Multifunctional Gelatin-Based Smart Films Integrating Thermochromic Encryption, Temperature-Regulated Photothermal Management, Reprocessability, and Biodegradability for Sustainable Applications

Multifunctional gelatin-based smart films are engineered by incorporating hyperbranched polyglycerol (HBPG) as a plasticizer, dialdehyde β-cyclodextrin (Da-β-CD) as a crosslinker, and thermochromic microcapsules (TCMs). Structural analyses, including FTIR, XPS, and NMR, confirm the formation of covalent Schiff base linkages between Da-β-CD and gelatin, alongside hydrogen bonding reorganization facilitated by HBPG. The optimized film (GHBT₂-CD) exhibits enhanced tensile strength (28.7 MPa), hydrophobicity (water contact angle of 116°), UV-blocking capability (>97%), and complete (100%) bacterial inhibition. Crucially, these films demonstrate programmable thermochromism for multilevel information encryption, enabling features such as laser-writing, temperature-gated message display (e.g., “SUST” decryption), and numeral switching (9→7→8) using TCMs with distinct transition temperatures (18°C, 28°C, and 38°C). Furthermore, they achieve dual-modal encryption by combining the intrinsic fluorescence of gelatin (emission at 340 nm) with thermochromism, which enables four-state displays (e.g., showing “Accept”). Additionally, the films provide self-adaptive temperature regulation: their black state below 28°C significantly boosts solar heating (achieving a ΔT of +27°C in a 4°C ambient environment), while their pale-yellow state above 28°C mitigates overheating (keeping the surface below 56°C in a 30°C ambient), an effect augmented by the phase-change latent heat buffering of the TCMs. Finally, the films embody closed-loop sustainability. The presence of dynamic Schiff base and hydrogen bond networks enables over 91% self-healing efficiency using stimuli like water, heat, or vapor, facilitates physical reprocessing, and allows for tunable degradation rates dependent on pH or soil conditions (complete degradation within 24 h at pH = 2, and within 12 days in sludge). This work pioneers an all-in-one smart materials platform that bridges optical security, thermal management, and the principles of a circular economy.