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Conducting polymers are promising materials for applications including bioelectronics and soft robotics, but little is known about how morphology affects mixed conduction. Here, the authors show how bulk ionic/electronic transport is affected by changes in nano- and meso-scale structure in PEDOT:PSS films.

Although organic transistors have many advantages, they are not typically known for their high performance. Khodagholy et al. report the fabrication of organic electrochemical transistors that combine high transconductance with mechanical flexibility, and are attractive for biosensor applications.

The detrimental effects of charge trapping in organic semiconductors can be minimized by diluting the electroactive polymer in an insulating host.

Electrically conductive hydrogels based on conducting polymers often rely on covalent and therefore irreversible crosslinking mechanisms. Here, the authors report a thermo-responsive conducting polymer that undergoes a fully reversible non-covalent crosslinking at 35 °C within less than a minute to form conductive hydrogels.

The recent demonstration that highly disordered polymer films can transport charges as effectively as polycrystalline semiconductors has called into question the relationship between structural order and mobility in organic materials. It is now shown that, in high-molecular-weight polymers, efficient charge transport is allowed due to a network of interconnected aggregates that are characterized by short-range order.

Cell seeding on scaffolds for regenerative tissue engineering presents a regulatory and manufacturing hurdle. Here, the authors report on the development of a cell free conductive scaffold which can support tissue regeneration without cell seeding, demonstrating application in bladder repair.

Organic electrochemical transistors (OECTs) function as a result of ion injection from electrolytes into organic semiconductors. In this Review, the authors discuss OECT physics, organic materials and fabrication technologies, and the application of OECTs in circuits, bioelectronics and memory devices.

Oxygen is the most limiting factor in cell transplantation. Here, the authors present an on-site oxygen production platform for implantable cell therapeutics via electrocatalytic water electrolysis, demonstrating the maintenance of high cell loading in hypoxic incubation and a rat model.

Interfacing living systems with electronics for biosensing and biocomputing applications is challenging. Here, Gao et al. present hybrid transistors with electroactive bacteria capable of extracellular electron transfer, enabling transduction of biological computations to electrical readouts.

Stable anti-ambipolar organic materials are limited, thus preventing the design of integrated, tunable, and multifunctional neuromorphic systems. Here, the authors report a small form factor neuromorphic circuit based on organic anti-ambipolar materials, mimicking the pre-processing functions of the retina.

The optimization of organic mixed ionic-electronic conductor is critical to realize high performance organic electrochemical transistors. Here, the authors demonstrate the removal of residual palladium impurities to be the key factor to achieving a figure-of-merit of [μC*] of over 2000 V⁻¹ cm⁻¹ s⁻¹.

The distinctive interdependence in mixed ionic-electronic conductors emulates retinal pathway. Here, the authors develop a modular organic neuromorphic spiking circuit to replicate the interdependent functions of receptors, neurons and synapses that are chemically modulated by neurotransmitters.

Understanding mesoscale structure and dynamics in organic mixed ionic–electronic conductors is crucial. Mesoscale strain kinetics and structural hysteresis have been studied, and they uncover the coupling between charge carrier dynamics and mesoscale order in organic mixed ionic–electronic conductors.

Micropatterning of organic semiconductors by electron-beam exposure can be used to create vertical organic electrochemical transistor arrays and complementary logic circuits with densities of up to 7.2 million transistors per cm².

Organic electrochemical transistors transduce ionic to electronic signals in aqueous solutions, holding promise for biological sensing applications. Here, Giovannitti et al. report an ambipolar organic electrochemical transistor, based on a conjugated copolymer, which has a high stability in water.

Electrochemical aptamer-based sensors typically exhibit limited sensitivity especially when the area of electrode is reduced for miniaturization. Here, the authors demonstrate electrochemical transistors as universal on-site amplifiers for enhancement in sensitivity of over 3 orders of magnitude.

Vertical organic electrochemical transistors demonstrating unprecedented performances in both p- and n-type operation modes have been synthesized from new electro-active and ion-permeable semiconducting polymers by the interface engineering of electro-active blend layers.

Organic transistors that can simulate basic synaptic functions and act as biomimetic devices are advantageous for next generation bioelectronics. Here, the authors realize non-volatile organic electrochemical transistors with optimized performance required for associative learning circuits.

By tracking the electrochromic doping front, a hole-limited electrochemical doping mechanism is discovered in organic mixed ionic–electronic conductors.

The lack of understanding of mixed transport in ion-permeable conjugated polymer films hinders the advance of organic electrochemical transistors for bioelectronics. Here, the authors elucidate the structure-property-performance relationships for conventional and crystallized PEDOT:PSS films.

Improving electron transport and stability of n-type organic electrochemical transistors (OECTs) is required to realize a commercially-viable technology for bioelectronics applications. Here, the authors report water-stable doped n-type OECTs with enhanced transconductance and record stability.

Grain boundaries are already known to have a large effect on the charge-carrier mobility of molecular semiconductors. Several experimental and computational techniques now show that the orientation of grain boundaries in a perylene diimide semiconductor modulates carrier mobility by two orders of magnitude. The results provide important guidelines for producing device-optimized molecular semiconductors.

Organic materials that support both electronic and ionic transport hold promise for applications in bioelectronics and energy storage. Here, Inal et al. use transistors to quantify the materials performance of organic mixed conductors in terms of the product of charge mobility and volumetric capacitance.

Biohybrid devices based on engineered cells interfaced with bioelectronics combine the strengths of each field, resulting in constructs with properties that are not otherwise achievable. In this Review, we discuss the enabling mechanisms of communication between these domains and describe the approaches and challenges in assembling and deploying such systems.

From optoelectronic to biomedical and energy storage applications, the interest in organic mixed ionic–electronic conductors is expanding. This Review describes current understanding of the processes occurring in these materials and their structure–property relations.