Ionic Circuits Research Articles

Stretchable and transparent ionic diode and logic gates

Inspired by the asymmetric ion transport phenomenon commonly seen in biology, in this article, we develop transparent and stretchable hydrogel-based ionic diode and logic gates. The ionic diode is made up of polyacrylamide hydrogel doped by two different polyelectrolytes with opposite fixed charges. Due to the asymmetric distribution of the fixed charges, the hydrogel diode shows ion rectification capability. Our experiments further show that large rectification ratio of the diode can be maintained when the diode is under moderate stretch. Using the ionic diode, we further successfully construct two types of ionic logic gates: OR gate and AND gate, which can perform logic operations on binary inputs and produce a single output. We hope the transparent and stretchable ionic diode, as well as the logic gates, can be used as the basic functional elements for advanced and integrated stretchable ionic circuit system.

Self-healing electronic skins for aquatic environments

Gelatinous underwater invertebrates such as jellyfish have organs that are transparent, stretchable, touch-sensitive and self-healing, which allow the creatures to navigate, camouflage themselves and, indeed, survive in aquatic environments. Artificial skins that emulate such functionality could be used to develop applications such as aquatic soft robots and water-resistant human–machine interfaces. Here we report a bio-inspired skin-like material that is transparent, electrically conductive and can autonomously self-heal in both dry and wet conditions. The material, which is composed of a fluorocarbon elastomer and a fluorine-rich ionic liquid, has an ionic conductivity that can be tuned to as high as 10−3 S cm−1 and can withstand strains as high as 2,000%. Owing to ion–dipole interactions, it offers fast and repeatable electro-mechanical self-healing in wet, acidic and alkali environments. To illustrate the potential applications of the approach, we used our electronic skins to create touch, pressure and strain sensors. We also show that the material can be printed into soft and pliable ionic circuit boards. A transparent electronic skin, composed of an elastomer and an ionic liquid, can autonomously self-heal in both dry and wet conditions due to ion–dipole interactions.

An Ionic Capacitor for Integrated Iontronic Circuits

AbstractOrganic electronics, in combination with custom polyelectrolytes, enables solid‐ and hydrogel‐state circuit components using ionic charges in place of the electrons of traditional electronics. This growing field of iontronics leverages anion‐ and cation‐exchange membranes as analogs to n‐type and p‐type semiconductors, and conjugated polymer electrodes as ion‐to‐electron converters. To date, the iontronics toolbox includes ionic resistors, ionic diodes, ionic transistors, and analog and digital circuits comprised thereof. Here, an ionic capacitor based on mixed electron–ion conductors is demonstrated. The ionic capacitor resembles the structure of a conventional electrochemical capacitor that is inverted, with an electronically conducting core and two electrolyte ionic conductors. The device is first verified as a capacitor, and then demonstrated as a smoothing element in an iontronic diode bridge circuit driving an organic electronic ion pump (ionic resistor). The ionic capacitor complements the existing iontronics toolbox, enabling more complex and functional ionic circuits, and will thus have implications in a variety of mixed electron–ion conduction technologies.

Push-Button Method To Create Nanopores Using a Tesla-Coil Lighter.

Controlled dielectric breakdown (CDB) of silicon nitride thin films immersed in electrolyte solution has been used to fabricate single nanofluidic channels with ∼10 nm and smaller diameters, nanopores, useful in single-molecule sensing and ionic circuit construction. A hand-held Tesla-coil lighter was used to form nanofluidic ionic conductors through a ∼10 nm thick silicon nitride membrane. Modifications to the conventional approach were required by the low-overhead Tesla-coil-assisted method (TCAM): increased circuit resistance by including water in place of electrolyte and discrete rather than continuous voltage applications. The resulting ionic conductance could be tuned with the number of voltage applications. TCAM and conventional CDB produced nanopores with different conductance versus pH curves, suggesting different surface chemistry. Nevertheless, sensing experiments using the canonical test molecule, λ-DNA, produced signals comparable to translocation results through solid-state nanopores fabricated by other methods. Thus, the TCAM method offers flexibility in fabrication and in the properties and function of the nanoscale ionic conductors that it can generate.

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.

Solid-state nanopore hydrodynamics and transport.

The resistive pulse method based on measuring the ion current trace as a biomolecule passing through a nanopore has become an important tool in biotechnology for characterizing molecules. A detailed physical understanding of the translocation process is essential if one is to extract the relevant molecular properties from the current signal. In this Perspective, we review some recent progress in our understanding of hydrodynamic flow and transport through nanometer sized pores. We assume that the problems of interest can be addressed through the use of the continuum version of the equations of hydrodynamic and ion transport. Thus, our discussion is restricted to pores of diameter greater than about ten nanometers: such pores are usually synthetic. We address the fundamental nanopore hydrodynamics and ion transport mechanisms and review the wealth of observed phenomena due to these mechanisms. We also suggest future ionic circuits that can be synthesized from different ionic modules based on these phenomena and their applications.

A Stretchable Ionic Diode from Copolyelectrolyte Hydrogels with Methacrylated Polysaccharides

AbstractAs the demand for soft and flexible devices steadily increases, the ionic applications demonstrated with gel materials have come under the spotlight. Here, stretchable and wearable ionic diodes (SIDs) made from polyelectrolyte hydrogels are introduced. Polyelectrolyte hydrogels are mechanically modified using methacrylated polysaccharides while preserving the ion‐permselectivity of poly(sulfopropyl acrylate) potassium salt (PSPA) and poly([acrylamidopropyl]trimethylammonium chloride) (PDMAPAA‐Q), forming ionic copolymers. Then, SIDs composed of polyelectrolyte copolymer hydrogels are fabricated in VHB substrates as a stretchable and transparent insulating layer which is engraved by a laser. The SIDs show rectifying behaviors beyond the stretch of 3 with the aid of perfect adhesion between hydrogels and elastomeric substrates, and preserve their rectifications over hundreds of cycles. The operation of the SID is visualized by a wearable ionic circuit which rectifies ionic currents and lightens the LED under the forward bias during finger movements.

Asymmetric Nafion-Coated Nanopore Electrode Arrays as Redox-Cycling-Based Electrochemical Diodes.

Inspired by the functioning of cellular ion channels, pore-based structures with nanoscale openings have been fabricated and integrated into ionic circuits, for example, ionic diodes and transistors, for signal processing and detection. In these systems, the nonlinear current responses arise either because asymmetric nanopore geometries break the symmetry of the ion distribution, creating unequal surface charge across the nanopore, or by coupling unidirectional electron transfer within a nanopore electrode. Here we develop a high-performance redox-cycling-based electrochemical diode by coating an asymmetric ion-exchange membrane, that is, Nafion, on the top surface of a nanopore electrode array (Nafion@NEA), in which each pore in the array exhibits one or more annular electrodes. Nafion@NEAs exhibit highly sensitive and charge-selective electroanalytical measurements due to efficient redox-cycling reaction, the permselectivity of Nafion, and the strong confinement of redox species in the nanopore array. In addition, the top electrode of dual-electrode Nafion@NEAs can serve as a voltage-controlled switch to gate ion transport within the nanopore. Thus Nafion@NEAs can be operated as a diode by switching voltages applied to the top and bottom electrodes of the NEA, leading to a large rectification ratio, fast response times, and simplified circuitry without the need for external electrodes. By taking advantage of closely spaced and individually addressable electrodes, the redox-cycling electrochemical diode has the potential for application to large-scale production and electrochemically controlled circuit operations, which go well beyond conventional electronic diodes or transistors.

Programmable Hydrogel Ionic Circuits for Biologically Matched Electronic Interfaces.

The increased need for wearable and implantable medical devices has driven the demand for electronics that interface with living systems. Current bioelectronic systems have not fully resolved mismatches between engineered circuits and biological systems, including the resulting pain and damage to biological tissues. Here, salt/poly(ethylene glycol) (PEG) aqueous two-phase systems are utilized to generate programmable hydrogel ionic circuits. High-conductivity salt-solution patterns are stably encapsulated within PEG hydrogel matrices using salt/PEG phase separation, which route ionic current with high resolution and enable localized delivery of electrical stimulation. This strategy allows designer electronics that match biological systems, including transparency, stretchability, complete aqueous-based connective interface, distribution of ionic electrical signals between engineered and biological systems, and avoidance of tissue damage from electrical stimulation. The potential of such systems is demonstrated by generating light-emitting diode (LED)-based displays, skin-mounted electronics, and stimulators that deliver localized current to in vitro neuron cultures and muscles in vivo with reduced adverse effects. Such electronic platforms may form the basis of future biointegrated electronic systems.

Voltage-Induced Modulation of Ionic Concentrations and Ion Current Rectification in Mesopores with Highly Charged Pore Walls.

It is believed that ion current rectification (ICR), a property that assures preferential ionic transport in one direction, can only be observed in nanopores when the pore size is comparable to the thickness of the electric double layer (EDL). Rectifying nanopores became the basis of biological sensors and components of ionic circuits. Here we report that appreciable ICR can also occur in highly charged conical, polymer mesopores whose tip diameters are as large as 400 nm, thus over 100-fold larger than the EDL thickness. A rigorous model taking into account the surface equilibrium reaction of functional carboxyl groups on the pore wall and electroosmotic flow is employed to explain that unexpected phenomenon. Results show that the pore rectification results from the high density of surface charges as well as the presence of highly mobile hydroxide ions, whose concentration is enhanced for one voltage polarity. This work provides evidence that highly charged surfaces can extend the ICR of pores to the submicron scale, suggesting the potential use of highly charged large pores for energy and sensing applications. Our results also provide insight into how a mixture of ions with different mobilities can influence current-voltage curves and rectification.

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.

A universal design of field-effect-tunable microfluidic ion diode based on a gating cation-exchange nanoporous membrane

Based on the continuum mechanics theory, we propose herein a universal design of microfluidic ionic diode based on external concentration polarization of a gating ion-selective medium embedded in the microfluidic network with four power terminals. This micro/nanofluidic hybrid chip employs a cation-exchange nanoporous membrane (CEM) coupled with both a control and output microfluidic channel. Under the action of a vertical electric field throughout the CEM, nanoscale surface conduction of excessive counterions within the charged nanopores is converted to the propagation of either enriched or depleted boundary toward the opposing electrode-terminal in phase with the electroconvective flow, thereby making an adjustment in the electrical conductance of output microchannel for achieving high-flux field-effect current control and diode functionality. Three basic working states, including the “on,” “transition,” and “off” statuses, are distinguished in different ranges of source voltage magnitude. The rectification factor of the proposed ionic circuit platform can attain one hundred-fold even at small source and gate voltages. The presented field-effect-tunable microfluidic ion diode is easily scalable, permits appreciable fluid flow due to an intrinsically small hydrodynamic resistance, and holds promise for producing high-flux ion current rectification in next-generation integrated circuits.

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.

Batteries and Fuel Cells with Convective Mobile Electrolyte

Ultralow resistance electrode architecture, previously described by the presenters in (MS #JESP-17-0147R), is here configured for use with mobile convective electrolyte using unique porous electrode structure in a novel oxygen breathing battery and direct-hydrocarbon oxidation AFC. Apollo Mission or F. Bacon mobile KOH-electrolyte H2/O2 fuel cells were competitive power producers by today’s standards, but have been replaced by PEMFC due to historic BOP issues. Porous electrode permeation described within was not an option and inter-electrode electrolyte flow was replaced by static electrolyte as multiple cell series connection caused inter-cell electrolyte pathway ionic short circuits. PEM replacement uses noble metal catalyst, expensive membranes, high electrode area ohmic resistance, low current and power density at low temperature associated with high overvoltage among other issues. Historic fuel cells used Ni anode and NiO cathode catalyst at 230 oC. Apollo limited pressure to 0.34 MPa in 75% KOH. Bacon used 4.5 MPa and 30% KOH to produce 1 A/cm2 at 0.8 volt; indicating an efficacy of non-noble catalyst for AFC at temperatures too high for PEM. Raney-Ni raised catalytic activity with greater surface density; here greatly improved as strongly coupled coatings of CNT in low resistance electrode architecture. 80% KOH, 300 oC and 0.7 MPa is useful for H2/air catalysis due to high solubility and low vapor pressure of KOH. The electrodes are also stable at 700 oC with molten carbonate electrolyte. Here, mobile electrolyte permeates both anode and cathode, preferably in that order, to move convectively between and through electrodes in a single pathway circulation loop. The concept is first described as a convection battery where CNT fibers are coated by or lodge and immobilize entrained nanoscale faradaic particles within nonwoven web-like nanoporous CNT structures attached to metal as described within. It is then expanded to include fuel/gas injection to create highly concentrated foam menisci that coat catalyst surfaces while preserving combined gas/electrolyte mobility in a fuel cell configuration. Sources demonstrate that catalyzed redox activity occurs mainly within thin menisci. The concept radically departs from all fuel cell architectural paradigms that use a separating membrane to stop cross-electrode fuel contamination. Virtually complete fuel oxidation and 1-way convection obviates the need for an ion selective semipermeable membrane. Electrode volumetric redox activity in mobile electrolyte is several orders of magnitude higher due, in the example, to higher active catalytic/faradaic surface density exposed to convective electrolyte. Exchange current density, io is concomitantly significantly increased. The purpose is high area specific electrode current density, i operating close to io with less voltage decrement. In batteries that translates into faster recharge, less heat and in fuel cells, higher power. LIB charge current must be less than 10 mA/cm2 where high ohmic resistance, e.g. i2R is a real hazard and iR lowers voltage. PEMFC operates at 0.7 volt and 0.5 A/cm2 or peak power of 3.5 W/cm2. If the popular LIB form could produce 0.5 A/cm2 current density it would charge 50 times faster and as previously shown by the authors, produce negligible ohmic heat. It is a common mistake to characterize Nernstian Tafel resistance, r = ∂V/∂i, influenced by ion diffusion in electrolyte as ohmic resistance due to electron conduction in metal. When either r or R is increased, i is reduced but only R produces heat. Electrolyte cannot be an electrical conduction path or it would short circuit any cell in which it is used. Ultralow resistance electrodes refer to ohmic resistance at less than 10-4 Ω-cm2 to permit 0.5 and 5 A/cm2 unprecedented area specific current density respectively, for batteries and fuel cells; uniquely possible using electrolyte convection, e.g. 1 cm/sec or 103times faster than ion diffusion in electrolyte. A stack of paired nested concentric annular bi-polar convection electrodes also comprise a stack of unit-cells separated by insulation and connection in series of for high voltage. Electrolyte flows from the central lumen of an inner annular anode, through a gap separating anode and outer annular cathode and into a chamber from which electrolyte exits the stack. The described electrode layered structure forces normal vector convective flow through electrolyte permeable membranes and transversely between layers. The stack uses a single electrolyte circulation loop that simplifies electrolyte processing, restoration and stack drainage for long shelf-life storage; pump hardware is also minimal. In spite of simplified electrolyte circulation the classic problem of back emf through one or more electrolyte conduits is completely eliminated. Anodes and cathodes comprise nonwoven CNT membranes compressed between square weave metal screens brazed to one another so that all screens lie in window registry. Figure 1

Viscosity and Conductivity Tunable Diode-like Behavior for Meso- and Micropores.

Rectifying pores, which transport ions mainly in one direction blocking the ionic flow in the other, were shown to be important in the preparation of chemical sensors, components of ionic circuits, and mimics of biological channels. Ionic rectification has been shown with various engineered systems, but pores with similar opening diameters often rectify to a various uncontrolled extent. In this Letter we present a system of single meso-pores, whose current-voltage curves and rectification can be tuned with great precision via viscosity and conductivity gradients of solutions placed on both sides of the membrane. The mechanism of rectification is based on electroosmotically induced flow, which fills the entire volume of the pore with a single solution from either side of the membrane. The highly predictable rectifying system can find various applications, including measuring viscosity of unknown media and tuning electrokinetic passage of particles.

An optically-gated AuNP-DNA protonic transistor.

Bio-interface transistors, which manipulate the transportation of ions (i.e. protons), play an important role in bridging physical devices with biological functionalities, because electrical signals are carried by ions/protons in biological systems. All available ionic transistors use electrostatic gates to tune the ionic carrier density, which requires complicated interconnect wires. In contrast, an optical gate, which offers the advantages of remote control as well as multiple light wavelength selections, has never been explored for ionic devices. Here, we demonstrate a light-gated protonic transistor fabricated from an Au nanoparticle and DNA (AuNP-DNA) hybrid membrane. The device can be turned on and off completely by using light, with a high on/off current ratio of up to 2 orders of magnitude. Moreover, the device only responds to specific light wavelengths due to the plasmonic effect from the AuNPs, which enables the capability of wavelength selectivity. Our results open up new avenues for exploring remotely controlled ionic circuits, in vivo protonic switches, and other biomedical applications.

Polarization of Gold in Nanopores Leads to Ion Current Rectification.

Biomimetic nanopores with rectifying properties are relevant components of ionic switches, ionic circuits, and biological sensors. Rectification indicates that currents for voltages of one polarity are higher than currents for voltages of the opposite polarity. Ion current rectification requires the presence of surface charges on the pore walls, achieved either by the attachment of charged groups or in multielectrode systems by applying voltage to integrated gate electrodes. Here we present a simpler concept for introducing surface charges via polarization of a thin layer of Au present at one entrance of a silicon nitride nanopore. In an electric field applied by two electrodes placed in bulk solution on both sides of the membrane, the Au layer polarizes such that excess positive charge locally concentrates at one end and negative charge concentrates at the other end. Consequently, a junction is formed between zones with enhanced anion and cation concentrations in the solution adjacent to the Au layer. This bipolar double layer together with enhanced cation concentration in a negatively charged silicon nitride nanopore leads to voltage-controlled surface-charge patterns and ion current rectification. The experimental findings are supported by numerical modeling that confirm modulation of ionic concentrations by the Au layer and ion current rectification even in low-aspect ratio nanopores. Our findings enable a new strategy for creating ionic circuits with diodes and transistors.

New prototypes for the isolation of the anodic chambers in microbial fuel cells

This work has been focused on the assessment of new prototypes of MFC in which a more strict separation of the anode and cathode compartments is looked for, in order to attain strict anaerobic conditions in the anode chamber and hence, avoid lack of efficiency due to the prevalence of non-electrogenic competing microorganisms and to optimize composition of the anolyte and catholyte. A cylinder reactor with an inner chamber with graphite bars acting as anodes and the outer one with a stainless steel tube acting as cathode was used in three different configurations and results obtained during lifetests are compared in terms of electricity production, cathode oxygen consumption and anode COD depletion. Separation between compartments was obtained by (a) a two faces cathode (prototype R1, in which the cathode faces both compartments of the MFC), (b) a single cathode (prototype R2, in which the surface facing the anode compartment is electrically isolated and the ionic circuit is virtually detached) and (c) a conventional PEM membrane (prototype R3). Results show that the three prototypes perform as MFC from the electric point of view and they are efficient in the degradation of the COD contained in wastewater. However, the efficiencies in the production of electricity are very different and the greater the isolation of the anodic compartment, the lower are the current intensities produced. These results are explained because of the higher resulting ohmic loses. This large increase in the cell potential mask the potential benefits searched for with the isolation of the anode chamber but, at the same time, it gives significant information about performance of MFC that can be used for future MFC miniaturization studies.

High-flux ionic diodes, ionic transistors and ionic amplifiers based on external ion concentration polarization by an ion exchange membrane: a new scalable ionic circuit platform.

A microfluidic ion exchange membrane hybrid chip is fabricated using polymer-based, lithography-free methods to achieve ionic diode, transistor and amplifier functionalities with the same four-terminal design. The high ionic flux (>100 μA) feature of the chip can enable a scalable integrated ionic circuit platform for micro-total-analytical systems.

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

Iontronic circuits are built using components which are analogous to those used in electronic circuits, however they involve the movement of ions in an electrolyte rather than electrons in a metal or semiconductor. Developments in these circuits' performance have led to applications in biological sensing, interfacing and drug delivery. While transistors, diodes and elementary logic circuits have been demonstrated for ionic circuits if more complex circuits are to be realized, the precident set by electrical circuits suggests that a component which is analogous to an electrical capacitor is required. Herein, an ionic supercapacitor is reported, our experiments show that charge may be stored in a conductive porous reduced graphene oxide film that is contacted by two isolated aqueous solutions and that this concept extends to an arbitrary polarizable sample. Parametric studies indicate that the conductivity and porosity of this film play important roles in the resultant device's performance. This ionic capacitor has a specific capacitance of 8.6 F cm-3 at 1 mV s-1 and demonstrates the ability to filter and smooth signals in an electrolyte at a variety of low frequencies. The device has the same interfaces as a supercapacitor but their arrangement is changed, hence the name inside-out supercapacitor.