Flexographic Printed Flexible Thermochromic Stickers for Smart Sensing Applications

The growing demand for thermochromic materials in areas such as food packaging requires the development of fast-response and extended-life materials. The use of metal cation-ionic liquid complexes is increasing due to the easiness to tailor their properties, leading to changes in thermochromic response. This work presents the synthesis and introduction of thermochromic 1-butyl-3-methylimidazolium chloride and 4-chloronickelate ([BMIm]₂[NiCl₄]) complexes into water-based inks. Formulations with varying concentrations of complexes (10–40 wt.%) are printed on tags thorugh a roll-to-roll process and tested between 30–45 °C. Also, films with one to five layers are analyzed for color contrast and different encapsulating materials are studied to improve the thermochromic films' robustness. The optimal films, with three layers of ink with 40 wt.% complexes encapsulated with Fixomull Transparent present enhanced color contrast and fast response time. Red-green-blue analysis is performed on these films, revealing a blue/red ratio of 1.7 after heating at 40 °C for 30 min. This work paves the way for developing flexible stickers with fast response and high contrast. The innovative and straightforward thermochromic inks' production process and compatibility with large-scale manufacturing show their promising future as cost-effective stickers for smart packaging applications, such as visual indicators of ideal food consumption temperatures.