Temperature Indicator with Electrochemical Switch

Abstract

A temperature-activated temperature indicator includes a substrate and an indicator material supported by the substrate. The indicator material includes a mixture of a side-chain crystalline polymer and conductive particles and provides an electrical indication upon exposure to elevated temperature.

Description

[0001] Field

[0002] This disclosure pertains to the field of temperature indicators. More specifically, some embodiments include electronically readable temperature-activated temperature indicators.

[0003] Background

[0004] Many vaccines, drugs, foods, and other products are temperature-sensitive or prone to spoilage and may lose quality over time, with the rate of quality loss being affected by the product temperature, which is closely related to the ambient temperature and the temperature around the product. Time-temperature indicators are known that can provide a simple visual indication of the history of a subject product's exposure to heat, such as cumulative or peak heat exposure. Indicators provided near or on the product packaging are exposed to temperatures similar to those experienced by the product itself over time. The visual indication can be used to provide a signal as to whether the product may have lost quality or freshness. Some time-temperature indicators can integrate historical temperature exposure over time under various conditions in a predictable, quantitative manner and can be used to monitor cumulative heat exposure to indicate the effective shelf life of a perishable subject product or for other purposes.

[0005] Known time-temperature indicators can provide a color change at a predetermined endpoint to indicate that the subject product may have lost quality or freshness. The color change can be displayed in a suitable label or the like and optically read, for example, by a human observer or electronically via a device such as a barcode reader or a mobile phone by visual means. The color change can be colored or colorless, or provided by another visually detectable optical parameter change. The temperature response parameter of the time-temperature indicator over time can be configured to be related to the degradation characteristics of the subject product to appropriately coordinate the color change with the possible condition of the subject product.

[0006] Some known time-temperature indicators use diacetylenic monomer compounds that polymerize in response to environmental conditions (such as temperature exposure) to provide a color change. See, for example, U.S. Patent Application Publication Nos. 2009 / 0131718; 2011 / 0086995; and 2008 / 0004372; and U.S. Patent Nos. 4,789,637; 4,788,151; 5,254,473; 5,053,339; 5,045,283; 4,189,399; 4,384,980; and 3,999,946.

[0007] Some other time-temperature indicators employ diffusion techniques, such as U.S. Patent Nos. 6,741,523; 6,614,728; and 5,667,303; and U.S. Patent Application Publication No. 2003 / 0053377.

[0008] Moreover, for certain perishable products such as vaccines and sensitive drugs, as well as some food and other products, including some industrial products, their quality or safety may be compromised by relatively brief exposure to temperatures exceeding a predetermined threshold. Various proposals for monitoring such temperature exposure are known, including those in U.S. Patent Nos. 7,517,146; 5,709,472; and 6,042,264.

[0009] However, there is a need for improved temperature indicators that can be read without relying on visually detectable changes in optical parameters. SUMMARY OF THE INVENTION

[0010] Disclosed herein are temperature-activatable temperature indicators that utilize conductivity to provide an indication of exposure to elevated temperatures, which indication can be a reversible indication or an irreversible indication. In some embodiments, the indicator is a small, flexible, low-cost temperature event recorder that can be easily manufactured, for example, by printing, and is suitable for attachment to small items such as vaccine vials. For example, the indicator can be printed directly on a product label or packaging.

[0011] Some embodiments of the present invention include a temperature-activatable temperature indicator that includes a substrate and an indicator material supported by the substrate. The indicator material includes a mixture of a side-chain crystalline polymer and conductive particles.

[0012] Some embodiments of the present invention include a temperature-activatable temperature indicator that includes a substrate, a first layer supported by the substrate, and a second layer that includes a side-chain crystalline polymer. The first layer includes conductive particles and is located between the substrate and the second layer.

[0013] According to some embodiments, a perishable host product includes a temperature indicator as disclosed herein, the temperature indicator being associated with the host product to monitor the exposure of the host product to ambient temperature.

[0014] Some embodiments of the present invention include a method of manufacturing a temperature-activatable temperature indicator, including: melting a side-chain crystalline polymer, adding conductive particles to form a mixture, and applying the mixture to a substrate.

[0015] These and other features are disclosed in more detail in the drawings and the detailed description of the invention below.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1A is a plan view of an example of a temperature indicator according to an example embodiment of the present disclosure.

[0018] Figure 1B Is a plan view of another example of a temperature indicator according to an example embodiment of the present disclosure.

[0019] Figure 2 Is a graph showing the temperature-viscosity curves of three temperature indicator materials according to the present disclosure.

[0020] Figure 3A Is an example of a perishable host product associated with a temperature indicator to monitor the exposure of the host product to ambient temperature, wherein the temperature indicator is electrically connected to the antenna of an RFID tag.

[0021] Figure 3B Is an example of a perishable host product associated with a temperature indicator to monitor the exposure of the host product to ambient temperature, wherein the temperature indicator is electrically connected to the chip of an RFID tag. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention relates to a temperature-activatable temperature indicator composed of a substrate and an indicator material supported by the substrate. The indicator material can be coated or printed or otherwise adhered to the substrate. The disclosed temperature indicator provides an indication of exposure to elevated temperature, particularly an electrical indication of exposure to elevated temperature. The temperature indicator provides a record of the specific temperature events to which the indicator (and thus any product to which it is attached) has been exposed, which in some cases is irreversible. In some cases, a perishable host product includes a temperature indicator as disclosed herein, which is associated with the host product to monitor the exposure of the host product to ambient temperature.

[0024] The substrate can be made of a variety of materials, such as polymer films, paper, cardboard, or fabrics, or similar materials. The substrate can include a pressure-sensitive adhesive for attaching the temperature indicator to the host product, packaging, or container. If desired, the adhesive can be protected with a removable liner that can be removed before attachment.

[0025] The indicator material is configured to undergo a change in conductivity in response to the indicator material being exposed to a temperature above a threshold temperature. In some cases, the change in conductivity occurs almost immediately or after a relatively short period of time after the indicator material is exposed to a temperature above the threshold temperature. In such cases, the conductivity changes after the indicator material is exposed to a temperature above the threshold temperature for about 30 seconds or less, such as about 25 seconds or less, about 20 seconds or less, about 15 seconds or less, about 10 seconds or less, about 5 seconds or less, or about 2 seconds or less. In some cases, the conductivity changes after the indicator material is exposed to a temperature above the threshold temperature for a relatively long period of time. In such cases, the conductivity changes after the indicator material is exposed to a temperature above the threshold temperature for about 1 minute to about 48 hours, such as about 1 minute to about 2 minutes, about 2 minutes to about 5 minutes, about 5 minutes to about 10 minutes, about 10 minutes to about 30 minutes, about 30 minutes to about 1 hour, about 1 hour to about 90 minutes, about 1 hour to about 2 hours, about 2 hours to about 5 hours, about 5 hours to about 10 hours, about 10 hours to about 24 hours, about 24 hours to about 36 hours, or about 24 hours to about 48 hours.

[0026] In response to being exposed to a temperature above a threshold temperature, the indicator material can undergo a change in conductivity from a first conductivity before being exposed to a temperature above the threshold temperature to a second conductivity after being exposed to a temperature above the threshold temperature. In some cases, the first conductivity is greater than the second conductivity. In some cases, the second conductivity is greater than the first conductivity.

[0027] The change in conductivity of the indicator material can be a change from a conductive state to an insulating state, or from an insulating state to a conductive state. In some cases, the indicator material can be conductive before being exposed to a temperature above the threshold temperature and then insulating after being exposed to a temperature above the threshold temperature. In some cases, the indicator material can be insulating before being exposed to a temperature above the threshold temperature and then conductive after being exposed to a temperature above the threshold temperature. Whether the material is conductive or insulating can be determined by measuring the sheet resistance of the material. Generally, the sheet resistance of a suitable conductive material is about 100 ohms per square or less, such as about 70 ohms per square or less, about 50 ohms per square or less, about 30 ohms per square or less, about 20 ohms per square or less, or about 10 ohms per square or less. Generally, the sheet resistance of an insulating material is about 100,000 ohms per square or higher, such as about 500,000 ohms per square or higher, or such as about 1,000,000 ohms per square or higher.

[0028] The change in conductivity can be irreversible or reversible. In the case of an irreversible change in conductivity, the change in conductivity persists after the indicator material is no longer exposed to a temperature above the threshold temperature. In such a case, after the conductivity changes and then after subsequent exposure to a temperature below the threshold temperature, the indicator material retains the changed conductivity. Generally, when the change in conductivity still persists after exposure to a temperature below the threshold temperature for at least 48 hours, for example, after exposure to a temperature below the threshold temperature for at least 72 hours, the change in conductivity will be considered irreversible. In the case where the change in conductivity is reversible, the change in conductivity no longer persists after the indicator material is no longer exposed to a temperature above the threshold temperature. In such a case, after the conductivity changes and then after subsequent exposure to a temperature below the threshold temperature, the indicator material does not retain the changed conductivity and can revert to its initial conductivity before it was exposed to a temperature above the threshold temperature. Generally, when the change in conductivity no longer persists after exposure to a temperature below the threshold temperature for 48 hours or less, the change in conductivity will be considered reversible. For example, subsequent exposure to a temperature below the threshold temperature can be from about 1 minute to about 48 hours, such as about 1 minute to about 2 minutes, about 2 minutes to about 5 minutes, about 5 minutes to about 10 minutes, about 10 minutes to about 30 minutes, about 30 minutes to about 1 hour, about 1 hour to about 90 minutes, about 1 hour to about 2 hours, about 2 hours to about 5 hours, about 5 hours to about 10 hours, about 10 hours to about 24 hours, about 24 hours to about 36 hours, or about 24 hours to about 48 hours.

[0029] The indicator material includes conductive particles. In some cases, the indicator material is present in a first layer and a side-chain crystalline polymer is present in a second layer, with the first layer being located between the substrate and the second layer.

[0030] In some cases, the indicator material includes a side-chain crystalline (SCC) polymer and conductive particles. In some cases, the side-chain crystalline polymer and the conductive particles are present together in a single layer as a mixture or blend. In some cases, the conductive particles are dispersed or dissolved in the side-chain crystalline polymer, or are partially or completely encapsulated by the side-chain crystalline polymer. The conductive particles can be distributed in the side-chain crystalline polymer either substantially uniformly or non-uniformly. In some cases, the indicator material is present as a single-layer film supported by a substrate. In some cases, the thickness of the film is less than about 5 mils (where one mil is 0.001 inches, ~25.4 microns), such as from about 1 mil to about 4 mils, about 2 mils to about 3 mils, or about 3 mils. Figure 1A An embodiment of the temperature indicator 10 is illustrated in a plan view. The indicator 10 includes a substrate 12. The substrate supports a layer containing a mixture of conductive particles 14 and a side-chain crystalline polymer 16.

[0031] Figure 1B Another embodiment of the temperature indicator 20 is shown in a plan view. The indicator 20 includes a substrate 12. The substrate supports a layer containing conductive particles 14 and a matrix material 18. Suitable matrix materials include, but are not limited to, adhesives, surfactants, coating additives, and solvents. The temperature indicator 20 also includes a layer containing a side-chain crystalline polymer 16. As Figure 1B shown, the layer containing conductive particles 14 and matrix material 18 is located between the substrate and the layer containing side-chain crystalline polymer 16. When the side-chain crystalline polymer melts, it flows into the layer containing conductive particles 14 and matrix material 18 to effect a change in the conductivity of the layer containing conductive particles 14.

[0032] The side-chain crystalline polymer and the conductive particles can be present in the indicator material in various weight ratios. In some cases, the weight ratio of the side-chain crystalline polymer to the conductive particles in the indicator material is from about 50:50 to about 20:80, about 45:55 to about 20:80, about 40:60 to about 20:80, about 35:65 to about 20:80, about 30:70 to about 20:80, about 25:75 to about 20:80, about 40:60 to about 25:75, about 35:65 to about 25:75, about 30:70 to about 25:75, about 40:60 to about 30:70, about 35:65 to about 30:70, about 30:70, about 35:65, about 40:60, about 45:55, or about 50:50. In some cases, the weight ratio of the side-chain crystalline polymer to the conductive particles in the indicator material is from about 1:1.2 to about 1:4, such as about 1:1.3 to about 1:3, about 1:1.3 to about 1:2.5, about 1:1.4 to about 1:2.4, about 1:1.5 to about 1:2.3, about 1:1.6 to about 1:2.4, about 1:1.7 to about 1:2.5, about 1:1.8 to about 1:2.6, about 1:1.9 to about 1:2.7, about 1:2 to about 1:2.6, about 1:2.1 to about 1:2.5, about 1:2.2 to about 1:2.4, or about 1:2.3.

[0033] Advantageously, the side-chain crystalline polymer can provide a relatively sharp transition from the solid state to the liquid state. The sharp transition can be used to correlate the temperature-responsive properties of the polymer with the temperature-responsive properties of the host product to facilitate monitoring of the temperature exposure of the host product. The melting range of the side-chain crystalline polymer generally represents the sharpness of the solid-to-liquid transition. In some cases, a relatively narrow melting range may be useful, such as a melting range of about 10 °C, about 5 °C, about 2 °C, about 1 °C, or about 0.5 °C. The side-chain crystalline polymer may also have a relatively low melting temperature, such as about 55 °C to about 65 °C, about 45 °C to about 55 °C, about 35 °C to about 45 °C, about 25 °C to about 35 °C, about 65 °C or lower, about 60 °C or lower, about 55 °C or lower, about 50 °C or lower, about 45 °C or lower, about 40 °C or lower, about 35 °C or lower, about 30 °C or lower, or about 25 °C or lower.

[0034] Without wishing to be bound by theory, it is believed that after exposure to a threshold temperature for a sufficient length of time, the side-chain crystalline polymer melts and thus separates or reorients the conductive particles, causing the indicator material to undergo a change in conductivity.

[0035] As used herein, the term "melting temperature" or "melting point" refers to the temperature at which, as determined by differential scanning calorimetry, the material exhibits a peak heat absorption per degree Celsius. Above its melting temperature, the material can exhibit liquid properties, while below its melting temperature, the material can exhibit solid properties.

[0036] As used herein, the term "melting temperature range" refers to the temperature range from the onset melting temperature of the material to the melting temperature.

[0037] As used herein, the term "onset melting temperature" refers to the temperature at which, as determined by differential scanning calorimetry, a meltable material begins to exhibit an increase in heat absorption per degree Celsius. Below its onset melting temperature, the material can be solid.

[0038] The side-chain crystalline polymer can have a melting temperature close to the threshold temperature of the temperature indicator. Thus, for example, the threshold temperature can be about 55 °C to about 65 °C, about 45 °C to about 55 °C, about 35 °C to about 45 °C, about 25 °C to about 35 °C, about 65 °C or below, about 60 °C or below, about 55 °C or below, about 50 °C or below, about 45 °C or below, about 40 °C or below, about 35 °C or below, about 30 °C or below, or about 25 °C or below.

[0039] Suitable side-chain crystalline polymers include polymers and copolymers of methacrylates and acrylates, which have straight-chain aliphatic side chains capable of becoming crystalline at temperatures in the target temperature range, such as from about 25°C to about 65°C. The side chain can have at least 10 carbon atoms, such as from about 10 to about 30 carbon atoms, for example 10 to 30 carbon atoms, 10 to 24 carbon atoms, 10 to 22 carbon atoms, 10 to 20 carbon atoms, 12 to 18 carbon atoms, 14 to 16 carbon atoms, 14 to 18 carbon atoms, 16 to 18 carbon atoms, 12 to 16 carbon atoms or 14 to 16 carbon atoms. Some examples of such polymers include poly(alkyl methacrylate), such as poly(hexadecyl methacrylate) or poly(octadecyl methacrylate); poly(alkyl acrylate), such as poly(tetradecyl acrylate), poly(hexadecyl acrylate) or poly(dodecyl acrylate); copolymers, such as a copolymer of hexadecyl acrylate and octadecyl methacrylate, poly(hexyl-co-dodecyl acrylate), a copolymer of tetradecyl acrylate and octadecyl acrylate, a copolymer of hexadecyl methacrylate and octadecyl methacrylate, and a copolymer of tetradecyl acrylate and hexadecyl acrylate. Mixtures of two or more of any of the side-chain crystalline polymers described herein can also be used. Examples of side-chain crystalline polymers are also disclosed in U.S. Patent No. 9,546,911, which is incorporated herein by reference in its entirety.

[0040] The properties of the side-chain crystalline polymer can be adjusted by introducing crosslinks into the polymer structure by including one or more crosslinking agents during the polymerization process. For example, a difunctional acrylate or methacrylate, or other suitable compounds, such as hexanediol diacrylate, can be included during the polymerization process to act as a crosslinking agent and produce a crosslinked polymer product. For example, the side-chain crystalline polymer can be crosslinked to increase the persistence of the altered conductivity, i.e., to increase the period of time the indicator material can be exposed to temperatures below the threshold temperature before the altered conductivity ceases to persist. In some cases, a relatively low crosslink density (e.g., about 0.01 to about 0.09 intermolecular crosslinks per polymer chain) or weight average molecular weight can be used.

[0041] As used herein, unless the context indicates a number average molecular weight, the term "molecular weight" refers to the weight average molecular weight. A side chain crystalline polymer can have a molecular weight of at least about 1000 Da, such as at least about 1,500 Da, at least about 2,000 Da, or at least about 5,000 Da. In some cases, the molecular weight of the side chain crystalline polymer is in the range of: about 2,000 Da to about 300,000 Da, such as about 3,000 Da to about 300,000 Da, about 5,000 Da to about 250,000 Da, about 10,000 Da to about 200,000 Da, about 15,000 Da to about 150,000 Da, about 20,000 Da to about 120,000 Da, about 30,000 Da to about 100,000 Da, about 50,000 Da to about 80,000 Da, about 2,000 Da to about 20,000 Da, about 3,000 Da to about 15,000 Da, about 4,000 Da to about 10,000 Da, about 4,000 Da to about 5,000 Da, about 5,000 Da to about 6,000 Da, about 6,000 Da to about 7,000 Da, about 7,000 Da to about 8,000 Da, about 8,000 Da to about 9,000 Da, about 9,000 Da to about 10,000 Da, about 2,000 Da to about 10,000 Da, about 10,000 Da to about 20,000 Da, about 20,000 Da to about 30,000 Da, about 30,000 Da to about 50,000 Da, about 50,000 Da to about 100,000 Da, about 100,000 Da to about 150,000 Da, about 150,000 Da to about 200,000 Da, about 200,000 Da to about 250,000 Da, or about 250,000 Da to about 300,000 Da.

[0042] Generally, compared to a corresponding side chain crystalline polymer with a lower molecular weight, a higher molecular weight side chain crystalline polymer can have a higher viscosity when in a liquid state at a temperature close to its melting point. Thus, the properties of the side chain crystalline polymer can also be adjusted by changing the molecular weight of the side chain crystalline polymer.

[0043] The conductive particles may include any suitable conductive material. In some cases, the conductive particles include copper, silver, gold, aluminum, or a mixture or combination of one or more of the foregoing. The conductive particles may have various particle sizes and shapes. In some cases, the average particle size of the conductive particles may be the diameter of a volume-equivalent sphere, from about 500 nm to about 1000 μm, such as from about 500 nm to about 100 μm, from about 500 nm to about 50 μm, from about 500 nm to about 10 μm, from about 500 nm to about 5 μm, from about 500 nm to about 1 μm, from about 1 μm to about 2 μm, from about 2 μm to about 3 μm, from about 3 μm to about 4 μm, from about 4 μm to about 5 μm, from about 1 μm to about 1000 μm, from about 10 μm to about 500 μm, from about 20 μm to about 200 μm, from about 25 μm to about 100 μm, from about 30 μm to about 50 μm, or about 40 μm. In some cases, the conductive particles have a spherical shape. In some cases, the conductive particles are in the form of flakes, and may further be characterized in that the aspect ratio of the flake thickness to the flake diameter is, for example, from about 1:50 to about 1:1000, from about 1:50 to about 1:500, or from about 1:100 to about 1:250.

[0044] The properties of the indicator material can be adjusted by further including a binder and / or one or more other additives. Suitable binders include, but are not limited to, starch, cellulose, natural and synthetic gelatin, methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyacrylic acid, polymethacrylate, polyurethane, epoxy resin, copolymers of vinyl chloride and vinyl acetate, polybutyl methacrylate, and aqueous emulsions of polystyrene. Other suitable additives include, but are not limited to, rheology modifiers, surfactants, wetting agents, or slip agents.

[0045] A method of manufacturing a temperature indicator is also disclosed according to the present invention. The method includes melting a side-chain crystalline polymer as described herein, adding conductive particles as described herein to form a mixture, and applying the mixture to a substrate as described herein.

[0046] A switchable RFID tag comprising an indicator material as described herein is further disclosed. In some cases, the switchable RFID tag includes an antenna and a switch electrically connected to the antenna. In some cases, the switchable RFID tag includes a chip and a switch electrically connected to the chip. In some cases, the switch includes an indicator material that includes a mixture of a side-chain crystalline polymer and conductive particles as described herein. In some cases, the switch includes an indicator material and a side-chain crystalline polymer, and the indicator material includes conductive particles. In some cases, the switchable RFID tag further includes an integrated circuit electrically connected to the antenna.

[0047] Figure 3AShows an embodiment of a perishable subject product 30 that is associated with a temperature indicator 24 to monitor the exposure of the subject product to ambient temperature. In one embodiment, the perishable subject product is a vaccine contained in a vial. The temperature indicator 24 can be electrically connected to the antenna 22 of an RFID tag, and the RFID tag can also include a chip.

[0048] Figure 3B Illustrates an embodiment of a perishable subject product 30 that is associated with a temperature indicator 24 to monitor the exposure of the subject product to ambient temperature. In one embodiment, the perishable subject product is a vaccine contained in a vial. The temperature indicator 24 can be electrically connected to the chip 26 of an RFID tag, which can also include an antenna 22. Examples

[0049] Example 1

[0050] Ink formulations were prepared with different ratios of side-chain crystalline (SCC) polymers to conductive Cu / Ag particles (particle size 40 μm), and the physical properties of the formulations were evaluated. The SCC polymers include an acrylate polymer backbone with C14 and C16 side-chain groups and have a molecular weight of 5000 - 7000 Da. The formulations were prepared by melting the SCC polymers, adding the conductive particles, and mixing to form a mixture of SCC polymers and conductive particles. Formulations were prepared with weight ratios of SCC polymer to conductive particles of 30:70, 40:60, and 50:50. The onset melting temperature, melting peak temperature, and Bingham viscosity at 25 °C (in cps) were measured and the values are listed in Table 1.

[0051] Table 1

[0052]

[0053] The Bingham viscosity of the formulations was also measured at different temperatures and the results are given in Figure 2 The results show that the viscosity decreases with increasing temperature. The results also show that in a temperature range, formulation 1 (30:70 SCC polymer:conductive particles) exhibits a higher viscosity than formulation 2 (40:60 SCC polymer:conductive particles), and formulation 2 exhibits a higher viscosity than formulation 3 (50:50 SCC polymer:conductive particles).

[0054] Example 2

[0055] Using a Gardco bird applicator, the formulation according to Example 1 was coated onto a polyethylene terephthalate (PET) substrate at 35 °C and 50 °C. The wet film thickness of the coated film was approximately 3 mils. The film was cured at room temperature (ambient temperature of ~20 °C). The sheet resistance of the coated film was measured at room temperature. Thereafter, the sample was placed on a hot plate at different temperatures. Then the film was removed from the hot plate, allowed to cool and re-cured for about 15 minutes. Subsequently, sheet resistance measurements were carried out to study the effect of temperature on the conductivity of the SCC polymer. The sheet resistance values are given in Table 2.

[0056] Table 2

[0057]

[0058]

[0059] The results showed that the resistance of the film containing Formulation 1 increased significantly after heating, such that the initially conductive film effectively became non-conductive after heating. After allowing the heated samples to return to room temperature for 48 hours, the resistance of each of Samples 1A, 1B and 1C was measured and found that these resistance values had returned to their initial values before heating. The results also showed that the resistance of the film containing Formulation 2 increased significantly after heating to 40 °C, such that the initially conductive film effectively became non-conductive after heating. When the initially conductive film containing Formulation 2 was heated to 35 °C, 30 °C or 25 °C, the resistance of the film also increased, but to a lesser extent, by approximately 50-fold, approximately 20-fold and approximately 2-fold increases, respectively. After allowing the heated samples to return to room temperature for 48 hours, the resistance of each of Samples 2A, 2B, 2C and 2D was measured and found that these resistance values had returned to their initial values before heating. The results also showed that the resistance of the film containing Formulation 3, which was initially substantially non-conductive, remained substantially non-conductive after heating.

[0060] Example 3

[0061] An irreversible temperature indicator was prepared by the following steps: melting the SCC polymer, adding conductive particles such as Cu / Ag particles (particle size 40 μm) and a binder such as epoxy resin (for solvent-based systems) or water-reducible epoxy (for water-based systems), and mixing to form a mixture of SCC polymer, conductive particles and binder. Formulations were prepared with weight ratios of SCC polymer to conductive particles such as 30:70, 40:60 and 50:50.

[0062] An irreversible temperature indicator is also prepared by the following steps: melting the crosslinked SCC polymer, adding conductive particles such as Cu / Ag particles (particle size of 40 μm), and mixing to form a mixture of the crosslinked SCC polymer and the conductive particles. Formulations are prepared with a weight ratio of crosslinked SCC polymer to conductive particles of, for example, 30:70, 40:60, and 50:50.

[0063] An irreversible temperature indicator is also prepared by the following steps: melting an SCC polymer having a molecular weight of at least about 4000 Da, adding conductive particles such as Cu / Ag particles (particle size of 40 μm), and mixing to form a mixture of the SCC polymer and the conductive particles. Formulations are prepared with a weight ratio of SCC polymer to conductive particles of, for example, 30:70, 40:60, and 50:50. The side-chain crystalline polymer may also have a molecular weight of at least about 5,000 Da. In some cases, the side-chain crystalline polymer may have a molecular weight in the following ranges: about 4,000 Da to about 200,000 Da, about 4,000 Da to about 10,000 Da, about 10,000 Da to about 25,000 Da, about 25,000 Da to about 50,000 Da, about 50,000 Da to about 75,000 Da, about 75,000 Da to about 100,000 Da, about 100,000 Da to about 125,000 Da, about 125,000 Da to about 150,000 Da, about 150,000 Da to about 175,000 Da, and / or about 175,000 Da to about 200,000 Da.

[0064] The formulation is coated onto a PET substrate at 35 °C and 50 °C using a Gardco bird applicator. The wet film thickness of the coated film is about 3 mils. The film is cured at room temperature (∼20 °C). The sheet resistance of the coated film is measured at room temperature and the film is found to be conductive. Thereafter, the sample is placed on a hot plate at different temperatures. Then the film is removed from the hot plate, cooled, and re-cured. Subsequently, sheet resistance measurements are carried out to study the effect of temperature on the film conductivity.

[0065] The sheet resistance values indicate that the resistance of the film increases significantly after heating, such that the initially conductive film effectively becomes non-conductive after heating. After the heated sample is returned to room temperature for 48 hours, the resistance is measured and found to still remain non-conductive.

[0066] Example 4

[0067] The irreversible temperature indicator is prepared by the following steps: forming a bottom coating comprising about 90 wt% conductive particles (flakes) such as Cu / Ag particles (particle size 40 μm) and about 10 wt% binder such as epoxy binder. Then adding a top coating comprising a blend of about 25 wt% conductive particles (spherical / near-spherical) and about 75 wt% SCC polymer and an alkane such as hentriacontane (C21 alkane). The sheet resistance of the indicator was measured before melting and found to be >1,000,000 ohms / square (i.e., the indicator is effectively non-conductive). The indicator was then heated and the sheet resistance after heating was found to be <100 ohms / square (i.e., the initially non-conductive indicator becomes conductive after heating).

[0068] Embodiments of the present invention

[0069] Embodiment 1. A temperature-activated temperature indicator comprising:

[0070] a substrate; and

[0071] an indicator material supported by the substrate, the indicator material comprising a side-chain crystalline polymer and conductive particles.

[0072] Embodiment 2. A temperature-activated temperature indicator comprising:

[0073] a substrate;

[0074] an indicator material supported by the substrate, the indicator material comprising conductive particles; and

[0075] a side-chain crystalline polymer;

[0076] wherein the indicator material is present in a first layer and the side-chain crystalline polymer is present in a second layer, the first layer being located between the substrate and the second layer.

[0077] Embodiment 3. The indicator of Embodiment 1 or 2, wherein the indicator material undergoes a change in conductivity in response to exposure to a temperature above a threshold temperature.

[0078] Embodiment 4. The indicator of Embodiment 3, wherein the indicator material retains the changed conductivity after subsequent exposure to a temperature below the threshold temperature.

[0079] Embodiment 5. The indicator of Embodiment 3, wherein the indicator material does not retain the changed conductivity after subsequent exposure to a temperature below the threshold temperature.

[0080] Embodiment 6. The indicator of Embodiment 3, 4 or 5, wherein the indicator material has a first conductivity before exposure to a temperature above the threshold temperature and a second conductivity after exposure to a temperature above the threshold temperature, the first conductivity being greater than the second conductivity.

[0081] Embodiment 7. The indicator of Embodiment 3, 4 or 5, wherein the indicator material has a first conductivity before being exposed to a temperature above the threshold temperature and has a second conductivity after being exposed to a temperature above the threshold temperature, and the second conductivity is greater than the first conductivity.

[0082] Embodiment 8. The indicator of Embodiment 3, 4 or 5, wherein the indicator material is conductive before being exposed to a temperature above the threshold temperature and is insulating after being exposed to a temperature above the threshold temperature.

[0083] Embodiment 9. The indicator of Embodiment 3, 4 or 5, wherein the indicator material is insulating before being exposed to a temperature above the threshold temperature and is conductive after being exposed to a temperature above the threshold temperature.

[0084] Embodiment 10. The indicator of Embodiment 3, 4, 5, 6, 7, 8 or 9, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 30 seconds or less, such as about 25 seconds or less, about 20 seconds or less, about 15 seconds or less, about 10 seconds or less, about 5 seconds or less, or about 2 seconds or less.

[0085] Embodiment 11. The indicator of Embodiment 3, 4, 5, 6, 7, 8 or 9, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 1 minute to about 48 hours, such as about 1 minute to about 2 minutes, about 2 minutes to about 5 minutes, about 5 minutes to about 10 minutes, about 10 minutes to about 30 minutes, about 30 minutes to about 1 hour, about 1 hour to about 2 hours, about 2 hours to about 5 hours, about 5 hours to about 10 hours, about 10 hours to about 24 hours, about 24 hours to about 36 hours or about 24 hours to about 48 hours.

[0086] Embodiment 12. The indicator of Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, wherein the side-chain crystalline polymer has a molecular weight of at least about 1,000 Da.

[0087] Embodiment 13. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, wherein the molecular weight of the side-chain crystalline polymer is in the range of from about 2,000 Da to about 300,000 Da, such as from about 3,000 Da to about 300,000 Da, from about 5,000 Da to about 250,000 Da, from about 10,000 Da to about 200,000 Da, from about 15,000 Da to about 150,000 Da, from about 20,000 Da to about 120,000 Da, from about 30,000 Da to about 100,000 Da, from about 50,000 Da to about 80,000 Da, from about 2,000 Da to about 20,000 Da, from about 3,000 Da to about 15,000 Da, from about 4,000 Da to about 10,000 Da, from about 4,000 Da to about 5,000 Da, from about 5,000 Da to about 6,000 Da, from about 6,000 Da to about 7,000 Da, from about 7,000 Da to about 8,000 Da, from about 8,000 Da to about 9,000 Da, from about 9,000 Da to about 10,000 Da, from about 2,000 Da to about 10,000 Da, from about 10,000 Da to about 20,000 Da, from about 20,000 Da to about 30,000 Da, from about 30,000 Da to about 50,000 Da, from about 50,000 Da to about 100,000 Da, from about 100,000 Da to about 150,000 Da, from about 150,000 Da to about 200,000 Da, from about 200,000 Da to about 250,000 Da, or from about 250,000 Da to about 300,000 Da.

[0088] Embodiment 14. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, wherein the side-chain crystalline polymer comprises a methacrylate polymer, a methacrylate copolymer, an acrylate polymer or an acrylate copolymer comprising monomer units having crystallizable straight-chain aliphatic side chains, said crystallizable straight-chain aliphatic side chains having at least 10 carbon atoms, such as 10 to 30 carbon atoms, 10 to 24 carbon atoms, 10 to 22 carbon atoms, 10 to 20 carbon atoms, 12 to 18 carbon atoms, 14 to 16 carbon atoms, 14 to 18 carbon atoms, 16 to 18 carbon atoms, 12 to 16 carbon atoms or 14 to 16 carbon atoms.

[0089] Embodiment 15. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 14, wherein the side-chain crystalline polymer is selected from: poly(alkyl methacrylate), poly(tetradecyl acrylate), poly(hexadecyl methacrylate), poly(octadecyl methacrylate), poly(alkyl acrylate), poly(hexadecyl acrylate), poly(dodecyl acrylate), a copolymer of hexadecyl acrylate and octadecyl methacrylate, poly(hexyl-co-dodecyl acrylate), a copolymer of tetradecyl acrylate and octadecyl acrylate, a copolymer of hexadecyl methacrylate and octadecyl methacrylate, and a copolymer of tetradecyl acrylate and hexadecyl acrylate.

[0090] Embodiment 16. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 14 or 15, wherein the conductive particles comprise copper, silver, gold, aluminum, or a combination thereof.

[0091] Embodiment 17. An indicator according to Embodiment 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16, wherein the weight ratio of the side-chain crystalline polymer to the conductive particles in the indicator material is from about 1:1.2 to about 1:4, such as from about 1:1.3 to about 1:3, from about 1:1.3 to about 1:2.5, from about 1:1.4 to about 1:2.4, from about 1:1.5 to about 1:2.3, from about 1:1.6 to about 1:2.4, from about 1:1.7 to about 1:2.5, from about 1:1.8 to about 1:2.6, from about 1:1.9 to about 1:2.7, from about 1:2 to about 1:2.6, from about 1:2.1 to about 1:2.5, from about 1:2.2 to about 1:2.4 or about 1:2.3.

[0092] Embodiment 18. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or 17, wherein the average particle size of the conductive particles is from about 500 nm to about 1000 μm, such as from about 500 nm to about 100 μm, from about 500 nm to about 50 μm, from about 500 nm to about 10 μm, from about 500 nm to about 5 μm, from about 500 nm to about 1 μm, from about 1 μm to about 2 μm, from about 2 μm to about 3 μm, from about 3 μm to about 4 μm, from about 4 μm to about 5 μm, from about 1 μm to about 1000 μm, from about 10 μm to about 500 μm, from about 20 μm to about 200 μm, from about 25 μm to about 100 μm, from about 30 μm to about 50 μm, or about 40 μm.

[0093] Embodiment 19. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18, wherein the conductive particles have a spherical shape.

[0094] Embodiment 20. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18, wherein the conductive particles are in the form of flakes.

[0095] Embodiment 21. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20, wherein the indicator material further comprises an adhesive.

[0096] Embodiment 22. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or 21, wherein the side-chain crystalline polymer is crosslinked.

[0097] Embodiment 23. An indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 or 22, wherein the indicator material exists as a single-layer film.

[0098] Embodiment 24. An indicator according to Embodiment 23, wherein the thickness of the film is less than about 5 mils, such as about 1 mil to about 4 mils, about 2 mils to about 3 mils, or about 3 mils.

[0099] Embodiment 25. A perishable host product comprising a temperature indicator according to Embodiment 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or 24, the temperature indicator being associated with the host product to monitor the exposure of the host product at ambient temperature.

[0100] Embodiment 26. A method of manufacturing a temperature indicator, comprising: melting a side-chain crystalline polymer, adding conductive particles to form a mixture, and applying the mixture onto a substrate.

[0101] Embodiment 27. A switchable RFID tag, comprising: an antenna and a switch electrically connected to the antenna, the switch comprising an indicator material, the indicator material comprising a mixture of a side-chain crystalline polymer and conductive particles.

[0102] Embodiment 28. The RFID tag according to Embodiment 27, further comprising an integrated circuit electrically connected to the antenna.

[0103] Embodiment 29. A switchable RFID tag, comprising: a chip and a switch electrically connected to the chip, the switch comprising an indicator material, the indicator material comprising a mixture of a side-chain crystalline polymer and conductive particles.

[0104] Embodiment 30. The RFID tag of Embodiment 29, further comprising an antenna and an integrated circuit electrically connected to the antenna.

[0105] Embodiment 31. A switchable RFID tag, comprising: an antenna and a switch electrically connected to the antenna, the switch comprising an indicator material and a side-chain crystalline polymer, the indicator material comprising conductive particles.

[0106] Embodiment 32. The RFID tag of Embodiment 32, further comprising an integrated circuit electrically connected to the antenna.

[0107] Embodiment 33. A switchable RFID tag, comprising: a chip and a switch electrically connected to the chip, the switch comprising an indicator material and a side-chain crystalline polymer, the indicator material comprising conductive particles.

[0108] Embodiment 34. The RFID tag of Embodiment 33, further comprising an antenna and an integrated circuit electrically connected to the antenna.

[0109] The authors of the present disclosure specifically contemplate these and other embodiments.

[0110] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, etc. used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and the appended claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. While the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviations found in their respective testing measurements. In one embodiment, the terms “about” and “approximately” mean a numerical parameter within 10% of the specified range.

[0111] As used in the context of describing embodiments of the present disclosure (particularly in the context of the following claims), the terms "a," "an," "the," and similar referents are to be construed to cover both the singular and the plural forms, unless otherwise specified herein or clearly contradicted by the context. The recitation of a range of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each separate value is incorporated into the specification as if it were individually recited herein. Unless otherwise specified herein or clearly contradicted by the context, all methods described herein can be performed in any suitable order. The use of any and all examples, or exemplary language (e.g., "such as") provided herein is merely intended to better illuminate embodiments of the present disclosure and does not pose a limitation on the scope of the present disclosure. Any language in the specification should not be construed as indicating any non-claimed element as essential to the practice of the embodiments of the present disclosure.

[0112] The grouping of alternative elements or embodiments disclosed herein should not be construed as a limitation. Each member of a group can be referred to and claimed individually or in any combination with other members of that group or other elements found herein. For convenience and / or patentability reasons, it is anticipated that one or more members of a group may be included in or deleted from a group. When any such inclusion or deletion occurs, the specification is deemed to include the modified group, thereby satisfying the written description of all Markush groups used in the appended claims.

[0113] Certain embodiments of the invention are described herein, including the best mode known to the inventors for practicing the embodiments of the present disclosure. Of course, variations of these described embodiments will become apparent to those of ordinary skill in the art after reading the above description. The inventors expect those skilled in the art to adopt such variations as appropriate, and the inventors intend for the embodiments of the present disclosure to be practiced in a manner different from that specifically described herein. Accordingly, the present disclosure includes all modifications and equivalents of the subject matter recited in the appended claims as permitted by applicable law. In addition, unless otherwise specified herein or clearly contradicted by the context, the present disclosure covers any combination of all possible variations of the above elements.

[0114] Specific embodiments of the invention disclosed herein may be further limited in the claims by the language "consisting of" or "consisting essentially of." When used in a claim, whether submitted or added by amendment, the transitional term "consisting of" excludes any element, step, or ingredient not specified in the claim. The transitional term "consisting essentially of" limits the scope of the claim to the specified materials or steps and those that do not materially affect the basic and novel characteristics. Embodiments of the present disclosure so claimed are inherently or explicitly described and enabled herein.

[0115] In addition, if any references to patents and printed publications are made throughout the present disclosure, each of these references and printed publications is hereby incorporated by reference in its entirety.

[0116] Finally, it should be understood that the embodiments disclosed herein are illustrative of the principles of the present invention. Other modifications that may be employed are within the scope of the present disclosure. Thus, by way of example and not limitation, alternative configurations of the embodiments of the present invention may be used in accordance with the teachings herein. Accordingly, the present disclosure is not limited to precisely as shown and described.

Claims

1. A temperature - activatable temperature indicator, comprising: A substrate; And An indicator material supported by the substrate, the indicator material comprising a mixture of a side - chain crystalline polymer and conductive particles, wherein the side - chain crystalline polymer melts when exposed to a temperature above a threshold, and the conductivity of the indicator material increases in response to the melting of the side - chain crystalline polymer, and when the temperature returns below the threshold, the indicator material retains the changed conductivity.

2. A temperature - activatable temperature indicator, comprising: A substrate; An indicator material supported by the substrate, the indicator material comprising conductive particles; And A side - chain crystalline polymer; Wherein the indicator material is present in a first layer and the side - chain crystalline polymer is present in a second layer, the first layer being located between the substrate and the second layer, wherein the side - chain crystalline polymer melts when exposed to a temperature above a threshold, and the conductivity of the indicator material increases in response to the melting of the side - chain crystalline polymer, and when the temperature returns below the threshold, the indicator material retains the changed conductivity.

3. The temperature indicator according to claim 1, wherein the threshold is in the range of about 25 °C to about 65 °C.

4. The temperature indicator according to claim 2, wherein the threshold is in the range of about 25 °C to about 65 °C.

5. The temperature indicator according to any one of claims 1 to 4, wherein the indicator material is insulating before being exposed to a temperature above the threshold temperature and is conductive after being exposed to a temperature above the threshold temperature.

6. The temperature indicator according to any one of claims 1 to 4, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 30 seconds or less.

7. The temperature indicator according to claim 6, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 5 seconds or less.

8. The temperature indicator according to any one of claims 1 to 4, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 1 minute to about 48 hours.

9. The temperature indicator according to claim 8, wherein the conductivity changes after being exposed to a temperature above the threshold temperature for about 1 hour to about 2 hours.

10. The temperature indicator according to any one of claims 1 to 4, wherein the side - chain crystalline polymer has a molecular weight of at least 1,000 Da.

11. The temperature indicator according to any one of claims 1 to 4, wherein the molecular weight range of the side - chain crystalline polymer is from about 2,000 Da to about 300,000 Da.

12. The temperature indicator according to claim 11, wherein the molecular weight range of the side - chain crystalline polymer is from about 4,000 Da to about 10,000 Da.

13. The temperature indicator according to claim 11, wherein the molecular weight range of the side - chain crystalline polymer is from about 3,000 Da to about 5,000 Da, from about 5,000 Da to about 6,000 Da or from about 6,000 Da to about 7,000 Da.

14. The temperature indicator according to any one of claims 1 to 4, wherein the side-chain crystalline polymer comprises a methacrylate polymer or an acrylate polymer, the monomer units of which comprise a crystallizable straight-chain aliphatic side chain having at least 10 carbon atoms.

15. The temperature indicator according to claim 14, wherein the side-chain crystalline polymer comprises a methacrylate copolymer or an acrylate copolymer, the monomer units of which comprise a crystallizable straight-chain aliphatic side chain having at least 10 carbon atoms.

16. The temperature indicator according to claim 14, wherein the crystallizable straight-chain aliphatic side chain has 30 carbon atoms, 24 carbon atoms, 22 carbon atoms, 20 carbon atoms, 18 carbon atoms, 16 carbon atoms, 14 carbon atoms, 12 carbon atoms or 10 carbon atoms.

17. The temperature indicator according to claim 15, wherein the crystallizable straight-chain aliphatic side chain has 30 carbon atoms, 24 carbon atoms, 22 carbon atoms, 20 carbon atoms, 18 carbon atoms, 16 carbon atoms, 14 carbon atoms, 12 carbon atoms or 10 carbon atoms.

18. The temperature indicator according to claim 14, wherein the side-chain crystalline polymer is selected from: copolymers of polyalkyl methacrylates, polyalkyl acrylates, hexadecyl acrylate and octadecyl methacrylate, polyhexyl-co-dodecyl acrylate, copolymers of tetradecyl acrylate and octadecyl acrylate, copolymers of hexadecyl methacrylate and octadecyl methacrylate, and copolymers of tetradecyl acrylate and hexadecyl acrylate.

19. The temperature indicator according to claim 18, wherein the polyalkyl methacrylates are selected from polyhexadecyl methacrylate and polyoctadecyl methacrylate; and the polyalkyl acrylates are selected from poly(tetradecyl acrylate), poly(hexadecyl acrylate) and poly(dodecyl acrylate).

20. The temperature indicator according to any one of claims 1 to 4, wherein the conductive particles comprise copper, silver, gold, aluminum or a combination thereof.

21. The temperature indicator according to any one of claims 1 to 4, wherein the weight ratio of the side-chain crystalline polymer to the conductive particles in the indicator material is from about 1:1.2 to about 1:

4.

22. The temperature indicator according to claim 21, wherein the weight ratio of the side-chain crystalline polymer to the conductive particles in the indicator material is from about 1:1.6 to about 1:2.

4.

23. The temperature indicator according to any one of claims 1 to 4, wherein the average particle size of the conductive particles is from about 500 nm to about 1000 μm.

24. The temperature indicator according to claim 23, wherein the average particle size of the conductive particles is about 500 nm, about 100 μm, about 50 μm, about 10 μm, about 5 μm, about 1 μm, about 2 μm, about 3 μm, about 4 μm, about 1000 μm, about 500 μm, about 20 μm, about 200 μm, about 25 μm, about 30 μm or about 40 μm.

25. The temperature indicator according to any one of claims 1 to 4, wherein the conductive particles have a spherical shape.

26. The temperature indicator according to any one of claims 1 to 4, wherein the conductive particles are in the form of flakes.

27. The temperature indicator according to any one of claims 1 to 4, wherein the indicator material further comprises a binder.

28. The temperature indicator according to any one of claims 1 to 4, wherein the side-chain crystalline polymer is crosslinked.

29. The temperature indicator according to any one of claims 1 to 4, wherein the indicator material exists as a single-layer film.

30. The temperature indicator according to claim 29, wherein the thickness of the film is less than 5 mils.

31. The temperature indicator according to claim 30, wherein the thickness of the film is about 2 mils to about 3 mils.

32. A perishable host product comprising the temperature indicator according to any one of claims 1 to 31, the temperature indicator being associated with the host product to monitor the exposure of the host product at ambient temperature.

33. A method of manufacturing a temperature indicator according to any one of claims 1 to 31, comprising: Melt the side-chain crystalline polymer, add conductive particles to form a mixture, and apply the mixture to a substrate.

34. A switchable RFID tag, comprising: An antenna and a switch electrically connected to the antenna, the switch comprising the temperature indicator according to any one of claims 1 to 31.

35. The switchable RFID tag according to claim 34, further comprising an integrated circuit electrically connected to the antenna.

36. A switchable RFID tag, comprising: A chip and a switch electrically connected to the chip, the switch comprising the temperature indicator according to any one of claims 1 to 31.

37. The switchable RFID tag according to claim 36, further comprising an antenna and an integrated circuit electrically connected to the antenna.

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