Photonic Crystal Hydrogel Film with Responsive Structural Color for Multifunctional Sensing

Responsive photonic crystal materials that can change their structural color in response to external stimuli have attracted growing attention in the research area of flexible electronics. This study presents a conductive structural color hydrogel film featuring an inverse opal structure designed for use in wearable sensors. The hydrogel film consists of a double-network structure, where the first network is formed by acrylic acid and acrylamide (P(AA-co-AM)), and the second network is composed of quaternized chitosan. By incorporating multiwall carbon nanotubes (MWCNTs) into the hydrogel, not only is the conductivity enhanced but also the tensile strength of the hydrogel is significantly improved (0.45 MPa). Due to the excellent stretchability and inverse opal architecture of the hydrogel matrix, the film exhibited vibrant and tunable structural colors in response to applied stress or strain. Leveraging these attributes, the hydrogel film not only visualized color changes but also provided feedback on electrical signals in response to strain or stress stimuli, thereby functioning as a dual-signal soft sensor for the real-time monitoring of optical and electrical signals related to human motion. In addition, the presence of chitosan, known for its excellent water absorption properties, allows the film to swell in water while shrinking in ethanol. This characteristic enables the film to quantitatively respond to ethanol through noticeable color changes. This work demonstrates a strategy of synergistic enhancement of the mechanical properties of hydrogels through a combination of MWCNTs and a double-network structure. The presented conductive structural color hydrogel film, featuring a dual-signal output, exhibits remarkable potential for applications in wearable devices and multifunctional sensors.