Highly Stretchable Photonic Crystal Hydrogels for a Sensitive Mechanochromic Sensor and Direct Ink Writing

Abstract

Photonic crystals, which are materials with periodic dielectric constants on the submicroscale, have been the focus of research for an extended period. Photonic soft materials have been extensively developed for use as colorimetric indicators and mechanochromic sensors, but their limited mechanical properties and molding characteristics only suitable for films restrict their practical implementation. Herein we report an approach to synthesize highly stretchable photonic soft materials based on a hydrogel system that is cross-linked by a crystalline colloidal array. The intrinsic inhomogeneous submicroscale structure is exploited for effective reinforcement in the multiphase system of the photonic crystals. The photonic hydrogels exhibit a high deformation capacity, with a stretching deformation above 2800% and compression above 98%. The gel has a full-color tunable range and shows 460 nm photonic shifts that can be reversibly actuated by a small compressive stress (kPa level) and can be ink-written to form patterns and freestanding structures. Mechanochromic sensors are constructed based on the three-dimensional and two-dimensional Bragg diffraction of the gel. Owing to its mechanical strength, formability, and tunable colors, the gel can be used in wearable optical devices, colorimetric tactile sensors, and full-color displays.

Supramolecular Photonic Elastomers with Brilliant Structural Colors and Broad‐Spectrum Responsiveness

“…[ 10 ] Rheological investigation in Figure 1d showed that the amino‐PDMS polymer demonstrated a typical viscous liquid behavior with its loss modulus ( G ″) higher than the storage modulus ( G ′). [ 2g ] With the introduction of Fe 3 O 4 @C NPs, the G ′ significantly increased from 0.07 Pa of amino‐PDMS polymer to 12 900 Pa of Fe 3 O 4 @C/PDMS PE50 at angular frequency 1 rad s −1 , and was higher than its G ″, indicating the presence of the cross‐linked networks in the Fe 3 O 4 @C/PDMS PEs. In addition, compared with the glass transition temperature ( T g ) of the amino‐PDMS polymer, a slight increase (0.32 °C) of the T g for the Fe 3 O 4 @C/PDMS PEs was also observed from differential scanning calorimetry (DSC) analysis (Figure S1, Supporting Information), demonstrating the resultant supramolecular interactions between the NPs and polymer chains.…”

“…[ 1 ] Structural colors of PEs can be tuned by adjusting the lattice spaces of incorporated colloidal particles through mechanical strain, which have demonstrated great promise in many applications, especially in visualized strain sensors. [ 1b,2 ] In general, the structural colors produced by PEs can be divided into two categories depending on the arrangements of the colloidal arrays: 1) Delicate long‐range ordered arrangements of the colloidal particles usually produce vivid color because of the well‐defined periodic structures, while the as‐produced color is variable with the viewing angle due to the anisotropically crystal structure (e.g., angle‐dependent structural color). [ 1b,e,3 ] 2) Instead, amorphous arrays of colloidal particles with short‐range order would generate angle‐independent structural colors due to the isotropic structures, which could reflect the light with the same wavelength in all directions and exhibit consistent structural colors independent of the viewing angle.…”

“…[ 6 ] However, for the commonly used colloidal particles and polymers, the as‐generated PEs usually exhibited low RI difference (<0.2) and created nearly invisible reflection peak. [ 1b,2g,4h ] When being stretched or placed on white substrates, these colors are difficult to be identified by naked eyes and even spectrophotometer, limiting their applications for accurate detection and sensing. Therefore, it still remains a challenge to further improve the color saturation of PEs with angle‐independent structural color for the practical applications.…”

“…Generally, the color‐shifting capability of the PEs depends on the alterations of lattice spacing under large deformation according to Bragg's law. [ 2 ] Kim and co‐workers prepared PEs with increased individual NP distance and enhanced the responsiveness by assembling silica NPs into a colloidal array in the presence of a liquid monomer, followed by a photoinitiated polymerization of the monomer. [ 3b ] More recently, our group demonstrated a facile yet effective route for in situ construction of PEs via assembly of NPs with the help of supramolecular interactions.…”

“…With this strategy, the interparticle distance of the colloidal particles within the supramolecular polymeric matrix can be effectively enlarged, and the reversible interaction in supramolecular polymer allowed the PE to self‐heal their mechanical damages. [ 1b,2g ]…”