Ionic Circuits Research Articles

Unlocking Nanoprecipitation: A Pathway to High Reversibility in Nanofluidic Memristors.

Solid-state nanochannels have emerged as a promising platform for the development of ionic circuit components with analog properties to their traditional electronic counterparts. In the last years, nanofluidic devices with memristive properties have attracted special interest due to their applicability in, for example, the construction of brain-like computing systems. In this work, an asymmetric track-etched nanofluidic channel with memory-enhanced ion transport is reported. The results illustrate that the formation of nanoprecipitates on the channel walls induces memory effects in ion transport, leading to characteristic hysteresis loops in the current-voltage curves, a hallmark of memristive behavior. Notably, these memristive properties are achievable with a straightforward experimental setup that combines an aqueous solvent and a relatively low-soluble inorganic salt. The various conductance states can be rapidly and reversibly tuned over prolonged time scales. Furthermore, under appropriate measurement conditions, the nanofluidic device can alternate between different iontronic regimes and states, encompassing ion current rectification, ON-OFF states, and memristor-like behavior. These findings provide insights into the design and optimization of nanofluidic devices for bioinspired ionic circuit components.

Direct Ionic Ink Writing on and Penetrating into Elastomer for Patternable, Waterproof, and Wear‐Resistant Ionic Circuits

Direct Ionic Ink Writing on and Penetrating into Elastomer for Patternable, Waterproof, and Wear‐Resistant Ionic Circuits

Hydrogel ionic diode with ultra-high rectification ratio for ionic circuit

Organisms transmit signals through complex ion systems, and signal transmission in artificial electronic circuits mainly depends on electrons. The development of rectifier ionic diode, grounded in ion transport principles, paves ways for artificial bionic circuit and seamless human–machine interaction. Among its pivotal performance indicators, the ion rectification ratio (IR) is of primal importance, facilitating enhanced ion selectivity, bolstered energy efficiency, and robust, high-performing operations. Here, hybrid hydrogel ionic diode based on chitosan (CS) doped with 5,10,15,20-tetrakis(4-aminophenyl)-21,23H-porphyrin (TAPP)/sodium polyacrylate (PAAs) doped with 5,10,15,20-tetrakis(4-sulfonatophenyl)-21,23H-porphyrin (TPPS) was constructed to achieve a high IR. This design achieved a remarkable feat by elevating the IR from 5.57 to an unprecedented 158.64, surpassing previous benchmarks set by hydrogel-based ionic diodes. Moreover, this study emphasizes the selectivity of polyelectrolytes to different porphyrin structures, as well as the charge properties of the groups carried by porphyrins. It delves into the fundamental mechanisms underpinning their interactions, providing a comprehensive understanding of their behavior. In order to demonstrate the excellent rectification characteristics and practical applicability of the CS-TAPP/PAAs-TPPS ionic diode, we successfully integrated it into the full-wave rectification and logic gate system. The full-wave rectifier can convert a 1.2 V sinusoidal voltage input to a 0.8 V full-wave voltage output. This shows the advantages of high IR ionic diodes. It offers insights for the development of high-performance hydrogel-based ionic diodes, and promotes the advancement of novel biomimetic devices and ionic circuits.

Frequency-dependent electrochemical breakdown of hydrogel ionotronics

When an ionic conductor meets an electronic conductor, ions and electrons couple at the interface between ionic and electronic circuits, enabling a class of devices: ionotronics. Such a coupling gives innovations but can cause electrochemical breakdown. Rapid advances in hydrogel ionotronics, in which hydrogel serves as stretchable, transparent, ionic conductor, highlight an urgent need: a general approach and criterion to study electrochemical breakdown under AC condition. Here we study the breakdown behavior of a metal-hydrogel-metal ionotronic system subject to alternating voltages of various frequencies and amplitudes. First, we apply a sinusoidal voltage to the ionotronic system which is connected in series to a load resistor, and compare the waveforms of the applied voltage and the output voltage between the two ends of the load resistor at various frequencies and amplitudes. Electrochemical breakdown tends to take place at low frequencies and high amplitudes, and causes distortion of waveform of the output voltage. Next, we develop an electric circuit model, in which each hydrogel-metal junction is modeled as a constant phase element in parallel with a leakage resistor and the hydrogel is modeled as a resistor. Our experiments show that the electrochemical window is insensitive to the frequency and amplitude of applied voltage, as well as to the concentration of ions in the hydrogel. Through a combination of experiment and theory, we further propose a frequency-amplitude phase diagram. Different from DC condition, the breakdown behavior under AC condition depends not only on the amplitude of applied voltage, but also on the frequency of applied voltage. We also show that electrochemical reaction can be retarded by stretching the hydrogel and by increasing the frequency. It is hoped that this work will guide the development of stable and reliable hydrogel ionotronic devices.

Ultrathin Ionic Diodes with Electrostatically Heterogeneous Hybrid Interfaces of Nanoporous SiO2 Nanofilms and Polymer Layer-by-Layer Multilayers.

Nanofluidic ionic diodes have attracted much attention due to their unique functions as unidirectional ion transportation ability and promising applications from molecular sensing, and energy harvesting to emerging neuromorphic devices. However, it remains a challenge to fabricate diode-like nanofluidic systems with ultrathin film thickness <100nm. Herein the formation of ultrathin ionic diodes from hybrid nanoassemblies of nanoporous (NP) SiO2 nanofilms and polyelectrolyte layer-by-layer (LbL) multilayers is described. Ultrathin ionic diodes are prepared by integrating polyelectrolyte multilayers onto photo-oxidized NP SiO2 nanofilms obtained from silsesquioxane-containing block copolymer thin films as a template. The obtained ultrathin ionic diodes exhibit ion current rectification (ICR) properties with high ICR factor = ≈20 under low ionic strength and asymmetric pH conditions. It is concluded that this ICR behavior arises from effective ion accumulation and depletion at the interface of NP SiO2 nanofilms and LbL multilayers attributed to high ion selectivity by combining the experimental data and theoretical calculations using finite element methods. These results demonstrate that the hybrid nano assemblies of NP SiO2 nanofilms and polyelectrolyte LbL multilayers have potential applications for (bio)sensing materials and integrated ionic circuits for seamless connection of human-machine interfaces.

Rectification in Ionic Field Effect Transistors Based on Single Crystal Silicon Nanopore

AbstractIonic FETs have enormous potential for energy conversion, sensing, and ionic circuits due to their efficient regulation of the nanochannel. Here ionic FETs based on single‐crystal silicon nanopores and the rectification of the fabricated devices are studied. The electrical characterization results demonstrated that since the silicon‐based nanopores have the advantage of modulating the surface charge due to their semiconductor nature and benefitting from the effective 3D gating effect on the nanochannel, the magnitude and polarity of surface charge can be modulated by the gate voltage. The rectification effect can be adjusted by applying a certain voltage and fulfilling a transition between anion selectivity and cation selectivity when the surface charge polarity is reversed. Moreover, current–voltage characteristics of the reported ionic FET can be switched between ohmic and diode‐like regimes. The proposed ionic FETs supply a novel platform to study the ionic properties and have great potential to be applied in large‐scale ionic circuits due to their excellent performance. Finally, simulation results prove the surface charge modulated by the gate voltage determines the magnitude and direction of rectification, which is consistent with the reported experiment result.

Bioinspired carbon nanotube-based nanofluidic ionic transistor with ultrahigh switching capabilities for logic circuits.

The voltage-gated ion channels, also known as ionic transistors, play substantial roles in biological systems and ion-ion selective separation. However, implementing the ultrafast switchable capabilities and polarity switching of ionic transistors remains a challenge. Here, we report a nanofluidic ionic transistor based on carbon nanotubes, which exhibits an on/off ratio of 104 at operational gate voltage as low as 1 V. By controlling the morphology of carbon nanotubes, both unipolar and ambipolar ionic transistors are realized, and their on/off ratio can be further improved by introducing an Al2O3 dielectric layer. Meanwhile, this ionic transistor enables the polarity switching between p-type and n-type by controlled surface properties of carbon nanotubes. The implementation of constructing ionic circuits based on ionic transistors is demonstrated, which enables the creation of NOT, NAND, and NOR logic gates. The ionic transistors are expected to have profound implications for low-energy consumption computing devices and brain-machine interfacing.

Bio‐inspired Two‐dimensional Nanofluidic Ionic Transistor for Neuromorphic Signal Processing

AbstractVoltage‐gated ion channels prevalent in neurons play important roles in generating action potential and information transmission by responding to transmembrane potential. Fabricating bio‐inspired ionic transistors with ions as charge carriers will be crucial for realizing neuro‐inspired devices and brain‐liking computing. Here, we reported a two‐dimensional nanofluidic ionic transistor based on a MXene membrane with sub‐1 nm interlayer channels. By applying a gating voltage on the MXene nanofluidic, a transmembrane potential will be generated to active the ionic transistor, which is similar to the transmembrane potential of neuron cells and can be effectively regulated by changing membrane parameters, e.g., thickness, composition, and interlayer spacing. For the symmetric MXene nanofluidic, a high on/off ratio of ~2000 can be achieved by forming an ionic depletion or accumulation zone, contingent on the sign of the gating potential. An asymmetric PET/MXene‐composited nanofluidic transitioned the ionic transistor from ambipolar to unipolar, resulting in a more sensitive gate voltage characteristic with a low subthreshold swing of 560 mV/decade. Furthermore, ionic logic gate circuits, including the “NOT”, “NAND”, and “NOR” gate, were implemented for neuromorphic signal processing successfully, which provides a promising pathway towards highly parallel, low energy consumption, and ion‐based brain‐like computing.

Bio-inspired Two-dimensional Nanofluidic Ionic Transistor for Neuromorphic Signal Processing.

Voltage-gated ion channels prevalent in neurons play important roles in generating action potential and information transmission by responding to transmembrane potential. Fabricating bio-inspired ionic transistors with ions as charge carriers will be crucial for realizing neuro-inspired devices and brain-liking computing. Here, we reported a two-dimensional nanofluidic ionic transistor based on a MXene membrane with sub-1 nm interlayer channels. By applying a gating voltage on the MXene nanofluidic, a transmembrane potential will be generated to active the ionic transistor, which is similar to the transmembrane potential of neuron cells and can be effectively regulated by changing membrane parameters, e.g., thickness, composition, and interlayer spacing. For the symmetric MXene nanofluidic, a high on/off ratio of ~2000 can be achieved by forming an ionic depletion or accumulation zone, contingent on the sign of the gating potential. An asymmetric PET/MXene-composited nanofluidic transitioned the ionic transistor from ambipolar to unipolar, resulting in a more sensitive gate voltage characteristic with a low subthreshold swing of 560 mV/decade. Furthermore, ionic logic gate circuits, including the "NOT", "NAND", and "NOR" gate, were implemented for neuromorphic signal processing successfully, which provides a promising pathway towards highly parallel, low energy consumption, and ion-based brain-like computing.

Molecular Structure Effects on Ionic Diode Performance in Desalination: Ultrahigh Rectification in Butylated Intrinsically Microporous Polyamine (PIM‐EA‐TB)

AbstractIonic diodes are components in ionic circuits for AC‐electricity driven desalination and ion extraction processes. Independent of the ionic diode type/mechanism, achieving ionic current rectification at high ionic strengths is challenging but important, for example in seawater desalination and treatment of brine. Here, the butylation of a molecularly rigid (glassy) polymer of intrinsic microporosity (PIM‐EA‐TB) is shown to give anionic diodes with ultrahigh rectification effects (associated with interfacial resistivity) even in high ionic strength aqueous 2 M NaCl solution. The effect is rationalised based on polymer structure affecting ion transport.

Nanoscale electrohydrodynamic ion transport: Influences of channel geometry and polarization-induced surface charges.

Electrohydrodynamic ion transport has been studied in nanotubes, nanoslits, and nanopores to mimic the advanced functionalities of biological ion channels. However, probing how the intricate interplay between the electrical and mechanical interactions affects ion conduction in asymmetric nanoconduits presents further obstacles. Here, ion transport across a conical nanopore embedded in a polarizable membrane under an electric field and pressure is analyzed by numerically solving a continuum model based on the Poisson, Nernst-Planck, and Navier-Stokes equations. We report an anomalous ionic current depletion, of up to 75%, and an unexpected rise in current rectification when pressure is exerted along the external electric field. Membrane polarization is revealed as the prerequisite to obtain this previously undetected electrohydrodynamic coupling. The electric field induces large surface charges at the pore tip due to its conical shape, creating nonuniform electrical double layers (EDL) with a massive accumulation of electrolyte ions near the orifice. Once applied, the pressure distorts the quasiequilibrium distribution of the EDL ions to influence the nanopore conductivity. Our fundamental approach to inspect the effect of pressure on the channel EDL (and thus ionic conductance) in contrast to its effect on the current arising from the hydrodynamic streaming of ions further explains the pressure-sensitive ion transport in different nanochannels and physical regimes manifested in past experiments, including the hitherto inexplicit mechanism behind the mechanically activated ion transport in carbon nanotubes. This enhances our broad understanding of nanoscale electrohydrodynamic ion transport, yielding a platform to build nanofluidic devices and ionic circuits with more robust and tunable responses to electrical and mechanical stimuli.

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.

Ion Concentration Polarization Tunes Interpore Interactions and Transport Properties of Nanopore Arrays

AbstractBiological processes require concerted function of many channels embedded in the cell membrane. While single solid‐state nanopores are already designed to mimic properties of individual biological channels, it is not yet known how to connect the pores to achieve biomimetic ionic circuits with interacting components. To identify fundamental processes that control interactions between nanopores embedded in the same membrane, a model system of minimal arrays consisting of two and three nanopores in silicon nitride films is designed. The constituent nanopores have an opening diameter &lt;10 nm, and the interpore spacing is tuned between 15 and 200 nm. The experimental and modeling results reveal that nanopores in an array interact with each other via overlapping depletion zones created by the process of concentration polarization. The interactions can be further controlled by salt concentration and voltage. These results showcase a possibility of tuning interactions between nanopores and transport properties of arrays by chemical modification of the pore walls. Arrays consisting of nanoporous ionic diodes feature depletion zones with higher concentrations, and lower current suppression than homogeneously charged pores. These experiments and modeling provide the first steps to leave the constraints of single nanopores and to design biomimetic ionic circuits.

Dynamic asymmetric mechanical responsive carbon nanotube fiber for ionic logic gate

Inspired by biological ion channels, numerous artificial asymmetric ion channels have been synthesized to facilitate the fabrication of ionic circuits. Nevertheless, the creation of biomimetic asymmetric ion channels necessitates expensive scientific apparatus and intricate material processing procedures, which constrains its advancement within the realm of ionic devices. In this study, we have devised dynamic asymmetric ion channels with mechanical responsiveness by combining polymers of varying elastic modulus along the longitudinal axis of carbon nanotube fiber (CNTF). The ion rectification can be modulated via the disparate response of CNTF-based ion channels to mechanical stress. We have effectively employed these asymmetric ion channels with mechanical sensitivity in the design of a logic gate device, achieving logic operations such as “AND” and “OR”. The conception of these dynamic asymmetric ion channels with mechanical sensitivity offers a straightforward, cost-effective, and versatile approach for generating ion channels, highlighting their potential application in intricate, highly integrated ionic circuits.

Stretchable, Programmable and Magnet-Insensitive Protonic Display Based on Integrated Ionic Circuit.

As a new approach to "More than Moore", integrated ionic circuits serve as a possible alternative to traditional electronic circuits, yet the integrated ionic circuit composed of functional ionic elements and ionic connections is still challenging. Herein, a stretchable and transparent ionic display module of the integrated ionic circuit has been successfully prepared and demonstrated by pixelating a proton-responsive hydrogel. It is programmed to excite the hydrogel color change by a Faraday process occurring at the electrode at the specific pixel points, which enables the display of digital information and even color information. Importantly, the display module exhibits stable performance under strong magnetic field conditions (1.7T). The transparent and stretchable nature of such ionic modules also allows them to be utilized in a broad range of scenarios, which paves the way for integrated ionic circuits.

Ultrasensitive Flexible Thermal Sensor Arrays based on High-Thermopower Ionic Thermoelectric Hydrogel.

Ionic circuits using ions as charge carriers have demonstrated great potential for flexible and bioinspired electronics. The emerging ionic thermoelectric (iTE) materials can generate a potential difference by virtue of selective thermal diffusion of ions, which provide a new route for thermal sensing with the merits of high flexibility, low cost, and high thermopower. Here, ultrasensitive flexible thermal sensor arrays based on an iTE hydrogel consisting of polyquaternium-10 (PQ-10), a cellulose derivative, as the polymer matrix and sodium hydroxide (NaOH) as the ion source are reported. The developed PQ-10/NaOH iTE hydrogel achieves a thermopower of 24.17mV K-1 , which is among the highest values reported for biopolymer-based iTE materials. The high p-type thermopower can be attributed to thermodiffusion of Na+ ions under a temperature gradient, while the movement of OH- ions is impeded by the strong electrostatic interaction with the positively charged quaternary amine groups of PQ-10. Flexible thermal sensor arrays are developed through patterning the PQ-10/NaOH iTE hydrogel on flexible printed circuit boards, which can perceive spatial thermal signals with high sensitivity. A smart glove integrated with multiple thermal sensor arrays is further demonstrated, which endows a prosthetic hand with thermal sensation for human-machine interaction.

Gated ion transport in disjoint carbon nanotubes by a water bridge

Nanofluidic ion transport in carbon nanotubes (CNTs) has numerous applications, including membrane separation, energy conversion, and ionic circuits. To imitate the natural gated ion channel systems, CNTs require a more complex structure beyond a single intact tube. In this study, we present molecular dynamics simulations demonstrating the gated ion transport in two disjoint CNTs with a nanogap. The observation of nonlinear current-voltage characteristics, featuring onset and plateau voltages for ion transport, indicates that the nanogap serves as a gate. Ions cannot transport through the disjoint CNTs below the onset voltage. Interestingly, a water bridge forms occasionally within the nanogap above the onset voltage, allowing ions to transport through coupling with water molecules. Above the plateau voltage, a stable water bridge forms and ions can transport in disjoint CNTs with the same efficiency as in the intact CNT. The formation of water bridges is due to that the electric stress on polarized water molecules near the nanogap overcomes the surface tension. Furthermore, the gated ion transport behavior is dependent on the nanogap length, CNT diameter, and CNT wettability, but not on the ion concentration. This work offers a viable approach for developing CNT-based nanofluidic devices with finely tuned ion transport efficiency.

Photoexcited Anhydrous Proton Conductivity in Coordination Polymer Glass

Optically switchable proton-conductive materials will enable the development of artificial ionic circuits. However, most switchable platforms rely on conformational changes in crystals to alter the connectivity of guest molecules. Guest dependency, low transmittance, and poor processability of polycrystalline materials hinder overall light responsiveness and contrast between on and off states. Here, we optically control anhydrous proton conductivity in a transparent coordination polymer (CP) glass. Photoexcitation of tris(bipyrazine)ruthenium(II) complex in CP glass causes reversible increases in proton conductivity by a factor of 181.9 and a decrease in activation energy barrier from 0.76 eV to 0.30 eV. Modulating light intensity and ambient temperature enables total control of anhydrous protonic conductivity. Spectroscopies and density functional theory studies reveal the relationship between the presence of proton deficiencies and the decreasing activation energy barrier for proton migrations.

Designing with Iontronic Logic Gates─From a Single Polyelectrolyte Diode to an Integrated Ionic Circuit.

This article presents the implementation of on-chip iontronic circuits via small-scale integration of multiple ionic logic gates made of bipolar polyelectrolyte diodes. These ionic circuits are analogous to solid-state electronic circuits, with ions as the charge carriers instead of electrons/holes. We experimentally characterize the responses of a single fluidic diode made of a junction of oppositely charged polyelectrolytes (i.e., anion and cation exchange membranes), with a similar underlying mechanism as a solid-state p- and n-type junction. This served to carry out predesigned logical computations in various architectures by integrating multiple diode-based logic gates, where the electrical signal between the integrated gates was transmitted entirely through ions. The findings shed light on the limitations affecting the number of logic gates that can be integrated, the degradation of the electrical signal, their transient response, and the design rules that can improve the performance of iontronic circuits.

High‐Performance Integrated Iontronic Circuits Based on Single Nano/Microchannels

Recently, artificial channel-based ionic diodes and transistors are extensively studied to mimic biological systems. Most of them are constructed vertically and are challenging to be further integrated. Several examples of ionic circuits with horizontal ionic diodes are reported. However, they generally require nanoscale channel sizes to meet the demand for ion-selectivity, resulting in low current output and restricting potential applications. In this paper, a novel ionic diode is developed based on multiple-layer polyelectrolyte nanochannel network membranes. Both bipolar and unipolar ionic diodes can be achieved by simply switching the modification solution. Ionic diodes with a high rectification ratio of ≈226 are achieved in single channels with the largest channel size of 2.5µm. This design can significantly reduce the channel size requirement and improve the output current level of ionic devices. The high-performance ionic diode with a horizontal structure enables the integration of advanced iontronic circuits. Ionic transistors, logic gates, and rectifiers are fabricated on a single chip and demonstrated for current rectification. Furthermore, the excellent current rectification ratio and the high output current of the on-chip ionic devices highlight the promise of the ionic diode as a component of complex iontronic systems for practical applications.