An Integrated Capacitive Pressure Sensor Based on Gradient Charge Distribution with High Sensitivity and Fast Response

Hydrogel-based electronic skin (E-skin) has attracted great research interest because of their promising applications in skin-attachable devices for health monitoring, prosthetics, and robotics. However, the design and fabrication of hydrogel-based E-skin devices with desirable mechanical properties, alongside simultaneous high sensitivity and a broad detection range, remains a significant challenge. In this study, we successfully synthesize an integrated capacitive sensor based on a gradient charge distribution hydrogel with a sandwich-like structure (cation-rich layer, dielectric layer, and anion-rich layer). This synthesis is accomplished via a process entailing the migration of ions in solution under a high-voltage electrostatic field, followed by UV curing. Fluorescent nanospheres are utilized to mimic ion motion, as indicated by the alterations in fluorescence intensity and micromorphology observed on both sides of the hydrogel. The capacitive sensor exhibits impressive mechanical performance, with a compressive strength of 987 kPa at 70% compression strain. Besides, owing to the electrical double layer (EDL) constructed by the ion gradient and the nanospine structure fabricated on the hydrogel surface, the sensor achieves both high sensitivity (increased by more than 20 times) and wide detection range at the same time. Moreover, the flexible sensors demonstrate fast response time (31 ms), allowing for immediate detection of both subtle and robust physiological activities. This strategy provides a convenient method to fabricate gradient-ion hydrogels and develops the applications in monitoring subtle vital signals and human movements.