Organic electrochemical transistors for neural recording and modulation: From mixed‐conduction physics to closed‐loop biointerfaces

Next‐generation neural interfaces increasingly aim to couple neural recording with local processing and adaptive modulation. Conventional electrode‐ and silicon‐based platforms have enabled major advances, but they still face limitations in mechanical biointegration, multimodal access, and compact low‐power front‐end operation. Organic electrochemical transistors (OECTs) offer a complementary route based on mixed ionic–electronic conduction, where physiological ionic signals modulate electronic currents through volumetric electrochemical coupling. This mechanism enables low‐voltage operation, intrinsic local amplification, and soft coupling to hydrated tissue, while also providing access to neurochemical/ionic sensing, temporal filtering, nonlinear thresholding, and event‐based encoding. These capabilities make OECTs attractive as front‐end building blocks for closed‐loop biointerfaces. However, the same ionic processes also introduce constraints in bandwidth, drift, state retention, and long‐term stability, requiring coordinated design across materials, device geometry, circuits, and biointerfaces. This review discusses OECTs from a device‐to‐system perspective. We summarize the mixed‐conduction mechanisms that determine transduction gain, temporal response, and ionic memory, then examine their translation into electrophysiological recording, deep‐tissue and soft interfaces, neurochemical/ionic sensing, scalable arrays, and near‐sensor processing. Finally, we discuss electrical and electrochemical modulation strategies and outline system‐level challenges for robust closed‐loop neural interfaces.