Toward Complementary Ionic Circuits: The npn Ion Bipolar Junction Transistor

In-Situ Gas Permeation-Driven Ionic Current Rectification of Heterogeneously Charged Nanopore Arrays

Ionic diodes provide ionic current rectification (ICR), which is useful for micro-/nanofluidic devices for ionic current-mediated applications. However, the modulation of ICR is not fully developed, and current challenges include limited active control and localized modulation for further multiplexing of micro-/nanofluidic ionic diodes. Herein, a microfluidic device integrated with particle-assembly-based ionic diodes (PAIDs) and a gas-flow channel above them is presented. Exploiting in-situ gas permeation through a polymeric film, precise control over the physiochemical conditions of the nanopores within the PAIDs, leading to the modulation of ICR is demonstrated. The investigation not only characterizes the rectification properties of the PAIDs but also unveils their capacitor-like behavior and the ability to actively modulate ICR using various gas flows. Furthermore, the reversible modulation of ICR through dynamic switching of gas-dissolved solutions, enabling ion-signal amplification is showcased. This pioneering approach of in situ gas-permeation offers programmable manipulation of ion transport along PAIDs, thereby positioning ionic diodes as versatile nanofluidic components. Looking ahead, the development of multiplexed PAIDs in an addressable manner on a chip holds promise for practical applications across diverse fields, including ion signaling, ion-based logic, chemical reactors, and (bio)chemical sensing.

Study on the Rectification of Ionic Diode Based on Cross-Linked Nanocellulose Bipolar Membranes

Nanocellulose-based membranes have attracted intense attention in bioelectronic devices due to their low cost, flexibility, biocompatibility, degradability, and sustainability. Herein, we demonstrate a flexible ionic diode using a cross-linked bipolar membrane fabricated from positively and negatively charged cellulose nanofibrils (CNFs). The rectified current originates from the asymmetric charge distribution, which can selectively determine the direction of ion transport inside the bipolar membrane. The mechanism of rectification was demonstrated by electrochemical impedance spectroscopy with voltage biases. The rectifying behavior of this kind of ionic diode was studied by using linear sweep voltammetry to obtain current-voltage characteristics and the time dependence of the current. In addition, the performance of cross-linked CNF diodes was investigated while changing parameters such as the thickness of the bipolar membranes, the scanning voltage range, and the scanning rate. A good long-term stability due to the high density cross-linking of the diode was shown in both current-voltage characteristics and the time dependence of current.

Ionic Transistors

Biological voltage-gated ion channels, which behave as life's transistors, regulate ion transport precisely and selectively through atomic-scale selectivity filters to sustain important life activities. By this inspiration, voltage-adaptable ionic transistors that use ions as signal carriers may provide an alternative information processing unit beyond solid-state electronic devices. This review provides a comprehensive overview of the first generation of biomimetic ionic transistors, including their operating mechanisms, device architecture development, and property characterizations. Despite its infancy, significant progress has been made in the applications of ionic transistors in fields such as DNA detection, drug delivery, and ionic circuits. Challenges and prospects of full exploitation of ionic transistors for a broad spectrum of practical applications are also discussed.

Iontronic memories based on ionic redox systems: operation protocols

A recently developed, new ionic device called the ionic voltage effect soft triode (IVEST) was optimized, tuned and embedded into a memory application concept. The device is an electrochemical micro-cell, consisting of a top electrode and two bottom electrodes. The device controls the concentration and diffusion of ions via the voltage applied on the top electrode. The device showed a memory effect lasting up to 6 hours. Despite the remarkably large stability time, the memory contrast was small in the first device versions. Now, we have increased the memory contrast by introducing a new external electrical circuit layout combined with a new operation protocol. This new investigation also reveals peculiarities of the memory and shows that the IVEST can be used in memory applications. These iontronic memories show a secondary information storage connected with the read-out frequency.

An Aqueous Analog MAC Machine

Using ions in aqueous milieu for signal processing, like in biological circuits, may potentially lead to a bioinspired information processing platform. Studies, however, have focused on individual ionic diodes and transistors rather than circuits comprising many such devices. Here a 16 × 16 array of new ionic transistors is developed in an aqueous quinone solution. Each transistor features a concentric ring electrode pair with a disk electrode at the center. The electrochemistry of these electrodes in the solution provides the basis for the transistor operation. The ring pair electrochemically tunes the local electrolytic concentration to modulate the disk's Faradaic reaction rate. Thus, the disk current as a Faradaic reaction to the disk voltage is gated by the ring pair. The 16 × 16 array of these transistors performs analog multiply-accumulate (MAC) operations, a computing modality hotly pursued for low-power artificial neural networks. This exploits the transistor's operating regime where the disk current is a multiplication of the disk voltage and a weight parameter tuned by the ring pair gating. Such disk currents from multiple transistors are summated in a global reference electrode to complete a MAC task. This ionic circuit demonstrating analog computing is a step toward sophisticated aqueous ionics.

Ionic transistor using ion exchange membranes

Ionic transistors can be used to modulate ionic current in a way that is analogous to their electronic counterparts. An ionic transistor can reversibly change its ionic conduction to control ionic current by injecting electrical charges. To facilitate its applications in biomedical devices (e.g., controlled drug delivery, rectification of ionic current, and signal processing), an ionic transistor should maintain high performance of ionic current control within physiological solutions (e.g., 0.9% NaCl) for long durations. Here, we introduce an ionic transistor using cation and anion exchange membranes (CEM and AEM). It could impose a 10× impedance change in a channel filled with 0.9% NaCl solution and we observed a stable modulation of ionic current throughout a test of 1000 cycles of on/off switching of the ionic transistor.

A purely ionic voltage effect soft triode

We report on the construction and characterization of an ionic soft triode intended to be based on interfacial ion adsorption and redox oxidizer depletion. The soft triode was built in a simple manner with no need for sophisticated or expensive materials. It does not utilize the control of a semiconducting channel, but an electrolyte. In different electrical circuit configurations, it can show amplification or memory effects. The device had an electrical current amplification reaching 52 and memory effects in the electrical resistance lasting for up to 6 h. These values were achieved by tuning the electrode interface, the electrolyte and diffusion properties. They are promising for neuromorphic applications.

In Vivo Organic Bioelectronics for Neuromodulation

The nervous system poses a grand challenge for integration with modern electronics and the subsequent advances in neurobiology, neuroprosthetics, and therapy which would become possible upon such integration. Due to its extreme complexity, multifaceted signaling pathways, and ∼1 kHz operating frequency, modern complementary metal oxide semiconductor (CMOS) based electronics appear to be the only technology platform at hand for such integration. However, conventional CMOS-based electronics rely exclusively on electronic signaling and therefore require an additional technology platform to translate electronic signals into the language of neurobiology. Organic electronics are just such a technology platform, capable of converting electronic addressing into a variety of signals matching the endogenous signaling of the nervous system while simultaneously possessing favorable material similarities with nervous tissue. In this review, we introduce a variety of organic material platforms and signaling modalities specifically designed for this role as “translator”, focusing especially on recent implementation in in vivo neuromodulation. We hope that this review serves both as an informational resource and as an encouragement and challenge to the field.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Regulating the Self-Discharge of Flexible All-Solid-State Supercapacitors by a Heterogeneous Polymer Electrolyte

Supercapacitors with high power density and an ultralong cyclic lifetime have been intensively investigated. However, the crucial challenge of their rapid self-discharge process is often neglected in most cases. A heterogeneous interface formed between two layers of polymer electrolytes is designed, in which a polyanion and a polycation are added into a common matrix of polymer electrolyte, respectively. By using the heterogeneous polymer electrolyte (HPE) as the separator simultaneously, the resultant supercapacitors exhibit comparable electrochemical performance to that of devices based on traditional polymer electrolytes. The HPE-based supercapacitors using both electric double-layer capacitive and pseudocapacitive electrodes show at least one time longer self-discharge time than that of devices based on homogenous polymer electrolyte, especially for the electrode in an electrolyte containing polyanion served as a positive pole during the charging process. Because of the same polymer matrix used, the heterojunction structure of the HPE exhibits excellent stability without obvious phase separation during thousands of charge/discharge and repeated bending cycles. This novel strategy by interface engineering of electrolyte to suppress the self-discharge behavior of supercapacitors is very meaningful to promote their practical applications.

Modulation Doping: A Strategy for 2D Materials Electronics

It remains a remarkable challenge to develop practical techniques for controllable and nondestructive doping in two-dimensional (2D) materials for their use in electronics and optoelectronics. Here, we propose a modulation doping strategy, wherein the perfect n-/p-type channel layer is achieved by accepting/donating electrons from/to the defects inside an adjacent encapsulation layer. We demonstrate this strategy in the heterostructures of BN/graphene, BN/MoS₂, where the previously believed useless deep defects, such as the nitrogen vacancy in BN, can provide free carriers to the graphene and MoS₂. The carrier density is further modulated by engineering the surroundings of the encapsulation layer. Moreover, the defects and carriers are naturally separated in space, eliminating the effects of Coulomb impurity scattering and thus allowing high mobility in the 2D limit. This doping strategy provides a highly viable route to tune 2D channel materials without inducing any structural damage, paving the way for high-performance 2D nanoelectronic devices.

Hydrogel-Based Iontronics on a Polydimethylsiloxane Microchip

In response to the extensive utilization of ionic circuits, including in iontronics and wearable devices, a new method for fabricating a hydrogel-based ionic circuit on a polydimethylsiloxane (PDMS) microchip is reported. Prolonged UV/ozone oxidation combined with proper surface functionalizations and a novel microchip bonding method using thiol-epoxy click reaction enable the robust attachment of the photopolymerized hydrogel to the microchannel surface for eventual operation in electrolytes as an ionic circuit. The stretchable ionic diode constructed on the PDMS microchip shows a superior rectification ratio even under tensile stress and long-term storage stability. Furthermore, the combination of the ionic circuit and unique material properties of PDMS allows us to maximize the versatility and diversify the functionalities of the iontronic device, as demonstrated in a pressure-driven ionic switch and chip-integrated ionic regulator. Its iontronic signal transmission mimicking the excitatory and inhibitory synapses also evinces the potential of the hydrogel-based iontronics on the PDMS microchip as developed toward an aqueous neuromimetic information processor while opening up new opportunities for various bioinspired applications.

Toward Biodegradable Electronics: Ionic Diodes Based on a Cellulose Nanocrystal-Agarose Hydrogel

Bioderived cellulose nanocrystals (CNCs) are used to create light, flexible, biocompatible, and biodegradable electronic devices. Herein, surface modification of cellulose nanocrystals was employed to fabricate cationic and anionic CNCs. Subsequently, we demonstrated rectification behavior from a fixed junction between two agarose hydrogels doped with cationic and anionic cellulose nanocrystals. The current rectification ratio reaches 70 reproducibly, which is significantly higher than that for analogous diodes generated with microfibrillated cellulose (∼15) and the first polyelectrolyte gel diode (∼40). The current-voltage characteristics of the CNC-hydrogel diode are influenced by concentration, gel thickness, scanning frequency, and applied voltage. The high surface area of CNC resulted in high charge density after surface modification, which in turn resulted in good rectification behavior from only small amounts of dopant material.

Ionic circuitry with nanofluidic diodes

Ionic circuits composed of nanopores functionalized with polyelectrolyte chains can operate in aqueous solutions, thus allowing the control of electrical signals and information processing in physiological environments. We demonstrate experimentally and theoretically that different orientations of single-pore membranes with the same and opposite surface charges can operate reliably in series, parallel, and mixed series-parallel arrangements of two, three, and four nanofluidic diodes using schemes similar to those of solid-state electronics. We consider also different experimental procedures to externally tune the fixed charges of the molecular chains functionalized on the pore surface, showing that single-pore membranes can be used efficiently in ionic circuitry with distinct ionic environments.

Ion-to-ion amplification through an open-junction ionic diode

As biological signals are mainly based on ion transport, the differences in signal carriers have become a major issue for the intimate communication between electrical devices and biological areas. In this respect, an ionic device which can directly interpret ionic signals from biological systems needs to be designed. Particularly, it is also required to amplify the ionic signals for effective signal processing, since the amount of ions acquired from biological systems is very small. Here, we report the signal amplification in ionic systems as well as sensing through the modified design of polyelectrolyte hydrogel-based ionic diodes. By designing an open-junction structure, ionic signals from the external environment can be directly transmitted to an ionic diode. Moreover, the minute ionic signals injected into the devices can also be amplified to a large amount of ions. The signal transduction mechanism of the ion-to-ion amplification is suggested and clearly verified by revealing the generation of breakdown ionic currents during an ion injection. Subsequently, various methods for enhancing the amplification are suggested.

Organic Heterojunctions Formed by Interfacing Two Single Crystals from a Mixed Solution

Organic heterojunctions are widely used in organic electronics and they are composed of semiconductors interfaced together. Good ordering in the molecular packing inside the heterojunctions is highly desired but it is still challenging to interface organic single crystals to form single-crystalline heterojunctions. Here, we describe how organic heterojunctions are formed by interfacing two single crystals from a droplet of a mixed solution containing two semiconductors. Based on crystallization of six organic semiconductors from a droplet on a substrate, two distinct crystallization mechanisms have been recognized in the sense that crystals form at either the top interface between the air and solution or the bottom interface between the substrate and solution. The preference for one interface rather than the other depends on the semiconductor-substrate pair and, for a given semiconductor, it can be switched by changing the substrate, suggesting that the preference is associated with the semiconductor-substrate molecular interaction. Furthermore, simultaneous crystallization of two semiconductors at two different interfaces to reduce their mutual disturbance results in the formation of bilayer single crystals interfaced together for organic heterojunctions. These single-crystalline heterojunctions exhibit ambipolar charge transport in field-effect transistors, with the highest electron mobility of 1.90 cm² V^-1 s^-1 and the highest hole mobility of 1.02 cm² V^-1 s^-1. Hence, by elucidating the interfacial crystallization events, this work should greatly harvest the solution-grown organic single-crystalline heterojunctions.

Ion Electron-Coupled Functionality in Materials and Devices Based on Conjugated Polymers

The coupling between charge accumulation in a conjugated polymer and the ionic charge compensation, provided from an electrolyte, defines the mode of operation in a vast array of different organic electrochemical devices. The most explored mixed organic ion-electron conductor, serving as the active electrode in these devices, is poly(3,4-ethyelenedioxythiophene) doped with polystyrelensulfonate (PEDOT:PSS). In this progress report, scientists of the Laboratory of Organic Electronics at Linköping University review some of the achievements derived over the last two decades in the field of organic electrochemical devices, in particular including PEDOT:PSS as the active material. The recently established understanding of the volumetric capacitance and the mixed ion-electron charge transport properties of PEDOT are described along with examples of various devices and phenomena utilizing this ion-electron coupling, such as the organic electrochemical transistor, ionic-electronic thermodiffusion, electrochromic devices, surface switches, and more. One of the pioneers in this exciting research field is Prof. Olle Inganäs and the authors of this progress report wish to celebrate and acknowledge all the fantastic achievements and inspiration accomplished by Prof. Inganäs all since 1981.

Macroscale Biomolecular Electronics and Ionics

The conduction of ions and electrons over multiple length scales is central to the processes that drive the biological world. The multidisciplinary attempts to elucidate the physics and chemistry of electron, proton, and ion transfer in biological charge transfer have focused primarily on the nano- and microscales. However, recently significant progress has been made on biomolecular materials that can support ion and electron currents over millimeters if not centimeters. Likewise, similar transport phenomena in organic semiconductors and ionics have led to new innovations in a wide variety of applications from energy generation and storage to displays and bioelectronics. Here, the underlying principles of conduction on the macroscale in biomolecular materials are discussed, highlighting recent examples, and particularly the establishment of accurate structure-property relationships to guide rationale material and device design. The technological viability of biomolecular electronics and ionics is also discussed.

A miniaturized solid salt reverse electrodialysis battery: a durable and fully ionic power source

A novel pump-free miniaturized reverse electrodialysis (RED) system was designed to provide lasting power transduced from salinity gradients, named solid salt RED (ssRED), and this quasi-battery uses a solid salt instead of electrolyte solution for streamlined usage. It is portable, flexible, comparable in size to a universal serial bus flash drive, and easily activated with a small amount of water. It maintains a constant ionic concentration gradient through precipitation reactions between a pair of different salts. This precipitation-assisted solid salt RED (PssRED) is an unprecedented ionic power source as it can generate steady electricity in the absence of a driving pump. The PssRED was successfully coupled with bipolar electrode (BPE) microchip sensors which require stable ionic electricity and a polyelectrolyte ionic diode to realize a fully ionic circuit. It is envisioned that the range of application could be expanded to supply electromotive force to various devices through an ionic charge flow.

Stretchable Ionics - A Promising Candidate for Upcoming Wearable Devices

As many devices for human utility aim for fast and convenient communication with users, superb electronic devices are demonstrated to serve as hardware for human-machine interfaces in wearable forms. Wearable devices for daily healthcare and self-diagnosis offer more human-like properties unconstrained by deformation. In this sense, stretchable ionics based on flexible and stretchable hydrogels are on the rise as another means to develop wearable devices for bioapplications for two main reasons: i) ionic currents and choosing the same signal carriers for biological areas, and ii) the adoption of hydrogel ionic conductors, which are intrinsically stretchable materials with biocompatibility. Here, the current status of stretchable ionics and future applications are introduced, whose positive effects can be magnified by stretchable ionics.

Electrokinetic ion transport in nanofluidics and membranes with applications in bioanalysis and beyond

Electrokinetic transport of ions between electrolyte solutions and ion permselective solid media governs a variety of applications, such as molecular separation, biological detection, and bioelectronics. These applications rely on a unique class of materials and devices to interface the ionic and electronic systems. The devices built on ion permselective materials or micro-/nanofluidic channels are arranged to work with aqueous environments capable of either manipulating charged species through applied electric fields or transducing biological responses into electronic signals. In this review, we focus on recent advances in the application of electrokinetic ion transport using nanofluidic and membrane technologies. We start with an introduction into the theoretical basis of ion transport kinetics and their analogy to the charge transport in electronic systems. We continue with discussions of the materials and nanofabrication technologies developed to create ion permselective membranes and nanofluidic devices. Accomplishments from various applications are highlighted, including biosensing, molecular separation, energy conversion, and bio-electronic interfaces. We also briefly outline potential applications and challenges in this field.

Ionic Circuits Powered by Reverse Electrodialysis for an Ultimate Iontronic System

Despite numerous reports on iontronic devices, there has been no whole circuit working in aqueous media including even power source. Herein, we introduce complete ionic circuits powered by reverse electrodialysis (RED) for the first time without employing any electronic components. The RED-driven polyelectrolyte diode successfully shows rectification behavior which is verified by monitoring dynamic ion distribution through fluorescence in real-time. We can also turn on and off the voltage applied to the circuit, and apply an arbitrary voltage by precisely manipulating the pressure imposed to an elastic connection tube filled with electrolyte. Furthermore, this new concept containing ionic power source advances to a more sophisticated ionic OR logic gate. The proposed system paves the way to develop not only passive iontronic devices (e.g. current ionic diode), but active ones requiring a source of energy, particularly such as a neuron-like information processor powered by fully ionic systems, and thereby aqueous computers.

Cross-Linked Polyelectrolyte for Improved Selectivity and Processability of Iontronic Systems

On-demand local release of biomolecules enables fine-tuned stimulation for the next generation of neuromodulation therapies. Such chemical stimulation is achievable using iontronic devices based on microfabricated, highly selective ion exchange membranes (IEMs). Current limitations in processability and performance of thin film IEMs hamper future developments of this technology. Here we address this limitation by developing a cationic IEM with excellent processability and ionic selectivity: poly(4-styrenesulfonic acid-co-maleic acid) (PSS-co-MA) cross-linked with polyethylene glycol (PEG). This enables new design opportunities and provides enhanced compatibility with in vitro cell studies. PSSA-co-MA/PEG is shown to out-perform the cation selectivity of the previously used iontronic material.

Redox Active Binary Logic Gate Circuit for Homeland Security

Bipolar junction transistors are at the frontiers of modern electronics owing to their discrete voltage regulated operational levels. Here we report a redox active binary logic gate (RLG) which can store a "0" and "1" with distinct operational levels, albeit without an external voltage stimuli. In the RLG, a shorted configuration of half-cell electrodes provided the logic low level and decoupled configuration relaxed the system to the logic high level due to self-charge injection into the redox active polymeric system. Galvanostatic intermittent titration and electrochemical quartz crystal microbalance studies indicate the kinetics of self-charge injection are quite faster and sustainable in polypyrrole based RLG, recovering more than 70% signal in just 14 s with minor signal reduction at the end of 10000 cycles. These remarkable properties of RLGs are extended to design a security sensor which can detect and count intruders in a locality with decent precision and switching speed.

Water-Gated n-Type Organic Field-Effect Transistors for Complementary Integrated Circuits Operating in an Aqueous Environment

The first demonstration of an n-type water-gated organic field-effect transistor (WGOFET) is here reported, along with simple water-gated complementary integrated circuits, in the form of inverting logic gates. For the n-type WGOFET active layer, high-electron-affinity organic semiconductors, including naphthalene diimide co-polymers and a soluble fullerene derivative, have been compared, with the latter enabling a high electric double layer capacitance in the range of 1 μF cm-2 in full accumulation and a mobility-capacitance product of 7 × 10-3 μF/V s. Short-term stability measurements indicate promising cycling robustness, despite operating the device in an environment typically considered harsh, especially for electron-transporting organic molecules. This work paves the way toward advanced circuitry design for signal conditioning and actuation in an aqueous environment and opens new perspectives in the implementation of active bio-organic interfaces for biosensing and neuromodulation.

Organic Bioelectronics: Bridging the Signaling Gap between Biology and Technology

The electronics surrounding us in our daily lives rely almost exclusively on electrons as the dominant charge carrier. In stark contrast, biological systems rarely use electrons but rather use ions and molecules of varying size. Due to the unique combination of both electronic and ionic/molecular conductivity in conducting and semiconducting organic polymers and small molecules, these materials have emerged in recent decades as excellent tools for translating signals between these two realms and, therefore, providing a means to effectively interface biology with conventional electronics-thus, the field of organic bioelectronics. Today, organic bioelectronics defines a generic platform with unprecedented biological recording and regulation tools and is maturing toward applications ranging from life sciences to the clinic. In this Review, we introduce the field, from its early breakthroughs to its current results and future challenges.

The inside-out supercapacitor: induced charge storage in reduced graphene oxide

By turning the standard supercapacitor geometry inside out, an ionic capacitor is made for use in ionic circuits.

Flexible Organic Electronics in Biology: Materials and Devices

At the convergence of organic electronics and biology, organic bioelectronics attracts great scientific interest. The potential applications of organic semiconductors to reversibly transmit biological signals or stimulate biological tissues inspires many research groups to explore the use of organic electronics in biological systems. Considering the surfaces of movable living tissues being arbitrarily curved at physiological environments, the flexibility of organic bioelectronic devices is of paramount importance in enabling stable and reliable performances by improving the contact and interaction of the devices with biological systems. Significant advances in flexible organic bio-electronics have been achieved in the areas of flexible organic thin film transistors (OTFTs), polymer electrodes, smart textiles, organic electrochemical ion pumps (OEIPs), ion bipolar junction transistors (IBJTs) and chemiresistors. This review will firstly discuss the materials used in flexible organic bioelectronics, which is followed by an overview on various types of flexible organic bioelectronic devices. The versatility of flexible organic bioelectronics promises a bright future for this emerging area.

Iontronics

Iontronics is an emerging technology based on sophisticated control of ions as signal carriers that bridges solid-state electronics and biological system. It is found in nature, e.g., information transduction and processing of brain in which neurons are dynamically polarized or depolarized by ion transport across cell membranes. It suggests the operating principle of aqueous circuits made of predesigned structures and functional materials that characteristically interact with ions of various charge, mobility, and affinity. Working in aqueous environments, iontronic devices offer profound implications for biocompatible or biodegradable logic circuits for sensing, ecofriendly monitoring, and brain-machine interfacing. Furthermore, iontronics based on multi-ionic carriers sheds light on futuristic biomimic information processing. In this review, we overview the historical achievements and the current state of iontronics with regard to theory, fabrication, integration, and applications, concluding with comments on where the technology may advance.

A hybrid living/organic electrochemical transistor based on the Physarum polycephalum cell endowed with both sensing and memristive properties

A hybrid bio-organic electrochemical transistor was developed by interfacing an organic semiconductor, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate), with the Physarum polycephalum cell. The system shows unprecedented performances since it could be operated both as a transistor, in a three-terminal configuration, and as a memristive device in a two terminal configuration mode. This is quite a remarkable achievement since, in the transistor mode, it can be used as a very sensitive bio-sensor directly monitoring biochemical processes occurring in the cell, while, as a memristive device, it represents one of the very first examples of a bio-hybrid system demonstrating such a property. Our system combines memory and sensing in the same system, possibly interfacing unconventional computing. The system was studied by a full electrical characterization using a series of different gate electrodes, namely made of Ag, Au and Pt, which typically show different operation modes in organic electrochemical transistors. Our experiment demonstrates that a remarkable sensing capability could potentially be implemented. We envisage that this system could be classified as a Bio-Organic Sensing/Memristive Device (BOSMD), where the dual functionality allows merging of the sensing and memory properties, paving the way to new and unexplored opportunities in bioelectronics.

Polyphosphonium-based ion bipolar junction transistors

Advancements in the field of electronics during the past few decades have inspired the use of transistors in a diversity of research fields, including biology and medicine. However, signals in living organisms are not only carried by electrons but also through fluxes of ions and biomolecules. Thus, in order to implement the transistor functionality to control biological signals, devices that can modulate currents of ions and biomolecules, i.e., ionic transistors and diodes, are needed. One successful approach for modulation of ionic currents is to use oppositely charged ion-selective membranes to form so called ion bipolar junction transistors (IBJTs). Unfortunately, overall IBJT device performance has been hindered due to the typical low mobility of ions, large geometries of the ion bipolar junction materials, and the possibility of electric field enhanced (EFE) water dissociation in the junction. Here, we introduce a novel polyphosphonium-based anion-selective material into npn-type IBJTs. The new material does not show EFE water dissociation and therefore allows for a reduction of junction length down to 2 μm, which significantly improves the switching performance of the ion transistor to 2 s. The presented improvement in speed as well the simplified design will be useful for future development of advanced iontronic circuits employing IBJTs, for example, addressable drug-delivery devices.

Modeling of charge transport in ion bipolar junction transistors

Spatiotemporal control of the complex chemical microenvironment is of great importance to many fields within life science. One way to facilitate such control is to construct delivery circuits, comprising arrays of dispensing outlets, for ions and charged biomolecules based on ionic transistors. This allows for addressability of ionic signals, which opens up for spatiotemporally controlled delivery in a highly complex manner. One class of ionic transistors, the ion bipolar junction transistors (IBJTs), is especially attractive for these applications because these transistors are functional at physiological conditions and have been employed to modulate the delivery of neurotransmitters to regulate signaling in neuronal cells. Further, the first integrated complementary ionic circuits were recently developed on the basis of these ionic transistors. However, a detailed understanding of the device physics of these transistors is still lacking and hampers further development of components and circuits. Here, we report on the modeling of IBJTs using Poisson's and Nernst-Planck equations and the finite element method. A two-dimensional model of the device is employed that successfully reproduces the main characteristics of the measurement data. On the basis of the detailed concentration and potential profiles provided by the model, the different modes of operation of the transistor are analyzed as well as the transitions between the different modes. The model correctly predicts the measured threshold voltage, which is explained in terms of membrane potentials. All in all, the results provide the basis for a detailed understanding of IBJT operation. This new knowledge is employed to discuss potential improvements of ion bipolar junction transistors in terms of miniaturization and device parameters.

Effect of interactions with the chaperonin cavity on protein folding and misfolding

Recent experimental and computational results have suggested that attractive interactions between a chaperonin and an enclosed substrate can have an important effect on the protein folding rate: it appears that folding may even be slower inside the cavity than under unconfined conditions, in contrast to what we would expect from excluded volume effects on the unfolded state. Here we examine systematically the dependence of the protein stability and folding rate on the strength of such attractive interactions between the chaperonin and substrate, by using molecular simulations of model protein systems in an idealised attractive cavity. Interestingly, we find a maximum in stability, and a rate which indeed slows down at high attraction strengths. We have developed a simple phenomenological model which can explain the variations in folding rate and stability due to differing effects on the free energies of the unfolded state, folded state, and transition state; changes in the diffusion coefficient along the folding coordinate are relatively small, at least for our simplified model. In order to investigate a possible role for these attractive interactions in folding, we have studied a recently developed model for misfolding in multidomain proteins. We find that, while encapsulation in repulsive cavities greatly increases the fraction of misfolded protein, sufficiently strong attractive protein-cavity interactions can strongly reduce the fraction of proteins reaching misfolded traps.

Assembling surface mounted components on ink-jet printed double sided paper circuit board

Printed electronics is a rapidly developing field where many components can already be manufactured on flexible substrates by printing or by other high speed manufacturing methods. However, the functionality of even the most inexpensive microcontroller or other integrated circuit is, at the present time and for the foreseeable future, out of reach by means of fully printed components. Therefore, it is of interest to investigate hybrid printed electronics, where regular electrical components are mounted on flexible substrates to achieve high functionality at a low cost. Moreover, the use of paper as a substrate for printed electronics is of growing interest because it is an environmentally friendly and renewable material and is, additionally, the main material used for many packages in which electronics functionalities could be integrated. One of the challenges for such hybrid printed electronics is the mounting of the components and the interconnection between layers on flexible substrates with printed conductive tracks that should provide as low a resistance as possible while still being able to be used in a high speed manufacturing process. In this article, several conductive adhesives are evaluated as well as soldering for mounting surface mounted components on a paper circuit board with ink-jet printed tracks and, in addition, a double sided Arduino compatible circuit board is manufactured and programmed.

Electronic and optoelectronic materials and devices inspired by nature

Inorganic semiconductors permeate virtually every sphere of modern human existence. Micro-fabricated memory elements, processors, sensors, circuit elements, lasers, displays, detectors, etc are ubiquitous. However, the dawn of the 21st century has brought with it immense new challenges, and indeed opportunities-some of which require a paradigm shift in the way we think about resource use and disposal, which in turn directly impacts our ongoing relationship with inorganic semiconductors such as silicon and gallium arsenide. Furthermore, advances in fields such as nano-medicine and bioelectronics, and the impending revolution of the 'ubiquitous sensor network', all require new functional materials which are bio-compatible, cheap, have minimal embedded manufacturing energy plus extremely low power consumption, and are mechanically robust and flexible for integration with tissues, building structures, fabrics and all manner of hosts. In this short review article we summarize current progress in creating materials with such properties. We focus primarily on organic and bio-organic electronic and optoelectronic systems derived from or inspired by nature, and outline the complex charge transport and photo-physics which control their behaviour. We also introduce the concept of electrical devices based upon ion or proton flow ('ionics and protonics') and focus particularly on their role as a signal interface with biological systems. Finally, we highlight recent advances in creating working devices, some of which have bio-inspired architectures, and summarize the current issues, challenges and potential solutions. This is a rich new playground for the modern materials physicist.

Polyphosphonium-based bipolar membranes for rectification of ionic currents

Bipolar membranes (BMs) have interesting applications within the field of bioelectronics, as they may be used to create non-linear ionic components (e.g., ion diodes and transistors), thereby extending the functionality of, otherwise linear, electrophoretic drug delivery devices. However, BM based diodes suffer from a number of limitations, such as narrow voltage operation range and/or high hysteresis. In this work, we circumvent these problems by using a novel polyphosphonium-based BM, which is shown to exhibit improved diode characteristics. We believe that this new type of BM diode will be useful for creating complex addressable ionic circuits for delivery of charged biomolecules.

Organic bioelectronics for electronic-to-chemical translation in modulation of neuronal signaling and machine-to-brain interfacing

BACKGROUND

A major challenge when creating interfaces for the nervous system is to translate between the signal carriers of the nervous system (ions and neurotransmitters) and those of conventional electronics (electrons).

SCOPE OF REVIEW

Organic conjugated polymers represent a unique class of materials that utilizes both electrons and ions as charge carriers. Based on these materials, we have established a series of novel communication interfaces between electronic components and biological systems. The organic electronic ion pump (OEIP) presented in this review is made of the polymer-polyelectrolyte system poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The OEIP translates electronic signals into electrophoretic migration of ions and neurotransmitters.

MAJOR CONCLUSIONS

We demonstrate how spatio-temporally controlled delivery of ions and neurotransmitters can be used to modulate intracellular Ca(2+) signaling in neuronal cells in the absence of convective disturbances. The electronic control of delivery enables strict control of dynamic parameters, such as amplitude and frequency of Ca(2+) responses, and can be used to generate temporal patterns mimicking naturally occurring Ca(2+) oscillations. To enable further control of the ionic signals we developed the electrophoretic chemical transistor, an analog of the traditional transistor used to amplify and/or switch electronic signals. Finally, we demonstrate the use of the OEIP in a new "machine-to-brain" interface by modulating brainstem responses in vivo.

GENERAL SIGNIFICANCE

This review highlights the potential of communication interfaces based on conjugated polymers in generating complex, high-resolution, signal patterns to control cell physiology. We foresee widespread applications for these devices in biomedical research and in future medical devices within multiple therapeutic areas. This article is part of a Special Issue entitled Organic Bioelectronics-Novel Applications in Biomedicine.

Ion diode logics for pH control

Electronic control over the generation, transport, and delivery of ions is useful in order to regulate reactions, functions, and processes in various chemical and biological systems. Different kinds of ion diodes and transistors that exhibit non-linear current versus voltage characteristics have been explored to generate chemical gradients and signals. Bipolar membranes (BMs) exhibit both ion current rectification and water splitting and are thus suitable as ion diodes for the regulation of pH. To date, fast switching ion diodes have been difficult to realize due to accumulation of ions inside the device structure at forward bias--charges that take a long time to deplete at reverse bias. Water splitting occurs at elevated reverse voltage bias and is a feature that renders high ion current rectification impossible. This makes integration of ion diodes in circuits difficult. Here, we report three different designs of micro-fabricated ion bipolar membrane diodes (IBMDs). The first two designs consist of single BM configurations, and are capable of either splitting water or providing high current rectification. In the third design, water-splitting BMs and a highly-rectifying BM are connected in series, thus suppressing accumulation of ions. The resulting IBMD shows less hysteresis, faster off-switching, and also a high ion current rectification ratio as compared to the single BM devices. Further, the IBMD was integrated in a diode-based AND gate, which is capable of controlling delivery of hydroxide ions into a receiving reservoir.

A quasi-liquid iontronic-electronic light-harvesting hybrid photodetector with giant response

A heterostructure formed by a layer of agarose gel drop-cast on a hydrothermally grown layer of ZnO nanorods on fluorine-doped tin oxide (FTO)-coated glass is examined for photoresponse with a top platinum tip contact. This ionic-gel-based hybrid device shows three orders of magnitude higher photocurrent as compared to the case of bare ZnO nanorods film.

A polysaccharide bioprotonic field-effect transistor

In nature, electrical signalling occurs with ions and protons, rather than electrons. Artificial devices that can control and monitor ionic and protonic currents are thus an ideal means for interfacing with biological systems. Here we report the first demonstration of a biopolymer protonic field-effect transistor with proton-transparent PdH(x) contacts. In maleic-chitosan nanofibres, the flow of protonic current is turned on or off by an electrostatic potential applied to a gate electrode. The protons move along the hydrated maleic-chitosan hydrogen-bond network with a mobility of ~4.9×10(-3) cm(2) V(-1) s(-1). This study introduces a new class of biocompatible solid-state devices, which can control and monitor the flow of protonic current. This represents a step towards bionanoprotonics.