Smart Core-Shell Nanostructures for Force, Humidity, and Temperature Multi-Stimuli Responsiveness

A force, humidity, and temperature-responsive electronic skin is presented by combining piezoelectric zinc oxide (ZnO) and poly-N-vinylcaprolactam-co-di(ethylene glycol) divinyl ether hydrogel into core-shell nanostructures using state-of-the-art dry vapor-based techniques. The proposed concept is realized with biocompatible materials in a simplified design that delivers multi-stimuli sensitivity with high spatial resolution, all of which are prerequisites for an efficient electronic skin. While the piezoelectricity of ZnO provides sensitivity to external force, the thermoresponsiveness of the hydrogel core provides sensitivity to surrounding temperature and humidity changes. The hydrogel core exerts mechanical stress onto the ZnO shell, which is translated to a measurable piezoelectric signal. A localized force sensitivity of 364 ± 66 pC N⁻¹ is achieved with very low cross talk between 0.25 mm² pixels. Additionally, the sensor's sensitivity to humidity is demonstrated at 25 and 40 °C, i.e., above and below the hydrogel's lower critical solution temperature (LCST) of 34 °C. The largest response to temperature is obtained at high humidity and below the hydrogel's LCST. The sensor response to force, humidity, and temperature is significantly faster than the system's intrinsic or excitation-induced time scale. Finally, the sensor response to touch and breath demonstrates its applicability as e-skin in real-life environment.