A d33 piezoionic hydrogel with bioinspired multi-gradient structure for enhanced mechano-iontronic transduction

Mechanoelectric transduction based on piezoionic polarization mechanisms diverges from piezoelectric polarization, and is promising for various human-interfaced applications; yet, strategies are urgently demanded to enrich the device design beyond the state-of-the-art d₃₁ mode and also to enhance the electrical outputs. Herein, inspired by the mechanoionic conversion of natural articular cartilage, we realize a d₃₃ piezoionic hydrogel with multi-gradient structure and enhance its output by more than an order of magnitude. The geometry and modulus gradients are designed to amplify the deformation-induced convective ionic current, and the charge gradient is introduced to enlarge the cation-anion transfer rate difference. By synergizing with these three gradients, a multi-gradient piezoionic hydrogel exhibits significantly improved electrical outputs under uniform compression, achieving a d₃₃ coefficient of 27.9 μC N^-1. Then, piezoionic hydrogel arrays are fabricated for the demonstration of applications in self-powered electrostimulation-promoted wound healing. Therefore, we present general principles and practical materials-engineering approaches for enhancing piezoionic effect of hydrogels, which will greatly promote its future applications.