second) thermocouple elements <NUM>, <NUM> are elongate and parallel to both the connector tracks 36A, 36B. Preferably these are half way between connector tracks 36A, 36B. The proximal and distal ends of the second thermocouple element <NUM> meets the periphery <NUM> at two opposing locations (see <FIG>) and the same is true for the underlying first thermocouple element <NUM>. The first and second connector tracks <NUM>, <NUM> each meet the periphery <NUM> at an opposite location to each other. Indeed, in <FIG> first connector track <NUM> does not quite meet the periphery <NUM>, as it begins where second thermocouple element <NUM> terminates but it would be preferable if it did. Such overlaps and slight variations in design details and layout arrangements may depend on the tolerances of the deposition methods selected.

It can also be seen from <FIG> that the thermocouple junction <NUM> is preferably formed in such a way that any voltage change across the thermocouple junction <NUM> due to the voltage applied to the heater element <NUM> via connector tracks 36A, 36B is very small indeed, preferably close to zero and more preferably zero (volts). This embodiment of the present invention achieves this by both sides (first and second thermocouple elements <NUM>, <NUM>) of the thermocouple junction <NUM> and in particular second thermocouple element <NUM> being at the same point (or almost the same point) within the resistive heater element <NUM>. In other words, first thermocouple element <NUM>, defined by the later addition of second thermocouple element <NUM>, preferably follows a line of expected constant temperature and/or preferably also follows a line of expected equipotential in the resistive heater element <NUM> between first and second connector tracks 36A, 36B. Indeed, the first and second thermocouple elements are placed directly on top of each other as shown in <FIG>. Furthermore, preferably the second thermocouple element <NUM> is located along a line of symmetry in the geometry of the resistive heater element <NUM> and tracks 36A, 36B so that the voltage (field lines immediately adjacent to thermocouple element <NUM> are the same or substantially the same along each respective edge (of the second thermocouple element <NUM> facing tracks 36A, 36B) so no voltage gradient is formed along second thermocouple element <NUM>.

The geometry of the invention, for example the first thermocouple element <NUM> and, indeed, the second thermocouple element <NUM>, lying along a line of equipotential and/or temperature (or rather along a region symmetrically either side of a line of equipotential and/or temperature) on heater element <NUM>, assists in ensuring that voltage difference developed along the thermal gradients from the 'hot' to the 'cold' junctions measured across contact pads <NUM> and <NUM> is independent of the voltage applied across contact pads 38A and 38B. Indeed, the 'ends' of the thermocouple junction <NUM> typically preferably meet the periphery <NUM> at the same location or at two locations that are at the same potential and/or at the same temperature (e.g. on opposing parts of the periphery). Various arrangements and geometries can be used, as would be apparent to those skilled in the art from this disclosure.

The voltage generated at the remote ('cold' or 'reference') ends of each connector track <NUM>, <NUM> is dependent on the thermal gradient from each heated thermocouple element <NUM>, <NUM> to the unheated remote ends of each respective unheated ('cold') connector track <NUM>, <NUM>. Absent other voltage inputs, the difference in the voltage generated at each remote end of the connector track <NUM>, <NUM> will depend on the Seebeck effect of the first and second materials and the thermal gradient along each from hot to cold.

To achieve more accurate measurements the two first ends (first and second thermocouple elements <NUM>, <NUM>) need to be at the same temperature and a thermocouple junction <NUM> at which these overlap and contact one another is a good way of achieving this, as well as providing a measurement circuit.

On resistive heater element <NUM>, the first and second thermocouple elements <NUM>, <NUM>, are preferably evenly heated i.e. there is no voltage developed due to the Seebeck effect along either element. Once these start to cool, i.e. once the thermocouple elements <NUM>,<NUM> or tracks <NUM>, <NUM>, as appropriate, leave the heater <NUM> and for a while beyond it, there is a thermal gradient (two in fact, one in each of the first and second thermocouple elements and respective conductive tracks). Each element and track develops a voltage gradient (EMF) along this thermal gradient, which in effect produces a voltage difference at their second remote e.g. 'cold' ends.

It can be seen that the invention does not depend on the order of lay down of the two materials or the particular process used to lay down the materials.

The invention allows assays to be done at optimum and controlled temperatures at both ambient and under possibly changing conditions. It also provides the means to have multiple assay areas on the same strip with different temperature profiles if required. The invention is amenable to printing on paper as some recent low cost NATs and is consistent with need for low cost.

Referring now to <FIG>, a test sensor comprising a suitable substrate, such as a generally or substantially planar substrate of a suitable stiff material, is shown in plan view suitable substrate materials include polyester, polycarbonate, nylon, polyethylenenaphthalate, polyimide, polyvinyl chloride, polyethylene terephthalate, polypropylene, polystyrene, ceramic, glass, FR-<NUM> board, paper, silicon. A typical strip width is <NUM> and length is <NUM>. A first layer <NUM> of carbon has been laid down on the substrate by a wet deposit technique, here screen printing, using carbon ink. First layer <NUM> comprises a rectangular resistive heater element <NUM>, a first thermocouple element <NUM> comprising both a portion of resistor heater element <NUM> and a portion of conductive track extending beyond the periphery <NUM> of heater element <NUM>, a connector track <NUM> connected to (here contiguous with) the first thermocouple element <NUM> and terminating in a contact pad <NUM>, and additional separate contact pads 138A, 138B and <NUM>. In <FIG>, during a second step, a silver layer <NUM> has been printed, again here using screen printing and silver ink (as known in the art), comprising heater connector tracks 36A and 36B, heater connector track extensions 136A and 136B, a second thermocouple element <NUM> and a second connector track <NUM>. An optional contact pad 128A of silver may also be provided in second layer <NUM>. Alternatively, as shown in <FIG>, a contact pad comprising material of the first layer may be used, as any such resulting additional 'junctions' being at the same temperature as the cold junction contribute nothing to the measured voltage across contact pads <NUM>, <NUM>.

As can be seen in <FIG>, the resistive heater element <NUM> between heater connector tracks 36A and 36B provides a well-defined heated sample area A for receiving sample that can be uniformly heated and accurately heated to a desired temperature. The heated sample area A may be substantially two-dimensional but more usually is delimited by walls of a sample chamber or flowpath (see side walls of spacer layer <NUM> in <FIG>). Such a sample flowpath or chamber may be provided with a lid. Preferably it is sized and shaped to be filled by capillary action. The second layer <NUM> may be laid down first and the first layer may be non-soluble or water proof to facilitate formation of a sample test area A into which liquid sample can be received.

Heater connector track extensions 136A and 136B terminate in carbon contact pads 138A and 138B. Second thermocouple element <NUM> lies over a portion of resistive heater element <NUM> and over a portion of a connector track outside the periphery <NUM> of heater element <NUM> forming a thermocouple junction <NUM> that, in this embodiment, spans the periphery <NUM> of heater element <NUM>. The remaining portions of connector tracks <NUM> and <NUM> that do not overlap one another connect first and second thermocouple elements <NUM>, <NUM> to connector pads <NUM> and <NUM> respectively. Conductive second thermocouple element <NUM> will aid conduction of heat from heater element <NUM> to first thermocouple element <NUM>.

The resistive heater element <NUM> reaches different temperatures depending upon the voltage applied as a result of resistive Joule heating. Preferably it is of homogenous construction, e.g. of homogeneous material laid down in a layer of substantially constant depth, to provide substantially uniform resistive heating across it.

The temperatures seen in a prototype built according to <FIG> are a good match with the temperature range required for diagnostic assays including NATs. This is shown in <FIG> in which the measured temperature in °C, as measured by an external temperature gauge, e.g. an off-the-shelf thermometer or thermocouple, is shown against applied voltage in Volts. The surface temperature of the initial device was measured externally. It can be seen that the resistive element <NUM> behaves well in a linear fashion with respect to applied voltage.

<FIG> shows a plot of voltage difference across the thermocouple <NUM> contact pads <NUM>, <NUM> (i.e. voltage difference developed along the thermal gradients from the 'hot' to the 'cold' junctions) against the externally measured temperature using the external temperature gauge. It can be seen from <FIG> that the temperature-dependent voltage from the thermocouple <NUM> can also be measured that is unaffected by the voltage across the heater pad. In order to provide the measured temperature shown in <FIG>, increasing voltages were applied to resistive heater element <NUM> but nevertheless, there is a linear relationship between the voltage in mV developed across the contact pads <NUM>, <NUM> (i.e. voltage difference developed along the thermal gradients from the 'hot' to the 'cold' junctions) against temperature. It is quite remarkable that a thermocouple voltage in mV can be measured indicative of temperature at a thermocouple junction <NUM> touching, and preferably lying on, a resistive heater element across which voltages of units and tens of Volts (a thousand to ten thousand times higher) is present.

<FIG> shows a plot of amplified voltage difference across the thermocouple contact pads <NUM>, <NUM> (i.e. voltage difference developed along the thermal gradients from the 'hot' to the 'cold' junctions) during two on/off cycles of the resistive heater element <NUM> over <NUM> seconds. It can be seen that, as the heater switches on, the voltage developed across the contact pads <NUM>, <NUM> as a result of heating 'hot' thermocouple junction <NUM> increases quickly at first before settling down to a constant temperature. Similarly, upon the switching off of the resistive heater element <NUM>, the voltage drops quickly before settling down to be a smaller constant value.

In a further example, the prototype strip was used with a test rig built using an Arduino microcontroller to determine if a stable on-strip temperature could be provided. An off-the-shelf thermocouple amplifier (MAX <NUM>) was used to provide an output to the microcontroller which was set to run a proportional integral derivative (PID) control algorithm. The heater was powered using a 9V battery and controlled via one of the microcontrollers' PWM outputs to an optocoupler. The ambient temperature and a number derived from the strip thermocouple output (calibrated to reflect strip temperature) were measured An off-the-shelf thermocouple was taped to the strip surface and heat sink paste was used to establish a good thermal connection.

<FIG> shows a plot of the test rig display (i.e. the desired temperature calibrated to a known temperature) and the measured strip surface temperature measured by an external temperature gauge, here an off-the-shelf thermocouple, over time. The squares show the externally measured strip surface temperature. The diamonds show the printed thermocouple output. It can be seen that the printed thermocouple output matches the externally measured strip surface temperature well.

<FIG> shows the initial response of the printed thermocouple output against time. It can be seen that the time for the strip to heat up is about <NUM> from ambient temperature to just under <NUM>°. It has to be remembered that the sample area being heated is physically very well-defined as being the area of the resistive heater element <NUM>. Now the temperature of the sample area can also be very well controlled and measured at the same time. This on-board strip measurement/control is a big advantage over controlling a remote heat element and assuming good heat transfer or applying voltage to a printed element and hoping it provides the same heating effect each time. The size of the resistive heater element <NUM> shown in <FIG> is quite large but this can be reduced from the prototype shown e.g. to create heated sections of an assay flow channel. Multiple heaters on a single device could also be envisaged with various arrangements of sample chambers and/or flow paths. Nevertheless, the number of manufacturing steps required to provide heating and temperature control and functionality is just two.

<FIG> show plan and cross-section views of multiple, here two, heating areas within a single sample channel <NUM>. Two sample areas A1 and A2 are provided overlaying respective resistive heater elements <NUM> between heater connector tracks 36A and 36B. Thus, two heaters <NUM>-<NUM> and <NUM>-<NUM> are provided in a single sample channel. Respective thermocouple junctions <NUM>-<NUM> and <NUM>-<NUM> are provided by narrow overlapping rectangular first and second thermocouple elements <NUM> and <NUM> preferably equi-spaced, and symmetrically centrally located with respect to respective connector members 36A and 36B. Sample fluids <NUM> may be drawn along the sample channel by capillary action and heated to a first temperature in sample area A1 where a first sample measurement may be made e.g. a photometric measurement before arriving in sample area A2 where the same measurement may be conducted at a different temperature. Other types of measurement that would benefit from the invention might include potentiometric, amperometric, conductimetric, impedimetric, optical , calorimetric, acoustic, and mechanical measurements, e.g. where it is desirable that temperature is well controlled.

<FIG> show two sample areas <NUM> defined by side walls of a thick spacer-type layer <NUM>. For example, this may be formed from cut-outs in a film, or by wet or dry deposition techniques. The composition and/or thickness of this layer <NUM> will depend on the requirements of the application. The layer <NUM> may be used to separate one test area from another and/or to create a test well to hold more sample. Here, each area is heated by respective, resistive heating elements <NUM>. Sample areas <NUM> have a width "W" defined by the separation between side walls of insulating layer <NUM> and a length "L" similarly defined. Typically, sample areas <NUM> are defined by respective apertures and side walls of insulation layer <NUM>. Sample areas <NUM> are open and sample may be received into sample areas <NUM> from above. Each sample area <NUM> has a respective thermocouple junction <NUM>-<NUM>, <NUM>-<NUM> and a respective heater <NUM>-<NUM>, <NUM>-<NUM> formed from respective first and second thermocouple elements <NUM>, <NUM> and respective resistive heater elements <NUM> and connector tracks 36A and 36B. The two sample areas <NUM> are entirely distinct and separate with no fluid connection from one to the other.

<FIG> show a sample channel <NUM> with two side channels (not labelled) for receiving sample fluid <NUM> from a main channel. Reservoirs <NUM> are typically capillary fill reservoirs which draw fluid along flow path <NUM> into and past heated sections <NUM>-<NUM> and <NUM>-<NUM> on each a respective side channel. Respective thermocouple junctions <NUM>-<NUM> and <NUM>-<NUM> enable, in effect, the same fluids to be heated to different temperatures before a measurement is taken or for a sample test to be repeated multiple times on (in effect) the same sample but under different or the same temperature conditions.

<FIG> show a further embodiment in which a resistive heater element <NUM> and contiguous track portion leading to connector track <NUM> are overlaid by a second thermocouple element <NUM> (here shorter than the width of the resistive heater element <NUM>) defining a first thermocouple element <NUM>, part of which lies within the periphery <NUM> of first resistive element <NUM> and part of which lies beyond periphery <NUM> and comprises a portion of a track in the first layer leading to connector track <NUM>. This can be seen more clearly in <FIG> where the portion of overlapping thermocouple element <NUM> within the periphery <NUM> of resistive heater element <NUM> is shown having an area A1 and the part outside periphery <NUM> is shown having an area A2. Thus, thermocouple junction <NUM> formed at the interface between first and second thermocouple elements <NUM> and <NUM> has a portion of area A1 which is heated and a portion of area A2 which is not heated by heater resistive element <NUM>. This is not ideal and it is preferred that area A2 outside the heating element <NUM> is minimised. This is because the uneven heating/cooling in each of the first and second thermocouple elements <NUM>, <NUM> may result. The measured temperature will be closely related to the temperature where the first and second thermocouple elements separate, which here may be slightly different to that on the actual resistive heater element <NUM> where the sample is to be heated. It will also be noted in this embodiment that the field lines near the terminal free end second thermocouple element <NUM>, will no longer be uniform, again with possible uneven heating of each thermocouple element <NUM>, <NUM> with respect to the other as a result.

One way of achieving a small size of overlap area A2 is shown in <FIG>. Here, thermocouple elements <NUM>, <NUM> lead away from one another immediately at the periphery <NUM> of resistive heater element <NUM> providing a much smaller overlap A2 that lies outside periphery <NUM>.

This may be further improved as shown in <FIG>, in which connector tracks <NUM> and <NUM> each intersect the periphery <NUM> of resistive element <NUM> at opposing portions (edges) of the periphery. Thus, the area of the resistive heater element <NUM> overlapped by second thermocouple element <NUM> to form first thermocouple element <NUM> lies entirely within the periphery <NUM> (and here, for example, does not extend beyond the - here right hand side - of periphery <NUM>) and there is minimal and typically near zero, overlap that does not lie within periphery <NUM> of resistive heater element <NUM>. This, coupled with the shape of connector tracks 36A and 36B (not shown, but as in <FIG>) and the shape of resistive heater element <NUM> and the resulting uniform electric field extending between the connector tracks 36A and 36B, means that the entire resistive heater element <NUM> has the same resistance from connector track 36A to connector track 36B at each point along their respective length so that the current flowing and, therefore Joule heating will be comparable. This means that the resistance, and so resistive heating effect, across the sample area defined by resistive heating element <NUM> is generally or substantially uniform, particularly away from its periphery <NUM>.

It is preferably to provide a uniform field (and so uniformly distributed field lines (across resistive heater element <NUM> so as to facilitate for (e.g. generally or substantially) even heating across heater element <NUM>. The embodiments in <FIG>, <FIG>, and 14A facilitate this by providing an elongate (here rectangular) second thermocouple element <NUM> along a line of equipotential with elongate edges parallel to opposing edges of tracks 36A and 36B. There will be other configurations (physical arrangements) that could be envisaged from the teaching in this application that could be used to provide uniform electric field in further embodiments of the invention as would be understood by those skilled in the art.

However, <FIG> and <FIG> and <FIG> facilitate provision of a uniform electric field across most of heater element <NUM> (typically at least <NUM>% of the area of heater element <NUM>) by providing a short overlap of second thermocouple element <NUM> on heater element <NUM>. Some use a rectangular second thermocouple element <NUM> but this is not essential.

In <FIG> a very short overlap of second thermocouple element <NUM> within the periphery <NUM> of heater element <NUM> is provided. Here, the thermocouple element has a distal free end with a rounded profile (here semi-circular). The resultant field lines are shown and are equispaced in around <NUM>-<NUM>% of the area lying between connector tracks 36A, 36B indicating uniform electric field. Where rounded second thermocouple element <NUM> protrudes (overlaps) slightly onto the uppermost surface of heater element <NUM>, the field lines F approach its rounded distal end in various orthogonal directions and the electric field is slightly perturbed in this region. The area of overlap of second thermocouple element <NUM> on resistive heater element may be small, ≤ <NUM>%, or ≤ <NUM>%, or ≤ <NUM>% of the total area of resistive heater element, but it is preferably non-zero. Nevertheless, embodiments can be envisaged in which the second thermocouple element <NUM> touches a portion of a side wall of resistive heater element <NUM>.

The material requirements for a heater element, using Joule effect, are for a (relatively) low resistance material for the connector tracks and a (relatively) high resistance material for the heater element itself. Typically, the tracks have a resistance of less than a few Ω whereas the heating element requires a resistance of around <NUM>Ω or greater depending on the target temperature range and available voltage. Some other material properties may be dictated by the chosen lay down process.

The material requirements for a thermocouple element, using the Seebeck Effect are for the two materials forming the thermocouple with different Seebeck coefficient e.g. relative Seebeck coefficients of <NUM>-65µV/K, or <NUM>-50µV/K, or <NUM>-25µV/K, or <NUM> to 20µV/K or <NUM>µV/K. For the thermoelectric effect to measure temperature, it is desirable to have two materials with Seebeck coefficients as different as possible.

In some embodiments, the choice of materials is more limited to those which can be used in test sensors, for a particular application, and/or to those usable in the desired manufacturing techniques.

For the two separate functions of heating and temperature measurement, it is necessary to have materials with charge mobility carriers, hence the selection of the materials is restricted to conductive and semi-conductive materials. Nevertheless, there remains a challenge in how these are to be used, so the high voltage required for the heater has low enough (or preferably minimal) effect on the thermocouple behaviour and performance to allow accurate temperature measurement. The present invention seeks to address these problem(s).

In our experiments, silver and carbon have been used but other materials can be used to increase the effects, for example, alternatively or in addition to silver, any one or more materials such as copper, gold, aluminium and nickel can be used for the highly conductive tracks while, alternatively or in addition to carbon, any one or more materials such as bismuth, constantan, silicon, germanium, antimony, iron, nichrome, molybdenum could be used for the heater element.

It is therefore theoretically possible to use the same materials for both the thermocouple and the heater but this still leaves the issue of the heater voltage being many times larger than the voltage signal needing to be measured from the thermocouple. Therefore as described elsewhere in this disclosure, arrangements have been shown to have the thermocouple output independent of the applied heater voltage without separating the heater element and thermocouple with an insulating layer. This enables not only the use of the same materials but also a reduction of lay down steps for the materials concerned.

For convenience, in our experiments we have used an electrically floated measuring system to detect the voltage changes in the thermocouple, but it is clear that if the use of electrically isolated power supplies and circuits is to be avoided, it would be possible to use a differential measurement by, for example, connecting the thermocouple to an instrumentation amplifier.

It has been found that if a specific geometry of the devices is used, combined with appropriate material selection, the number of different materials and lay down steps can be reduced to two:.

The invention does not depend on the order of lay down of the two materials or the particular process used to lay down the materials.

A prototype strip (see <FIG>) was built using screen printing as the material lay down method as follows:
Strip width is <NUM>. The outside silver tracks/contacts are the heater pad connectors and the middle silver and carbon tracks form a thermocouple on the heater pad.

The heater element reaches different temperatures depending on the voltage applied. The temperatures seen in the first prototype are a good match with the temperature range required for diagnostic assays including NATs.

A temperature dependent voltage can also be measured from the thermocouple that is unaffected by the voltage across the heater pad. The response times seen in <FIG> indicate that it is temperature not the voltage applied to the heater element that determines the thermocouple output.

To see if this could be used to control a stable on-strip temperature a test rig was built using an Arduino microcontroller to use the output from an off-the-shelf thermocouple amplifier (MAX31855 from adafruit. com) and run a "proportional-integral-derivative" (PID) control algorithm. The heater is powered from a 9V battery and controlled via one of the Arduino's PWM outputs to an optocoupler. The display shows the ambient temperature (Int. Temp) and a number derived from the strip thermocouple output (°C) that was calibrated to reflect strip temperature. The required temperature is set in the Arduino software code. By taping a thermocouple to the strip surface and trying to get a good thermal connection with heat sink paste the temperature of the strip surface was measured while turning the control on and off (<FIG>. Strip heat up time is about <NUM> seconds from ambient temperature to just under <NUM>. The on-board measurement/control is a big advantage over controlling a remote heater element and assuming good heat transfer or applying a voltage to a printed element and hoping it's the same each time.

Two further examples of integrated temperature control devices were built and tested. The heater/thermocouple designs tested are shown in <FIG>. The heater/thermocouple were printed in a two layer print and a third insulation layer (not shown) printed over the heater/thermocouple to protect it from the sample fluid. The devices were built into a diagnostic test strip format by lamination with a spacer layer and lid film layer to form a capillary channel (e.g. similar to that shown in plan view in <FIG> (but with two heater/thermocouple devices rather than the one shown in each Figure here) and in cross section in <FIG> and photographed in <FIG>).

In <FIG>, thermocouple element <NUM> is a small shaped disk or 'blob' of (here) silver ink, of nearly circular shape, connected to a slightly thinner connector track <NUM> leading away from it. This configuration helps to reduce and in some embodiments minimise the overlap outside the periphery of heater element <NUM>.

Carbon connector track <NUM> connects to an extended connector track <NUM> of silver forming an 'additional' junction <NUM> that does not affect the measured thermocouple voltage as mentioned elsewhere. Similarly the contact pads <NUM>, <NUM>, 138A, 138B may be formed of both carbon and silver (here seen from underneath with carbon on an outermost exposed surface for robustness).

<FIG> shows an arrangement similar to that in <FIG> and <FIG>, save that here the contact tracks <NUM>, <NUM> both leave from the same side of the periphery and lead away from each other (spreading out) so as to reduce any overlap lying outside the periphery of heater <NUM>.

<FIG> shows a portion of a test sensor with a flow path <NUM> of relatively narrow width traversing (see vertical white lines) an e.g. square shaped heater <NUM> similar to that seen in <FIG>. The broad white band is an insulation layer defining the flow path and separating the fluid in flow path from the heater (and from the integrated thermocouple).

<FIG> shows a schematic block diagram of a circuit <NUM> for controlling a test strip comprising a PC <NUM>, an LCD display <NUM>, a microcontroller <NUM> (e.g. Arduino Uno), an optocoupler/phototransistor <NUM> e.g. Vishay SFH618A, a thermocouple amplifier <NUM> (e.g. an Adafruit MAX31856 breakout) containing a 'cold' junction, a battery <NUM>, a strip port connector <NUM> and a test sensor <NUM> (here a test strip) inserted in strip port connector <NUM>.

Devices such as those shown in <FIG> were tested in a test rig such as that seen in <FIG>. The microcontroller <NUM> reads the input from the thermocouple amplifier <NUM> and outputs a voltage via pulse width modulation (PWM) to the phototransistor <NUM> which in turn controls the voltage applied to the test strip heater.

In a feedback control loop, the microcontroller <NUM> determines the thermocouple voltage (e.g. developed between the 'hot' thermocouple junction <NUM> and a 'cold' thermocouple junction inside the thermocouple amplifier <NUM>) and as a result determines how much voltage to provide to the battery to maintain that temperature.

The microcontroller <NUM> was used to program a temperature cycle between <NUM> and <NUM> at <NUM> second intervals using a device of the design shown in <FIG>. <FIG> shows a plot of the PWM Output and temperature of the heater with time. It can be seen that the heater <NUM> achieves the set temperature within about <NUM> seconds. The oscillation seen is indicative of the high sensitivity (in this example) of the heater <NUM> (which overshoots) to small changes in the voltage developed in the thermocouple <NUM> used to control it via the feedback loop.

A device of the design shown in <FIG> was filled with a liquid sample and the response to a setpoint of <NUM> for <NUM> minute recorded as shown in <FIG>.

Claim 1:
A test sensor comprising a heater and thermocouple device (<NUM>), the heater and thermocouple device (<NUM>) comprising:
a substrate (<NUM>);
on the substrate (<NUM>), a first layer (<NUM>) of a first conductive material of a first conductivity comprising:
- a first thermocouple element (<NUM>);
- a first connector track (<NUM>) connected to the first thermocouple element (<NUM>);
- a resistive heater element (<NUM>);
- and in which at least part of the first thermocouple element (<NUM>) is comprised of a portion of the resistive heater element (<NUM>);
a second layer (<NUM>) of a second conductive material of a second conductivity comprising:
- a second thermocouple element (<NUM>) in contact with the first thermocouple element (<NUM>);
- a second connector track (<NUM>) connected to the second thermocouple element (<NUM>); and,
- two heater connector tracks (36A, 36B) spaced apart and connected to respective portions of the resistive heater element (<NUM>), each on a respective side of the thermocouple elements (<NUM>, <NUM>).