TIME-TEMPERATURE INDICATING DEVICE

Abstract

The present invention is generally in the field of measuring and indicating techniques and relates to a time-temperature indicating device and methods of manufacturing and reading this device. More specifically, the time-temperature indicator (TTI) device comprises at least one active reactant being at least a part of a component that is configured to be either an electrical component or transformable into an electrical component, said at least one active reactant being selected to be affectable by a chemical and/or physical reaction effecting a change in at least an electrical property of said electrical component at a rate that is time-temperature dependent.

Claims

1-16. (canceled)

17. A time-temperature indicator device comprising at least one active reactant being at least a part of an electrical component, said at least one active reactant is selected such that it intrinsically changes at least an electrical property of said electrical component at a rate that is time-temperature dependent.

18. A time-temperature indicator device according to claim 17, comprising: (i) at least one active reactant being at least a part of an electrical component, said at least one active reactant being selected to be affectable by a chemical and/or physical reaction effecting a change in at least an electrical property of said electrical component at a rate that is time-temperature dependent; and (ii) at least one first passive reactant in the form of a viscous substance, said chemical and/or physical reaction being a reaction between said at least one active reactant and said at least one first passive reactant and wherein said at least one first passive reactant is selected such that it causes time-temperature dependent development of said chemical and/or physical reaction.

19. A time-temperature indicator device according to claim 17, comprising: (i) at least one active reactant being at least a part of an electrical component, said at least one active reactant being selected to be affectable by a chemical and/or physical reaction effecting a change in at least an electrical property of said electrical component at a rate that is time-temperature dependent; (ii) at least one first passive reactant in the form of a viscous substance; and (iii) at least one second passive reactant, wherein said chemical and/or physical reaction is a reaction between said at least one active reactant and said at least one second passive reactant and wherein said at least one first passive reactant is selected such that it causes time dependent transfer of heat towards said at least one second passive reactant, thereby causing time-temperature dependent development of said chemical and/or physical reaction.

20. The time-temperature indicator device according to claim 17, wherein said electrical component is selected from the group consisting of resistor, capacitor, diode, inductance coil and antenna and wherein said electrical component is preferably configured as at least one element of an RF circuit.

21. The time-temperature indicator device according to claim 18, wherein said chemical and/or physical reaction is selected from the group consisting of acid-base reaction, oxidation-reduction reaction and salt forming reaction.

22. The time-temperature indicator device according to claim 17, wherein said at least one active reactant is a polymer transformable from its initial non-electrically conductive state to an electrically conductive state.

23. The time-temperature indicator device according to claim 18, wherein said at least one active reactant is polythiophene and the passive reactant is iodine.

24. The time-temperature indicator device according to claim 18, wherein said at least one active reactant is polyaniline and the passive reactant is a peroxydisulfate species.

25. The time-temperature indicator device according to claim 17, wherein said electrical component is a capacitor, preferably being configured as an element of an RF circuit and wherein said at least one active reactant is a dielectric material presenting a dielectric spacer in said capacitor.

26. The time-temperature indicator device according to claim 18, wherein said chemical and/or physical reaction consists of mixing said at least one active reactant and said at least one first passive reactant, thereby causing a change in permeability of the at least one active reactant.

27. The time-temperature indicator device according to claim 17, configured as a multi-layer structure, including a substrate layer of an electrically insulating, material, carrying a layer structure configured to form said electrical component and, preferably, wherein said layer structure comprises first and second electrode layers spaced by a dielectric layer, thereby forming the capacitor-type electric component.

28. The time-temperature indicator device according to claim 18, configured as a two-part device, wherein one of the two parts includes at least the at least one active reactant, and the other part includes the at least one first and/or second passive reactant, the two parts being configured to be attachable to one another to thereby induce said chemical and/or physical process and thereby put the device in operation.

29. A method of time temperature indication comprising the step of causing a chemical and/or physical reaction between at least one active reactant and at least one passive reactant in the form of a viscous substance effecting a change in at least an electrical property of an electrical component wherein said at least one active reactant is at least a part of a component that is configured to be either an electrical component or transformable into an electrical component and wherein said at least one passive reactant is selected such that, when being exposed to a temperature higher than a certain temperature specific for said at least one passive reactant, thereby causing time-temperature dependent development of said chemical and/or physical reaction.

30. A printing ink or printing ink concentrate, comprising an active reactant that intrinsically changes at least an electrical property of an electrical component at a rate that is time-temperature dependent, a passive reactant affectable by a chemical and/or physical reaction with an active reactant, wherein the passive reactant causes time-temperature dependent development of said chemical and/or physical reaction that intrinsically changes at least an electrical property of an electrical component and/or a passive reactant that causes time dependent transfer of heat towards another passive reactant thereby causing time-temperature dependent development of a chemical and/or physical reaction between the other passive reactant and an active reactant that intrinsically changes at least an electrical property of an electrical component.

31. A packaging material or a label, comprising the time-temperature indicator device according to claim 17.

32. The time-temperature indicator device according to claim 19, wherein said chemical and/or physical reaction is selected from the group consisting of acid-base reaction, oxidation-reduction reaction and salt forming reaction.

33. The time-temperature indicator device according to claim 19, wherein said at least one active reactant is polythiophene and the passive reactant is iodine.

34. The time-temperature indicator device according to claim 19, wherein said at least one active reactant is polyaniline and the passive reactant is a peroxydisulfate species.

35. The time-temperature indicator device according to claim 19, wherein said chemical and/or physical reaction consists of mixing said at least one active reactant and said at least one first passive reactant, thereby causing a change in permeability of the at least one active reactant.

36. The time-temperature indicator device according to claim 19, configured as a two-part device, wherein one of the two parts includes at least the at least one active reactant, and the other part includes the at least one first and/or second passive reactant, the two parts being configured to be attachable to one another to thereby induce said chemical and/or physical process and thereby put the device in operation.

Description

[0098] In order to better understand the invention and to see how it may be carried out in practice, further preferred embodiments will now be described by way of non-limiting examples and with reference to the accompanying drawings, in which:

[0099] FIG. 1A is a schematic illustration of a TTI structure according to one embodiment of the invention, utilizing passive and active reactants, with the active reactant being an electrically conductive material forming a resistor component or RF circuit;

[0100] FIGS. 1B to 1D schematically illustrate different examples of assembling and triggering a TTI device according to the present invention;

[0101] FIGS. 2A to 2H illustrate experimental results of the technique of the present invention showing the time-temperature dependent changes in the reflectivity and resistivity of an electrically conductive active reactant, where FIG. 2A shows an experimental TTI structure 100, FIG. 2B shows the time-temperature development of a mixing process between the active and passive reactants, FIGS. 2C-2H show the time and temperature dependent changes in the active reactant.

[0102] FIG. 3 schematically illustrates a system for reading a TTI device of the present invention utilizing an active reactant in the form of an RF circuit;

[0103] FIGS. 4A-4C, 4D and 4E exemplify different configurations, respectively, of a TTI structure utilizing an active reactant as a part of a capacitor component;

[0104] FIG. 5 is a schematic illustration of yet another example of a TTI structure utilizing an active reactant in the form of an electrically conductive layer affectable by the ambient; and

[0105] FIG. 6 is a schematic illustration of a TTI structure according to yet another embodiment of the invention utilizing an active reactant in the form of a material transformable into an electrically conductive material.

[0106] Referring to FIG. 1A, there is schematically illustrated a TTI structure of the present invention, generally designated 10. Generally, the TTI structure includes an active reactant of the kind that is capable of inducing or undergoing a physical and/or chemical reaction affecting the electrical property of the active reactant or an electric component with which the active reactant is associated. The active reactant may be an electrically conductive material (metal or semiconductor) that reduces its conductivity at a rate that is temperature dependent; or may be a non-conductor that increases its conductivity at a rate that is temperature dependent. In the present example of FIG. 1A, the TTI structure 10 includes a first, active reactant R.sub.1 and a second, passive reactant R.sub.2, which in the present example are located adjacent to one another to be in contact. The active reactant R.sub.1 is initially an electrically conductive layer (e.g., silver) presenting a resistor component. The active reactant R.sub.1 is located on an electrically insulating substrate S. It should be noted that such an initially electrically conductive active reactant may be configured as an RF circuit component or one of the RF circuit's features (e.g., antenna); or may be the electrode of a capacitor component in which case the passive reactant R.sub.2 may be a dielectric spacer of the capacitor. The passive reactant R.sub.2 disintegrates the electrically conductive material of the active reactant (affects the homogeneity thereof) at a rate proportional to the time and temperature changes developing in the passive reactant R.sub.2, thus affecting the electrical property (resistance) of the active reactant.

[0107] The TTI structure 10 preferably also includes a viscous material (termed herein viscoelastic component) R.sub.3 serving as a second passive reactant. The viscoelastic component is selected to exhibit a change in viscosity, or a phase transfer (from solid to liquid), that is dependent upon the temperature of the surroundings to which the viscoelastic component is exposed.

[0108] The TTI structure is preferably appropriately sealed such as to avoid interaction between the passive and active components and water from the surroundings.

[0109] The substrate S is a substantially transparent layer, such as glass, polymer film, etc., preferably formed with an adhesive coating on its outer surface to allow attachment of the TTI to a specific item.

[0110] The passive reactant R.sub.2 may be a salt layer (such as Kl, KCl, NaCl, KOH, NaOH, carbonate salts etc.). The viscoelastic layer R.sub.3 may for example be a polymer, e.g., polymer having an ionic character, such as polystyrenesulfonate derivatives.

[0111] The electrically conductive layer R.sub.1 may be deposited on the substrate S using evaporation and/or electroless deposition and/or electrical means, etc. The passive reactant R.sub.2 layer may for example be deposited atop the first reactant containing layer by evaporation (any other means may be used as well).

[0112] A viscoelastic material is one which exhibits elastic and viscous properties simultaneously. Viscoelastic materials are sometimes classified as either viscoelastic solids, i.e., elastic solids which exhibit some viscous effects during deformation, or viscoelastic liquids, i.e., viscous liquids which exhibit some elastic effects. A viscoelastic liquid can be identified as a viscoelastic material which continues to deform indefinitely when subjected to a shearing stress. A viscoelastic material may exhibit a transition from an immobile, glassy state to a viscoelastic liquid state at a temperature known as the glass transition temperature. It may also exhibit a transition from a partially crystalline state to an amorphous state at the temperature at which the crystalline material melts.

[0113] A viscoelastic material may also be chemically crosslinked, rendering it a viscoelastic solid. It may also be physically crosslinked by the presence of crystalline or glassy dispersed phases which are chemically coupled to the matrix phase. It may also exhibit viscoelastic solid properties because of the presence of ionic bonding or hydrogen bonding between polymer molecules (John D. Ferry, Viscoelastic Properties of Polymers, John Wiley & Sons, Inc, 1980.

[0114] The viscoelastic material used with the device of the present invention may be a liquid or a solid material. A viscoelastic liquid state is one which remains liquid at all temperatures to which the object to be monitored will be exposed. Such a viscoelastic material has all such thermal transitions at temperatures below the anticipated range of temperatures to which the object to be monitored will be exposed. This allows for an indicator which will be in its activated state upon contacting the viscoelastic material with the porous matrix. This also allows the viscoelastic material to migrate into the matrix throughout the entire anticipated temperature range. In this manner, the indicator will be able to provide continuous integration of time-temperature exposure over the entire range of temperatures to which the object to be monitored is exposed.

[0115] Solid viscoelastic materials are those which function when the modulus of the material is low enough for it to deform and penetrate entirely through the porous matrix under the influence of capillary action or other driving forces present in the device.

[0116] Non-limiting examples of viscoelastic materials which may be suitable for use with the indicator of the present invention include natural rubber, butyl rubber, polybutadiene and its copolymers with acrylonitrile and styrene, poly alpha olefins such as polyhexene, polyoctene, and copolymers of these and others, polyacrylates, polychloroprene, silicone pressure sensitive adhesives, and block copolymers such as styrene-isoprene block copolymers, and mixtures of any of the above.

[0117] The viscoelastic component may or may not have a solid-to-liquid transition at temperatures that are relevant to the specific application, and consequently the use of the appropriately selected viscoelastic component allows for monitoring either partial or full time-temperature history of the TTI. The viscoelastic component is selected to be characterized by that when being exposed to temperatures higher than a certain threshold temperature specific for said viscoelastic component, it undergoes a change in its mobility and ability to dissolve and transport other chemical substances and propagate in porous solids. Such a change may cause a phase change process in the other passive reactant R.sub.2 thereby causing time dependent development of the chemical and/or physical reaction between the active reactant R.sub.1 and the passive reactant R.sub.2.

[0118] Thus, the viscoelastic component (e.g., polymer) R.sub.3, used with the device of the present application, is selected to be characterized by a certain temperature T.sub.g, such that at a temperature T.sub.1 below this temperature T.sub.g, the polymer is characterized by a substantially low mobility and substantially high viscosity, and at a temperature T.sub.2 higher than T.sub.g the mobility is relatively high and viscosity relatively low. The viscosity of the polymer varies with temperature as ii is exposed to temperatures higher than T.sub.g. When the viscosity of the polymer layer reduces, it causes dissolution of the salt layer R.sub.2 in the polymer R.sub.3, thus causing the dissolved salt to slowly penetrate towards the electrically conductive reactant R.sub.1 and act to replace parts of the electrically conductive material or to react therewith thereby disintegrating the electrically-conductive material.

[0119] Thus, at temperatures T.sub.1, little or practically no reaction between the passive and active reactant layer materials R.sub.2 and R.sub.1 occurs, and the status of the TTI 10 is practically time independent. At temperatures T.sub.2 that are higher than the T.sub.g, the reaction occurs at a rate that is proportional to the time-temperature conditions history, and the status of the TTI 10 (at least its electrical properly) is time dependent. At these temperatures, the electrically conductive layer R.sub.1 (e.g., silver) disintegrates and its electrical property (conductivity), as well as the optical property (reflectivity), varies as the function of the aggregated time-temperature history. It should be understood that depending on whether partial or full time-temperature indication is needed, the viscoelastic component is appropriately chosen to have its threshold temperature T.sub.g, respectively, within the temperature range of interest for a specific application, or outside the temperature range (taking into account whether an increase or decrease of temperature is expected when the device is exposed to the temperature changes).

[0120] It should be noted that the provision of two passive reactants is optional, and the single passive reactant R.sub.2 may be directly exposed to the environmental changes. A viscous substance, when used, may actually serve as the single passive reactant (thus eliminating the need for the other reactant R.sub.2 in FIG. 1A), such that a time and temperature change in the viscoelasticity thereof effects a change in the detectable electrical properties of the active reactant R.sub.1 or the component with which the active reactant is associated, e.g., RF circuit as will be exemplified further below.

[0121] The reaction between the passive and active reactant materials may be physical and/or chemical in nature. Physical reactions may for example be, but are not limited to, such reactions in which a result is a replacement, disintegration, dissolution, dislocation, segregation, mixing or insertion of the passive reactant, usually a soluble salt, into the substructure of the active reactant, typically the material making the conductive layer. In other embodiments of the invention, the meaning of physical processes/reactions may be the process of wetting and progression of a liquid material into a porous/absorbing static material or the mixing of two liquids, etc. Chemical reactions may for example be acid-base reactions, oxidation-reduction reactions, salt forming reactions, and other reactions which may or may not be reversible in nature.

[0122] Generally, the active reactant material(s) may be any reactant(s) capable of inducing or undergoing physical or chemical reaction affecting an electrical property of the active reactant. The active and passive reactants may for example be, respectively, a metal and an oxidizing agent; a metal oxide and a reducing agent; a metal or semiconductor and a disintegrating agent such as Kl, HCl, KOH, NaOH, and the like; metal salt and an agent such as oxidizer, reducer, or disintegrating agent. Examples of metals and metal oxides suitable to be used in the active reactant include but are not limited to silver, gold, aluminum, copper, nickel, etc. and oxides or salts thereof. It should be understood that in the embodiment of the present invention where the active reactant includes an electrically conductive material, the only requirements to this electrically conductive active-reactant material are: capability of being oxidized or reduced, or disintegrated by a selective material for the passive reactant, which may and may not be a physical component of the TTI (as will be described further below).

[0123] According to some other embodiments of the invention, the active reactant is a dielectric material, as will be described further below.

[0124] It should also be noted that the term disintegration used herein refers to a change in the homogeneity of the material, which may be caused by one of the following effects: oxidation, reduction, material removal (e.g., dissolution).

[0125] Turning back to the example of FIG. 1A, the reaction between the active and passive reactant materials results in affecting the homogeneity of the electrically-conductive layer material R.sub.1, and consequently affects the electrical characteristics of this material, especially the electrical resistance changes dramatically, and also affects the light response of this layer, i.e., the reflectivity of the layer reduces. Since the reaction is expressed in the dissolution of the metal layer, the electrical resistance changes from values that are typical for conductors to values that exhibit insulating materials.

[0126] Reference is made to FIGS. 1B to 1D exemplifying the assembling and triggering of a TTI device according to the invention.

[0127] In the example of FIG. 1B, a TTI structure is formed by an active reactant (or an electrical component including an active reactant as one of its elements) and possibly also a passive reactant (shown in the figure in dashed lines as its provision is optional), and is located in a container C, which is initially placed in a sealed enclosure SE. Considering the TTI structure formed only by active reactant, the reactant is an electrically conductive material. The sealed enclosure is configured such as to allow breaking or removing or puncturing the enclosure SE to thereby put the TTI device in operation, i.e., expose the TTI structure to the environmental changes. It should be understood that if the TTI structure includes active and passive reactants, the passive reactant may be liquid with the active reactant being embedded therein.

[0128] In the example of FIG. 1C, a TTI structure is a two-part structure: one part P.sub.1 is by itself inactive and in order to put the TTI device in operation is to be assembled with the other part P.sub.2. The inactive part P.sub.1 includes an active reactant (or an electrical component including an active reactant as one of its elements) and possible also a first passive reactant (shown in the figure in dashed lines as its provision is optional). The other part P.sub.2 includes a viscoelastic polymer or liquid serving as passive reactant. The viscoelastic polymer or liquid is associated with a separate label L, namely is either placed on a sticky label or a sticky viscoelastic polymer is selected (as mentioned above). Applying the sticker L atop the inactive TTI outs the TTI device in operation.

[0129] FIG. 1D exemplifies a TTI structure configured as a two-part device, including an initially inactive part P.sub.1 having an active reactant (or an electrical component including an active reactant as one of its elements) and possibly also a first passive reactant (shown in the figure in dashed lines as its provision is optional) located in a container C.sub.1 having an inlet opening O.sub.1; and the other part P.sub.2 including a viscoelastic component, which is initially solid and is located in a container C.sub.2 having an outlet opening O.sub.2 and a sealed enclosure SE. The latter is configured to allow breaking, removing or puncturing thereof and to be attachable to the inactive-part container so as to define a liquid passage between the two containers via inlet and outlet openings.

[0130] FIGS. 2A to 2H illustrate experimental results of the technique of the present invention showing the time-temperature dependent changes in the reflectivity of an electrically conductive active reactant. FIG. 2A shows an experimental TTI structure 100 formed by a transparent electrically insulating substrate layer S such as glass; a first passive reactant layer R.sub.2, which is typically a 100 nm Kl that is evaporated atop the substrate layer S; an active reactant layer R.sub.1 which is typically a 100 nm silver mirror layer that covers the entire layer of Kl also from the sides to avoid contact between Kl and humidity; and a second passive reactant layer R.sub.3 which is a viscoelastic polymer. The structure 100 also includes an uppermost transparent layer.

[0131] The viscoelastic polymer R.sub.3 mixes the materials of layers R.sub.1 and R.sub.2 at a rate that is a function of the temperature. FIG. 2B shows the time-temperature development of such a mixing process. FIGS. 2C-2F show the temperature dependent changes on the active reactant layer R.sub.1 (temperature varies from 35 C. to (2) C. for respectively, the following time points: the starting point (time=0), and 41, 400, and 1700 hours thereafter. FIGS. 2G and 2H show the results of the active reactant changes after 90 hours of the reaction development for, respectively, 45 and 20 C. of the environmental conditions and different viscoelastic mediators.

[0132] The inventors have conducted experiments with different viscoelastic polymers. The results were different time temperature profiles. The reflectivity of the electrically conductive active reactant decreases in response to rising temperatures and time.

[0133] It should be understood that by replacing the glass substrate S in the structure 100 by a polymer coated ITO, a capacitor arrangement is formed by two electrodes (layer R.sub.1 and polymer coated ITO substrate S) and a dielectric spacer between them formed by the Kl layer R. In this case, mixing of layer materials R.sub.1 and R.sub.2 would affect the capacitance of this structure in a manner which can easily be detectable.

[0134] Referring to FIGS. 1 and 2A, it should also be understood that the electrically conductive active reactant layer R.sub.1 may be patterned to form an RF circuit to thereby operate as an RF tag. The construction and operation of RF tags are known per se and therefore need not be described in details, except to note the following. Generally, RF tags can be active (i.e., utilizing an internal energy source incorporated with the tag) or passive that function using the energy of an external interrogation signal. RF tags are dependant on energy supplied from a tag reader or an external device. RF tag includes an antenna attached to a resonance or oscillatory circuit (typically including capacitive, inductive and resistive elements), which is energized (e.g., by the received interrogation signal) and which, when energized, excites the antenna to transmit an RF response signal at a resonance frequency of the circuit. Antennas used in an RF ID tag are generally constituted by loops of wire or metal etched or plated on the tag surface.

[0135] FIG. 3 schematically illustrates a system 200 for reading a TTI device 300 of the present invention. In the present example, the device 300 includes a TTI structure similar to that of FIG. 1, but it should be understood that the structure configuration of FIG. 2A can also be used, as well as a structure having a single passive reactant.

[0136] The RF tag pattern may have various code or memory configurations. The most simple is the single code tag (such as typically utilized in EAS systems). Such a tag emits a single response when activated by a reader. The response is a simple YES or NO, indicating whether or not the tag is present or activated. Alternatively, the RF tag pattern may define a plurality of resonant circuits, each for outputting a response signal at a predetermined frequency in response to an interrogating signal. The response signals define a response code of the entire tag, which is determined by the number of individual circuits and the manner of their operation. RF configurations suitable to be used in the present invention are disclosed for example in U.S. Pat. Nos. 6,104,311 and 6,304,169. Thus, generally, the RF tag includes at least one resonance circuit. The elements of the RF tag can be printed on the substrate layer (in a known manner by using conductive inks), or the continuous electrically conductive layer is deposited on the substrate and then patterned to define the elements of the tag. The magnitudes of the capacitive, inductive and resistive elements can be predefined as a part of the printing/patterning process, in accordance with a frequency representing a code element of the RF tag.

[0137] The system 200 includes an interrogating antenna 202A, an RF ID reader 204 and a control unit 206. While the TTI device 300 (a product with which the TTI is associated) is exposed to the time-temperature environmental changes, a reaction between the active and passive reactants develops, thus effecting a change in the electrical property of at least one RF Lag feature and accordingly effecting a change in the RF tag response to the interrogating field. To detect this change, the antenna 202 energizes (interrogates) the TTI by a reading field and the TTI response is detected by the reader 204, which generated data indicative of the detected response and transmits this data to the control unit 206. The principles of the RF tag reading are known per se and therefore need not be described in details.

[0138] Reference is made to FIGS. 4A to 4E illustrating yet another examples of a TTI structure of the present invention including an electrical component configured as a capacitor. In the example of FIGS. 4A-4C, a TTI structure 400A includes a capacitor component formed by two electrodes E.sub.1 and E.sub.2 and a dielectric spacer between them in the form of a porous material; and a reservoir containing a viscoelastic polymer or any other viscous material. The reservoir is located adjacent to the capacitor component and has an outlet allowing the viscoelastic polymer passage to the capacitor component. FIG. 4A shows the starting point, or inoperative position of the TTI device (for example, the reservoir may have a sealed enclosure to be broken, removed or punctured to put the device in operation). FIG. 4B shows the device after some time, during which the viscoelastic component penetrates into the porous material, thus causing a change in the capacitance, indicating the time-temperature history of the device. FIG. 4C shows the device after some more time: the viscoelastic component completely fills the porous material, the capacitance changes and indicates longer time-temperature history. It should be noted that the electrode(s) may be transparent for the provision of simple visual time-temperature progression as well, by measuring a distance the viscous liquid penetrated the porous material.

[0139] The viscoelastic component penetration into the dielectric spacer affects the dielectric permeability of the spacer material thus affecting the capacitance, in which case a dielectric spacer of the capacitor presents an active reactant, namely has a time-temperature varying dielectric permeability thereby effecting a change in the electrical properties of the capacitor component. Additionally, the dielectric spacer material may be selected similar to the above-described example of FIG. 1A such that, while being dissolved in the viscoelastic component it affects the homogeneity of the electrode layer, which presents an active reactant.

[0140] In the example of FIG. 4D, a TTI structure 400D includes an electrically insulating substrate layer S (e.g., glass); an electrically conductive layer on top of the substrate patterned to define electrodes E.sub.1 and E.sub.2; a dielectric spacer layer D between the electrodes presenting an active reactant R.sub.1, which is a porous and insulating layer such as even a paper layer or a porous metal oxide layer; a viscoelastic layer R.sub.2; and a removable enclosure. As the viscoelasticity of layer R.sub.2 changes with time and temperature (as described above), this viscoelastic liquid penetrates the layer R.sub.1 and creates time-temperature changes in the capacitance, as a result of a change in the dielectric permeability of the spacer layer R.sub.1. In the example of FIG. 4E, a TTI structure 400E includes an insulating substrate S coated with an electrically conductive layer presenting a first electrode E.sub.1; a porous and insulating layer such as even a paper layer to be a dielectric spacer of a capacitor and presenting an active reactant R.sub.1; a viscoelastic layer (passive reactant R.sub.2); and a second electrode E.sub.2 layer (which may or may not be transparent) on top of the viscoelastic layer R.sub.2. The viscoelastic layer R.sub.2 is exposed to the time and temperature changes of the environment via the electrode E.sub.2 and/or via regions of layer R.sub.2 outside the electrode E.sub.2.

[0141] It should be understood that two additional insulating layers may be used in the structures 400A and 400B that insulate the conductive layers from any ionic conductance that may occur if the viscoelastic layer is an ionic or charge conductor.

[0142] It should also be noted that the capacitor component of FIGS. 4A-4E may be an element of an RF circuit, in which case the time-temperature changes in the dielectric spacer R.sub.1 of the capacitor effect a change in the RF signal from the RF circuit.

[0143] Starting from a RF circuit with a first normal capacitor (C1), a resistor (R1), an antenna (L1) and a second time temperature responsive capacitor, the capacitance of C2 varies with time at a rate that is temperature dependent. It may be that the temperature dependency is continuous or it has a threshold so that below a given temperature the evolution of the capacitance in time is negligible.

[0144] Assuming that R1 is a constant resistor, the resonance frequency is

[00001]$2.Math..Math..Math.f_{\mathrm{Not}.Math..Math.\mathrm{spoiled}}=\sqrt{\frac{1}{L\left(C.Math..Math.1+C.Math..Math.2\right)}}$

at time=0 from activation. When the capacitance of C2 drops to zero or to any pre defined value C2, the frequency reaches the frequency that defines a spoiled good

[00002]$\left(2.Math..Math..Math.f_{\mathrm{Spoild}}=\sqrt{\frac{1}{L\left(C.Math..Math.1+C.Math..Math.2^{}\right)}}\right).$

[0145] Capacitance change is the only factor, which needs to be considered. To this end, a capacitor is used with the following architecture: [0146] Viscous layer with de-doping agent [0147] Metal grid [0148] Doped conjugated polymer [0149] Insulating layer [0150] Metallic electrode

[0151] Upon placing the viscous layer that contains the de-doping agent atop the metal grid that is positioned on top of the doped conjugated polymer, said viscous layer that contains the de-doping agent causes the de-doping of said doped conjugated polymer at a rate that is time temperature dependent, thus changing the capacitance of C2 and the resonance frequency of said circuit.

[0152] Other electric and/or electronic properties of the device such as the resistance of the doped conjugated layer when it acts as the resistor R1, its AC and DC response may be changed at a rate that is temperature dependent. In the case of the resistance of R1, the effect is mainly expressed in the Q factor of the resonating circuit.

[0153] It should be clear that the term a rate that is time temperature dependent may mean that the outcome is a gradual change in the electronic properties but in other embodiments, it may result in a sudden change in the electronic properties.

[0154] Yet another example of inducing time-temperature changes in electrical properties of the TTI via the dielectric spacer of a capacitor is by using a series of capacitors, rather a single capacitor, that break when a viscoelastic polymer penetrates to them, in which case the time-temperature profile is measured by the number of capacitors that have been affected (i.e., penetrated by the viscoelastic component). It should be understood that the same viscoelastic polymer may be used for penetrating the capacitors one after the other; different viscoelastic polymers, having different penetration rates, may be used; different porous materials within the capacitors may be used; and/or different spacings in between the electrodes of the capacitors may be used.

[0155] The device of the present invention may be manufactured using any known suitable techniques, including for example ink jet printing, offset printing, gravure, etc.

[0156] Reference is made to FIG. 5, showing yet another embodiment of the invention. Here, a TTI structure 500 is configured to utilize, as its constructional element, only an active reactant R.sub.1 in the form of an electrically conductive layer of a kind undergoing oxidation when exposed to the ambient. This may be Cu, GaAs, etc. A passive reactant R.sub.2 (oxygen) comes from the environment. The time-temperature dependent oxidation will thus result in a change of conductivity of the layer R.sub.1 as well as the reflectance thereof. Here again, if the layer R.sub.1 is patterned to form an RF circuit, the oxidation of this layer will effect a change in the circuit response to an interrogation field.

[0157] FIG. 6 illustrates yet another example of the invention. A TTI structure 600 is shown including an active reactant layer R.sub.1 (on top of a substrate) which is a polymer layer of the kind that is initially non-conductive and is transformable into an electrically conductive material; and a passive reactant R.sub.2 selected to affect the electrical conductivity of the active reactant R.sub.1 so as to transfer it into an electrically conductive material. The active reactant R.sub.1 may be polythiophene, and the passive reactant R.sub.2 may be iodine (I.sub.2) or a complex of iodine (say iodine complexed to starch) or iodine that is dissolved in a polymeric matrix. When exposed to the ambient, iodine R.sub.2 slowly sublimes and penetrates through the polythiophene layer R.sub.1. This induces electrical conductivity to the polythiophene layer R.sub.1. In this case, the TTI state change can be detected by measuring an electric current through the active reactant layer R.sub.1, and can also be detected visually since the polythiophene while becoming a conductor changes its color to black.