Ionic Devices Research Articles

Iontronics: Aqueous ion-based engineering for bioinspired functionalities and applications

Iontronics is an artificial platform using ions or molecules as signal carriers in an aqueous environment and is inspired by biological systems and their operating principles. Applications of iontronics have been primarily developed to mimic the characteristics of biological systems or to form seamless biointerfaces for communication. This review provides a comprehensive description of such endeavors in iontronics over the recent decades, as well as demonstrations pertaining to biomimetic nonlinear behaviors and ionic chemical delivery devices. The research highlights and applications are discussed based on the types of charge-selective materials used and their underlying principles. As iontronics is still at the early stage of development and diversification, a brief overview of its historical aspects and origin is first provided, followed by theoretical discussions regarding each iontronic material and its related applications. Finally, the review is concluded with some perspectives regarding future developments of iontronics in relation to natural systems in living organisms.

High-resolution patterning of silica nanoparticle-based ionogels by reverse-offset printing and its characterization

In this study, nanoparticle-based, high-resolution patternable ionogels are presented to provide a route for realizing printed solid-state ionic devices. By incorporating an ionic liquid (IL) into a spherical silica nanoparticle suspension, a quasi-solid ionogel layer compatible with reverse-offset printing (ROP) with a spatial resolution of approximately 5 μm was realized. In situ near-infrared (NIR) spectroscopic analysis revealed the drying kinetics of the ionogel ink during printing, and a temporal margin for successful patterning in relation to its dry state was provided. In contrast to polymer-based gels, the present ionogel can be regarded as a porous medium of silica filled with ionic liquids with a certain degree of saturation. By optimizing the ink formulations, ROP patterning was successful for saturation up to 102%, indicating the nanoscale pores between silica nanoparticles can be fully used as an ion-conductive phase in the proposed patternable gel. The conductivity depends drastically on saturation, with a saturation exponent of approximately −7 according to Archie’s law. From a complementary scratch test, an ionogel at a saturated condition still exhibited fragile but solid-like characteristics. As a demonstration, planar micro-supercapacitors fully printed with reverse-offset printable ionogel and carbon inks were fabricated. A comparison with a drop-casted IL showing a similar capacitance indicates a limited ability of the carbon nanoparticle material used here, while a relatively high resistance of the silica-nanoparticle-based ionogel hinders a fast cyclic voltammetry response.

Dynamic Nonlinear Behavior of Ionic Liquid-Based Reservoir Computing Devices.

Herein, a physical reservoir device that uses faradaic currents generated by redox reactions of metal ions in ionic liquids was developed. Synthetic time-series data consisting of randomly arranged binary number sequences (“1” and “0”) were applied as isosceles-triangular voltage pulses with positive and negative voltage heights, respectively, and the effects of the faradaic current on short-term memory and parity-check task accuracies were verified. The current signal for the first half of the triangular voltage-pulse period, which contained a much higher faradaic current component compared to that of the second half of the triangular voltage-pulse period, enabled higher short-term memory task accuracy. Furthermore, when parity-check tasks were performed using a faradaic current generated by asymmetric triangular voltage-pulse levels of 1 and 0, the parity-check task accuracy was approximately eight times higher than that of the symmetric triangular voltage pulse in terms of the correlation coefficient between the output signal and target data. These results demonstrate the advantage of the faradaic current on both the short-term memory characteristics and nonlinear conversion capabilities and are expected to provide guidance for designing and controlling various physical reservoir devices that utilize electrochemical reactions.

Soft Ionics: Governing Physics and State of Technologies

Soft ionic materials combine charged mobile species and tailored polymer structures in a manner that enables a wide array of functional devices. Traditional metal and silicon electronics are limited to two charge carriers: electrons and holes. Ionic devices hold the promise of using the wide range of chemical and molecular properties of mobile ions and polymer functional groups to enable flexible conductors, chemically specific sensors, bio-compatible interfaces, and deformable digital or analog signal processors. Stand alone ionic devices would need to have five key capabilities: signal transmission, energy conversion/harvesting, sensing, actuation, and signal processing. With the great promise of ionically-conducting materials and ionic devices, there are several fields working independently on pieces of the puzzle. These fields range from waste-water treatment research to soft robotics and bio-interface research. In this review, we first present the underlying physical principles that govern the behavior of soft ionic materials and devices. We then discuss the progress that has been made on each of the potential device components, bringing together findings from a range of research fields, and conclude with discussion of opportunities for future research.

Angstrofluidics: Walking to the Limit

Angstrom-scale fluidic channels are ubiquitous in nature and play an important role in regulating cellular traffic, signaling, and responding to stimuli. Synthetic angstrom channels are now a reality with the emergence of several cutting-edge bottom-up and top-down fabrication methods. In particular, the use of atomically thin 2D materials and nanotubes as components to build fluidic conduits has pushed the limits of fabrication to the angstrom scale. Here, we provide an overview of recent developments in the fabrication methods for nano- and angstrofluidic channels while categorizing them on the basis of dimensionality (0D pores, 1D tubes, 2D slits), along with the latest advances in measurement techniques. We discuss the ion transport governed by various stimuli in these channels and the variation of ionic mobility, streaming power, and osmotic power with pore size across all the dimensionalities. Finally, we highlight unique future opportunities in the development of smart ionic devices.

Principles of creating the devices that are able to control the current flow in the second class conductors

From the position of conceptions inherent to semiconductor physics and semiconductor technology, the processes occurring in conductors of the second class, which take place due to the current flow in them, have been considered in this paper. It has been shown that using the several composite materials consisting of a porous matrix filled with an electrolyte enables to obtain multilayer structures, in which potential barriers appear in the electrolyte medium. These barriers are caused by the difference in properties inherent to layers of porous matrix. Considered here has been the prospect of creating the new devices based on the second-class conductors capable to control the direction and magnitude of the current flow in these conductors that are in a liquid state (in electrolytes). In other words, it was considered the possibility to obtain the ionic devices that are closest in their functional properties to basic electronic devices, such as diodes, transistors, etc.

Transmembrane transport characterization across ionic redox transistors using surface-tracked scanning ion conductance microscopy

A fundamental understanding of ion transport at the nanoscale is critical to the development of efficient chemical separation membranes, catalysts, ionic/bio-inspired materials, and its scale up into multi-functional ionic devices. Electrochemical imaging using scanning probe microscopy hardware has provided a method to visualize and understand processes that occur at the surface of ionic active materials. The suite of scanning probe microscopy techniques developed over the last few years are limited to imaging surface-level phenomena and have not been applied to investigate transmembrane properties of synthetic and natural membranes with high spatial and temporal resolution. In this article, we demonstrate the application our recently developed ‘surface-tracked scanning ion conductance microscopy’ technique to characterize voltage-regulated ion transport in an ionic redox transistor. The ionic redox transistor exhibits controlled transmembrane ion transport as a function of its electrochemical redox state. The technique presented in this article uses shear force measured between the nanopipette and ionic substrate to image topography of the porous substrate and simultaneously characterize topography-correlated transmembrane transport through the ionic redox transistor. The transmembrane conductance measured across an array of pores within the ionic redox transistor varies from 0.004 μS/cm (OFF state) to 0.015 μS/cm (ON state). We anticipate that the spatial correlation of transmembrane ion transport in the ionic redox transistor would result in a scale up into smart membrane separators for energy storage, neuromorphic circuits, and desalination membranes.

Synaptic Current Response of a Liquid Ga Electrode via a Surface Electrochemical Redox Reaction in a NaOH Solution.

An ionic device using a liquid Ga electrode in a 1 M NaOH solution is proposed to generate artificial neural spike signals. The oxidation and reduction at the liquid Ga surface were investigated for different bias voltages at 50 °C. When the positive sweep voltage from the starting voltage (VS) of 1 V was applied to the Ga electrode, the oxidation current flowed immediately and decreased exponentially with time. The spike and decay current behavior resembled the polarization and depolarization at the influx and extrusion of Ca2+ in biological synapses. Different average decay times of ∼81 and ∼310 ms were implemented for VS of −2 and −5 V, respectively, to mimic the synaptic responses to short- and long-term plasticity; these decay states can be exploited for application in binary electrochemical memory devices. The oxidation mechanism of liquid Ga was studied. The differences in Ga ion concentration due to VS led to differences in oxidation behavior. Our device is beneficial for the organ cell–machine interface system because liquid Ga is biocompatible and flexible; thus, it can be applied in biocompatible and flexible neuromorphic device development for neuroprosthetics, human cell–machine interface formation, and personal health care monitoring.

Ultra-stretchable yet tough, healable, and biodegradable triboelectric devices with microstructured and ionically crosslinked biogel

To reduce the environmental impact of non-biodegradable electronic waste, developing sustainable technology with biomass-derived biodegradable materials are essential. However, the insufficient mechanical and electrical performances of the conventional biodegradable materials with planar structures often limit their use in bioelectronics. Here, we develop a high-performance ionic biogel device based on three-dimensional (3D) microstructured design of completely healable yet fully biodegradable biogel by using ionically cross-linked biomass resource, gelatin. The stress-absorbing geometry of 3D microstructure improves the mechanical resilience and facilitates highly elastic (~4000%), notch-tolerable and extremely tough (~ 10,998 J/m2) ionic biogels. In addition, the interlocked feature of 3D architecture provides the ionic diode characteristics of the biogel that enhances the triboelectric energy harvesting capability from external stimuli of pressure and temperature, even under an extreme stretching condition. Our triboelectric nanogenerator based on 3D ionic biogels exhibits excellent power output (~325 mW/m2), superior energy conversion efficiency (~70.7%) and high-resolution mechano- (~ 9 Pa) as well as thermo- (~ 0.03 K) transduction functionalities with long-term stability. The 3D ionic biogel recovers its original electrical properties even after mechanical damage through self-healing. For proof-of-concept demonstrations, the gelatin biogel serve in soft and conformable electronic skins to monitor low-frequency vital signs and high-frequency acoustic waves, for haptic perception of surface textures, and in robotic tactile skins, providing a new benchmark as a clean and green technology for soft bio-electronic devices with zero waste.

Ionic diode-based self-powered ionic skins with multiple sensory capabilities

Ionic diode consisting of a pair of polycation and polyanion conductors can serve as self-powered ionic skin capable of transforming external stimuli into the potential signals through ions like human skin. The major barriers to their applications include unclear mechanoelectric conversion mechanism, limited transparency and stretchability, and singular function of pressure perception. Herein, we present a versatile strategy for constructing transparent and stretchable ionic diode-based self-powered multifunctional skins composed of a hydrogel-based ionic diode sandwiched between two ionic hydrogel electrodes. Benefiting from such sandwich structure, the self-powered skins can detect small variations in not only pressure/strain, but temperature, salinity and pH with high sensitivity and reproducibility based on thermodiffusion or salinity gradient-induced change of potential. In particular, we demonstrate for the first time that the ionic diode-based devices constitute two mechanoelectric conversion mechanisms, namely the thickness-dependent self-induced potentials of ionic diodes and varying potential losses at the diode/electrode interfaces. This work provides a route to the construction of high-performance bionic ionic sensory systems.

Ionic liquid multistate resistive switching characteristics in two terminal soft and flexible discrete channels for neuromorphic computing

By exploiting ion transport phenomena in a soft and flexible discrete channel, liquid material conductance can be controlled by using an electrical input signal, which results in analog neuromorphic behavior. This paper proposes an ionic liquid (IL) multistate resistive switching device capable of mimicking synapse analog behavior by using IL BMIM FeCL4 and H2O into the two ends of a discrete polydimethylsiloxane (PDMS) channel. The spike rate-dependent plasticity (SRDP) and spike-timing-dependent plasticity (STDP) behavior are highly stable by modulating the input signal. Furthermore, the discrete channel device presents highly durable performance under mechanical bending and stretching. Using the obtained parameters from the proposed ionic liquid-based synaptic device, convolutional neural network simulation runs to an image recognition task, reaching an accuracy of 84%. The bending test of a device opens a new gateway for the future of soft and flexible brain-inspired neuromorphic computing systems for various shaped artificial intelligence applications.

Self-powered and high sensitivity ionic skins by using versatile organogel

Based on organogels, the new ionic skins with self-powered function are highly desirable to develop sensors, human-machine interaction and flexible wearable devices but remain an enormous challenge to develop. Here, we report an ionic skin device based on organogel with surface microstructure, which is flexible, highly sensitive and conductive. It can convert mechanical energy into electrical energy in the case of compression and release. The ionic skin device can not only be made into a flexible pressure sensor with high sensitivity (6.64 kPa−1), but also be assembled into a tensile sensor with high sensitivity (43.02 MPa−1) and high linearity (99.41%) in a wide detection range (0–1.1 MPa). what’s more, the device generated stable electrical signal outputs (current ~ 1.7 mA, voltage ~ 60 mV), which can be used as ionic skin with self-powered function, which has never been reported on other sensors so far. On the basis of eliminating the external energy storage devices, it is safer and easier to carry. In addition, the organogel can be used as flexible electrodes in capacitive sensor, and good adhesion properties can help to improve the performance of sensors. The organogel has also the function of 3D printing, and can design different structures according to requirements. It shows good performance response in monitoring human physiological signals and motion signals, which provides a new direction for the development of intelligent devices.

Monolithic In‐Plane Integration of Gate‐Modulated Switchable Supercapacitors

Monolithic integration of iontronic devices is a key challenge for future miniaturization and system integration. The G‐Cap, a novel iontronic element, is a switchable supercapacitor with gating characteristics comparable to transistors in electronic circuits, but switching relies on ionic currents and ion electroadsorption. The first monolithic in‐plane G‐Cap integration through 3D‐inkjet printing of nanoporous carbon precursors is reported. The printed G‐Cap has a three‐electrode architecture integrating a symmetric “working” supercapacitor (W‐Cap) and a third “gate” electrode (G‐electrode) that reversibly depletes/injects electrolyte ions into the system, effectively controlling the “working” capacitance. The symmetric W‐Cap operates with a proton‐conducting hydrogel electrolyte PVA/H2SO4 and shows a high capacitance (1.6 mF cm−2) that can be switched “on” and “off” by applying a DC bias potential (−1.0 V) at the G‐electrode. This effectively suppresses AC electroadsorption in the nanoporous carbon electrodes of the W‐Cap, resulting in a high capacitance drop from an “on” to an “off” state. The new monolithic structures achieve high rate performance, reversible on‐off switching with an off‐value reaching 0.5 %, which even surpasses recently reported values. Establishing technologies and device architectures for functional ionic electroadsorption devices is crucial for diverse fields ranging from microelectronics and iontronics to biointerfacing and neuromodulation.

Ultra-stretchable and anti-freezing conductive organohydrogel reinforced with ionic clusters for wearable strain sensors

Ionic conductive hydrogels have attracted extensive attention as a substitute for traditional rigid metal conductors, especially in wearable electronic devices. However, to prolong the service life of ionic conductive hydrogel-based devices, the development of highly elastic ionic conductive hydrogels that can remain good performance under extreme environmental conditions (below 0 °C or dry) is still facing great challenges. Herein, a novel double-network (DN) organohydrogel with high tensile strength (~0.9 MPa), elongation at break (~1097%), compressive strength (~6.96 MPa), toughness (~4.75 MJ m−3), transparency (~97%), anti-freezing and ionic conductivity (1.07 mS cm−1) is prepared by photoinitiation polymerization, complexation of transition metal ions, and solvent exchange method using polyacrylamide (PAM) and sodium carboxymethyl cellulose (CMCNa) in dimethyl sulfoxide-water binary solvent system. It is noteworthy that the DN gel design is more compact, and zinc carboxylate complexes composed of CMCNa and Zn2+ form nanoscale metal ionic clusters through coulomb force interaction in the binary solvent environment, which proves the synergistic effect on mechanical properties and conductivity. Moreover, the prepared organohydrogels used as strain sensors have obvious recovery effect, flexibility, high sensitivity (GF=4.38) and stable electrical properties (0.6 mS cm−1 at 30 °C) under the temperature range (−20 to 60 °C). Combined with the above advantages, this conductive organohydrogel promotes the application prospect in different fields, such as soft robots, human-machine interaction, artificial sensors, ionic skin, and optical electronics devices, energy-storage devices, and so on.

Skin-inspired electrochemical tactility and luminescence

Animal skin is deformable, yet capable of consistently sensing external stimuli (mechanical, thermal, chemical) without morphological and functional deterioration. This high deformability and signal stability are ensured because the receptors and the signal transmission systems are based on the electrochemical change in the embedded cells. Further, in the skin of various nocturnal animals and deep-sea creatures, electrochemical changes triggered by mechanical stimuli lead to bioluminescence, which can be effective or direct visual feedback that responses to environmental stress in the biological system. In this regard, recently, researches to artificially imitate electrochemical mechanisms of tactile sensation and luminescence in the skin have made remarkable advances in the field of electronic skin (e-skin). This perspective article introduces recent pivot advances in the ion-based tactile sensors and electrochemiluminescent skins, and suggests the technological challenges, then the article ends with perspectives to the skin-inspired ionic artificial devices.

Bioinspired multifunctional self-powered ionic receptors derived by gradient polyelectrolyte hydrogels

Emerging ionic diode-based self-powered ionic devices are intriguing in mimicking human perceptual systems. However, challenges remain for their applications such as undesired delamination between diode components, single function of press perception, and limited stretchability and transparency caused by metal/carbonous electrodes. Herein, we demonstrate a versatile strategy for constructing multifunctional self-powered ionic receptors/sensors. The gradient polyelectrolyte hydrogel has been firstly demonstrated to hold a pressure-sensitive built-in potential analogous to ionic diodes, but can avoid their defect of delamination. The receptor is assembled by two stretchable ionic hydrogel electrodes and a sandwiched gradient polyelectrolyte hydrogel, resembling the structure of electrolyte/cell membrane/electrolyte of human receptors. Benefitting from such sandwich structure, the as-prepared self-powered receptors are highly transparent, stretchable, and multifunctional. They not only can sensitively perceive tiny pressure change based on the thickness-dependent potentials of gradient hydrogels, but temperature, salinity and pH stimuli according to the potential variations induced by thermodiffusion and salinity gradient. The strategy can inspire the design and fabrication of high-performance self-powered sensors consisting entirely of ionic conductors, contributing to the development of complex humanlike sensing systems in the future.

Modulation of ionic current behaviors based on a dual-channel micro/nano-pipette with ternary-form-charged model

• Ternary-Formed-Charged model is proposed for simulation of ionic behaviors of a dual-channel pipette such as a resistor, diodes, and a bipolar junction transistor. • The uneven distribution and location of surface charge density are confirmed to cause asymmetric bipolar junction transistor behavior and pseudo-bipolar junction transistor behavior. • Scale ratio of radius of the pipette to liquid bridge and the effect of surface charge at the interface between liquid bridge and air is evaluated. Dual-channel micro- or nanopipettes can be used to perform various ionic current behaviors such as a resistor, diodes, and a bipolar junction transistor (BJT) via chemical modification. These ionic current devices could be modulated by charge pattern of modification, pH, size and scale ratio of pipette to the liquid bridge, whose intrinsic mechanism is indistinct. Herein, a universal Ternary-Form-Charged (TFC) model is proposed, in which the pipettes contain three individual tunable regions with different scale, location, surface charge density while ions can flux across the charged liquid bridge connected with them. Based on the TFC model, we could obtain three basic ionic behaviors mentioned above of dual-channel pipettes and demonstrate that these basic ionic behaviors of dual-channel pipette are dependent on the number of transition zone. Transformation from the BJT behavior to a pseudo-BJT behavior is attributed to a narrow middle charged region or a thick liquid bridge. Asymmetric BJT behavior is also observed under asymmetric distribution of location and quantity of surface charge density. Furthermore, we provide the approximate size of liquid bridge: 5–8 nm and 15–20 nm for dual-channel nanopipette and micropipette, respectively. In short, the TFC model and our simulation results shed light on the mechanism of dual-channel pipette based ionic circuits and provide a useful guide to design, modify and develop novel applications of dual-channel pipettes in nanofluidic sensors.

Polymer‐Based Composites for Engineering Organic Memristive Devices

AbstractMemristive materials play a key role in the development of neuromorphic technology given that they can combine information processing with volatile or nonvolatile memory storage in a single computational component. Both functionalities are strictly required for the design and implementation of neuromorphic circuits. Many of these bioinspired materials emulate the characteristics of memory and learning processes that happen in the brain. The memristive properties of a two‐terminal (2‐T) organic device based on ionic migration mediated by an ion‐transport polymer are reported here. The material possesses unique memristive properties: it is reversibly switchable, shows tens of conductive states, presents Hebbian learning demonstrated by spiking time dependent plasticity, and behaves with both short‐ and long‐term memory in a single device. The origin and synergy of both learning phenomena are theoretically explained by means of the chemical interaction between ionic electrolytes and the ion‐conductive mediator. Further discussion on the transport mechanism is included to explain the dynamic behavior of these ionic devices under a variable electric field. This polymer‐based composite as an outstanding neuromorphic material is proposed for being tunable, cheap, flexible, easy to process, reproducible, and more biocompatible than their inorganic analogs.

Temperature Driven Current–Voltage Characteristics of Ionic Liquid Type Intelligent Connection Device

The temperature dependence of an intelligent connection (IConnect) device, in which an ionic liquid (IL) plays an essential role in a memristive function is presented in this study. An appropriate choice of IL and dissolved metal ion species can control the volatility of the IL-IConnect device. The temperature dependence of the IL-IConnect device was strong, although the change in current-voltage characteristics was reversible in response to temperature variations. The device’s potential as a physical reservoir-computing device is studied.

Nanopores in Graphene and Other 2D Materials: A Decade's Journey toward Sequencing.

Nanopore techniques offer a low-cost, label-free, and high-throughput platform that could be used in single-molecule biosensing and in particular DNA sequencing. Since 2010, graphene and other two-dimensional (2D) materials have attracted considerable attention as membranes for producing nanopore devices, owing to their subnanometer thickness that can in theory provide the highest possible spatial resolution of detection. Moreover, 2D materials can be electrically conductive, which potentially enables alternative measurement schemes relying on the transverse current across the membrane material itself and thereby extends the technical capability of traditional ionic current-based nanopore devices. In this review, we discuss key advances in experimental and computational research into DNA sensing with nanopores built from 2D materials, focusing on both the ionic current and transverse current measurement schemes. Challenges associated with the development of 2D material nanopores toward DNA sequencing are further analyzed, concentrating on lowering the noise levels, slowing down DNA translocation, and inhibiting DNA fluctuations inside the pores. Finally, we overview future directions of research that may expedite the emergence of proof-of-concept DNA sequencing with 2D material nanopores.