Organic Mixed Conductors in Electrochemical Transistors for Bioelectronic Applications

Organic semiconductors have emerged as promising materials for facilitating communication between electronic systems and biological entities due to their distinctive advantages, such as structural similarity to biological substances, biocompatibility, tailorability, and mechanical flexibility. Organic bioelectronics mainly focuses on developing devices capable of sensing biological substances and signals, as well as stimulating or regulating biological processes. This interdisciplinary field encompasses various applications, ranging from healthcare monitoring and diagnostics to neuroprosthetics and human–machine interfaces.

Among various organic devices, organic electrochemical transistors (OECTs) have gained significant attention in bioelectronics due to their effective coupling of electronic and ionic transports. OECTs utilize organic mixed ionic–electronic conductors (OMIECs) as ion-permeable channel materials, enabling ion doping throughout the entire channel. This unique volumetric doping gives OECTs ultrahigh transconductance at low working voltages, making them advantageous for highly sensitive biosensing and reliable recording of electrophysiological signals with enhanced signal-to-noise ratios. The properties of OMIECs play a crucial role in determining the device performance and the application scenarios, leading to considerable interest in recent decades.

Understanding the relationship between material figures of merit and specific applications is crucial for guiding material design and selection. This account focuses on the recent advances in OMIECs development for OECTs and highlights their impact on bioelectronic applications. First, we introduce the operation of OECTs, emphasizing the coupling of electronic and ionic circuits and the unique bulk doping mechanism that sets them apart from conventional field-effect transistors. Potential factors influencing transconductance and transient behavior are discussed. Then, we delve into the historical perspective on OMIECs development in OECTs, underscoring material design strategies that enable mixed conduction, including the introduction of glycolated side chains and the utilization of emerging 2D nanoporous structures. Subsequently, we explore the beneficial traits of OMIECs for bioelectronic applications. We discuss the strategies to harness the high transconductance originating from OMIECs for achieving high-performance biosensors and recording electrophysiological signals with superior signal-to-noise ratios. Additionally, we critically examine the latest strategies employed in the realization of stretchable, self-healing, and bioadhesive OMIECs. These innovative features have made significant contributions to wearable and implantable applications. The integration of stretchability ensures compatibility with the dynamic nature of biological entities, enabling robust and reliable performance. The self-healing capabilities of OMIECs exhibit a remarkable ability to autonomously repair damage or degradation, thereby prolonging the lifespan and functionality of bioelectronic devices. Moreover, the bioadhesive properties of OMIECs enable secure attachment to biological surfaces, establishing intimate contact for improved signal acquisition and stability. Finally, we discuss the challenges and opportunities in the further development of OMIECs in OECTs. This article provides an overview of recent advancements in OMIECs and their potential to revolutionize bioelectronic applications. With continuous innovation, OMIECs hold great promise for shaping the future of bioelectronics.