Patent Publication Number: US-2022228975-A1

Title: Test strip, monitoring device and method for fabricating a test strip

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the national stage entry of International Patent Application No. PCT/EP2020/070564, filed on Jul. 21, 2020, and published as WO 2020/023508 A1 on Feb. 11, 2021, which claims the benefit of priority of European Patent Application No. 19190268.3, filed on Aug. 6, 2019, all of which are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The present disclosure is related to a test strip, a monitoring device and a method for fabricating a test strip. 
     BACKGROUND OF THE INVENTION 
     A test strip is a part of a monitoring device. The part can be inserted before a test or measurement and removed after the test or measurement. The monitoring device is typically portable and, thus, can be used for point-of-care applications for medical diagnostics and environmental tests. 
     The test strip can be used for a lateral flow test. The test strip uses the capillary action of a porous material and the ability of the porous material to bind marker molecules. 
     A sample liquid such as water, urine, blood or another liquid is provided to the test strip. The sample liquid flows using the capillary effect of the porous material and performs a chemical reaction. Typically, the chemical reaction results in a change of color at a predetermined active area of the porous material. The active area may have the form of a narrow or broad line. Often there are reactions at two active areas, respectively lines, of the porous material. Typically, change of color is monitored by visual inspection. These class of tests have several advantages, but there are still some disadvantages related e.g. to sensitivity and multi-analyte detection. Finding a solution for these drawbacks may bring more tests to a home environment. 
     It is an object to provide a test strip, a monitoring device and a method for fabricating a test strip that allow an efficient read-out of measurement results. 
     These objects are achieved by the subject-matter of the independent claims. Further developments and embodiments are described in the dependent claims. 
     SUMMARY OF THE INVENTION 
     In an embodiment, a test strip comprises a porous material, a photodetector and a substrate with a first and a second side. The porous material is attached to the first side of the substrate. The photodetector is attached to the second side of the substrate. 
     Advantageously, the porous material and the photodetector are both fixed at the substrate during production of the test strip. Advantageously, a distance from the photodetector to the porous material is short and the location of the photodetector is predetermined with respect to the porous material resulting in a high efficiency and reproducibility for detecting light transmitted or emitted by the porous material. 
     In an embodiment, the test strip comprises conducting lines arranged at the second side of the substrate. The photodetector comprises contact areas which are electrically connected to the conducting lines at the second side of the substrate. Advantageously, the substrate with the conducting lines can provide electrical power and signals to the photodetector. The only electrical connections to the photodetector run over the conducting lines on the substrate. 
     In an embodiment, the test strip comprises an anisotropic conducting film that mechanically connects the photodetector to the second side of the substrate and electrically connects the contact areas of the photodetector to the conducting lines at the second side of the substrate. Advantageously, the anisotropic conducting film allows a stable fixation of the photodetector on the substrate with low thickness. Advantageously, the anisotropic conducting film realizes a conductive connection of one contact area to one of the conducting lines and isolates this contact area to the other conducting lines. Advantageously, the electrical connection of the photodetector is free of a bond wire. The anisotropic conducting film acts as an adhesive underfiller. 
     In an embodiment, the conducting lines comprise a power supply line, a reference potential line and at least one bus line. Advantageously, electrical power can be provided to the photodetector from a power supply using the power supply line and the reference potential line. Data can be transmitted from the photodetector to other circuits of a monitoring device via the at least one bus line. 
     The porous material may comprise nitrocellulose. The porous material may be bibulous. The porous material may be covered by a film. The film may protect the porous material. The film may be transparent or translucent. Transparent may be named also optically clear. The porous material may have a specific ratio of pore size, porosity and thickness plus a dedicated chemical treatment. 
     In an embodiment, the substrate is transparent or translucent. Thus, light emitted or transmitted from the porous layer can reach the photodetector via the substrate. 
     In an alternative embodiment, the substrate is opaque and comprises a transparent or a translucent area. The transparent or translucent area may be realized as a window of the substrate. The photodetector is located at this transparent or translucent area. 
     In an embodiment, the porous material, in particular comprising nitrocellulose, is operable to transfer a liquid from an input point of the porous material to an active area of the porous material. The active area of the porous material is provided with a chemical substance, typically a chemical compound, which reacts with a component of the liquid. The component may be an analyte included in the liquid. The photodetector is arranged adjacent to the active area. 
     In an embodiment, the porous material comprises at least two active areas. Thus, two components of the liquid or two analytes may be detected at the at least two active areas. 
     In an embodiment, the photodetector comprises at least two pixels which are arranged adjacent to the at least two active areas. A pixel can be named photodetector element. A pixel may be realized as photodiode. 
     In an alternative embodiment, the photodetector is arranged adjacent to one of the at least two active areas. An additional photodetector of the test strip is arranged adjacent to another one of the at least two active areas. 
     In an embodiment, the photodetector is implemented as a spectral sensor that is operable to detect light in at least two different wavelength regions, in particular to separately detect light in at least two different wavelength regions. Advantageously, two colors may be generated at one active area; each of the colors can be measured separately by the spectral sensor. Thus, two components or two analytes may be detected at one active area. The spectral sensor may be operable to detect light in more than two, more than four or more than eight different wavelength regions. The different wavelength regions may be in the visible range or in the visible plus infrared range or in the visible plus near-infrared range. 
     In an embodiment, the active area may have the form of a line, a square, a rectangle, a dot, a circle or an ellipse. The line may be narrow or broad. 
     In an embodiment, the active areas may each have the form of a line, a square, a rectangle, a dot, a circle or an ellipse. 
     The active areas may be arranged like a matrix, e.g. as a matrix of dots or squares. The active areas may have different colors or may have identical colors. At least two active areas may have different colors. At least two active areas may have identical colors. 
     In an embodiment, the test strip comprises a sample pad for absorbing liquid and providing the liquid to the porous material directly or via a conjugate pad of the test strip. The conjugate pad includes e.g. a chemical substance (typically a chemical compound) designed for reaction with an analyte included in the liquid. 
     In an embodiment, the test strip comprises an absorbing pad for absorbing excess liquid from the porous material. 
     In an embodiment, the sample pad, the conjugate pad and the absorbing pad are also realized by a porous film or a porous layer, such as made of nitrocellulose. 
     In an embodiment, a monitoring device comprises the test strip, a light source and a control circuit connected to the photodetector and to the light source. The porous material and the substrate are located between the light source and the photodetector. The monitoring device may be named reader. 
     In an embodiment, the monitoring device is operable such that the test strip is inserted into the device and removed again. Thus, the test strip is used once, namely for a single test. The monitoring device is designed to be used several times, e.g. with different test strips for detecting different analytes or with test strips for detecting the same analyte. 
     In an embodiment, the control circuit is operable to detect whether the test strip is inserted or not and to provide an enable signal when the test strip is inserted. 
     Advantageously, by generating the enable signal, the monitoring device may be able to detect the correct insertion of the test strip into the monitoring device or/and may be able to detect whether the correct test strip is inserted. 
     In an embodiment, the monitoring device is operable to perform a lateral flow test, abbreviated LFT, or a lateral flow immunochromatographic assay. 
     In an embodiment, a method for fabricating a test strip comprises
         providing a substrate with a first and a second side,   attaching a photodetector to the second side of the substrate, and   attaching a porous material to the first side of the substrate.       

     Advantageously, the attachment of the porous material and of the photodetector on the two sides of the substrate results in a compact and stable stack which allows a high accuracy of measurement of light provided or transmitted by the porous material. 
     In an embodiment, the photodetector is attached to the second side of the substrate by flip-chip assembly. Thus, a reliable connection of the photodetector to the substrate is achieved. 
     In an embodiment, an anisotropic conducting film is provided between the photodetector and the second side of the substrate. Advantageously, a lead frame or a bond wire is not required for electrically contacting the photodetector. 
     The method for fabricating a test strip may be implemented, for example, by the test strip and the monitoring device according to one of the embodiments defined above. 
     The test strip with embedded spectral sensor is configured for an optical assay reading device. The test strip is implemented as a smart test strip. The smart test strip is with an embedded spectral sensor. The test strip is configured for a lateral flow test system, abbreviated as LFT system. The LFT system performs refracted and/or absorbance measurements. 
     The disclosure applies to the field of lateral-flow-test for point-of-care (abbreviated PoC). The test strip reacts to a certain substance that is present in the liquid under test, and color of the porous active area, realized e.g. by depositing conjugated antibodies on the nitrocellulose membrane, changes accordingly when analyte bind to conjugated antibodies. The liquid under test may be named sample, sample liquid or analyte. Alternatively, the substance to be detected is called analyte. The substance to be detected may be a chemical element or a chemical compound. 
     An example of an application is a home pregnancy test. The test is e.g. able to detect human chorionic gonadotropin (HCG) in urine of a pregnant women. The test assay utilizes the capillary action of porous paper and the ability to bind marker proteins to the cellulose. Usually a two line pattern is used. The first line generates a yes/no signal (pregnant or not). The second line indicates if the test is successful or not. Point-of-care tests (PoC tests) have the ability to test a patient at the point where the care is necessary. This allows a faster diagnosis, hence a faster treatment. 
     Normally the LFT is read (analyzed) by the human eye, and therefore the ability is lacking to measure variation in concentrations accurately. Lateral flow tests also known as lateral flow immunochromatographic assays are effective devices intended to detect the presence (or absence) of a target analyte in a sample (matrix) without the need for specialized and costly equipment, though many lab based applications exist that are supported by reading equipment. Typically, these tests are used for medical diagnostics either for home testing, point of care testing or laboratory use. 
     Advantageously, the test strip described in the present disclosure aims at an improvement of system performance, e.g. 
     a further increase of the reading accuracy and better quantitative analysis by an improved location of the spectral sensor with respect to the membrane area that needs to be analysed. Moreover, the LFT PoC reading device may be improved, e.g. by a cost reduction on the reader side. 
     The following description of figures of embodiments may further illustrate and explain aspects of the test strip, the monitoring device and the method for fabricating a test strip. Devices, areas and layers with the same structure and the same effect, respectively, appear with equivalent reference symbols. Insofar as devices, areas and layers correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a test strip; 
         FIGS. 2A to 2D  show examples of details of a test strip; 
         FIGS. 3A and 3B  show further examples of a test strip 
         FIGS. 4A to 4C  show additional examples of a test strip and of a monitoring device; and 
         FIGS. 5A to 5D  show simulation results. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a test strip  10 . The test strip  10  comprises a substrate  11  with a first and a second side  12 ,  13 , a porous material  14  and a photodetector  15 . In  FIG. 1 , a sketch of a possible assembly layout of the test strip  10  is elucidated. The porous material  14  is attached to the first side  12  of the substrate  11 . The photodetector  15  is attached to the second side  13  of the substrate  11 . The porous material  14  may be directly and permanently connected to the first side  12  of the substrate  11 . The substrate  11  is translucent or transparent. The substrate  11  may be realized as a printed circuit board, abbreviated PCB. The substrate  11  may be made of plastic or a polymer such as polystyrene, polyethylene, polyethylene terephthalate (abbreviated PET), vinyl, polyester, polyimide, acrylamide, epoxy resin or a woven fiberglass cloth impregnated with an epoxy resin (usually named FR-4 glass epoxy) or glass. 
     The substrate  11  may have a thickness between 20 μm to 200 μm or between 50 μm to 100 μm or between 60 μm to 90 μm. For example, a PET based substrate  11  may have a thickness out of these ranges. A PET based substrate  11  is transparent and is well suitable for copper deposition and/or etching for conductive traces placement. A PET based substrate  11  works well for direct flip chip attach. 
     Alternatively, the substrate  11  includes a PCB based material. Such a PCB based substrate  11  is suitable. Optionally, the substrate  11  includes at least a window or via corresponding to the area of the photodetector  15  or to the area of the pixels or pixels of the photodetector  15  (because normally a PCB is are not very transparent). 
     Correspondingly, the photodetector  15  may be directly and permanently connected to the second side  13  of the substrate  11 . 
     The porous material  14  is translucent or transparent. The porous material  14  has the form of a layer, membrane, film or sheet. The porous material  14  may be made of nitrocellulose. The porous material  14  may be fabricated as a nitrocellulose membrane. The porous material  14  may have an active area  16 . The porous material  14  is configured such that a liquid can laterally flow in the porous material  14 . The direction of flow F is indicated by an arrow. The flow F of the liquid is performed using a capillary effect in the porous material  14 . The liquid may be named sample liquid. 
     The photodetector  15  may be implemented as a spectral sensor. The photodetector  15  may be fabricated as a spectral sensor integrated circuit. Since the pixels and contact areas of the photodetector  15  are on a first side  17  of the photodetector  15  which is a bottom side of the photodetector  15 , the pixels and contact areas cannot be seen in the view shown in FIG.  1 . The photodetector  15  has a second side  18  shown in  FIG. 1 . 
     The test strip  10  comprises conducting lines  20  to  24  arranged at the second side  13  of the substrate  11 . Thus, the conducting lines  20  to  24  face the first side  17  of the photodetector  15 . The conducting lines  20  to  24  can also be named “traces”. The conducting lines  20  to  24  are made out of a metal such as copper, aluminum or silver (e.g. fabricated as printed silver ink). The conducting lines  20  to  24  made out of copper or aluminum may be etched. The conducting lines  20  to  24  may be designed partially as straight lines and partially as curved lines. The conducting lines  20  to  24  may run parallel. The conducting lines  20  to  24  are aligned with contact areas of the photodetector  15  (e.g. as shown in 
       FIGS. 2A and 2B ) and conductive pads at the end of the test strip  10  (to mechanically align with a strip holder or socket e.g. as shown in  FIG. 4C ). The conducting lines  20  to  24  may run parallel to the arrow indicating the direction of the flow F of the liquid. The conducting lines  20  to  24  may be e.g. rectangular. For example, a first conducting line  20  may be realized as a power supply line. A second conducting line  21  may be implemented as a reference potential line. A third line  22  may be designed as a bus line. A fourth and/or a fifth conducting line  23 ,  24  may also be realized as bus lines. Thus, the conducting lines  20  to  24  comprise at least one bus line  22  to  24 . The bus lines  22 ,  23  may be realized as inter-integrated circuit bus lines, abbreviated I 2 C lines. 
       FIG. 2A  shows an example of the photodetector  15  that can be inserted in the test strip  10  shown in  FIG. 1 . In  FIG. 2A , a perspective view on the photodetector  15  is illustrated. The photodetector  15  comprises at least one pixel  30  on the first side  17  of the photodetector  15 . The pixel  30  may be called photodetector element. The photodetector  15  may comprise a photodetector array  31  of pixels  30  on the first side  17  of the photodetector  18 . The photodetector array  31  may be an n·m array of pixels. In the example shown in  FIG. 2A , the photodetector array  31  is a  4 · 4  array. The pixels  30  of the photodetector array  31  may be sensitive for different regions of light. The pixels  30  may be realized as photodiodes. Thus, the photodetector array  31  realizes a spectral sensor. Additionally, the photodetector  15  may comprise an additional pixel  32  and a further pixel  33  that are larger than a pixel  30  of the photodetector array  31  and are located adjacent to the photodetector array  31 . 
     Moreover, the photodetector  15  comprises contact areas  40  to  47  on the first side  17  of the photodetector  15 . The contact areas  40  to  47  are arranged at a border or two borders of the photodetector  15 . The contact areas  40  to  46  have a distance to the photodetector array  31 . The contact areas  40  to  46  can be realized as bond pads or chip bumps. 
       FIG. 2B  shows another example of the photodetector  15  that is a further development of the photodetector  15  shown in  FIGS. 1 and 2A . The photodetector array  31  is in the center or nearly in the center of the photodetector  15 . A first contact area  40  is realized as a power supply contact area for receiving a supply voltage VCC. A second contact area  41  is implemented as reference potential contact area for receiving a reference potential GND. At least one contact area  42  is designed as a bus contact area. For example, a third and a fourth contact area  42 ,  43  are implemented as bus contact areas, for example for an inter-integrated circuit bus, abbreviated I 2 C bus. For example the third and the fourth contact area  42 ,  43  receive signals or provides signals of the I 2 C bus such as a data signal SDA and a clock signal SCL. The fifth contact area  44  may be designed for a bus or for another purpose and receives a signal INT. 
     The contact areas  40  to  44  are not arranged at exactly one border of the photodetector  15 . The photodetector  15  has a first to a fourth border  51  to  54 . The first border  51  is opposite to the second border  52 . The first border  51  runs parallel to the second border  52 . The third border  53  is opposite to the fourth border  44 . The third border  53  runs parallel to the fourth border  54 . The first contact area  40  may be located at the first border  51 . The second contact area  41  may be located near the first border  51  but has a distance D to the first border  51 . The first contact area  40  has a distance smaller than the distance D to the first border  51 . 
     The third contact area  42  is located adjacent to the second border  52 . The fourth and the fifth contact area  43 ,  44  are adjacent to the second border  52  and have distances D 1 , D 2  to the second border  52 . The third contact area  42  has a distance smaller than the distances D 1  and D 2  to the second border  52 . The fourth contact area  43  may have the distance D 1  to the second border  52  and the fifth contact area  44  may have the distance D 2  to the second border  52 . The distance D 1  may be larger than the distance D 2 . Advantageously, the contact areas  40  to  44  are distributed between the first border  51  and the second border  52 . The photodetector  15  is free of a contact area that has the same or approximately the same distance to the first border  51  as any other contact area. The contact areas  40  to  44  have different distances to the first border  51  and, thus, also to the second border  52 . At least two of the contact areas  40  to  44  have different distances also to the third border  53  and, thus, also to the fourth border  54 . 
     Thus, when looking also at  FIG. 1 , the first side  17  of the photodetector  15  can be attached to the second side  13  of the substrate  11  such that the first contact area  40  is in contact with the first conducting line  20  and has a distance to each other conducting line  21  to  24 . Thus, there is only an electrical contact of the first contact area  40  to the first conducting line  20 . The first contact area  40  is isolated from the other conducting lines  21  to  24 . Correspondingly, the second contact area  42  is only in contact with the second conducting line  21 . Similarly, the third to the fifth contact areas  42  to  44  are in electrical contact with their respective conducting lines  22  to  24 . Consequently, each of the contact areas  40  to  44  is in mechanical and electrical contact only to exactly one conducting line and is isolated from the other conducting lines. Thus, the conducting lines  20  to  24  may be realized as metal lines. The conducting lines  20  to  24  may be fabricated without an isolation layer on top of the conducting lines  20  to  24  which comprises a contact opening for an electrical contact to the respective contact areas. 
     Advantageously, the locations of the contact areas  40  to  44  on the first side  17  of the photodetector  15  allow an easy contacting of the conducting lines  20  to  24  on the second side  13  of the substrate  11 . 
     The photodetector  15  may be realized as a spectral sensor. The photodetector  15  may have a layout optimized for direct bonding (or any other bonding process). The photodetector  15  may have a layout optimized for direct flip chip attach. 
     In an alternative embodiment, the conducting lines  20  to  24  are covered by an isolating layer that has openings at the respective locations for the realization of an electrical contact of the conducting lines  20  to  24  to the contact areas  40  to  43 . 
       FIGS. 2C and 2D  show details of a cross-section of the test strip  10  that is a further development of the test strip shown in the figures above. In the example shown in  FIG. 2C , the first to the third conducting lines  20  to  22  are fixed on the second side  13  of the substrate  11 . The test strip  10  may comprise further conducting lines, not shown. The surface of the conducting lines  20  to  22  rises above the surface of the second side  13  of the substrate  11 . 
     The photodetector  15  is implemented as an integrated circuit, integrated circuit chip or integrated circuit die. In the example shown in  FIG. 2C , the photodetector  15  comprises the first to the third contact area  40  to  42 . The photodetector  15  may comprise further contact areas. The contact areas  40  to  42  are realized as chip bumps. Thus, the surface of the contact areas  40  to  42  rises above the other parts of the surface of the photodetector  15 . 
     Moreover, the test strip  10  comprises a film  55  that is arranged at the second side  13  of the substrate  11 . The film  55  is located at the conducting lines  20  to  22 . Thus, the film  55  is located at the surfaces of the conducting lines  20  to  22  and on those areas of the second side  13  of the substrate  11  which are not covered by the conducting lines  20  to  22 . The film  55  may be an adhesive film. The film  55  is realized as an anisotropic conducting film. The film  55  comprises a non-conducting polymer and conducting particles  56 . The particles may be realized as spheres. 
     In an alternative embodiment, not shown, the film  55  is arranged at the first side  17  of the photodetector  15 . Thus, the film  55  is located at the surfaces of the contact areas  40  to  42  (that means at the surfaces of the chip bumps) and also on the areas on the first side  17  of the photodetector  15  which are not covered by the contact areas  40  to  42 . 
       FIGS. 2C and 2D  show the test strip  10  before and after the process step of fixating the photodetector  15  to the substrate  11 . Whereas in  FIG. 2C  the test strip  10  is shown before attachment of the photodetector  15  to the substrate  11 , the test strip  10  is shown after the fixation of the photodetector  15  to the substrate  11  in  FIG. 2D . 
     As shown in  FIG. 2D , during the attachment process of the photodetector  15  to the substrate  11 , pressure P or force is applied to a stack including the photodetector  15  and the substrate  11 . Thus, the film  55  is squeezed. The stack may be heated during the attachment process. The conducting particles  56  may have a thin outer insulating layer which is broken by the pressure P. Thus, conducting particles  56 , which are located between the contact areas  50  to  52  of the photodetector  15  and the conducting lines  20  to  22  on the substrate  11 , form an electrical contact of the contact areas  50  to  52  to the conducting lines  20  to  22 . There is at least one conducting particle  56  between a contact area  40  to  42  and the corresponding conducting line  20  to  22 . The density of conducting particles  56  in the film  55  is so small that there is no lateral electrical connection of one contact area to an adjacent contact area via the conducting particles  56 . Thus, the contact areas  40  to  42  are isolated from each other and, correspondingly, also the conducting lines  20  to  22  are electrically isolated from each other. The film  55  fills the gap between the photodetector  15  and the substrate  11 . 
     The photodetector  15 , also called spectral sensor, is directly integrated on the substrate  11 . The substrate  11  is realized as plastic carrier. The photodetector  15  is attached with flip chip assembly technology or direct chip attach technology. The integrated circuit of the photodetector  15  is assembled in a way that the photodetector  15  is facing toward the nitrocellulose membrane through the carrier material (PET)  14  or any active area that need to be analyzed. By doing this it will be possible to control with high accuracy the alignment and optical between area that need to be analyzed and spectral sensor (basically they are all on the test strip  10 ). This will allow to increase sensitivity and signal-to-noise ratio during optical measurement, at the same time it will also simplify the hardware implementation of the PoC reader device. By having the photodetector  15  directly aligned with the active area  16  on the test strip  10 , most of the photons/light going through the active area  16  of nitrocellulose will be focused or caught by the photodetector  15 . Contrary to that, in a traditional system based on reflection measurements, the light/photons are scattered by the active areas and captured by the photodetector located in the same plane of the light source (in this case, if the test stripe is not perfectly aligned with the light source and the photodetector, then the photodetector may miss scattered energy and loses sensitivity). 
     The photodetector  15  is attached to the substrate  11  by using the anisotropic conducting film/glue (ACF) by applying small heat (depending by machine process and used ACF) and pressure to it. The photodetector  15  does not require any packaging in order to be directly attached on the test strip  10 . 
       FIG. 3A  shows a further example of the test strip  10  that is a further development of the above-shown examples. In  FIG. 3A , a cross-section of the test strip  10  is shown. The test strip  10  is configured with multiple spectral sensors and multiple active areas. The porous material  14  comprises the active area  16  and an additional active area  60 . Additionally, the porous material  14  may comprise a further active area  61 . Thus, the porous material  14  may comprise a first number L of active areas. In the example shown in  FIG. 3A , the first number L is  3 . The first number L may be larger than  1 , larger than  2  or larger than  3 . The first number L of active areas  16 ,  60 ,  61  may be rectangular, quadratic, circular or elliptical. The first number L of active areas  16 ,  60 ,  61  may reach e.g. from one border of the porous material  14  to the other border of the porous material  14 . 
     Correspondingly, the test strip  10  comprises the first number L of photodetectors  15 ,  61 ,  62 . The test strip  10  comprises the photodetector  15  and an additional photodetector  62  which is attached to the second side  13  of the substrate  11 . Moreover, the test strip  10  may comprise a further photodetector  63  fixed to the second side  12  of the substrate  11 . 
     The photodetector  15  is located adjacent to the active area  16  such that the photodetector  15  receives light from the active area  16 . Correspondingly, the additional photodetector  62  is located adjacent to the additional active area  60 . The further photodetector  63  is positioned adjacent to the further active area  61 . 
     Thus, a separate photodetector  15 ,  62 ,  63  is located adjacent to each of the first number L of active areas  16 ,  60 ,  61 . The additional and the further photodetector  62 ,  63  are implemented as spectral sensors. In the cross-section shown in  FIG. 3A , the flow F of the liquid in the porous material  14  is from the right side to the left side. 
     The test strip  10  comprises a sample pad  80 , a conjugate pad  81  and an absorbent pad  82 . The sample pad  80 , the conjugate pad  81  and the absorbent pad  82  are arranged at the first side  12  of the substrate  11  and enclose the side surfaces of the porous material  14 . The pads  80  to  82  will be further explained with respect to  FIG. 4A . 
       FIG. 3B  shows a further example of the test strip  10  that is based on the examples of the test strip shown in  FIGS. 1, 2C, 2D and 3A  and a light source  70 . The light source  70  emits light. The light source  70  may be implemented as a spectral source. The light is emitted in a broad angle by the light source  70  and reaches the active area  16 . Light of the light source  70  also reaches the additional and the further active area  60 ,  61 . Thus, light emitted by the light source  70  reaches the first number L of active areas  16 ,  60 ,  61 . The light source  70  may be fabricated as a broad band spectral source or black body spectral source, abbreviated BB spectral source. The light source  70  may emit light e.g. in the range from 390 nm to 1050 nm or from 390 nm to 750 nm. Alternatively, the light source  70  may be realized as a narrow band source (having e.g. a very limited bandwidth, such as for example a specific light color source or a pure near infrared source). 
     The substrate  11  is translucent or transparent. Additionally, the porous material  14  is translucent or transparent. Light from the active area  16  reaches the photodetector  15 . The optical characteristic of the active area  16  depends on the optical characteristics of the porous material  14 , of the chemical substances fixed at the porous material  14  in the active area  16  and of the concentration of an analyte in the liquid. For example, the light source  70  emits light in a broad spectrum. The active area  16  transmits light only in a small spectrum such as, for example, red light. The amount of light emitted or transmitted by the active area  16  depends on the concentration of the analyte in the liquid. The value of the light may rise with rising concentration of the analyte. The light transmitted by the active area  16  is detected by the photodetector  15 . 
     The additional photodetector  62  and the further photodetector  63  can detect light originating from the additional and the further active area  60 ,  61 . The first number L of active areas  16 ,  60 ,  61  may have different substances for reaction with the analytes of the liquid. Thus, the optical characteristics of the first number L of active areas  16 ,  60 ,  61  are different and are detected by the first number L of photodetectors  15 ,  62 ,  63 . 
     Each of the first number L of photodetectors  15 ,  62 ,  63  may comprise a photodetector array  31 , similar to  FIGS. 2A and 2B . Thus, each of the first number L of photodetectors  15 ,  62 ,  63  is configured to detect light emitted by the first number L of active areas  16 ,  60 ,  61  at different ranges of light spectrum. The first number L of photodetectors  15 ,  62 ,  63  is realized using separate integrated circuits or chips or dies. The test strip  10  is fabricated with multiple spectral sensors and multiple active areas. 
     The detection/measurement is not realized as reflected ray measurements, but will be a refracted/transmitted/absorbed measurement. The light source  70  will be placed on one side of the test strip  10  and the detection is done on the other side of the substrate  11  of the test strip  10  by the photodetectors  15 ,  62 ,  63 . 
     In an alternative embodiment, not shown, light is emitted by the light source  70  at one wavelength and is absorbed by a substance in the active area  16 , wherein the substance in the active area  16  emit light at another wavelength. Thus, the light is emitted by the active area  16  using fluorescence or phosphorescence. In this case, the light source  70  emits light in a small band. The amount of light emitted by the active area  16  depends on the amount of analyte that reacts with the substance fixed at the active area  16 . For example, the light source  70  is realized as a laser, a light-emitting diode or a vertical-cavity surface-emitting laser, abbreviated as VCSEL. 
     In an alternative embodiment, not shown, the light source  70  is omitted. Thus, a monitoring device is free from a light source. A reaction of a chemical substance of the active area  16  with an analyte in the liquid results in an emission of light. Thus, light is emitted by the active area  16  using chemo-luminescence. 
     In an alternative embodiment, not shown, the first number L of photodetectors  15 ,  62 ,  63  are realized on one die or chip. The first number L of photodetectors  15 ,  62 ,  63  may be implemented as pixels  30  or as photodetector arrays  31  on one die or chip. 
       FIG. 4A  shows an additional example of the test strip  10  which is a further development of the above-shown examples. The porous material  14  comprises the active area  16  and the additional active area  60 . The active area  16  may be realized as a test active area, e.g. as a test line. The additional active area  60  may be implemented as a control active area, e.g. as a control line. Thus, a change of the optical characteristics at the additional active area  60  indicates that the test is performed correctly, for example that a sufficient amount of liquid has been provided to the test strip  10 . A result of the test is detected by measurement of the optical characteristics at the active area  16 . The substrate  11  may be fabricated as a backing card, for example as a plastic adhesive backing card. 
     Additionally, the test strip  10  comprises the sample pad  80 . The sample pad  80  is configured to receive the liquid e.g. from a user or a liquid dispenser. The sample pad  80  is located on the first side  12  of the substrate  11 . Moreover, the test strip  10  comprises the conjugate pad  81 . The conjugate pad  81  is realized for providing a substance to the liquid. The conjugate pad  81  is located on the first side  12  of the substrate  11 . The conjugate pad  81  is arranged between the sample pad  80  and the porous material  14 . An overlap of the sample pad  80  is on the conjugate pad  81 . Thus, an effective transfer of liquid from the sample pad  80  to the conjugate pad  81  is achieved using the overlap. An overlap of the conjugate pad  81  is on the porous material  14 . An efficient transfer of liquid from the conjugate pad  81  to the porous layer  14  is achieved by the area of the overlap. The sample pad  80  and the conjugate pad  81  are located at a first end of the test strip  10 . 
     The test strip  10  comprises the absorbent pad  82  being located on the first side  12  of the substrate  11 . The absorbent pad  82  is arranged at a second end of the test strip  10 . The absorbent pad  82  is in contact with the porous material  14 . The absorbent pad  82  has an overlap with the porous material  14 . Thus, a transfer of liquid is achieved from the porous material  14  to the absorbent pad  82  by the overlap. As indicated by the arrow F, the liquid flows from the sample pad  80  via the conjugate pad  81  and the porous material  14  to the absorbent pad  82 . The liquid inserted on the sample pad  80  only partially reaches the absorbent pad  82 . The photodetectors  15 ,  62  detect the change of the optical characteristics at the active areas  16 ,  60 . 
     In general, the test strip  10  is built in a staked structure as following: The test strip  10  includes the substrate  11  which may be called carrier. The material of the substrate  11  is made e.g. of polystyrene, vinyl or polyester. In general, the substrate  11  is clear (that means transparent) or can be opaque too. An opaque substrate  11  may comprise a transparent or translucent window at the active area  16 . The window may be realized by inserting a transparent or translucent material or by reducing the thickness of the substrate  11 . The substrate  11  is used to hold the nitrocellulose membrane  14 . Then on one side or end of the membrane  14  a detection conjugate is placed on the conjugate pad  81  followed by the sample pad  80 . On the other side or end of the membrane  14 , the absorbent pad  82  is placed. 
     The two lines, control line and test line  16 ,  60 , show respectively the validity of the test and the test result. The disclosure refers how the carrier material is prepared prior nitrocellulose integration. 
       FIG. 4B  shows an example of the test strip  10  which is a further development of the above-shown examples. The test strip  10  comprises a housing  85 . The housing  85  may be fabricated as plastic holder. The liquid can be inserted via a sample port  86  of the housing  85  to the sample pad  80 . The housing  85  has at least one test opening  87 . Thus, light emitted by the light source  70  can be transmitted via the test opening  87  to the active area  16  and to the additional active area  60 . The test opening  87  has a form such that light emitted by the light source  70  reaches the number L of test areas  16 ,  60 . 
     The test strip  10  comprises the conducting lines  20  to  24  which can be contacted at a connecting area  89 . For example, the conducting lines  20  to  24  can be contacted at an end of the substrate  11 . Thus, the housing  85  comprises an opening  90  such that a part of the substrate  11  is located outside of the housing  85 . 
     In an alternative embodiment, not shown, the opening  90  of the housing  85  is on a bottom side of the housing  85 . Thus, contacts to the conducting lines  20  to  24  are realized from below. Thus, the sample port  86  and the test opening  87  are located on an upper side of the housing  85  and the opening  90  is realized on a lower side of the housing  85 . 
     In an alternative embodiment, not shown, the housing  85  comprises an additional opening. Light emitted by the light source  70  can be transmitted through the test opening  87  to the active area  16  and through the additional opening to the additional active area  60 . 
       FIG. 4C  shows an example of a monitoring device  100  that comprises the light source  70 . The test strip  10  as elucidated above can be inserted into the monitoring device  100  and can be removed again. The monitoring device  100  may be named reader or PoC reader. The monitoring device  100  includes a device housing  101 . The light source  70  and part of the test strip  10  are located in the device housing  101 . In  FIG. 4C , the monitoring device  100  is only drawn schematically and as an example. 
     The monitoring device  100  may comprise a socket  103  having pins or spring contacts. The pins or spring contacts of the socket  103  contact the conducting lines  20  to  24 . The monitoring device  100  may comprise guiding parts (not shown) to guide the test strip  10  e.g. into the socket  103 . The device housing  101  may include parts to provide a light shield which shields light from the external of the monitoring device  100  from penetrating into the interior of the device housing  101 . 
     Moreover, the monitoring device  100  comprises a control circuit  102  that is connected to the light source  70  and via the socket  102  and the conducting lines  20  to  24  to the at least one photodetector  15 ,  62 ,  63 . The control circuit  102  is configured to detect whether the test strip  10  is inserted or not and to provide an enable signal, when the test strip  10  is inserted. Additionally, the monitoring device  100  may comprise an interface  104  connected to the control circuit  102  for providing information gained by the monitoring device  100  to an external device. The monitoring device  100  may also comprise a display  105  for displaying information gained by the control circuit  102 . For example, the display  105  may display the enable signal to indicate to a user that the sample liquid can be applied to the test strip  10 . Moreover, the display  105  displays the result of the test. The monitoring device  100  may comprise a power supply  106  such as a battery. Additionally, the monitoring device  100  may comprise a user interface  107  such as a button to start the measuring process. 
     The monitoring device  100  is free of a complicate mechanical switch for detecting the test strip insertion into the monitoring device  100 . The test strip  10  will have electrical connection (termination) corresponding to the various signal and power lines on the substrate  11 . The electronics on the monitoring device  100  will be able to detect the insertion of the test strip  10  by detecting the spectral sensor  15  on the I 2 C lines and consequently enable the monitoring device  100  in a way that is ready to run measurements. 
     Optionally, a protection system can be implemented on the I 2 C line to avoid that counterfeiting strips are inserted. The test strip  10  may be masked in a way that only electronic reading is allowed. This can be done by completely covering the area where the photodetectors  15 ,  62 ,  63  are located or both side leaving only a small slit to allow the impinging light to reach the membrane  14 . 
     The test strip  10  may be configured to perform reactance measurements. To achieve proper measurements the test strip  10  may be equipped with a qualified spectrometer photodetector integrated circuit. The test strip  10  may be named test stripe. 
       FIGS. 5A and 5B  show two different schematic configurations of a monitoring device  100 ,  FIG. 5C  shows results of the simulation and  FIG. 5D  shows an example of the photodetector  15  that has been used for simulation. 
     In  FIG. 5A , a reflective configuration is illustrated. Light emitted by the light source  70  hits the active area  16  on the test strip  10 . Light reflected or emitted by the active area  16  is detected by the photodetector  15 . The light source  70  and the photodetector  15  are approximately in the same plane; the active area  16  is above the plane. A light barrier  110  protects the photodetector  15  from light generated by the light source  70 . The light source  70  may be a light-emitting diode, e.g. emitting white light or broad band white light. An example of a light path is shown in  FIG. 5A . 
     In  FIG. 5B , a configuration as shown in  FIGS. 3B, 4B or 4C  is illustrated. Light emitted by the light source  70  reaches the photodetector  15  via the active area  16  on the test strip  10 . The light source  70  may be a light-emitting diode, e.g. emitting white light or broad band white light. An example of a light path is shown in  FIG. 5B . 
     In  FIG. 5C , simulation results of the configurations shown in  FIGS. 5A and 5B  are listed. As shown in  FIG. 5D , the photodetector  15  has the photodetector array  31  with sixteen pixels  30 . The photodetector array  31  includes eight pairs of pixels  30 , indicated by F 1  to F 8 . The pixels  30  of a pair are sensitive for the same wavelength region in visible light. The additional pixel  32  detects a flicker of ambient light. The further pixel  33  is sensitive in the near-infrared range (abbreviated NIR range). The table in  FIG. 5C  shows the simulation results as total power in Watt for the eight pairs F 1  to F 8  and the NIR sensitive further pixel  33 . As shown in the table, four pairs of pixels F 2  to F 5  and the NIR sensitive further pixel  33  achieve better results in the configuration of  FIG. 5A  and also four pairs of pixels F 1 , F 6  to F 8  achieve better results in the configuration of  FIG. 5B . The differences are small. Thus, both configurations achieve comparable simulation results. For both configurations, a wort case distance between the light source  70  and the photodetector  15  on one side and the active area  16  on the other side is approximately 5 mm. 
     The embodiments shown in  FIGS. 1 to 5D  as stated represent example embodiments of the improved test strip, the monitoring device and method for fabricating a test strip, therefore they do not constitute a complete list of all embodiments according to the improved test strip, the monitoring device and method. Actual test strips and methods for fabricating may vary from the embodiments shown in terms of devices, layers, shape, size and materials, for example.