Bioinspired optical/electrical dual-modal flexible strain sensor with microcrack structure based on fluorescent hydrogel

AbstractInteractive stretchable sensors with dual electrical and optical signals have gained prominence due to their high detection accuracy, robust signal synergy stability in electromagnetic/optical interference environments, and visualization of mechanical stimuli. However, it remains a challenge to fabricate an observation angle-insensitive optical/electrical dual-modal sensor capable of quantifying strain degree through optical signal intensity. Inspired by cuttlefish skin and spider legs, an observation angle-independent optical/electrical dual-modal sensor with microcrack structure based on fluorescent hydrogel is developed that can quantify the magnitude of applied strain through fluorescent signals. A pH-responsive fluorescent rhodamine-derived monomer (Rho) is copolymerized into poly(acrylamide-co-2-hydroxyethyl methacrylate) network, yielding a fluorescent hydrogel that serves as the dual-modal sensor coated with a carbon nanotube (CNT) film. In the relaxed state, the intact CNT film shields the hydrogel''s fluorescence emission. Upon stretching, the mechanical mismatch between the flexible hydrogel and rigid CNT film induces microcracks in the latter, allowing fluorescence to penetrate and thus generating a detectable fluorescence signal. Herein, the fluorescence signal is insensitive to observation angles and exhibits a linear correlation with applied strain (R2 = 0.9853). Meanwhile, the strain-induced microcracks disrupt the conductive paths of the CNT film, leading to a progressive increase in resistance. In this way, the sensor achieves simultaneous optical/electrical dual-modal outputs through the evolution of strain-induced microcracks in the CNT film. The sensors are mounted on the dorsal surfaces of robotic fingers for detection of finger movements and gesture recognition. Besides, the sensor can be applied to dark-environment exploration and serve as a potential encryption device, demonstrating its versatility. This research advances the development of flexible strain sensors and expands their application fields.

https://www.sciencedirect.com/science/article/pii/S1385894726012416