Ultrasoft Iontronics: Stretchable Diodes Enabled by Ionically Conductive Bottlebrush Elastomers

Abstract Inspired by the controlled ion migration found in biological systems, ionic diodes that regulate ion flow in iontronic systems have shown great potential for applications in human–machine interfaces, wearable, and implantable devices. However, developing biointegrable ionic diodes with mechanical compliance to biological tissues remains challenging due to the limited availability of ultrasoft materials. Although hydrogel‐based diodes can achieve ultrasoftness, they suffer from dehydration, resulting in instability in mechanical and electrical performance. Here, a solvent‐free, ultrasoft, and stretchable ionic diode enabled by oppositely charged bottlebrush elastomers (BBEs) is presented. The nanostructure of bottlebrush polymers allows the crosslinked BBE diode to achieve tissue‐matched softness. Meanwhile, copolymerized ionic liquids ensure stable ionic conductivity by preventing leaching and thermal evaporation. The BBE diode achieves an ultralow Young's modulus (<23 kPa), stretchability exceeding 400%, and a high rectification ratio of 46. To our knowledge, this is the softest ionic diode ever reported. Its functionality in ionic circuits is demonstrated, including full‐wave rectifiers and logic gates. Furthermore, it is integrated into self‐powered biointerfacing devices for strain sensing, evaluated through finger flexion, eye blinking, and stomach volume changes in an ex vivo model. These results highlight its potential for soft iontronic systems and next‐generation bioelectronic interfaces.