Mechanochromic hydrogels, which can switch their color in response to the applied external force, have shown great potential in stress visualization and damage indication. However, the kinds of colors in the reported mechanochromic hydrogels are limited. It is challenging to develop mechanochromic hydrogels with new kinds of color changes. Herein, a kind of mechanochromic double network (DN) hydrogel is reported based on the hybrid phenol-rhodamine mechanophore. The hydrogels turn into orange color with an emission wavelength of around 566/574 nm in response to tensile and compressive stress. The DN hydrogels show great reversibility. The color of DN hydrogels vanishes slowly after releasing the stress. The stress sensitivity can be tailored by the crosslinking density and the mechanophore concentration of the first network. In addition, the influence of the pH on the mechanochromic properties of DN hydrogels is also studied. This study provides an insightful study in tuning the stress sensitivity in the mechanochromic hydrogel, which will be beneficial for the development of mechanochromic materials.

Cephalopods can change their color and patterns by activating the skin chromatophores for camouflage. However, in the man-made soft material systems, it is greatly challenging to fabricate the color-change structure in the desired patterns and shapes. Herein, we employ a multi-material microgel direct ink writing (DIW) printing method to make mechanochromic double network hydrogels in arbitrary shapes. We prepare the microparticles by grinding the freeze-dried polyelectrolyte hydrogel and immobilize the microparticles in the precursor solution to produce the printing ink. The polyelectrolyte microgels contain mechanophores as the cross-linkers. We adjust the rheological and printing properties of the microgel ink by tailoring the grinding time of freeze-dried hydrogels and microgel concentration. The multi-material DIW 3D printing technique is utilized to fabricate various 3D hydrogel structures which could change into a colorful pattern in response to applied force. The microgel printing strategy shows great potential in the fabrication of the mechanochromic device with arbitrary patterns and shapes.

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