No abstract

Time–temperature indicators (TTIs) can visually reflect the remaining shelf life of various products, especially in cold chain transportation, by taking consideration of both time and temperature influences. Though various TTIs have been developed, few have been commercialized. This review aims at providing a timely reference to scientific community and valuable guidance for the future design of TTIs to boost the commercialization. This review firstly introduces the general principle and models for TTIs including key parameters of reaction rate constants, activation energies and activation energy matching. Then, four classical types of TTIs covering diffusion‐based, polymer‐based, enzyme‐based, and microbial TTIs are introduced regarding their working mechanisms, advantages and disadvantages as well as commercialized TTI products as examples. The development trend of TTI design and fabrication is highlighted from three categories including nanoparticles, electrospun nanofibers and printable inks. Furthermore, the review summarizes recent application of TTIs for monitoring the quality and safety of various products (meat, seafood, dairy, vaccines and fruits and vegetables). We also point our concerns from the perspective of safety, accuracy, multifunctionality and commercialization for the future development of TTIs.

Flexible and stretchable electronic devices are indispensable parts of wearable devices. However, these electronics employ electrical transducing modes and lack the ability to visually respond to external stimuli, restricting their versatile application in the visualized human–machine interaction. Inspired by the color variation of chameleons’ skin, we developed a series of novel mechanochromic photonic elastomers (PEs) with brilliant structural colors and a stable optical response. Typically, these PEs with a sandwich structure were prepared by embedding PS@SiO2 photonic crystals (PCs)within the polydimethylsiloxane (PDMS) elastomer. Benefiting from this structure, these PEs exhibit not only bright structural colors, but also superior structural integrity. Notably, they possess excellent mechanochromism through lattice spacing regulation, and their optical responses are stably maintained even when suffering from 100 stretching–releasing cycles, showing superior stability and reliability and excellent durability. Moreover, a variety of patterned PEs were successfully obtained through a facile mask method, which provides great inspiration to create intelligent patterns and displays. Based on these merits, such PEs can be utilized as visualized wearable devices for detecting various human joint movements in real time. This work offers a new strategy for realizing visualized interactions based on PEs, showing huge application prospects in photonic skins, soft robotics, and human–machine interactions.