BACKGROUND

Many commercial products are sensitive to environmental stimuli, such as temperature, cumulative temperature, humidity, pH, and ultraviolet radiation, and may lose efficacy or quality under any of these conditions. Examples of such commercial products include certain pharmaceuticals, medical products, and foodstuffs as well as some industrial products. Conventional environmental indicators to detect such changes typically use a constant/fixed-color reference to show an end point appearance of the active indicator/area. The end point appearance may be an appearance such as a dark appearance that indicates a probable condition of an associated host product, for example, that the host product has lost efficacy or quality and should not be used.

SUMMARY

The present disclosure provides new and innovative color-changing environmental stimulus indicators having a color changing reference. In some embodiments, a color-changing environmental stimulus indicator includes a substrate, an active indicator placed above the substrate, and a reference component. The active indicator and the reference component are color changeable in response to an environmental stimulus. In some embodiments, the active indicator and the reference component change color in response to the same environmental stimulus. In other embodiments, the active indicator and the reference component change color in response to different environmental stimuli. The environmental stimulus includes one or more of temperature, cumulative temperature, humidity, pH, and ultraviolet radiation.

In some embodiments, the active indicator is configured to darken in response to exposure to the environmental stimulus and the reference component is configured to lighten in response to exposure to the environmental stimulus. In other embodiments, both the active indicator and the reference component are configured to darken (or lighten) in response to exposure to the environmental stimulus. In some embodiments, the active indicator or the reference component is a color changing ink, such as a diacetylene. In other embodiments, the active indicator or the reference component change color by using a technique selected from the group consisting of metal etching, dye diffusion, acid/base reactions, and combinations thereof. The color change is at least one of a change in color, color density, fluorescence, or opacity.

In some embodiments, the reference component is configured to change color with time. In some embodiments, the color change of the active indicator and the reference is irreversible and occurs after a predetermined cumulative exposure to the environmental stimulus. The reference component is configured to have a similar appearance to the active indicator at an end point. In some embodiments, the color-changing environmental stimulus indicator includes an ultraviolet radiation filter configured to protect the active indicator or the reference component from exposure to ultraviolet radiation. In some embodiments, an active ink is added to the reference component. The active ink is configured to change color quickly at the beginning of the exposure and stabilize at later times.

In some embodiments, both the active indicator and the reference component are configured to darken in response to exposure to cumulative temperature at different rates such that the color-changing environmental stimulus indicator has a higher effective activation energy than the active indicator. In other embodiments, the active indicator and the reference component are configured to darken in response to exposure to ultraviolet radiation and temperature. Then, a temperature-insensitive monomer is coated over the reference component to match the darkening of the active indicator in the presence of ultraviolet light.

In some embodiments, the active indicator is configured to change color in response to exposure to temperature, and the reference component is configured to change color in response to exposure to at least one of humidity, pH, and ultraviolet radiation. In some embodiments, the active indicator is configured to darken in response to exposure to the temperature and the reference component is configured to lighten in response to exposure to the at least one of humidity, pH, and ultraviolet radiation.

In some embodiments, the active indicator is configured to change color in response to exposure to temperature and low humidity, and the reference component is configured to change color in response to exposure to the low humidity to offset the effect of the low humidity to the active indicator. In some embodiments, the active indicator is configured to change color in response to exposure to cumulative temperature and the reference component is configured to change color in response to exposure to a temperature near or below a predetermined temperature.

In some embodiments, a color-changing environmental stimulus indicator may include a substrate, an indicator layer having an indicator material, and a reference layer having a reference material. The reference layer and the indicator layer may be either in the same layer or in parallel layers. The indicator material and the reference material may be color-changeable in response to an environmental stimulus. The environmental stimulus may be selected from the group consisting of peak temperature, cumulative temperature, humidity, pH, radiation, ultraviolet radiation, and combinations thereof. In some embodiments, the color change of the indicator material and the reference material may be irreversible and at least one of a change in color, color density, fluorescence, or opacity. The environmental stimulus indicator may be configured to provide an indication of an environmental stimulus that is not possible if only the indicator material were color-changeable and the reference material were not color-changeable.

In some embodiments, the indication of an environmental stimulus may be an extended period of time over which the color-changing environmental stimulus indicator is able to monitor the environmental stimulus. In other embodiments, the indication of an environmental stimulus may be a shortened response period during which the color-changing environmental stimulus indicator monitors the environmental stimulus. In some embodiments, the indication of an environmental stimulus may be the combined effect of at least two environmental stimuli. The two environmental stimuli may be selected from the group consisting of temperature and radiation, temperature and pH, and temperature and humidity.

These and other features are disclosed in greater detail in the accompanying figures and the Detailed Description below.

