Electrocardiographic recording with conformable organic electrochemical transistor fabricated on resorbable bioscaffold

Performance of Organic Electrochemical Transistors with Ionic Liquid Crystal Elastomers as Solid Electrolytes

Organic electrochemical transistors (OECTs) have emerged as attractive devices for bioelectronics, wearable electronics, soft robotics, and energy storage devices. The electrolyte, being a fundamental component of OECTs, plays a crucial role in their performance. Recently, it has been demonstrated that ionic liquid crystal elastomers (iLCEs) can be used as a solid electrolyte for OECTs. Their capabilities, however, have only been shown for relatively large size substrate-free OECTs. Here, we study the influence of the different alignments of iLCEs on steady state and transient behavior of OECTs using a lateral geometry with source, drain, and gate in the same plane. We achieve excellent electrical response with an ON/OFF switching ratio of >105 and minimal leakage current. The normalized maximum transconductance gm/w of the most sensitive iLCE was found to be 33 S m-1, which is one of the highest among all solid-state-based OECTs reported so far. Additionally, iLCEs show high stability and can be removed and reattached multiple times to the same OECT device without decreasing performance.

Soft Bioelectronics for Heart Monitoring

Cardiovascular diseases (CVDs) are a predominant global health concern, accounting for over 17.9 million deaths in 2019, representing approximately 32% of all global fatalities. In North America and Europe, over a million adults undergo cardiac surgeries annually. Despite the benefits, such surgeries pose risks and require precise postsurgery monitoring. However, during the postdischarge period, where monitoring infrastructures are limited, continuous monitoring of vital signals is hindered. In this area, the introduction of implantable electronics is altering medical practices by enabling real-time and out-of-hospital monitoring of physiological signals and biological information postsurgery. The multimodal implantable bioelectronic platforms have the capability of continuous heart sensing and stimulation, in both postsurgery and out-of-hospital settings. Furthermore, with the emergence of machine learning algorithms into healthcare devices, next-generation implantables will benefit artificial intelligence (AI) and connectivity with skin-interfaced electronics to provide more precise and user-specific results. This Review outlines recent advancements in implantable bioelectronics and their utilization in cardiovascular health monitoring, highlighting their transformative deployment in sensing and stimulation to the heart toward reaching truly personalized healthcare platforms compatible with the Sustainable Development Goal 3.4 of the WHO 2030 observatory roadmap. This Review also discusses the challenges and future prospects of these devices.

Microscale Sensor Arrays for the Detection of Dopamine Using PEDOT:PSS Organic Electrochemical Transistors

We present a sensor array of microscale organic electrochemical transistors (OECTs) using poly (3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) as the channel material. The devices show high sensitivity and selectivity to detect dopamine (DA) with platinum (Pt) as a pseudo-reference gate electrode. First, we describe the wafer-scale fabrication process for manufacturing the PEDOT:PSS OECTs, and then we introduce a dilution method to adjust the thickness of the PEDOT:PSS film. Next, we investigate the effect of the film thickness on the sensitivity of DA detection. Reducing the film thickness enhances the sensitivity of DA detection within the concentration range of 1 μM to 100 μM. The OECTs show impressive sensitivitywith a limit of detection (LoD) as low as 1 nM and a high selectivity against uric acid (UA) and ascorbic acid (AA). Finally, we modify the surface of the Pt gate electrode with chitosan to improve the selectivity of OECTs at high concentrations of up to 100 µM to expand the detection range.

Highly Stable Flexible Organic Electrochemical Transistors with Natural Rubber Latex Additives

Organic electrochemical transistors (OECTs) have attracted considerable interest in the context of wearable and implantable biosensors due to their remarkable signal amplification combined with seamless integration into biological systems. These properties underlie OECTs' potential utility across a range of bioelectronic applications. One of the main challenges to their practical applications is the mechanical limitation of PEDOT:PSS, the most typical conductive polymer used as a channel layer, when the OECTs are applied to implantable and stretchable bioelectronics. In this work, we address this critical issue by employing natural rubber latex (NRL) as an additive in PEDOT:PSS to improve flexibility and stretchability of the OECT channels. Although the inclusion of NRL leads to a decrease in transconductance, mainly due to a reduced carrier mobility from 0.3 to 0.1 cm2/V·s, the OECTs maintain satisfactory transconductance, exceeding 5 mS. Furthermore, it is demonstrated that the OECTs exhibit excellent mechanical stability while maintaining their performance even after 100 repetitive bending cycles. This work, therefore, suggests that the NRL/PEDOT:PSS composite film can be deployed for wearable/implantable applications, where high mechanical stability is needed. This finding opens up new avenues for practical use of OECTs in more robust and versatile wearable and implantable biosensors.

OECT - Inspired electrical detection

Organic Electrochemical Transistors (OECTs) are integral in detecting human bioelectric signals, attributing their significance to distinct electrochemical properties, the utilization of soft materials, compact dimensions, and pronounced biocompatibility. This review traverses the technological evolution of OECT, highlighting its profound impact on non-invasive detection methodologies within the biomedicalfield. Four sensor types rooted in OECT technology were introduced: Electrocardiogram (ECG), Electroencephalogram (EEG), Electromyography (EMG), and Electrooculography (EOG), which hold promise for integration into wearable detection systems. The fundamental detection principles, material compositions, and functional attributes of these sensors are examined. Additionally, the performance metrics and delineates viable optimization strategies for assorted physiological electrical detection sensors are discussed. The overarching goal of this review is to foster deeper insights into the generation, propagation, and modulation of electrophysiological signals, thereby advancing the application and development of OECT in medical sciences.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Flexible Organic Transistors for Biosensing: Devices and Applications

Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible-OTFT-based biosensors and their broad applications, herein, a timely and comprehensive review is provided. It starts with a detailed introduction to the features of various OTFTs including organic field-effect transistors and organic electrochemical transistors, and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, the applications of flexible-OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces are detailed. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, the remaining challenges and the future opportunities in this field are summarized.

Healthcare Monitoring Sensors Based on Organic Transistors: Surface/Interface Strategy and Performance

Organic transistors possess inherent advantages such as flexibility, biocompatibility, customizable chemical structures, solution-processability, and amplifying capabilities, making them highly promising for portable healthcare sensor applications. Through convenient and diverse modifications at the material and device surfaces or interfaces, organic transistors allow for a wide range of sensor applications spanning from chemical and biological to physical sensing. In this comprehensive review, the surface and interface engineering aspect associated with four types of typical healthcare sensors is focused. The device operation principles and sensing mechanisms are systematically analyzed and highlighted, and particularly surface/interface functionalization strategies that contribute to the enhancement of sensing performance are focused. An outlook and perspective on the critical issues and challenges in the field of healthcare sensing using organic transistors are provided as well.

Organic Electronics in Biosensing: A Promising Frontier for Medical and Environmental Applications

The promising field of organic electronics has ushered in a new era of biosensing technology, thus offering a promising frontier for applications in both medical diagnostics and environmental monitoring. This review paper provides a comprehensive overview of organic electronics' remarkable progress and potential in biosensing applications. It explores the multifaceted aspects of organic materials and devices, thereby highlighting their unique advantages, such as flexibility, biocompatibility, and low-cost fabrication. The paper delves into the diverse range of biosensors enabled by organic electronics, including electrochemical, optical, piezoelectric, and thermal sensors, thus showcasing their versatility in detecting biomolecules, pathogens, and environmental pollutants. Furthermore, integrating organic biosensors into wearable devices and the Internet of Things (IoT) ecosystem is discussed, wherein they offer real-time, remote, and personalized monitoring solutions. The review also addresses the current challenges and future prospects of organic biosensing, thus emphasizing the potential for breakthroughs in personalized medicine, environmental sustainability, and the advancement of human health and well-being.

Phosphorylcholine-Functionalized PEDOT-Gated Organic Electrochemical Transistor Devices for Ultra-Specific and Sensitive C-Reactive Protein Detection

Affinity-based organic electrochemical transistor (OECT) sensors offer an attractive approach to point-of-care diagnostics due to their extreme sensitivity and easy operation; however, their application in the real world is frequently challenged by the poor storage stability of antibody proteins and the interference from biofouling in complex biofluids. In this work, we developed an antibody-free and antifouling OECT biosensor to detect C-reactive protein (CRP) at ultra-high specificity and sensitivity. The key to this novel biosensor is the gate coated by phosphorylcholine-functionalized poly (3,4-ethylene dioxythiophene) (PEDOT-PC), which possesses large capacitance and low impedance, prevents biofouling of bovine serum albumin (BSA) and the fetal bovine serum (FBS), and interacts specifically with CRP molecules in the presence of calcium ions. This PEDOT-PC-gated OECT biosensor demonstrated exceptional sensitivity when detecting the CRP molecules at 10 pg/mL, while significantly depressing the signal from the nonspecific binding. This indicates that this biosensor could detect the CRP molecules directly without nonspecific binding blocking, the usual process for the earlier transistor sensors before detection. We envision that this PEDOT-PC-gated OECT biosensor platform may offer a potentially valuable tool for point-of-care diagnostics as it alleviates concerns about poor antibody stability and BSA blocking inconstancy.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

Tunable Organic Active Neural Probe Enabling Near-Sensor Signal Processing

Neural recording systems have significantly progressed to provide an advanced understanding and treatment for neurological diseases. Flexible transistor-based active neural probes exhibit great potential in electrophysiology applications due to their intrinsic amplification capability and tissue-compliant nature. However, most current active neural probes exhibit bulky back-end connectivity since the output is current, and the development of an integrated circuit for voltage output is crucial for near-sensor signal processing at the abiotic/biotic interface. Here, inkjet-printed organic voltage amplifiers are presented by monolithically integrating organic electrochemical transistors and thin-film polymer resistors on a single, highly flexible substrate for in vivo brain activity recording. Additive inkjet printing enables the seamless integration of multiple active and passive components on the somatosensory cortex, leading to significant noise reduction over the externally connected typical configuration. It also facilitates fine-tuning of the voltage amplification and frequency properties. The organic voltage amplifiers are validated as electrocorticography devices in a rat in vivo model, showing their ability to record local field potentials in an experimental model of spontaneous and epileptiform activity. These results bring organic active neural probes to the forefront in applications where efficient sensory data processing is performed at sensor endpoints.

Organic electrochemical transistors printed from degradable materials as disposable biochemical sensors

Transient electronics hold promise in reducing electronic waste, especially in applications that require only a limited lifetime. While various degradable electronic and physical sensing devices have been proposed, there is growing interest in the development of degradable biochemical sensors. In this work, we present the development of an organic electrochemical transistor (OECT) with degradable electrodes, printed on an eco- and bioresorbable substrate. The influence of the design and materials for the contacts, channel and gate of the transducer, namely poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) and carbon, is systematically evaluated for the development of OECT-based transient biosensors. The sensing capabilities of the electrochemical transistors are demonstrated with ionic solutions as well as for the enzyme-based detection of glucose. The disposable OECTs show comparable performance to their non-degradable counterparts. Their integration with highly conductive degradable and printable zinc tracks is studied for the realization of interconnects. These eco-friendly OECTs may find applications as disposable and sustainable biochemical sensors, and constitute a step towards bioresorbable biosensors.

Bionic Tactile-Gustatory Receptor for Object Identification Based on All-Polymer Electrochemical Transistor

Human sensory receptors enable the real world to be perceived effortlessly. Hence, massive efforts have been devoted to the development of bionic receptors capable of identifying objects. Unfortunately, most of the existing devices are limited to single sensory emulation and are established on solid-state electronic technologies, which are incompatible with the biological reactions occurring in electrolyte media. Here, an iontronic tactile-gustatory receptor using an all-polymer electrochemical transistor (AECT) is presented. The sensor is biocompatible with the operation voltage of 0.1 V, which is 1 to 2 orders lower than those of reported values. By this study, one receptor is able to accurately recognize various objects perceived by the human tactile and gustatory system without complex circuitry. Additionally, to promote its further application, flexible AECT arrays with channel length of 2 µm and density of 104 167 transistors cm^-2 (yield of 97%) are fabricated, 1 to 5 orders higher than those of related works. Finally, a flexible integrated network for electrocardiogram recording is successfully constructed. This study moves a step forward toward state-of-the-art bionic sensors.

Wearable Electronics Based on Stretchable Organic Semiconductors

Wearable electronics are attracting increasing interest due to the emerging Internet of Things (IoT). Compared to their inorganic counterparts, stretchable organic semiconductors (SOSs) are promising candidates for wearable electronics due to their excellent properties, including light weight, stretchability, dissolubility, compatibility with flexible substrates, easy tuning of electrical properties, low cost, and low temperature solution processability for large-area printing. Considerable efforts have been dedicated to the fabrication of SOS-based wearable electronics and their potential applications in various areas, including chemical sensors, organic light emitting diodes (OLEDs), organic photodiodes (OPDs), and organic photovoltaics (OPVs), have been demonstrated. In this review, some recent advances of SOS-based wearable electronics based on the classification by device functionality and potential applications are presented. In addition, a conclusion and potential challenges for further development of SOS-based wearable electronics are also discussed.

Ultrathin, Soft, Bioresorbable Organic Electrochemical Transistors for Transient Spatiotemporal Mapping of Brain Activity

A critical challenge lies in the development of the next-generation neural interface, in mechanically tissue-compatible fashion, that offer accurate, transient recording electrophysiological (EP) information and autonomous degradation after stable operation. Here, an ultrathin, lightweight, soft and multichannel neural interface is presented based on organic-electrochemical-transistor-(OECT)-based network, with capabilities of continuous high-fidelity mapping of neural signals and biosafety active degrading after performing functions. Such platform yields a high spatiotemporal resolution of 1.42 ms and 20 µm, with signal-to-noise ratio up to ≈37 dB. The implantable OECT arrays can well establish stable functional neural interfaces, designed as fully biodegradable electronic platforms in vivo. Demonstrated applications of such OECT implants include real-time monitoring of electrical activities from the cortical surface of rats under various conditions (e.g., narcosis, epileptic seizure, and electric stimuli) and electrocorticography mapping from 100 channels. This technology offers general applicability in neural interfaces, with great potential utility in treatment/diagnosis of neurological disorders.

Organic Bioelectronics Development in Italy: A Review

In recent years, studies concerning Organic Bioelectronics have had a constant growth due to the interest in disciplines such as medicine, biology and food safety in connecting the digital world with the biological one. Specific interests can be found in organic neuromorphic devices and organic transistor sensors, which are rapidly growing due to their low cost, high sensitivity and biocompatibility. This trend is evident in the literature produced in Italy, which is full of breakthrough papers concerning organic transistors-based sensors and organic neuromorphic devices. Therefore, this review focuses on analyzing the Italian production in this field, its trend and possible future evolutions.

Self-Powered Biosensors for Monitoring Human Physiological Changes

Human physiological signals have an important role in the guidance of human health or exercise training and can usually be divided into physical signals (electrical signals, blood pressure, temperature, etc.) and chemical signals (saliva, blood, tears, sweat). With the development and upgrading of biosensors, many sensors for monitoring human signals have appeared. These sensors are characterized by softness and stretching and are self-powered. This article summarizes the progress in self-powered biosensors in the past five years. Most of these biosensors are used as nanogenerators and biofuel batteries to obtain energy. A nanogenerator is a kind of generator that collects energy at the nanoscale. Due to its characteristics, it is very suitable for bioenergy harvesting and sensing of the human body. With the development of biological sensing devices, the combination of nanogenerators and classical sensors so that they can more accurately monitor the physiological state of the human body and provide energy for biosensor devices has played a great role in long-range medical care and sports health. A biofuel cell has a small volume and good biocompatibility. It is a device in which electrochemical reactions convert chemical energy into electrical energy and is mostly used for monitoring chemical signals. This review analyzes different classifications of human signals and different forms of biosensors (implanted and wearable) and summarizes the sources of self-powered biosensor devices. Self-powered biosensor devices based on nanogenerators and biofuel cells are also summarized and presented. Finally, some representative applications of self-powered biosensors based on nanogenerators are introduced.

Tafel-Kinetics-Controlled High-Speed Switching in a Electrochemical Graphene Field-Effect Transistor

Graphene field-effect transistors (FETs) have attracted tremendous attention owing to the single-atomic-layer thickness and high electron mobility for potential applications in next-generation electronics. With regards to switching methodology, the electric-field-induced metal-insulator transition offers a new strategy to produce a large on/off current ratio through reversible electrochemical hydrogenation of the graphene channels. Therefore, the performance of such electrochemical graphene FETs greatly relies on the kinetics of hydrogenation reaction. Here, we show that the switching time can be systemically controlled by the applied gate voltages and geometries of graphene channels. The turn-on and turn-off time display an exponential dependence on the gate voltages, manifesting the dominated Tafel-form kinetics of hydrogenation reaction in a two-dimensional limit. Moreover, the turn-off time is inversely proportional to the channel width but independent of the length, while the turn-on time relies on both the width and length, as well as the off-state gate voltage and duration. Our work improves the response time to the magnitude of tens of microseconds and advances the application of graphene-based electronic devices.

In operando nanomechanical mapping of PEDOT:PSS thin films in electrolyte solutions with bimodal AFM

Understanding the interplay between the nanomechanical properties of organic electronic materials and their electronic properties is central to developing sensors and transducers for applications ranging from immunosensing to e-skin. Controlling organic device operations in aqueous electrolyte solutions and their mechanical compliance with the host tissue or living systems, as for instance in active implants for the recording or stimulation of neural signals, is still largely unexplored. Here, we implemented bimodal AFM to map the nanomechanical and structural properties of thin films made from poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), the most widely used conducting polymer blend, during operation as a microelectrode in an electrolyte solution. Nanomechanical maps showed the film consisting of a granular structure made from PEDOT:PSS regions embedded in the PSS matrix. The film swelled upon immersion in an aqueous solution. In operando bimodal AFM data obtained by applying a sequence of doping/de-doping bias cycles showed a significant decrease in the modulus (70%) that saturated after about 10 cycles. A similar sequence of biases at the opposite polarity did not significantly influence the mechanical behaviour of PEDOT:PSS. The decrease in the modulus was explained by the development of persistent hydration, which was enhanced by the cations trapped inside the organic electronic material.

Thermodynamics of organic electrochemical transistors

Despite their increasing usefulness in a wide variety of applications, organic electrochemical transistors still lack a comprehensive and unifying physical framework able to describe the current-voltage characteristics and the polymer/electrolyte interactions simultaneously. Building upon thermodynamic axioms, we present a quantitative analysis of the operation of organic electrochemical transistors. We reveal that the entropy of mixing is the main driving force behind the redox mechanism that rules the transfer properties of such devices in electrolytic environments. In the light of these findings, we show that traditional models used for organic electrochemical transistors, based on the theory of field-effect transistors, fall short as they treat the active material as a simple capacitor while ignoring the material properties and energetic interactions. Finally, by analyzing a large spectrum of solvents and device regimes, we quantify the entropic and enthalpic contributions and put forward an approach for targeted material design and device applications.

Though models describing the operating mechanism of organic electrochemical transistors (OECTs) have been developed, these models are unable to accurately reproduce OECT electrical characteristics. Here, the authors report a thermodynamic-based framework that accurately models OECT operation.

Cyano-Functionalized n-Type Polymer with High Electron Mobility for High-Performance Organic Electrochemical Transistors

n-Type organic mixed ionic-electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop. As a result, the figure-of-merit (µC*) of n-type organic electrochemical transistors (OECTs) lags far behind the p-type analogs, restraining the development of OECT-based low-power complementary circuits and biosensors. Here, two n-type donor-acceptor (D-A) polymers based on fused bithiophene imide dimer f-BTI2 as the acceptor unit and thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. The cyanation of TVT enables polymer f-BTI2g-TVTCN with simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property. As a result, it is able to obtain a high volumetric capacitance (C*) of 170 ± 22 F cm^-3 and a record OECT electron mobility (μ_e,OECT ) of 0.24 cm² V^-1 s^-1 for f-BTI2g-TVTCN, subsequently achieving a state-of-the-art µC* of 41.3 F cm^-1 V^-1 s^-1 and geometry-normalized transconductance (g_m,norm ) of 12.8 S cm^-1 in n-type accumulation-mode OECTs. In contrast, only a moderate µC* of 1.50 F cm^-1 V^-1 s^-1 is measured for the non-cyanated polymer f-BTI2g-TVT. These remarkable results demonstrate the great power of cyano functionalization of polymer semiconductors in developing n-type OMIECs with substantial electron mobility in aqueous environment for high-performance n-type OECTs.

Highly Precise, Continuous, Long-Term Monitoring of Skin Electrical Resistance by Nanomesh Electrodes

The transepidermal water loss has been widely used as a method for directly evaluating the barrier function of the stratum corneum of the skin. However, transepidermal water loss could not be measured continuously for a long period of time, and there were no reports of continuous monitoring of skin barrier functions. Here, a method is reported to continuously monitor the skin electrical resistance by nanomesh electrodes for a long period of time while maintaining the natural skin condition that does not inhibit water evaporation. Simultaneous measurements of the skin electrical resistance by nanomesh electrodes and transepidermal water loss exhibits a linear fit with a high negative correlation. Furthermore, dynamics of skin physiological functions are successfully visualized by monitoring of the skin electrical resistance by nanomesh electrodes for 30 h in daily life.

Poly(3,4-ethylenedioxythiophene)-Based Neural Interfaces for Recording and Stimulation: Fundamental Aspects and In Vivo Applications

Next-generation neural interfaces for bidirectional communication with the central nervous system aim to achieve the intimate integration with the neural tissue with minimal neuroinflammatory response, high spatio-temporal resolution, very high sensitivity, and readout stability. The design and manufacturing of devices for low power/low noise neural recording and safe and energy-efficient stimulation that are, at the same time, conformable to the brain, with matched mechanical properties and biocompatibility, is a convergence area of research where neuroscientists, materials scientists, and nanotechnologists operate synergically. The biotic-abiotic neural interface, however, remains a formidable challenge that prompts for new materials platforms and innovation in device layouts. Conductive polymers (CP) are attractive materials to be interfaced with the neural tissue and to be used as sensing/stimulating electrodes because of their mixed ionic-electronic conductivity, their low contact impedance, high charge storage capacitance, chemical versatility, and biocompatibility. This manuscript reviews the state-of-the-art of poly(3,4-ethylenedioxythiophene)-based neural interfaces for extracellular recording and stimulation, focusing on those technological approaches that are successfully demonstrated in vivo. The aim is to highlight the most reliable and ready-for-clinical-use solutions, in terms of materials technology and recording performance, other than spot major limitations and identify future trends in this field.

Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic-Electronic Conductors for Organic Electrochemical Transistors

Expanding the toolbox of the biology and electronics mutual conjunction is a primary aim of bioelectronics. The organic electrochemical transistor (OECT) has undeniably become a predominant device for mixed conduction materials, offering impressive transconduction properties alongside a relatively simple device architecture. In this review, we focus on the discussion of recent material developments in the area of mixed conductors for bioelectronic applications by means of thorough structure-property investigation and analysis of current challenges. Fundamental operation principles of the OECT are revisited, and characterization methods are highlighted. Current bioelectronic applications of organic mixed ionic-electronic conductors (OMIECs) are underlined. Challenges in the performance and operational stability of OECT channel materials as well as potential strategies for mitigating them, are discussed. This is further expanded to sketch a synopsis of the history of mixed conduction materials for both p- and n-type channel operation, detailing the synthetic challenges and milestones which have been overcome to frequently produce higher performing OECT devices. The cumulative work of multiple research groups is summarized, and synthetic design strategies are extracted to present a series of design principles that can be utilized to drive figure-of-merit performance values even further for future OMIEC materials.

Organic Electrochemical Transistors for In Vivo Bioelectronics

Organic electrochemical transistors (OECTs) are presently a focus of intense research and hold great potential in expanding the horizons of the bioelectronics industry. The notable characteristics of OECTs, including their electrolyte-gating, which offers intimate interfacing with biological environments, and aqueous stability, make them particularly suitable to be operated within a living organism (in vivo). Unlike the existing in vivo bioelectronic devices, mostly based on rigid metal electrodes, OECTs form a soft mechanical contact with the biological milieu and ensure a high signal-to-noise ratio because of their powerful amplification capability. Such features make OECTs particularly desirable for a wide range of in vivo applications, including electrophysiological recordings, neuron stimulation, and neurotransmitter detection, and regulation of plant processes in vivo. In this review, a systematic compilation of the in vivo applications is presented that are addressed by the OECT technology. First, the operating mechanisms, and the device design and materials design principles of OECTs are examined, and then multiple examples are provided from the literature while identifying the unique device properties that enable the application progress. Finally, one critically looks at the future of the OECT technology for in vivo bioelectronic applications.

Fused Bithiophene Imide Dimer-Based n-Type Polymers for High-Performance Organic Electrochemical Transistors

The development of n-type organic electrochemical transistors (OECTs) lags far behind their p-type counterparts. In order to address this dilemma, we report here two new fused bithiophene imide dimer (f-BTI2)-based n-type polymers with a branched methyl end-capped glycol side chain, which exhibit good solubility, low-lying LUMO energy levels, favorable polymer chain orientation, and efficient ion transport property, thus yielding a remarkable OECT electron mobility (μ_e ) of up to ≈10^-2  cm²  V^-1  s^-1 and volumetric capacitance (C*) as high as 443 F cm^-3 , simultaneously. As a result, the f-BTI2TEG-FT-based OECTs deliver a record-high maximum geometry-normalized transconductance of 4.60 S cm^-1 and a maximum μC* product of 15.2 F cm^-1  V^-1  s^-1 . The μC* figure of merit is more than one order of magnitude higher than that of the state-of-the-art n-type OECTs. The emergence of f-BTI2TEG-FT brings a new paradigm for developing high-performance n-type polymers for low-power OECT applications.

Tuning Organic Electrochemical Transistor (OECT) Transconductance toward Zero Gate Voltage in the Faradaic Mode

In this work, we investigate material design criteria for low-powered/self-powered and efficient organic electrochemical transistors (OECTs) to be operated in the faradaic mode (detection at the gate electrode occurs via electron transfer events). To rationalize device design principles, we adopt a Marcus-Gerischer perspective for electrochemical processes at both the gate and channel interfaces. This perspective considers density of states (DOS) for the semiconductor channel, the gate electrode, and the electrolyte. We complement our approach with energy band offsets of relevant electrochemical potentials that can be independently measured from transistor geometry using conventional electrochemical methods as well as an approach to measure electrolyte potential in an operating OECT. By systematically changing the relative redox property offsets between the redox-active electrolyte and semiconducting polymer channel, we demonstrate a first-order design principle that necessary gate voltage is minimized by good DOS overlap of the two redox processes at the gate and channel. Specifically, for p-type turn-on OECTs, the voltage-dependent, electrochemically active semiconductor DOS should overlap with the oxidant form of the electrolyte to minimize the onset voltage for transconductance. A special case where the electrolyte can be used to spontaneously dope the polymer via charge transfer is also considered. Collectively, our results provide material design pathways toward the development of simple, robust, power-saving, and high-throughput OECT biosensors.

Electrolyte-gated transistors for enhanced performance bioelectronics

Electrolyte-gated transistors (EGTs), capable of transducing biological and biochemical inputs into amplified electronic signals and stably operating in aqueous environments, have emerged as fundamental building blocks in bioelectronics. In this Primer, the different EGT architectures are described with the fundamental mechanisms underpinning their functional operation, providing insight into key experiments including necessary data analysis and validation. Several organic and inorganic materials used in the EGT structures and the different fabrication approaches for an optimal experimental design are presented and compared. The functional bio-layers and/or biosystems integrated into or interfaced to EGTs, including self-organization and self-assembly strategies, are reviewed. Relevant and promising applications are discussed, including two-dimensional and three-dimensional cell monitoring, ultra-sensitive biosensors, electrophysiology, synaptic and neuromorphic bio-interfaces, prosthetics and robotics. Advantages, limitations and possible optimizations are also surveyed. Finally, current issues and future directions for further developments and applications are discussed.

Current-Driven Organic Electrochemical Transistors for Monitoring Cell Layer Integrity with Enhanced Sensitivity

In this progress report an overview is given on the use of the organic electrochemical transistor (OECT) as a biosensor for impedance sensing of cell layers. The transient OECT current can be used to detect changes in the impedance of the cell layer, as shown by Jimison et al. To circumvent the application of a high gate bias and preventing electrolysis of the electrolyte, in case of small impedance variations, an alternative measuring technique based on an OECT in a current-driven configuration is developed. The ion-sensitivity is larger than 1200 mV V-1 dec-1 at low operating voltage. It can be even further enhanced using an OECT based complementary amplifier, which consists of a p-type and an n-type OECT connected in series, as known from digital electronics. The monitoring of cell layer integrity and irreversible disruption of barrier function with the current-driven OECT is demonstrated for an epithelial Caco-2 cell layer, showing the enhanced ion-sensitivity as compared to the standard OECT configuration. As a state-of-the-art application of the current-driven OECT, the in situ monitoring of reversible tight junction modulation under the effect of drug additives, like poly-l-lysine, is discussed. This shows its potential for in vitro and even in vivo toxicological and drug delivery studies.

Sensing Inflammation Biomarkers with Electrolyte-Gated Organic Electronic Transistors

An overview of cytokine biosensing is provided, with a focus on the opportunities provided by organic electronic platforms for monitoring these inflammation biomarkers which manifest at ultralow concentration levels in physiopathological conditions. Specifically, two of the field's state-of-the-art technologies-organic electrochemical transistors (OECTs) and electrolyte gated organic field effect transistors (EGOFETs)-and their use in sensing cytokines and other proteins associated with inflammation are a particular focus. The overview will include an introduction to current clinical and "gold standard" quantification techniques and their limitations in terms of cost, time, and required infrastructure. A critical review of recent progress with OECT- and EGOFET-based protein biosensors is presented, alongside a discussion onthe future of these technologies in the years and decades ahead. This is especially timely as the world grapples with limited healthcare diagnostics during the Coronavirus disease (COVID-19)pandemic where one of the worst-case scenarios for patients is the "cytokine storm." Clearly, low-cost point-of-care technologies provided by OECTs and EGOFETs can ease the global burden on healthcare systems and support professionals by providing unprecedented wealth of data that can help to monitor disease progression in real time.

Flexible Electrodes for In Vivo and In Vitro Electrophysiological Signal Recording

A variety of electrophysiological signals (electrocardiography, electromyography, electroencephalography, etc.) are generated during the physiological activities of human bodies, which can be collected by electrodes and thus provide critical insights into health status or facilitate fundamental scientific research. The long-term stable and high-quality recording of electrophysiological signals is the premise for their further applications, leading to demands for flexible electrodes with similar mechanical modulus and minimized irritation to human bodies. This review summarizes the latest advances in flexible electrodes for the acquisition of various electrophysiological signals. First, the concept of electrophysiological signals and the characteristics of different subcategory signals are introduced. Second, the invasive and noninvasive methods are reviewed for electrophysiological signal recording with a highlight on the design of flexible electrodes, followed by a discussion on their material selection. Subsequently, the applications of the electrophysiological signal acquisition in pathological diagnosis and restoration of body functions are discussed, showing the advantages of flexible electrodes. Finally, the main challenges and opportunities in this field are discussed. It is believed that the further exploration of materials for flexible electrodes and the combination of multidisciplinary technologies will boost the applications of flexible electrodes for medical diagnosis and human-machine interface.

Decoding Electrophysiological Signals with Organic Electrochemical Transistors

The organic electrochemical transistor (OECT) has unique characteristics that distinguish it from other transistors and make it a promising electronic transducer of biological events. High transconductance, flexibility, and biocompatibility render OECTs ideal for detecting electrophysiological signals. Device properties such as transconductance, response time, and noise level should, however, be optimized to adapt to the needs of various application environments including in vitro cell culture, human skin, and inside of a living system. This review includes an overview of the origin of electrophysiological signals, the working principles of OECTs, and methods for performance optimization. While covering recent research examples of the use of OECTs in electrophysiology, a perspective is provided for next-generation bioelectric sensors and amplifiers for electrophysiology applications.

Reservoir computing with biocompatible organic electrochemical networks for brain-inspired biosignal classification

Early detection of malign patterns in patients' biological signals can save millions of lives. Despite the steady improvement of artificial intelligence-based techniques, the practical clinical application of these methods is mostly constrained to an offline evaluation of the patients' data. Previous studies have identified organic electrochemical devices as ideal candidates for biosignal monitoring. However, their use for pattern recognition in real time was never demonstrated. Here, we produce and characterize brain-inspired networks composed of organic electrochemical transistors and use them for time-series predictions and classification tasks using the reservoir computing approach. To show their potential use for biofluid monitoring and biosignal analysis, we classify four classes of arrhythmic heartbeats with an accuracy of 88%. The results of this study introduce a previously unexplored paradigm for biocompatible computational platforms and may enable development of ultralow-power consumption hardware-based artificial neural networks capable of interacting with body fluids and biological tissues.

Integrating Emerging Polymer Chemistries for the Advancement of Recyclable, Biodegradable, and Biocompatible Electronics

Through advances in molecular design, understanding of processing parameters, and development of non-traditional device fabrication techniques, the field of wearable and implantable skin-inspired devices is rapidly growing interest in the consumer market. Like previous technological advances, economic growth and efficiency is anticipated, as these devices will enable an augmented level of interaction between humans and the environment. However, the parallel growing electronic waste that is yet to be addressed has already left an adverse impact on the environment and human health. Looking forward, it is imperative to develop both human- and environmentally-friendly electronics, which are contingent on emerging recyclable, biodegradable, and biocompatible polymer technologies. This review provides definitions for recyclable, biodegradable, and biocompatible polymers based on reported literature, an overview of the analytical techniques used to characterize mechanical and chemical property changes, and standard policies for real-life applications. Then, various strategies in designing the next-generation of polymers to be recyclable, biodegradable, or biocompatible with enhanced functionalities relative to traditional or commercial polymers are discussed. Finally, electronics that exhibit an element of recyclability, biodegradability, or biocompatibility with new molecular design are highlighted with the anticipation of integrating emerging polymer chemistries into future electronic devices.

Multifunctional and Ultrathin Electronic Tattoo for On-Skin Diagnostic and Therapeutic Applications

Epidermal electronic systems for detecting electrophysiological signals, sensing, therapy, and drug delivery are at the frontier in man-machine interfacing for healthcare. However, it is still a challenge to develop multifunctional bioapplications with minimal invasiveness, biocompatibility, and stable electrical performance under various mechanical deformations of biological tissues. In this study, a natural silk protein with carbon nanotubes (CNTs) is utilized to realize an epidermal electronic tattoo (E-tattoo) system for multifunctional applications that address these challenging issues through dispersing highly conductive CNTs onto the biocompatible silk nanofibrous networks with porous nature to construct skin-adhesive ultrathin electronic patches. Individual components that incorporate electrically and optically active heaters, a temperature sensor (temperature coefficient of resistance of 5.2 × 10-3  °C-1 ), a stimulator for drug delivery (>500 µm penetration depth in skin), and real-time electrophysiological signal detectors are described. This strategy of E-tattoos integrated onto human skin can open a new route to a next-generation electronic platform for wearable and epidermal bioapplications.

In Vitro Models for Studying Respiratory Host-Pathogen Interactions

Respiratory diseases and lower respiratory tract infections are among the leading cause of death worldwide and, especially given the recent severe acute respiratory syndrome coronavirus-2 pandemic, are of high and prevalent socio-economic importance. In vitro models, which accurately represent the lung microenvironment, are of increasing significance given the ethical concerns around animal work and the lack of translation to human disease, as well as the lengthy time to market and the attrition rates associated with clinical trials. This review gives an overview of the biological and immunological components involved in regulating the respiratory epithelium system in health, disease, and infection. The evolution from 2D to 3D cell biology and to more advanced technological integrated models for studying respiratory host-pathogen interactions are reviewed and provide a reference point for understanding the in vitro modeling requirements. Finally, the current limitations and future perspectives for advancing this field are presented.

Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance

Donor-acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimize detrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts. We report three D-A polymers based on the diketopyrrolopyrrole unit that afford OECT performances similar to those of all-donor polymers, hence representing a significant improvement to the previously developed D-A copolymers. In addition to improved OECT performance, DFT simulations of the polymers and their respective hole polarons also reveal a positive correlation between hole polaron delocalization and steady-state OECT performance, providing new insights into the design of OECT materials. Importantly, we demonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers' conjugated backbone, thus paving the way for the development of even higher performing OECT polymers.

Organic Electrochemical Transistors (OECTs) Toward Flexible and Wearable Bioelectronics

Organic electronics have emerged as a fascinating area of research and technology in the past two decades and are anticipated to replace classic inorganic semiconductors in many applications. Research on organic light-emitting diodes, organic photovoltaics, and organic thin-film transistors is already in an advanced stage, and the derived devices are commercially available. A more recent case is the organic electrochemical transistors (OECTs), whose core component is a conductive polymer in contact with ions and solvent molecules of an electrolyte, thus allowing it to simultaneously regulate electron and ion transport. OECTs are very effective in ion-to-electron transduction and sensor signal amplification. The use of synthetically tunable, biocompatible, and depositable organic materials in OECTs makes them specially interesting for biological applications and printable devices. In this review, we provide an overview of the history of OECTs, their physical characterization, and their operation mechanism. We analyze OECT performance improvements obtained by geometry design and active material selection (i.e., conductive polymers and small molecules) and conclude with their broad range of applications from biological sensors to wearable devices.

Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability

A series of glycolated polythiophenes for use in organic electrochemical transistors (OECTs) is designed and synthesized, differing in the distribution of their ethylene glycol chains that are tethered to the conjugated backbone. While side chain redistribution does not have a significant impact on the optoelectronic properties of the polymers, this molecular engineering strategy strongly impacts the water uptake achieved in the polymers. By careful optimization of the water uptake in the polymer films, OECTs with unprecedented steady-state performances in terms of [μC^* ] and current retentions up to 98% over 700 electrochemical switching cycles are developed.

High-Resolution Electrochemical Transistors Defined by Mold-Guided Drying of PEDOT:PSS Liquid Suspension

Ion-sensitive transistors with nanoscale or microscale dimensions are promising for high-resolution electrophysiological recording and sensing. Technology that can pattern polymer functional materials directly from a solution can effectively avoid any chemical damage induced by conventional lithography techniques. The application of a mold-guided drying technique to pattern PEDOT:PSS-based transistors with high resolution directly from the water-based suspension is presented in this paper. Gold electrodes with short channels were first defined by creating high-resolution polymer lines with mold-guided drying followed by pattern transfer through a lift-off process. Then, PEDOT:PSS lines were subsequently created through an identical mold-guided drying process on the predefined electrodes. Small-scale transistor devices with both shortened channel length and width exhibited a good high-frequency response because of the weak capacitive effect. This is particularly advantageous for electrochemical transistors since the use of conventional fabrication techniques is extremely challenging in this case. In addition, modified polymer chain alignment of the assembled PEDOT:PSS lines during the drying process was observed by optical and electrical measurement. The mold-guided drying technique has been proven to be a promising method to fabricate small-scale devices, especially for biological applications.

Recent Progress in Enhancing Poly(Lactic Acid) Stereocomplex Formation for Material Property Improvement

The production and utilization of polymers have been widely implemented into diverse applications that benefit modern human society, but one of the most valuable properties of polymers, durability, has posed a long-standing environmental challenge from its inception since plastic waste can lead to significant contamination and remains in landfills and oceans for at least hundreds of years. Poly(lactic acid) (PLA) derived from renewable resources provides a sustainable alternative to traditional polymers due to its advantages of comparable mechanical properties with common plastics and biodegradability. However, the poor thermal and hydrolytic stability of PLA-based materials limit their potential for durable applications. Stereocomplex crystallization of enantiomeric poly (l-lactide) (PLLA) and poly (d-lactide) (PDLA) provides a robust approach to significantly enhance material properties such as stability and biocompatibility through strong intermolecular interactions between L-lactyl and D-lactyl units, which has been the key strategy to further PLA applications. This review focuses on discussing recent progress in the development of processing strategies for enhancing the formation of stereocomplexes within PLA materials, including thermal processing, additive manufacturing, and solution casting. The mechanism for enhancing SC formation and resulting material property improvement enabled by each method are also discussed. Finally, we also provide the perspectives on current challenges and opportunities for improving the understanding of processing-structure-property relationship in PLA materials that could be beneficial to their wide practical applications for a sustainable society.

Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Ion Exchange Gels Allow Organic Electrochemical Transistor Operation with Hydrophobic Polymers in Aqueous Solution

Conjugated-polymer-based organic electrochemical transistors (OECTs) are being studied for applications ranging from biochemical sensing to neural interfaces. While new polymers that interface digital electronics with the aqueous chemistry of life are being developed, the majority of high-performance organic transistor materials are poor at transporting biologically relevant ions. Here, the operating mode of an organic transistor is changed from that of an electrolyte-gated organic field-effect transistor (EGOFET) to that of an OECT by incorporating an ion exchange gel between the active layer and the aqueous electrolyte. This device works by taking up biologically relevant ions from solution and injecting more hydrophobic ions into the active layer. Using poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] as the active layer and a blend of an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and poly(vinylidene fluoride-co-hexafluoropropylene) as the ion exchange gel, four orders of magnitude improvement in device transconductance and a 100-fold increase in kinetics are demonstrated. The ability of the ion-exchange-gel OECT to record biological signals by measuring the action potentials of a Venus flytrap is demonstrated. These results show the possibility of using interface engineering to open up a wider palette of organic semiconductors as OECTs that can be gated by aqueous solutions.