Bioinspired Structural Color Hydrogel Skin from Nonclose-Packed Colloidal Crystal Arrays for Epidermal Sensing.

Developing multifunctional structural color hydrogel skin without sacrificing the unique periodic structure of photonic crystals is still a challenge due to the photonic bandgap limitation. Taking advantage of the synergistic effect of electrostatic repulsion and electronic conductivity, an intelligent structural color hydrogel skin with electrical and photonic sensing capabilities has been developed by doping MXene (TiCT) nanosheets and adhesive functional groups (nucleobases) into colloidal particle solutions. The introduction of MXene nanosheets could improve both the stability and electrical conductivity of the colloidal particle solutions, resulting in a conductive hydrogel with bright structural colors. With the help of functional groups of nucleobases, the resulting structural color hydrogel was also endowed with high biocompatibility and strong adhesion to different substrates, including the wet surfaces of tissues. It was demonstrated that the structural color hydrogel can not only realize visual sensing of tiny limb movements but also provide stable electrical sensing signals. The intelligent structural color hydrogel can be integrated into a capacitor device as a hydrogel electronic skin to simulate the sensory function of human skin. The results showed that such hydrogel skin can simulate the touch of human skin and perceive tiny movements on the body surface with both electrical and photonic signals. These features of the multifunctional structural color hydrogels make them potentially excellent value in bioinspired hydrogel skin electronics.