EP0771293A1

EP0771293A1 - Package having an electrochemical gas sensor

Info

Publication number: EP0771293A1
Application number: EP95925055A
Authority: EP
Inventors: Jerome Mcaleer; Martin Robert Ackland; Elliot Plotkin; Simon Balderson
Original assignee: ENVIROMED PLC
Current assignee: ENVIROMED PLC
Priority date: 1994-07-15
Filing date: 1995-07-14
Publication date: 1997-05-07
Also published as: WO1996002438A1; AU2932495A; JPH10506076A; GB9414360D0

Abstract

Packaging for foodstuffs or other products incorporates a gas impermeable barrier including an electrochemical sensor adapted to detect oxygen or other analyte in the interior of a package and adapted to generate a signal on the exterior of the package.

Description

PACKAGE HAVING AN ELECTROCHEMICAL GAS SENSOR

This invention relates to packaging particularly but not exclusively for foodstuffs and degradable products. The invention relates in particular to an electrochemical sensor incorporated into packaging to provide an indication of the condition of the packaged product.

Foodstuffs and other products are stored and distributed inside packages. These packages have several functions including containment, maintenance of a specific atmosphere and prevention of contamination and tampering. Packaging technology has advanced and new materials are available which allow increased storage life and increased protection for the packaged product. Tamper-proof packaging is also available.

Advances in packaging technology have involved the improvement of the barrier properties of the package. Such improvements have increasingly allowed the maintenance of an environment within the package which is different from the surrounding atmosphere. For example modified atmosphere packaging (MAP) , particularly for foodstuffs, involves flushing a package with a specific mixture of gases which inhibit bacterial growth. This can dramatically increase storage life. The development of polymer films with excellent gas barrier properties has allowed a modified atmosphere to be maintained by prevention of diffusion from inside or outside the package. The improvement in the effectiveness and performance of such packages has been reflected in the extension of the use of such packaging to a variety of products including pharmaceuticals, cosmetics and chemicals. In such cases the package provides an effective barrier containing a controlled artificial environment.

Although currently available packaging can provide an environment which will dramatically improve the storage life and safety of the packaged product, the storage life and safety of a product can be dramatically reduced if the package fails. For example a foodstuff stored under an atmosphere of carbon dioxide and nitrogen may have a storage life of up to 40 days. If the package is sealed incorrectly, broken, or tampered with, the modified atmosphere can be contaminated with air and the storage life can be reduced to as little as 6 days. Furthermore a similar loss of storage life can be observed if the package is not filled with the correct mixture of gases during manufacture. Incorrect gas flushing due to an equipment or operator error can result in storage difficulties.

Loss of package integrity and the consequent loss of a modified atmosphere or impairment of the barrier may not be evident by observation. Small holes which may be caused by incorrect sealing, breakage or tampering are usually invisible. Similarly the loss of a modified atmosphere may not result in any visual change to the product. The loss of storage life and compromised safety of a product may only be observed by a consumer, perhaps with unfortunate results.

The increasing importance of MAP and similar packaging techniques has resulted in a consequent need for manufacturers to test and modify the atmosphere inside the package to ensure that package integrity and any modified atmosphere has been maintained. However the very effectiveness of the barrier provided by current packages has made such testing difficult. It has been necessary to penetrate the package to remove a sample of the atmosphere for testing by an external instrument. This is a destructive procedure and the resulting package must be discarded. Destructive tests cannot be carried out on every package. Consequently a statistically significant quantity of packages must be tested and errors are inevitable. A negative result from a test of an individual package can cause considerable problems because it is not possible to assess other packages within the batch without destruction of all of them.

In addition to the problems associated with destructive testing of packages, gas samples removed from such packages must be analysed using sophisticated equipment. Such equipment is usually not portable and the testing can only be performed within the proximity of such equipment. Such tests are usually carried out by the packager but no further testing may be possible during the subsequent storage, distribution and retail procedures.

Gas sensitive materials which afford a colour change have been proposed. For example UK patent application 9401557.2 discloses use of reversible oxygen and carbon dioxide sensors for this purpose. Such sensors may indicate package integrity but they do not allow accurate measurement of the atmosphere within the package. In cases where the residual package oxygen level is important, a colour changing sensor may not allow sufficiently accurate interpretation. A colour change sensor cannot be used in an opaque package. Furthermore colour change sensors are unsuitable for measurement of small quantities of specific gases and they cannot be used to measure more than one gas.

According to the present invention a gas impermeable package for an article incorporates a gas impermeable barrier and includes an electrochemical sensor adapted to detect an analyte disposed in the interior of the package and adapted to generate a signal on the exterior of the package.

The electrochemical sensor may be adapted to detect one or more of a wide variety of analytes at precise concentrations. Destructive testing is avoided and the acceptability of every package may be ascertained. This is important for high value products, for example whole carcasses or other bulk foodstuffs.

A preferred package further comprises a detector adapted to provide a display in response to said signal. The detector may be integral with the package. In preferred embodiments the detector is releasably engagable with the sensor on the exterior of the package. The detector preferably incorporates a power supply. Preferred detectors may be hand held or are otherwise portable.

The electrochemical sensor may be conductimetric, potentiometric, amperometric or impedometric. Preferred analytes are gases or volatile liquids for example oxygen, carbon dioxide, hydrogen sulphide, ethanol, alkyl sulphide, ketones, aldehydes, esters, ammonia or amines.

Preferred electrochemical sensors may be manufactured by screen printing, lithographic or other thick film deposition processes, for example as disclosed in PCT/GB93/02076. Such sensors may be easy to fabricate and are inexpensive. Enzymic sensors may be employed. Dried stabilised enzymes may be provided together with a portion of aqueous solvent which may be disposed in a releasable container in the proximity of the electrode. A covering membrane may be used to prevent leaching or degradation of the constituents of the sensor.

Use of an electrochemical sensor in accordance with the invention affords the advantages of simple and economical manufacture and use, a high speed of response measurement and accuracy and affords the facility of measurement of a wide range of analytes. The analytes include gases, bacteria, pH, conductivity and additives. Multiple sensors responsive to a plurality of analytes may be afforded. Alternatively or in addition a multiplicity of single use sensors may permit multiple testing of a package so that an analyte may be monitored during a prolonged period, eg throughout the lifetime of a product.

An additive may be incorporated within a package in accordance with the invention to afford an indication of tampering or for verification of genuine goods. An electrochemical sensor adapted to detect a change in the amount of an additive gas within a package may afford a signal indicating escape of the gas from the package due to tampering or other loss of integrity. Such an additive may be a gas provided at a trace concentration for which the sensor may be particularly adapted. Alternatively an atmospheric gas may be provided at a different concentration to that of the surrounding atmosphere. An additive or physical property of the contents of a package may be selected to provide confirmation of genuine goods. Such an addition or property may be changed from time to time to inhibit unauthorised copying.

A preferred embodiment of the invention may serve to indicate that a product has been packaged correctly. For many foodstuffs for example the correct flushing of the package with a modified atmosphere is critical to maintain the product in a safe condition during storage and distribution. A low oxygen level, for example below 1% is required in many cases. A sensor responsive to oxygen may confirm that a suitably low concentration has been achieved.

Further preferred embodiments of the invention may indicate breakage or incorrect sealing of a package. Loss of the package atmosphere and replacement by air may be detected. Alternatively or in addition the sensor may be adapted to monitor the gas or humidity level within a package to indicate whether these are the same as the ambient atmosphere by comparison with an external sensor.

Provision of a portable detector allows testing of packages to be carried out at any convenient juncture during manufacture, distribution and retail. For example the condition and quality of a food product may be observed during transportation, storage or retail using packaging of the present invention.

The invention is further described by means of example but not in any limitative sense with reference to the accompanying drawings of which:

Figure 1 is a cross-sectional view of a sensor in accordance with the invention;

Figure 2 illustrates a Clark sensor in accordance with the invention;

Figure 3 illustrates a laminated sensor in accordance with the invention;

Figure 4 illustrates a sensor in accordance with the invention incorporating a metallised layer which acts as both gas barrier and electrode;

Figure 5 shows a plan view of the electrode shown in Figure 4;

Figure 6 illustrates an alternative embodiment of the invention;

Figure 7 illustrates a further embodiment of the invention;

Figure 8 illustrates a further embodiment of the invention;

Figure 9 is a cross sectional view of a sensor;

Figure 10 is a diagrammatic view of a sensor coupled to a monitor;

Figure 11 illustrates a typical hand held monitor in accordance with the invention;

Figure 12 illustrates the response of a sensor as described with reference to Figure 2; and

Figures 13 and 14 show the frequency dispersion of impedances of sensors in accordance with this invention.

Figure 1 shows a sensor element 1 attached to an impermeable packaging material 2, for example formed from a polymeric film, at a preformed hole 3. The sensor element 1 may be attached by welding, for example thermal or ultrasonic welding, by an adhesive or other bonding process so as to form a gas impermeable seal. The sensor l may comprise a polymeric substrate formed from polyester, polyethylene terephthalate or polycarbonate or a laminate of polymers which is physically compatible with the packaging material 2. Physical compatibility is important so that stresses are not imposed on the seal due to thermal or other environmental effects. Compatibility is also important to allow physical flexibility and resilience in use. Alternative materials include foil, metallised films and waxed papers. Electrical contacts 6 and 7 communicate with the exterior of the package and are connected to a sensing layer 8 incorporating electrodes. A protective layer 9 may overlie the sensing layer 8 and a spacer 14 may allow gas circulation in the proximity of the sensor in use.

Figure 2 shows an embodiment of the invention wherein the sensor is a Clark type electrode. Small holes 4 extending through the substrate 5 may be formed by laser photo-ablation. The holes 4 are filled with conducting materials 6 and 7 to form the sensor electrodes. For example a conducting ink may be deposited onto the substrate 5. Preferred inks may contain silver, gold or platinum particles together with a resin binder to provide a gas tight seal. Alternatively the electrodes may be incorporated into the patch substrate during manufacture, for example by inclusion of fine metal strands or particles to provide naturally conductive regions on the substrate 5. The electrodes are covered with an electrolyte layer 8 which can be deposited by any convenient means for example printing or spray coating. Preferred electrolyte layers include hydrogels, for example a gelatin/glycerol/water mixture. The glycerol or other hygroscopic material serves to retain the water content of the gel and ensure conductivity. Alternative hydrogels may include PVA or commercially available polymer electrolytes such as Nafion or Promeon.

An example of the response of such a sensor to alternating streams of nitrogen and air is shown in Figure 12. The "solid state" oxygen sensitive electrode was fabricated by dosing 2μl of a 5% Nafion solution in a mixture of alcohol and water onto a sensor comprising carbon and silver electrodes printed onto a polyester substrate. Currents were recorded under a polarisation potential of 800 mV.

Alternatively a highly water retentive membrane may be laminated onto the electrode to form an electrolyte layer. A gas permeable membrane 9 serves to isolate the electrolyte layer from the contents of the package. A Clark electrode of the kind described above measures oxygen directly.

Indirect oxygen measurement may also be employed. For example a redox active film for example Prussian blue may be employed. Further electrochemically active species which may be used to determine oxygen are tetraphenyl porphyrins . The use of such materials as disclosed in U C S Chem Comm 1990, 721 may give rise to higher sensitivity. Reduction of the porphyrin from the stable oxidised form to the oxygen sensitive reduced form provides an integral activation step which avoids the need for the sensor to be kept in an oxygen free environment prior to use.

Figure 3 illustrates a laminated structure wherein the sensor substrate 5 is formed in two parts separated by a metallic layer 10. The metallic layer 10 may be printed or vapour deposited. Such a configuration exhibits integrity against leakage. A plurality of metallic layers may be provided.

Figure 4 illustrates incorporation of an electrode as disclosed in PCT/GB93/02076 into a packaging arrangement in accordance with the invention. A metal coated polymer film 11 is demetalised in particular regions 12 to allow gas diffusion. The film is laminated across the hole 13 in the packaging film 2. An electrolyte layer 8 is protected from the external atmosphere by a gas impermeable material 2.

Figure 5 shows a plan view of the arrangement shown in Figure 4.

Figure 6 illustrates an alternative configuration to that shown in Figure 4. In Figure 6 an electrode array 6, 7 is printed onto the outer surface of the pack and small regions of the pack, adjacent the electrodes are rendered gas permeable. For example in a laminated pack laser etching may be employed. The electrode assembly is covered with an electrolyte layer and barrier film 2.

Figure 7 shows an alternative arrangement avoiding the need for a cut and patch arrangement. Electrical contact is made using very fine holes filled with resin based conductive inks which serve as a gas barrier. The risk of leakage around the contacts can be further minimised by use of evaporated metal coatings. This embodiment has the advantage that the sensor can be integrated into the packaging material at any convenient point of manufacture. The contact holes can be drilled by laser and filled and the electrolyte layer 8 and protective film 9 applied by an on-line procedure. Appropriate selection of the electrode materials can provide a sensor configured as a battery to drive a permanently attached external display 13. For example one electrode may be formed of zinc and the other of silver to generate a current in use. The total charge passed represents the integration of oxygen over time and can be used to trigger the external display.

Figure 8 shows an alternative arrangement shown in Figure 7 wherein the electrodes 6 and 7 are applied to the pack material 2 and holes are subsequently generated to expose the metal surfaces. Laser ablation may be used particularly because the aperture formed can be controlled by regulation of the number of pulses and wavelength of the laser. The exposed electrode can be made extremely small to minimise the perturbation of the atmosphere caused by each measurement.

The electrolyte layer 8 may comprise a conductor, the impedance of which changes as a consequence of the local environment. Conducting polymers such as polypyrrole, polyaniline or polythiophene may be employed. These materials are electrically conducting in the oxidised state but are non¬ conducting in the reduced state. The conductance of these materials also changes due to absorption of analytes such as ammonia and other nitrogen containing molecules. A plurality of sensors may be incorporated into a single device using the combined outputs to obtain a characteristic or fingerprint analysis of the contents of the package.

The protective layer 9 can be omitted. Alternatively the protective layer 9 can be arranged to allow transfer of specific species. One or more of the electrodes in a package may be sensitive to a target analyte. Alternatively a pH sensitive element may be incorporated using any of the several proton reversible reference electrode materials known to persons skilled in the art, for example oxides of antimony, bismuth or tantalum. Microbial activity may be detected via a pH change by means of a potentiometric sensor.

In an alternative embodiment an ion reversible electrode can be arranged to be poisoned to modify the response characteristic. For example silver electrodes which are sensitive to sulphur compounds and can be used to detect microbes which produce hydrogen sulphide or other sulphur containing metabolites.

Figure 9 illustrates a packaging configuration wherein a signal is provided without direct electrical connection between the interior and exterior of the package. A sensing circuit 15 is printed on the interior of a packaging film and a corresponding analytical circuit 16 is printed on the exterior of the film. A suitable wave form applied to the exterior circuit induces current flow in the interior circuit which in turn generates a back EMF limiting the current flow in the exterior circuit . The magnitude of the current flow in the inner circuit is limited chemically by an electrochemical sensor as described above. Analysis of the current-voltage relationship in the outer circuit allows the impedance of the inner circuit to be calculated.

The modification of the impedance of the inner circuit by the oxygen content of the atmosphere is clearly shown in Figures 13 and 14 which show the frequency dispersion of the magnitude and phase angle of the impedance of a sensor in a pack when the atmosphere is changed from nitrogen to air. In both cases the gas streams were water saturated reflecting the atmosphere inside a typical modified atmosphere package containing, for example, a carcass. Unlike the steady state current measurements referred to earlier, no DC bias was applied in this instance, the measurement being made at the equilibrium potential established between the two electrodes under those particular atmospheric conditions. In addition to the changes in impedance magnitude, the changes in maximum phase angle and roll-off frequency indicate that a simple tuned resonance device affords an economical measurement system.

Alternatively the configuration of the pack may constitute a capacitor as shown in Figure 10, the change in impedance at the inner plates of the capacitor 20 and 21 being monitored. Alternatively a change in the impedance of the sensing layer may be detected. A high relative permittivity and minimal thickness of the dielectric layer is preferred although the gas barrier must not be compromised. A high permittivity may be achieved by incorporation of a high dielectric material, for example barium stannate into the material and the thickness of the layer may be reduced by laser etching.

Figure 11 illustrates a measuring instrument 15 which is portable and comprises a display 16, a key pad to input parameters 17, connectors 18 and a port for data download ₁9. A memory (not shown) may be used to retain results of previous measurements and may serve to store data for future use. _A warning system (not shown) may be used to alert a user to an unacceptable reading or an unacceptable trend in readings.

Claims

1. A gas impermeable package for an article incorporating a gas impermeable barrier and including an electrochemical sensor adapted to detect an analyte disposed in the interior of the package and adapted to generate a signal on the extension of the package.

2. A package as claimed in Claim 1, further comprising a detector adapted to provide a display in response to said signal.

3. A package as claimed in Claim 2, wherein the detector is releasably engageable with the sensor on the exterior of the package.

4. A package as claimed in Claim 3, wherein the detector is portable.

5. A package as claimed in any preceding claim, wherein the analyte is a gas or volatile liquid.

6. A package as claimed in any preceding claim, wherein the sensor is manufactured by thick film deposition .

7. A package as claimed in any preceding claim, including a multiplicity of sensors.

8. A package as claimed in any preceding claim incorporating an additive, the sensor being adapted to provide a signal upon change in the amount of additive within the package.

9. A package as claimed in Claim 8, wherein the additive is a gas.

10. A package as claimed in claim 9, wherein the additive is an atmospheric gas, the gas being present at a different concentration within the package to that in the surrounding atmosphere.

11. A package as claimed in Claim 10, wherein the gas is oxygen.

12. A package as claimed in any preceding claim, wherein the article is a foodstuff.

13. A package as claimed in any preceding claim, wherein the signal affords an indication of tampering or loss of integrity of the package.