Heat history monitor: Combination of the sublimation and adsorption processes of specific dyes can be used as a measure of accumulated heat exposure. Mass transfer from the sublimation layer to the adsorption layer strongly depends on temperature and results in a gradual color change in the adsorption layer. The total color change reflects the accumulated heat exposure.

Time temperature indicators (TTIs) based on a dye sublimation process have been developed for full time monitoring of heat sensitive vaccines with shorter shelf-life during cold chain transportation and storage. The structure of TTI and the methods to make the TTI are introduced. TTI 1 and TTI 2 introduced in the article were made with methyl azulene-1-carboxylate and methyl 3-methylazulene-1-carboxylate as the functional dyes, respectively. The kinetic parameters of TTI 1 and TTI 2 were studied.