MXene‐Assisted Rapid Gelation and Foaming of Gradient Hydrogel as Human‐Machine Interfaces with Regulated Charge Accumulation/Dissipation and Enhanced Tactile Sensing Capability

Hydrogel-based tactile sensors, typically featuring a sandwiched structure of electrode/hydrogel/electrode with charges (electrons and ions) accumulated at both interfaces forming electrical double layers (EDLs), are considered promising candidates as human-machine interfaces (HMIs); however, the continuous dense structure limits the deformability of bulk hydrogels, hindering the achievement of high-sensitivity and wide detectable range. Moreover, the state-of-the-art strategies for enhancing sensitivity predominantly focus on enlarging the changes of hydrogel/electrode contact area upon external force, while overlooking the design of interfacial properties and ion transport kinetics. Herein, polyacrylamide/chitosan/MXene (PAM/CS/MXene, PCM) hydrogels with gradient porosity and tailorable softness are developed following the phase-transition-induced foaming mechanism. Ti₃C₂T_X MXene plays critical roles in promoting rapid gelation, stabilizing bubbles, regulating ion transport kinetics, and modulating charge accumulation/dissipation at the hydrogel/electrode interfaces. As a consequence, the obtained PCM foam (PCMF) tactile sensor exhibits an optimal sensitivity of 4267 kPa^-1 together with a wide detectable pressure range up to 100 kPa. Proof-of-concept applications are demonstrated for acquiring and identifying gesture commands to control a robotic arm for swab sampling and the movement of an electric car in a maze game by integrating PCMF sensors with a digital camera for visual feedback and machine learning for accurate recognition, respectively.