Tough and Transparent Photonic Hydrogel Nanocomposites for Display, Sensing, and Actuation Applications

Abstract

Thermosensitive hydrogels with periodic dielectric structures displaying tunable structural color by temperature stimuli have attracted much research interest recently. However, reported thermosensitive photonic hydrogels either using the poly(N-isopropylacrylamide) (PNIPAM) bulk hydrogel as the responsive matrix or using PNIPAM microgels as the photonic building blocks have poor mechanical strength. Moreover, the expensive and time-consuming preparation process also limits their further application. In this work, a different strategy for preparing PNIPAM-based photonic hydrogel nanocomposite films with strong mechanical strength is proposed by incorporating a PNIPAM microgel film with the polyacrylamide (PAM) hydrogel. The preparation process is simple but efficient, and the obtained nanocomposite films have tunable mechanical strength by changing the crosslinking degree of the PAM hydrogels. More interestingly, the structural color of the nanocomposite films can be retained up to 80 °C (at least), much higher than those of previously reported PNIPAM-based photonic materials. In addition to being thermochromic, the film is sensitive to humidity, and the structural color-changing process is similar to the molting process of cicadas. Furthermore, the nanocomposite films have synchronous shape deformations and color changes and can be used as color-tunable soft actuators. The microgel film-capped photonic hydrogels provide a new strategy for the preparation of thermoresponsive photonic hydrogels and structural color-tunable actuators.

Hydrogels for active photonics

“…The particle or porous structure allows optical modulation via the effective refractive index of the medium or optical path length 94 , 95 . The enhancement of the light-matter interaction by utilizing PC, which consisted of a particle-/pore-embedded hydrogel, was attempted 96 .…”

Thermoresponsive Structural Coloration of Hydrogel Fibers

“…In our previous work, close-contacted thermoresponsive microgels were used as an alternative route to generate a tunable structurally colored hydrogel film. The size change of the microgels, which is also the spacing distance changing between the microgels, resulted in color changes in the hydrogels. − …”

Biomimetic 3D Color-Changing Hydrogel Actuators Constructed Based on Soft Permeable Photonic Crystals

“…In nature, numerous organisms (e.g., cephalopods, chameleons, and frogs) can simultaneously change their colors and shapes in response to external stimuli for camouflage, communication, or reproduction. − These fascinating phenomena have inspired the development of materials with synergistic color-changing and shape-morphing capabilities for the applications in camouflaged soft robotics, , anticounterfeiting, , encryption, − display, , etc. In term of the imitation of color-shifting functionality, the photonic materials are the ideal candidates. − They can generate brilliant and nonfading structural colors by using periodically arranged fine microscopic structures to interfere with light. − Through controlling the distribution and size of periodically microstructures of photonic materials by multistep photolithography, , selective swelling, , and selective growth of polymer matrix, the multicolor patterns can be created. Similar to natural living, these structure color images can be altered reversibly under external stimuli due to the variation of lattice constant or refractive index of photonic materials. − Recently, by integrating the color-changing layer with an active/inert polymer layer into bilayer/gradient actuators, the simultaneous change of color and shape has been achieved. − Such impressive advances contribute to the development of powerful biomimetic color-changing soft robotics.…”