Colorimetric polymer nanofilm-based time-temperature indicators for recording irreversible changes of temperatures in cold chain

Monitoring and recording subzero temperatures and humidity are essential activities for pharmaceutical, food-beverage, and cold storage industries, as product quality can be hampered during storage and transportation due to temperature disruptions. Traditional electronic subzero time-temperature indicators (TTIs) can be energy-inefficient, fragile, non-recyclable, and susceptible to data breaches and cyber-attacks. Hydrophilic colorimetric polymer nanofilms have been developed as colorimetric temperature and relative humidity (RH) sensors, however, the reversible color-changing behavior of these films significantly limited their application as TTIs, as cannot record temperature changes in the past. Herein, the first colorimetric polymer nanofilm-based TTI for recording an irreversible change of temperatures is reported. This device has shown quick color response in temperature ranges from 23 °C to -30 °C in fewer than 50 s. Remarkably, when the device experiences temperature disruption above a certain threshold time (t), it shows irreversible color-changing behavior in response to the temperature change from subzero (-30 °C and -15 °C) to room temperature or above. It was demonstrated that t, from minutes to days, of the TTI device can be precisely tuned by adjusting moisture absorber type and weight, interior RH, and storage temperature. Several field tests have demonstrated good versatility and applicability of the device.