Fundamental Aspects of Stretchable Mechanochromic Materials: Fabrication and Characterization

Mechanochromic materials provide optical changes in response to mechanical stress and are of interest in a wide range of potential applications such as strain sensing, structural health monitoring, and encryption. Advanced manufacturing such as 3D printing enables the fabrication of complex patterns and geometries. In this work, classes of stretchable mechanochromic materials that provide visual color changes when tension is applied, namely, dyes, polymer dispersed liquid crystals, liquid crystal elastomers, cellulose nanocrystals, photonic nanostructures, hydrogels, and hybrid systems (combinations of other classes) are reviewed. For each class, synthesis and processing, as well as the mechanism of color change are discussed. To enable materials selection across the classes, the mechanochromic sensitivity of the different classes of materials are compared. Photonic systems demonstrate high mechanochromic sensitivity (Δnm/% strain), large dynamic color range, and rapid reversibility. Further, the mechanochromic behavior can be predicted using a simple mechanical model. Photonic systems with a wide range of mechanical properties (elastic modulus) have been achieved. The addition of dyes to photonic systems has broadened the dynamic range, i.e., the strain over which there is an optical change. For applications in which irreversible color change is desired, dye-based systems or liquid crystal elastomer systems can be formulated. While many promising applications have been demonstrated, manufacturing uniform color on a large scale remains a challenge. Standardized characterization methods are needed to translate materials to practical applications. The sustainability of mechanochromic materials is also an important consideration.

Swelling-Induced Chromotropism of Bionanocomposite Hydrogel Beads

Monodispersed gelatin hydrogel beads containing smectite with adsorbed cyanine dye exhibit chromotropic responses to compression and swelling/deswelling by solvent. Photoluminescence color of the beads changes by swelling in water (blue) and deswelling in ethanol (purple) reversibly. The forces generated by swelling/deswelling are thought to induce the transition between the J-aggregate and the monomer of cyanine dye adsorbed on smectite, giving the photoluminescent color changes.

Ionic crosslinking of alginate/carboxymethyl chitosan fluorescent hydrogel for bacterial detection and sterilization

Herein, a polysaccharide-based fluorescent hydrogel with multi-responsiveness simply implemented by concurrent effects of ionic crosslinking/rehydration processes is presented. Specifically, the alginate and carboxymethyl chitosan are chosen to prepare the interpenetrating polymer matrix while a pair of metal cations has been selectively sequentially integrated to alter hydrogel mechanical and fluorescent properties. Experimental results indicate the hydrogels show tunable fluorescent emission in response to multiple cations and pH conditions, and display a reversible "ON/OFF" fluorescent response to Mⁿ⁺/ethylenediaminetetraacetic acid. Moreover, this synergistic ionic crosslinking strategy is proved to be highly effective in preparing multifunctional metallohydrogels possessing robust/anisotropic mechanical properties, typical shape memory and cation/pH-responsive fluorescence performance, and a proof-of-application for bacterial detection and sterilization has also been demonstrated. Therefore, we believe this study would provide new insights into multifunctional luminescent hydrogels for advanced biomedical systems.