Facile Evaporation-Stamping Technique for Stimuli-Responsive Biopolymer-Supported Eutectogels Containing Opal Micropatterns

Biopolymer-supported deep eutectic solvent (DES)-based gels, also known as eutectogels, have emerged as promising alternatives to hydrogels and ionic-liquid-based gels for multiple applications in stretchable electronics and sensors due to many key advantages including their high ionic conductivity, tensile toughness, easy handling, simple synthesis, low cost, biocompatibility, and ultralow volatility. Particularly, gelatin-supported 1,2-propanediol (PD)-based eutectogels containing water have shown promise due to their hydrogel-like properties. They have low modulus values and biofriendly components, making them "skin-like" materials. They are optically transparent, which makes them ideal as user-friendly visual devices. Incorporation of color-tunable micropatterned opal structures into these novel gelatin-supported eutectogels enables the preparation of user-friendly, mechanically resilient, and stimuli-responsive materials for many applications via a simple color change. In this work, we utilize a simple and robust evaporative deposition-stamping technique to prepare eutectogels containing opal micropatterns to overcome limitations in existing fabrication techniques such as photolithography and soft lithography that suffer from costly equipment, harsh radical polymerization, and multistep processing and/or reliance on external forces. First, uniform and color-tunable opal micropatterns are formed via simple evaporative deposition. Scanning electron microscopy (SEM) images show the formation of a uniform hexagonal packing throughout the opal micropatterns. Next, the opal micropatterns are successfully transferred into gelatin-supported PD eutectogels via a simple hand-stamping technique to form opal eutectogels having uniform opal micropatterns due to the eutectogels' adhesive and mechanically resilient nature without the need for costly equipment. Photographs and dark-field optical micrographs, in combination with wavelength spectra measurements, illustrate the reliable nature of our simple evaporation-stamping method. Finally, sandwich eutectogels that fully encapsulate the opal micropatterns were produced by simply adding a secondary eutectogel layer to the top, yielding a reversible optical response to mechanical stimuli. We envision that this simple, reliable, and robust evaporation-stamping technique can be readily extended to manufacture biocompatible and user-friendly visual monitoring devices.