In the framework of VERITASCAN, the team has validated the concept of TTI based on erasable structural colour by producing a large number of samples of three basic prototypes. The basic structure of the devices consists of three layers, namely, a small molecular solvent which melting temperature is set to the critical temperature for the given application (e.g. 8 ºC for medicines that requires a fridge), an interlayer that prevents undesired activation of the device, and a semiconductor layer with strong colour shifting properties. When the ambient temperature increases above the critical temperature, the solvent melts and diffuses through the interlayer reaching the semiconductor and thus changing its color.
Prototype I responds to the need of having a very simple TTI in which a Yes/No answer is provided to the question: has this product made temperature excursions that make it unsafe to use? Besides the commercial value for specific markets of this prototype, the simplicity of the device architecture enabled us to make a very large screening of materials and processing parameters. Indeed, more than 25 different solvents, 6 semiconductor systems, 6 types of interlayers, and four types of substrates (glass, plastic, paper and aluminium foil) were investigated. This broad study helped to identify the best materials in terms of cost, processability and non-toxicity. In the second prototype, the semiconducting layer was pre-structured to include a given image, such as QR code, or an OK symbol. Following the same process, excursions above the critical temperature erase the printed image. Type III prototype delivers a more precise time indication and is of use for applications in which the accumulated time above the critical temperature needs to be recorded. In order to avoid the need to store the TTIs themselves at low temperatures, three activation mechanisms have also been investigated, lamination using PET, lamination using gel, and the use of a solvent reservoir blister which distributes the solvent upon applying pressure.
To improve readability and trust on the TTIs, we have also developed a software that can analyse a picture of the device and inform the user of the time that the product has exceeded the critical temperature. Finally, based on the software capabilities, we have devised a fourth prototype in which, for the first time in the power-free TTIs development, time and temperature can be independently determined. The mathematical framework to analyse dual TTIs with different architectures has been advanced.
