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Transistors, the foundation of modern electronics, are typically rigid, planar and 2D, which limits their integration with soft, irregular, 3D biological systems. This Perspective highlights the rise of hydrogel transistors in overcoming these challenges and how they provide an unprecedented opportunity for next-generation 3D, programmable and living bioelectronics.

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 enteric nervous system regulates gut function but is difficult to study due to sparse neuron distribution and gut motion. Here, the authors showcase a bioelectronic implant for electrophysiological monitoring of the enteric neural activity in freely-behaving animals.

Global regulation of synaptic strengths in neural systems is known as homeoplasticity. Here, Gkoupideniset al. use an electrolyte to connect and control an array of organic electrochemical devices, in order to demonstrate behaviour that resembles homeoplasticity phenomena in the brain.

Large area electrocorticography is limited by invasive craniotomies. Here, the authors combine soft robotic bioelectronics with origami inspired packaging to create a minimally invasive electrocorticography array.

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.

With the help of an orbital spinning technique, substrate-free open networks of imperceptible fibres can be created on a range of biological surfaces, providing on-skin sensors that can record electrocardiogram signals, skin-gated organic electrochemical transistors, and augmented touch and plant interfaces.

Nerve interfaces have the potential to deliver therapythroughout the body by monitoring and modulating function via the peripheral nervous system. Here, the authors present an ultraconformable cuff capable of high-resolution recording and stimulation of nerve activity in freely-moving conditions.

Current spread hampers the efficacy of neuromodulation, while existing animal, in vitro and in silico models have failed to give patient-centric insights. Here the authors employ 3D printing and machine learning to advance clinical predictions of current spread for cochlear implant patients.

Electrode arrays for neurological diagnosis and treatment carry a risk of nerve injury. Nerve cuffs with tiny voltage-controlled shape-reconfigurable electrode arrays have been reported, allowing active wrapping around delicate nerves.

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.

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.

Flexible organic electronic devices have the potential to serve as biosensors in living animals. Khodagholy et al. show that organic transistors can be used to record brain activity in rats and demonstrate that they have a superior signal-to-noise ratio compared with electrodes due to local signal amplification.

Quantum-dot-based infrared light-emitting diodes can achieve levels of brightness and efficiency that are competitive with state-of-the-art epitaxial devices by using linker molecules to control the distance between adjacent quantum dots.

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.

Electrochemical doping is assumed to be limited by ion motion due to large mass in mixed ionic-electronic conductors. Here, the authors reveal in a typical polythiophene that electrochemical doping speeds are limited by poor hole transport at low doping levels, leading to much slower switching speeds than expected.

Understanding charge-compensating interactions and ionic dynamics in organic mixed conductors can be challenging. Operando NMR spectroscopy is now used to quantify cation and water movement during doping/dedoping in mixed conductor films.

Large-area electrode arrays for epidermal electrophysiology offer new possibilities for the control of prosthetic devices and the monitoring of brain function.

The fabrication of thin-film poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) electrodes and electrochemical transistors via photolithography enables the reliable construction of mechanically compliant neural implants measuring less than ten micrometers in thickness.

In this technical report, Khodagholy and colleagues find that NeuroGrid, a planar, scalable and highly conformable electrode array, allows recordings of local-field potentials and stable single-unit activity from the surface of the rat cortex or hippocampus. The authors also validate NeuroGrid across species by showing that that it can capture LFP-modulated spiking activity intraoperatively in surgical patients, thus demonstrating its utility as tool for fundamental research on the human brain and in the clinic.

Electrolyte-gated transistors (EGTs) are fundamental building blocks of bioelectronics, which transduce biological inputs to electrical signals. This Primer examines the different architectures of EGTs, their mechanism of operation and practical considerations related to their wide range of applications.

This Review Article examines the development of organic neuromorphic devices, considering the different switching mechanisms used in the devices and the challenges the field faces in delivering neuromorphic computing applications.