A Double‐Layer Mechanochromic Hydrogel with Multidirectional Force Sensing and Encryption Capability

Abstract

Hydrogel-based soft mechanochromic materials that display colorimetric changes upon mechanical stimuli have attracted wide interest in sensors and display device applications. A common strategy to produce mechanochromic hydrogels is through photonic structures, in which mechanochromism is obtained by strain-dependent diffraction of light. Here, a distinct concept and simple fabrication strategy is presented to produce luminescent mechanochromic hydrogels based on a double-layer design. The two layers contain different luminescent species-carbon dots and lanthanide ions-with overlapped excitation spectra and distinct emission spectra. The mechanochromism is rendered by strain-dependent transmittance of the top-layer, which regulates light emission from the bottom-layer to control the overall hydrogel luminescence. An analytical model is developed to predict the initial luminescence color and color changes as a function of uniaxial strain. Finally, this study demonstrates proof-of-concept applications of the mechanochromic hydrogel for pressure and contact force sensors as well as for encryption devices.

Cephalopod‐Inspired High Dynamic Range Mechano‐Imaging in Polymeric Materials

“…Overlaying the two permits the two mechanochromic read‐outs to be monitored simultaneously; alternatively, the fluorescence signal of merocyanine offers the possibility to track the two strain regions in separate optical channels. While some mechanochromic hydrogels have recently been reported with large strain‐sensing ranges, [ 29,30 ] such materials tend to be soft or may have other limitations, such as a dependence of the optical response on the sample dimensions. Furthermore, the chemical mechanophore in the HDR‐MI system provides a snapshot of the molecular level stresses in the material.…”

Reversible data encryption–decryption using a pH stimuli-responsive hydrogel

“…To this end, recently, stimuli-responsive materials have received tremendous attention and have been considered as promising mediums for data security applications [12][13][14][15][16][17][18][19] . Exploiting the unique reactive properties of these smart materials, various data encryption platforms have been designed employing carbon dots composite 20 , mechanochromic hydrogel 21 and luminescent hydrogel 22 , etc. Among various intelligent reactive materials, pH-responsive hydrogel could be a suitable platform providing pH sensitivity for data manipulation and is operable in liquid environments [23][24][25][26][27] , yet few research has addressed their potentials for data security applications.…”

From Molecules to Polymers—Harnessing Inter‐ and Intramolecular Interactions to Create Mechanochromic Materials

“…Thus, the films displayed a straindependent decrease in the emission intensity ratio between the top and bottom layers (I 414 /I 544 ), which was visually discernible as a change in emission color from yellow-green to blue-green ( Figure 11c). [169] With the exception of this last example, the responses of mechanochromic materials with ML complexes is mediated by the intermolecular interactions between metal ions and ligand-functionalized polymers, allowing for a simultaneous control over both the optical and materials properties via the same motif. Alternatively, when transition metal-coordinating N-heterocyclic carbene (NHC) ligands are incorporated at the ends of a telechelic polymer, mechanically induced dissociation of these catalytically active transition-metal complexes can be harnessed to produce chemiluminescence and a striking turn-on fluorescence.…”