Exhalation-responsive optoelectronic clip: switchable non-contact wearable sensor enabled by nanocellulose-partially-encapsulated optical microfibers.

Tactile sensors, widely used in human-machine interactions, face limitations for individuals with mobility impairment or in touch-free scenarios. Humidity sensors, as non-contact alternatives, struggle with environmental interference and complex signal processing. To overcome these challenges, we developed an exhalation-triggered clip with a switchable humidity sensor enabled by a cellulose nanocrystal (CNC) partially-encapsulated optical microfiber (OMF). Thanks to the coffee-ring effect, CNC forms a wrinkle film on both sides of the OMF, leading to strong light scattering. The densely packed CNC within the film ensures that the light scattering remains impervious to ambient humidity. Only upon exhalation do water droplets condense on the hydrophilic film, which reduces the refractive index difference at the OMF/CNC/air tri-phase interface, thereby suppressing the light scattering and enhancing the light output of the OMF. As the water evaporates, the light intensity reversibly returns to its original state. The sensor achieves a switch point above 90% relative humidity and an ON/OFF ratio of ∼100. Benefiting from its rapid, switch-like response and high robustness, the clip enables remote alarm triggering, coded speech, and respiratory monitoring simply via human breath.