hydrogel devices. For example, the conductivity of hydrogels can be enhanced by adding ions or carbon nanomaterials. [20] Adding photochromic, thermochromic, or photonic materials endows hydrogel the ability to change color under certain external stimulations. [21][22][23][24] Using this method, we prepare a novel thermochromic hydrogel by embedding thermochromic capsule powders (TCPs) into the hydrogel (Figure 1, see details in the Experimental Section).TCPs change color in response to temperature. The particles are usually spheres with 3-5 μm in diameters as shown in images of the scanning electron microscope (SEM) (Figure 1a,b). The exterior of the particle is a 0.2-0.5 μm thick transparent shell that neither dissolves nor melts when environment temperature changes. The components of the TCPs are shown in Table S1 in the Supporting Information. The shell protects the components from being eroded by the external environment during the reversible reaction of coloration, so the thermochromic property is quite stable. [25] Besides the shell layer, the thermochromic components consist of two ingredients: a color agent (CA), and a solvent. The critical temperature for color changing is determined by the phase change temperature of the solvent: when the ambient temperature is lower than the critical temperature, the solvent is solid, and the CA displays color 1. As Figure 1c shows, when the temperature rises above the critical temperature, the electron transfer occurs between them resulting in the change of the molecular structure of the CA and the color displayed of the system. [26] Under a certain temperature, the coloration and discoloration reactions reach an equilibrium state, which determines the saturation of the color. Furthermore, the critical temperature can be changed by controlling the melting point of the solvent component. [27] The color of the prepared hydrogel can switch between two modes at a specific temperature with a prompt response. As TCPs can disperse well in water, the addition of a small amount of powders (5 mg mL −1 ) is enough to dye the hydrogel. This color-changing hydrogel has good mechanical properties and retains its ability of color changing even after cyclic loading. To explore the mechanical properties of the thermochromic hydrogel, one may use quasi-static stretching tests to measure the initial modulus of the hydrogel, and cyclic stretching tests to analyze the functional performance of the hydrogel. We characterize the color-changing ability by absorbance spectra at different temperatures through a spectrophotometer. In addition, we demonstrate that this thermochromic hydrogel Recently, hydrogels with coloration have attracted researchers from various fields, such as camouflage, anti-counterfeiting, and soft display. However, existing thermochromic hydrogels are limited by their weak color display performance and insufficient sensitivity. Here proposed is a new kind of thermochromic hydrogel which possesses bright colors, fast response time, and reliable results across a long lif...

Soft display has been intensively studied in recent years in the wake of rapid development of a variety of soft materials. The currently existing solutions for translating the traditional hard display into the more convenient soft display mainly include light-emitting diodes, liquid crystals, quantum dots, and phosphors. The desired soft display should take the advantages of facile fabrication processes and cheap raw materials. Besides, the device should be colorful, nontoxic, and not only flexible but also stretchable. However, the foregoing devices may not own all of the desired features. Here, a new type of soft display, which consists of dielectric elastomer and photonic crystals that cover all of the features mentioned above and can achieve the color change dynamically and in situ, is reported. In addition to the above features, the angle-dependent characteristic and the excellent mechanical reliability make it a great candidate for the next generation of soft display. Finally, the vast applications of the present concept in a variety of fields are also prospected.

Conductive hydrogels with ionic compounds possess great potential for the development of soft smart devices. A dielectric scarfskin is typically required for these devices to prevent short circuiting, leading to devices with lower stretchability than the hydrogel. Henceforth, commonly used dielectric materials, such as PDMS and Ecoflex, cannot be largely stretched. Hydrogel devices with ultrastretchability are required to accommodate hostile application environments. Herein, we propose a hydrogel fiber coated with a dielectric layer that can be stretched to over 2000% of its initial length. The fiber remains conductive when stretched to ~1300%. In addition, the core/sheath hydrogel fiber can be endowed with a variety of functional properties, such as electroluminescence (EL), photoluminescence (PL), and magnetic‐responsiveness, demonstrating scalability of the resultant fiber. The present work can pave the way for numerous next‐generation soft devices, such as smart textiles and wearable electronics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 272–280

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