The present invention relates to a device for visually indicating that an object has been stored for an excessive period or above a predetermined temperature or for indicating that a package or seal has been opened or tampered with.
Numerous devices have been proposed which provide a visual indication of the passage of a predetermined period of time, tampering, or storage under undesirable conditions. For example, Olsen et al, U.S. Pat. No. 3,344,670 (1967) describes a time/temperature indicator consisting of a cellulosic material such as filter paper treated with silver nitrate. This indicator exhibits a permanent color change depending solely upon the thermal history of the treated paper. Jackson, U.S. Pat. No. 3,480,402 (1969) discloses a time indicator formed of an absorbent carrier having absorbed thereon at least one chemical compound which changes color upon exposure to oxygen. The carrier and absorbed compound are protected from ambient oxygen by a barrier layer through which oxygen controllably diffuses over a preselected time. Thus, a color change is evident if recommended storage times are exceeded. Kydonieus et al, U.S. Pat. No. 4,212,153 (1980) discloses a time indicator in which migration of an agent from an interior layer to an exterior surface layer produces a visually perceptible color change. In one embodiment the migrating agent is a dye; in another it is an acid or base and the exterior layer contains a pH indicator. Other indicators are described in Yoshikawa et al, U.S. Pat. No. 4,169,811 (1979); Seiter, U.S. Pat. No. 3,967,579 (1976); and Youngren, U.S. Pat. No. 4,382,700 (1983).