The organic electrochemical transistor (OECT) is a device capable of simultaneously controlling the flow of electronic and ionic currents. This unique feature renders the OECT the perfect technology to interface man-made electronics, where signals are conveyed by electrons, with the world of the living, where information exchange relies on chemical signals. The function of the OECT is controlled by the properties of its core component, an organic conductor. Its chemical structure and interactions with electrolyte molecules at the nanoscale play a key role in regulating OECT operation and performance. Herein, the latest research progress in the design of active materials for OECTs is reviewed. Particular focus is given on the conducting polymers whose properties lead to advances in understanding the OECT working mechanism and improving the interface with biological systems for bioelectronics. The methods and device models that are developed to elucidate key relations between the structure of conducting polymer films and OECT function are discussed. Finally, the requirements of OECT design for in vivo applications are briefly outlined. The outcomes represent an important step toward the integration of organic electronic components with biological systems to record and modulate their functions.

The field of organic bioelectronics is advancing rapidly in the development of materials and devices to precisely monitor and control biological signals. Electronics and biology can interact on multiple levels: organs, complex tissues, cells, cell membranes, proteins, and even small molecules. Compared to traditional electronic materials like metals and inorganic semiconductors, conjugated polymers (CPs) have several key advantages for biological interactions; tunable physiochemical properties, adjustable form factors, and mixed conductivity (ionic and electronic). This review focuses on the use of CPs in five biologically oriented research topics: electrophysiology, tissue engineering, drug release, biosensing, and molecular bioelectronics. In electrophysiology, implantable devices with CP coating or CPonly electrodes are showing improvements in signal performance and tissue interfaces. CPbased scaffolds supply highly favorable static or even dynamic interfaces for tissue engineering.CPs also allow for delivery of drugs through a variety of mechanisms and form factors. For biosensing, CPs offer new possibilities to incorporate biological sensing elements in a conducting matrix. Molecular bioelectronics is today used to incorporate (opto)electronic functions in living tissue. Under each topic, limits of the utility of CPs are discussed and, overall, the review highlights the major challenges towards implementation of CPs and their devices to real-world applications.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))