Abstract:
Techniques and devices are presented for communicating exposure to a potentially harmful stimulus. The devices include a stimulus sensitive gel which changes volume in response to exposure to the stimulus and thereby presents a contrast in color that serves as a visual indicator that exposure to the stimulus has occurred.

Description:
This U.S. Utility Patent Application claims priority to U.S. Provisional Patent Application Ser. No. 60/675,882, filed Apr. 29, 2005, the content of which is hereby incorporated by reference in its entirety into this disclosure. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to stimulus-indicating techniques. More particularly, the present invention relates to devices, methods and kits for indicating when a product has been exposed to a given stimulus for a certain period of time. 
     2. Background of the Invention 
     It has long been recognized that the useful life of a perishable product is a function of its cumulative exposure to a harmful stimulus over a period of time. The types of stimuli that advance the degradation of stimulus-sensitive products over time include not only temperature, but also light, pH, humidity, electromagnetic radiation and radiation, to name a few. For example, the useful life of a temperature sensitive product is a function of its cumulative time-temperature exposure a combination of the temperature to which a product is exposed and the duration of the exposure. 
     For stimulus sensitive products, degradation generally occurs more rapidly at higher magnitudes of the stimulus (e.g., higher temperatures) than at lower magnitudes of the stimulus (e.g., lower temperatures). For example, often a temperature-perishable product will have a longer useful life if it is exposed to lower temperatures than if it is exposed to higher temperatures. However, the converse is also true, in that certain stimulus sensitive products will degrade more rapidly at a lower magnitude of stimulus (especially freezing temperatures) than at a higher magnitude of stimulus. For example, a malaria vaccine will degrade faster below 2° C. than if stored at 3° C.-7° C. Thus the rate of degradation is often stimulus and product specific. 
     Stimulus perishable products susceptible to degradation due to cumulative time-stimulus exposure include, but are not limited to, food, food additives, chemicals, biological materials, drugs, cosmetics, photographic supplies and vaccines, to name a few. Many manufacturers mark their products with printed expiration dates in an attempt to provide an indication of when the useful life of a perishable product lapses. However, these dates are only estimates and may be unreliable because they are based on assumptions about the stimulus history of the product that may not be true with respect to the particular unit or product within the package on which the expiration date appears. 
     Specifically, manufacturers compute expiration dates by assuming that the product will be kept within a specified stimulus range during the product&#39;s useful life. However, although the manufacturer may have certain control over the environmental conditions of the product while the product is in its possession, it cannot always predict or control the stimulus exposure of a product through each step of the supply chain that delivers the product from its possession to the consumer. If the product is actually exposed to stimuli greater in magnitude than those on which the expiration date is based, the perishable product may degrade or spoil well before the printed expiration date. When this happens, the expiration date may mislead the consumer into believing that the perishable product is still usable when, in fact, its useful life has lapsed. Such false belief of the usefulness/effectiveness of such a product could lead to medical harm, injury or death for a consumer. 
     Despite its limitations, marking a stimulus sensitive product with an expiration date is useful, nonetheless, because often a visual inspection of the perishable product does not warn a potential user that the exposure of the product to a harmful stimulus has caused it to degrade or spoil. Although there are certain time-stimulus indicators currently available, most are focused on temperature, and not on the other types of stimuli. Additionally, these time-temperature indicators require active agents to be kept separate from one another until the indicator is attached to the product it is monitoring. If the active ingredients are not kept separate they will begin to interact prematurely, thereby giving a premature indication of spoilage. As a result, these types of indicators require manual activation. However, manual activation is not always feasible, cost-effective or efficient when the indicator is being used with a product that is mass-produced in high-volume. 
     Thus, there is a need for a stimulus magnitude indicator that indicates when a stimulus sensitive product has been exposed to a deleterious stimulus extreme for a pre-determined amount of time. There is also a need for a time-stimulus indicator that does not contain active agents that will begin to interact prior to being attached to the product being monitored, thereby resulting in false indications of product expiration. Furthermore, there is a need for a stimulus indicator that does not require manual activation, and a stimulus indicator that can indicate exposure to two or more temperature magnitudes in a single device. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to stimulus sensitive devices and methods, which substantially overcome one or more of the above-mentioned limitations and disadvantages of the related art. Using the exemplary embodiments of the present invention, products having limited useable shelf lives may be better assessed in terms of effectiveness based on the individual package&#39;s history of exposure to potentially deleterious levels of harmful external stimulus. Thus, each particular package is individually considered in terms of effectiveness and usefulness based on the history of the package&#39;s exposure to various stimuli. Such external stimuli can include, but is not limited to, temperature, heat, light, oxygen, or some combination thereof. Such indication according to the present invention could be used in lieu of or in addition to conventional methods of the indication of an “expiration date” on a given product on the packaging of such product. 
     To achieve these and other advantages and in accordance with the purposes of the invention, as embodied and broadly described, one exemplary embodiment of the invention is a device that indicates when a stimulus sensitive product may have potentially or conclusively undergone a physical change in response to exposure to a predetermined stimulus extreme. 
     In accordance with the purposes of the invention, as embodied and broadly described, another exemplary embodiment of the invention is a device that indicates when a stimulus sensitive product has potentially or conclusively undergone a physical change in response to exposure to a predetermined stimulus extreme over a predetermined period of time. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings, in which: 
         FIG. 1A  is a top view of a stimulus-indicating device according to an exemplary embodiment of the present invention with two compartments and the stimulus-indicating gel in the expanded non-stimulated state. 
         FIG. 1B  is a top view of a stimulus-indicating device according to an exemplary embodiment of the present invention with two compartments and the stimulus-indicating gel in the contracted stimulated state. 
         FIG. 1C  is a side cross-sectional view of the embodiment shown in  FIG. 1A . 
         FIG. 1D  is a top view of a stimulus-indicating device according to another exemplary embodiment of the present invention with two compartments and the stimulus-indicating gel in the expanded non-stimulated state. 
         FIG. 2A  is a top view of the stimulus-indicating device according to an exemplary embodiment of the present invention with a single compartment and the stimulus-indicating gel in the expanded non-stimulated state. 
         FIG. 2B  is a top view of the stimulus-indicating device according to an exemplary embodiment of the present invention with a single compartment and the stimulus-indicating gel in the contracted stimulated state. 
         FIG. 2C  is a side cross-sectional view of the embodiment shown in  FIG. 2A . 
         FIG. 3  is a top view of a multiple compartment stimulus-indicating device according to an exemplary embodiment of the present invention for indicating exposure to at least two temperature thresholds. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a more accurate, more specific and more reliable technique for indicating the exposure history of a given product to levels of stimuli that could potentially accumulate to render the product ineffective, unreliable or even harmful. Each product packaging could be individually tagged with such an indicating device according to the present invention and individually assessed in terms of product usefulness. For example, two given pharmaceutical packages from the same manufacturing lot may have been exposed to different levels of temperatures according to the individual life histories of each package. If one package was inadvertently left out of a refrigerated compartment for a few hours, thereby rendering that product ineffective or unreliable, there would be no way for the product end-user to know of such change in the product because the conventional assumption is that the expiration date pre-printed on the product is the indicator of the product&#39;s usefulness. Thus, the use of that product can be unproductive, ineffective or even harmful, without the knowledge of the product end-user. The present invention addresses such circumstances and provides product end-users with more specific and more accurate assessment of each given product&#39;s potential effectiveness. 
     Referring now to the drawings, and more specifically to the embodiment of  FIGS. 1A and 1B , a top-down view of a stimulus-indicating device  100  according to an exemplary embodiment of the present invention is shown. This embodiment of the stimulus sensitive device  100  includes a first compartment  24  and a second compartment  25 . The first compartment  24  and the second compartment  25  are formed by the backing layer  21  on the bottom of the device  100  and an upper layer  23  on the top as shown in  FIG. 1C . The first compartment  24  and the second compartment  25  are additionally formed and differentiated from one another by a constricting region or neck portion  37 . Thus, the first compartment  24  and the second compartment  25  could be formed in the shape of an hour-glass wherein the two compartments are connected by the constricting region or neck portion  37 , as shown in  FIG. 1D , or the shape of two squares or rectangles connected by the constricting region or neck portion  37  formed as a channel as shown in  FIG. 1A . Other shapes are contemplated within the scope of the invention. A suitable liquid  27  as taught, for example, by the references incorporated herein, interacts with the gel  26  and can flow freely between the first compartment  24  and the second compartment  25  through the neck portion  37 , while the neck portion  37  provides a constricting region. 
     Both the backing layer  21  and the upper layer  23  may be constructed of a strong, resilient leak-proof material, such as plastic or other polymer material, so as to provide for the twisting or bending that might occur during transportation of the temperature sensitive product without tearing, breaking or leaking. The backing layer  21  and the upper layer  23  may also be made of a material that preferably allows them to be joined and sealed together, such as by heat stamping or other suitable means. The backing layer  21  and the upper layer  23  can both approximate the length and width of the first compartment  24  and second compartment  25  combined, although variations in these dimensions are within the scope of the present invention. The upper layer  23  can preferably be made of a clear material, such as plastic, so that the user of the device  100  can view at least some portion of the backing material  21 . The upper layer  23  can also be able to accept paint or ink so as to allow for coloring or concealing at least some portion of the first or second compartments  24  and  25 . 
     Although some portion of the upper layer  23  can also accept paint or ink for coloring, it is preferable that the display portion  29  of the upper layer  23  remain free from ink or coloring so that the user of the device  100  can observe the first compartment  24  through the display portion  29  as explained below. The backing layer  21  should also be able to accept paste, glue or other suitable adhesive  13  on the surface not facing the first compartment  24  and second compartment  25 . 
     In this exemplary embodiment, the indicator spot  31  located on the surface of the backing layer  21  (see also  FIG. 1B ) that is contained in the first compartment  24  is colored a first color, such as, for example, red. One of ordinary skill in the art will realize that while shown as a circular spot  31  in  FIGS. 1A and 1B , the indicator spot  31  could be a symbol (e.g., an “X”) or a word or numeric designation (e.g., “OK”, “Hot”, “Cold” or “2° C.”). Moreover, the gel  26  can be manufactured so that it is dyed with a colorant so that it possess a second color which is preferably visibly distinct from the first color located on the backing layer  21 . Alternatively, the gel  26  can be manufactured so that it is opaque in its unstimulated state. The gel  26  can also be designed and manufactured so that in its unstimulated-expanded state it contains a particle or metal colorant  28  that binds to the gel  26 . When the gel  26  contains a certain concentration of that particle or metal colorant  28 , the gel  26  takes on a color. For example, if the metal colorant  28  contained inside the gel  26  were copper, the gel  26  would take on a blue color. Other particle/metal and color combinations are also possible and within the purview of one of ordinary skill in the art after consideration of the present disclosure. One of ordinary skill in the art will also understand that the indicator spot  31  and the gel  26  do not necessarily need to be different colors. Instead the indicator spot  31  and the gel  26  can be the same color, so long as the gel  26  interacts with the indicator spot  31  so as to indicate to a viewer that a stimulus extreme has impacted the stimulus sensitive product to which the indicator  100  is attached. 
     As shown in  FIG. 1A  and  FIG. 1D , when the gel  26  in this embodiment has not been exposed to a stimulus extreme, such as temperature, the gel  26  is in its expanded state (contour shown by heavy dashed-lines  26   e ) and lies in both the first compartment  24  and the second compartment  25 . Thus, prior to being exposed to a critical temperature extreme, the gel  26   e  completely covers the indicator spot  31  located on the backing layer  21  and positioned inside the first compartment  24 . The gel  26  that is in its expanded state  26   e  is present in both the first compartment  24  and the second compartment  25  and is designed to be sensitive to a stimulus, and reacts to such stimulus by undergoing a noticeable decrease in volume. In other words, when the gel  26   e  is exposed to a predetermined stimulus, the liquid within the polymer network that gives the gel  26   e  its volume would be expelled. As the liquid is expelled, the gel  26   e  contracts and/or shrinks in size, resulting in a contracting and/or shrinking of the size and/or volume of the gel, to result in the shape as shown by the gel  26   c  in  FIG. 1B . The contracting and/or shrinking in size and/or volume of the gel  26  in this embodiment can be by any amount so long as it is noticeable and reveals, even partially, the indicator spot  31 . Preferably the contracting and/or shrinking in size and/or volume of the gel  26   e  in this embodiment is between at least a 1/10 contracting and/or shrinking in original size and/or volume, and preferably up to 500 times contracting and/or shrinking in original size and/or volume. 
     The type of gel used will determine the magnitude of decrease in size and/or volume that could be potentially experienced by the gel  26  within the device. For simplicity of discussion, it is understood that all references to expansion or contraction of the gels, as described herein, refer to changes in cross-sectional size and/or volume changes, even if not specifically identified as such. 
     Any of a number of different gels may be used in the exemplary embodiments shown herein and throughout this disclosure. Such gels can be any gel that produces a physical change in structure in response to a given physical stimulus. Such physical stimulus, as described above, may include, but is not limited to, temperature, light, humidity, electromagnetic radiation and radiation, pH, oxygen levels, CO 2 , CO, H 2 O, or any other type of stimulus that could be a source of potential damage, ineffectiveness, inertness or deterioration for a given product, device or composition. 
     By way of example only, a gel  26  (or other visual change agent) which changes in size and/or volume when it is exposed to a predetermined stimulus can be used with the present invention. Examples of such gels include, but are not limited to, those described in U.S. Pat. Nos. RE35,068, 5,403,893 and 4,732,930 to Tanaka et al. See also, Harmon et al., “A microfluidic actuator based on thermoresponsive hydrogels”, Polymer 44 (2003) at 4547-4556. These references are incorporated by reference herein in their entirety. Other gels may also be used. For sake of simplicity, reference will be made to the particular gels disclosed in the above cited and incorporated references. By manufacturing the gel  26  in the manner described in any of these references, the gel  26  will remain stable at its manufactured size and/or volume and color concentration until it is exposed to the predetermined stimulus. As disclosed in these references, the gel  26  is designed to work in conjunction with any suitable liquid  27  (for example, water) that aids in its decrease in size and/or volume change. In this embodiment the first compartment  24  and the second compartment  25  are interconnected so that the liquid  27  flows freely between the first compartment  24  and the second compartment  25 , and bathes the gel  26  in both its expanded unstimulated state (see  FIGS. 1A and 1D ) as well as its contracted stimulated state (see  FIG. 1B ) as described in further detail below. 
     These references also disclose a method to design the gel  26  so that it decreases in size and/or volume once the gel  26  is exposed to the predetermined stimulus. For example, the gel  26  in this embodiment can be designed so that it is stable so long as it stays above 20° C. However, once the gel is exposed to temperature less that 2° C., the gel  26  will begin to undergo its phase transition, resulting in the liquid  27  being expelled from the polymer network of the gel  26 . And as the liquid  27  is expelled, the gel  26  will contract and/or shrink in size and/or volume, resulting in the gel  26  withdrawing from the first compartment  24 , thereby revealing the indicator spot  31 . The converse is also possible, with the use of gels that are stable as long as the temperature is above a certain level, such as, for example, gels that are stable as long as they are below 20° C. Other magnitude and sensitivities of the gel are also possible and within the scope of the present invention. 
     As shown in  FIG. 1B , once the gel  26  has been exposed to the predetermined stimulus extreme for a predetermined amount of time, the gel  26  will collapse (the contour of which is shown by heavy dashed-lines  26   c ) to a point where the second color of the gel  26  is no longer visible because it has receded from the display portion  29  in the upper layer  23 , positioned above the first compartment  24 . At this point the first color (e.g., red) of the indicator spot  31  located on the backing material  21  in the first compartment  24  is fully visible to the user through the display portion  29  located in the upper layer  23 . It is the appearance of this first color in the indicator spot  31  that indicates to the user that the stimulus sensitive product to which the stimulus-indicating device  100  is attached, has been exposed to a deleterious stimulus extreme, and has either probably or definitely expired or lost effectiveness. 
     As the gel  26  is contracting and/or shrinking in size and/or volume, and hence retracting into the second compartment  25 , portions of the indicator spot  31  become revealed, thereby indicating the gradual ineffectiveness of the product to which device  100  is attached. In other words, if about half of the indicator spot  31  is exposed, then the product has been exposed to about half the deleterious levels of external stimuli that would render it ineffective. The time period needed to expose part or all of the indicator spot  31  is dependent on the product itself and would have to be calibrated accordingly with respect to each product. Indicator spot  31  size and shape is part of such calibration and would depend in large part to the particular gel  26  being used. Further, the gel  26  and device  100  used according to the present invention may be designed to indicate the exposure to a stimulus extreme or the time duration to a stimulus that may accumulate to render the product ineffective. For example, the device according to the present invention may indicate when the product has been exposed to a temperature of 40° C. for over 30 seconds, or it may indicate the total cumulative effect of exposure to temperatures above 10° C., or both (see  FIG. 3 ). A one time exposure (former case) and a cumulative effect (latter case) could either be a source of ineffectiveness for a product and the present invention could account for either or both scenarios. 
     In one exemplary embodiment, the gel  26  could be free-form or pliable enough so that it can withdraw from the first compartment  24  through the neck portion  37  as the liquid  27  is expelled and the gel  26  contracts and/or shrinks in size and/or volume. In the preferred embodiment the gel  26  would be attached at the attachment point  33  in the second compartment  25  to ensure that the gel  26  reveals the indicator spot  31  as the volume collapses. The attachment point  33  is made to either the upper layer  21 , the backing layer  23  or both, at one end of the first compartment  25 . Alternatively, rather than having the gel and liquid in a free-flowing state, the gel  26  may be pre-formed in a desired shape, matching the expanded shape  26   e . Thereafter, when exposed to the desired stimulus, and the gel  26  collapses into its smaller volume, it will not obscure the indicator spot  31 . 
     An exemplary gel  26  as used in the present invention may be reversible, meaning it reverts back to its original volume when the stimulus is removed. Thus, the embodiments described herein may include reversible and irreversible gels. It is helpful for this embodiment that the size and/or volume change that the gel  26  undergoes be irreversible. Therefore, when the stimulus is removed the gel  26  would not revert back to its original larger volume, since doing so would cover up the first color of the indicator spot  31  again, and thus falsely indicate that the stimulus sensitive product to which the device  100  is attached had not been exposed to a deleterious stimulus level. More preferably, the gel  26  must remain at its smaller volume so that the second color of the gel  26  is hidden under the upper layer  23 , and the indicator spot  31  is permanently visible to the user. For example, when the exemplary gel  26  heats back up to its non-critical temperature extreme, such as greater than or equal to 2° C., it should not revert back to its original larger volume, since doing so would cover up the first color of the indicator spot  31 , and thus falsely indicate that the stimulus sensitive product, such as a malaria vaccine vial to which the device  100  is attached, had not been exposed to a deleterious stimulus extreme. Instead the gel  26  must remain at its smaller volume so that the second color of the gel  26  is hidden under the upper layer  23 , and the indicator spot  31  is permanently visible to the user. 
     Even if the gels utilized in this invention are reversible (they revert back to their original shape upon removal of the stimulus), the compartment shapes  24  and  25  and/or the constricting region or neck portion  37  will restrict the flow of the gel  26 , so that the majority of the reversible gel remains in the second compartment  25 , even after the stimulus threshold is removed. 
     More specifically, once the gel  26  has migrated completely out of the first compartment  24  as shown in  FIG. 1B , the constricting region or neck portion  37  prevents the gel  26   c , which is totally gathered in the second compartment  25 , from moving back into the first compartment  24 , even after the stimulus extreme had been removed and the gel  26  expanded back to its original size and/or volume. In this exemplary embodiment the stimulus indicating device  100  is irreversible because the revealed indicator spot  31  can not be covered back up by the gel  26   c  because the gel  26   c  cannot reenter the first compartment  24  due to the constricting region or neck portion  37 , which prevents the movement of the gel  26   c  out of the second compartment  25 . Even if the deleterious stimulus extreme is removed and the gel  26  returns to its expanded state in the second compartment  25 , the gel  26  will not move back into the first compartment  24 . It will be understood that a similar embodiment is applicable to a situation where the deleterious temperature extreme only partially affects the gel  26 , thereby resulting in the gel  26  only partially contracting and/or shrinking in size and/or volume. This partial contraction of the gel  26  will result in the indicator spot  31  being only partially revealed. However, this partial uncovering of the indicator spot  31  will be permanent because the constricting region or neck region  37  prevents the gel  26  from expanding once the deleterious temperature extreme is removed. One of ordinary skill will recognize that if the device  100  is subjected to additional exposure to deleterious stimulus extremes, the liquid  27  will continue to be expelled from the gel  26 , which will result in the gel  26  further contracting and/or shrinking in size and/or volume. Moreover, the additional uncovering of the indicator spot  31  will also be permanent because of the constricting region or neck portion  37 . 
     As known with respect to some of the conventional gels described in the references cited above and incorporated herein, such as that disclosed in U.S. Pat. No. 6,030,442 to Kabra et al., the rate at which the gel  26  contracts can be manipulated by varying the size of the beads that comprise the gel  26 . This is because their volume change is controlled by the rate of heat transfer through the gel. As one of ordinary skill would appreciate, the smaller the bead size of the gel  26 , the faster the gel  26  will undergo its contraction and/or shrink in size and/or volume. Conversely, the larger the bead size of the gel  26 , the longer it will take the gel  26  to undergo its phase transition, and consequently its contraction and/or shrinking in size and/or volume; as well as its converse expansion and/or increase in size and/or volume. 
     Referring now to the exemplary embodiment of  FIGS. 2A and 2B , another stimulus-indicating device  200  is shown according to the present invention. In this embodiment the device  200  has only one compartment  25 , and the gel  26  is preferably manufactured in the shape of a sheet or layer. In this embodiment the sheet or layer of gel  26  is preferably attached at attachment point  33  to either the upper layer  21 , the backing layer  23  or both, at one end of the first compartment  25 . More preferably the gel  26  is attached to either the upper layer  21 , the backing material  23 , or both at the attachment point  33  at the end of the gel  26  that is opposite from the indicator spot  31  as shown in  FIG. 2A  and  FIG. 2B . By attaching the gel  26  to the attachment point  33 , the gel  26  colored a second color is ensured to contract and/or shrink in size and/or volume away from and reveal the first color of the indicator spot  31  through the display portion  29  in the upper layer  23  when the gel  26  reaches the pre-determined stimulus extreme. For example, when the gel  26  in  FIG. 2A  is at its non-deleterious stimulus extreme, such as 2° C. or warmer, or 9° C. or colder, the gel  26  is in its expanded state ( 26   e ) and covers the indicator spot  31 . However, when the gel  26  is exposed for the pre-determined period of time to a stimulus extreme that will cause the gel  26  to collapse, such as colder than 2° C. (or warmer than 9° C.) for longer than 1 hour, as shown in  FIG. 2B , the decrease in volume ( 26   c ) will reveal the indicator spot  31 , thus indicating that the stimulus sensitive product to which the device  100  is attached has probably or definitely been rendered ineffective because of exposure to a deleterious temperature extreme. Because the gel  26  is preferably fixed to the attachment point  33 , it cannot float freely in the first compartment  25  and potentially block the indicator spot  31  from view when in its contracted state. 
     It should also be understood that if a different type of gel  26  is used in the device  100 , another embodiment is possible where the same function of the device  100  can still be achieved (i.e., indicate exposure to a deleterious temperature extreme), but in reverse. In other words, rather than the process proceeding from an expanded gel state  26   e  to a contracted gel state  26   c  as shown in the sequence of  FIGS. 1A to 1B , the process would be reversed. The process would proceed from a contracted gel state  26   c  to an expanded gel state  26   e  as shown in the sequence of  FIGS. 1B to 1A . 
     For example, using gels such as those described herein and herein incorporated references, the gel  26  would be contracted and/or shrunk in size and/or volume ( 26   c ) so long as it was not exposed to a pre-determined stimulus extreme, such as less than 2° C. (or greater than 9° C.), and would be present in only the second compartment  25  as shown in  FIG. 1B . Preferably, the gel  26   c  would be attached to the upper layer  21  and lower layer  23  at the attachment point  33 . Thus the indicator spot  31  would be visible to the user through the display portion  29  and it would indicate that the temperature sensitive product to which the device  100  was attached had not been harmed by a deleterious stimulus extreme. However, once the stimulus extreme was experienced, such as when the temperature dropped below 2° C. (or climbed above 9° C.), as shown in  FIG. 1A , the gel  26   e  would increase in volume and cover the indicator spot  31  with its second color. Preferably the gel  26   e  would be manufactured to be permanently expanded (irreversible), and thus it would never recede from the first compartment  24  and would permanently indicate to the user that the stimulus sensitive product to which the device  100  was attached had been exposed and most likely harmed by a deleterious stimulus extreme. This same “reverse” embodiment of contraction to expansion can also be applied to the device  200  described above in regard to  FIGS. 2A and 2B . 
     In  FIGS. 1A and 1B , and  2 A and  2 B, the particular gel  26  either shrank or swelled in response to a cold temperature stimulus. One of ordinary skill in the art would understand that by selecting a particular gel, such as described in the previously cited references, the particular gel  26  could either shrink or swell in response to a hot or warmer temperature stimulus as well. 
     Another exemplary embodiment of the present invention, as shown in  FIG. 3 , is the combination of two separate and distinct gels that indicate exposure to two stimulus extremes in the same indicator. Any of the embodiments shown in  FIGS. 1 and 2  may be combined to provide such a dual indicator, although preferably one indicator will indicate exposure below a certain temperature threshold, and the other will indicate exposure above a certain temperature threshold. Also, more than two indicators may be combined to provide a visual indication at exposure temperatures above or below more than two thresholds. The two or more indicators can be oriented vertically, horizontally or any other relationship to each other. 
     For example, as shown in  FIG. 3 , the device  300  contains a first indicator  310  that operates, for example, as discussed above in regard to  FIGS. 1 and 2 , in that it indicates exposure to a certain stimulus extreme at one end of the stimulus spectrum (lower end), while the at least second indicator  330  operates as discussed above in regard to a device that indicates exposure to a certain stimulus extreme at the opposite end of the stimulus spectrum (upper end). 
     By way of example only, the first indicator  310  in device  300  would indicate exposure of the device  300  to a temperature extreme of less than or equal to 1° C. for a predetermined amount of time, and the second indicator  330  in device  300  would indicate exposure of the device  300  to a temperature extreme of greater than or equal to 9° C. for a predetermined amount of time. Thus, the indicator  300  would indicate, via a single indicator, whether or not the stimulus sensitive product to which the indicator  300  is attached had been exposed to a harmful stimulus on both sides of the safe stimulus spectrum. Other combinations are also possible, and may include different combinations of the stimuli discussed herein, for example temperature and pH. 
     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
     Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.