Wearable strain sensing skin with visual and stable structural color and electrical response for real-time monitoring

Abstract

Special animals in nature with structurally colored skin have the ability to alter their skin color in response to external stimuli through the processing of bioelectrical impulses. This natural adaptation serves as a sensory mechanism, achieved through an integrated and stretchable network that relays information. Herein, inspired by this remarkable phenomenon, the wearable strain sensing photonic-electric skin (PE-skin) was designed based on reduced graphene oxide(rGO)/polydimethylsiloxane (PDMS) black substrate adhered on SiO₂ photonic crystals/poly(ethylene glycol) phenyl ether acrylate (PEGPEA) films. The PE-skin contains a non-volatile and non-hygroscopic ionic liquid (IL), which enhances the sensitivity of the skin to electric stimuli. By optimizing the IL content, a dual response of the optical and electrical signal to stress is achieved. Under the optical, mechanical, and electrical properties investigations, the PE-skin treated with 2 s IL performs high stretch and sensitivity. Specifically, it can output distinct mechanochromic sensitivity (Δλ/Δε) of approximately 2.4 nm%⁻¹ and a gauge factor (GF) of around 1, with a color difference of roughly 160 nm, which shows advanced visual interaction sensing capability. Finally, a novel device was developed to detect a small current based on STM32 chip. To effectively integrate the PE-skin color change with the electrical signal, a current oscilloscope was specifically designed to visualize the current variation in a waveform representation. It is well suited to continuously outputting intuitive color-switching signals and discernible resistance signal changes under tensile strain. Additionally, it effectively monitors joint movements to provide intuitive and accurate feedback signals.