Multimodal Wearable Ionoskins Enabling Independent Recognition of External Stimuli Without Crosstalk.

Wearable ionoskins are one of the representative examples of the many useful applications offered by deformable stimuli-responsive sensory platforms. Herein, ionotronic thermo-mechano-multimodal response sensors are proposed, which can independently detect changes in temperature and mechanical stimuli without crosstalk. For this purpose, mechanically robust, thermo-responsive ion gels composed of poly(styrene-ran-n-butyl methacrylate) (PS-r-PnBMA, copolymer gelator) and 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMI][TFSI], ionic liquid) are prepared. The optical transmittance change arising from the lower critical solution temperature (LCST) phenomenon between PnBMA and [BMI][TFSI] is exploited to track the external temperature, creating a new concept of the temperature coefficient of transmittance (TCT). The TCT of this system (-11.5% °C-1 ) is observed to be more sensitive to temperature fluctuations than the conventional metric of temperature coefficient of resistance. The tailoring molecular characteristics of gelators selectively improved the mechanical robustness of the gel, providing an additional application opportunity for strain sensors. This functional sensory platform, which is attached to a robot finger, can successfully detect thermal and mechanical environmental changes through variations in the optical (transmittance) and electrical (resistance) properties of the ion gel, respectively, indicating the high practicality of on-skin multimodal wearable sensors.

Polymeric Ion Conductors Based on Sono-Polymerized Zwitterionic Polymers for Electrochromic Supercapacitors with Improved Shelf-Life Stability.

Monolithic electrochromic supercapacitors (ECSs) have attracted increasing interest in recent electrochemical electronics due to their simplicity and unique ability to visually indicate stored energy levels. One crucial challenge for practical use is the improvement of shelf-life. Herein, zwitterionic (ZI) ionogels are proposed as effective electrolytes to reduce the self-discharging decay of ECSs. All-in-one ZI electrochromic (EC) gels are produced by one-pot sono-polymerization. The presence of ZI moieties in the gel does not affect the EC characteristics of chromophores. In addition, excellent capacitive properties in areal capacitance and coulombic efficiency are presented owing to the alignment of ZI units under an electric field and the formation of ion migration channels where rapid ion transport is allowed. Furthermore, the shelf-life of the ZI gel-based ECS is significantly improved by adjusting the interaction between polymeric gelators and ion species. The ZI gel-based ECS is expected to be a key platform for future smart energy storage devices.

Solution-processable electrochemiluminescent ion gels for flexible, low-voltage, emissive displays on plastic.

Ion gels comprising ABA triblock copolymers and ionic liquids have received much attention as functional materials in numerous applications, especially as gate dielectrics in organic transistors. Here we have expanded the functionality of ion gels by demonstrating low-voltage, flexible electrochemiluminescent (ECL) devices using patterned ion gels containing redox-active luminophores. The ECL devices consisted only of a 30 µm thick emissive gel and two electrodes and were fabricated on indium tin oxide-coated substrates (e.g., polyester) simply by solution-casting the ECL gel and brush-painting a top Ag electrode. The triblock copolymer employed in the gel was polystyrene-block-poly(methyl methacrylate)-block-polystyrene, where the solvophobic polystyrene end blocks associate into micellar cross-links in the versatile ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). An ECL gel containing ~6.25 wt % Ru(bpy)3Cl2 (relative to [EMI][TFSI]) as the luminophore turned on at an AC peak-to-peak voltage as low as 2.6 V (i.e., -1.3 to +1.3 V) and showed a relatively rapid response (sub-ms). The wavelength of maximum emission was 610 nm (red-orange). With the use of an iridium(III) complex, Ir(diFppy)2(bpy)PF6 [diFppy = 2-(2',4'-difluorophenyl)pyridine; bpy = 2,2'-bipyridyl], the emitting color was tuned to a maximum wavelength of 540 nm (green). Moreover, when a blended luminophore system containing a 60:40 mixture of Ru(bpy)3(2+) and Ir(diFppy)2(bpy)(+) was used in the emissive layer, the luminance of red-orange-colored light was enhanced by a factor of 2, which is explained by the generation of the additional excited state Ru(bpy)3(2+)* by a coreactant pathway with Ir(diFppy)2(bpy)(+)* in addition to the usual annihilation pathway. This is the first time that enhanced ECL has been achieved in ion gels (or ionic liquids) using a coreactant. Overall, the results indicate that ECL ion gels are attractive multifunctional materials for printed electronics.

Unveiling the Impact of Tailoring Ionic Conductor Characteristics on the Performance of Wearable Triboelectric Nanogenerators.

This work reveals the correlation between the performance of triboelectric nanogenerators (TENGs) and the characteristics of deformable solid-state ionic conductors (referred to as ionogels). For this purpose, we modify ionogel characteristics by incorporating additional plasticizers (propylene carbonate) and solid salts (lithium bis(trifluoromethylsulfonyl)imide) into the ionogels. We conclude that the high capacitance of the ionogel is crucial for achieving a high-performance TENG platform. The optimized ionogel-based TENG (i-TENG) exhibits a power density of ∼372.4 mW·m-2 (based on 95 V and 36 mA·m-2 outputs) with outstanding long-term stability over 2 weeks. Additionally, successful demonstrations of wearable nanogenerators are performed by leveraging the high stretchability (up to ∼1000%) and optical transparency (∼90%) of the ionogels. Overall, the results provide insight into the design of deformable ionic conductors for high-performance, reliable, and wearable TENGs.

Low-Temperature, Universal Synthetic Route for Mesoporous Metal Oxides by Exploiting Synergistic Effect of Thermal Activation and Plasma.

Mesoporous metal oxides exhibit excellent physicochemical properties and are widely used in various fields, including energy storage/conversion, catalysis, and sensors. Although several soft-template approaches are reported, high-temperature calcination for both metal oxide formation and template removal is necessary, which limits direct synthesis on a plastic substrate for flexible devices. Here, a universal synthetic approach that combines thermal activation and oxygen plasma to synthesize diverse mesoporous metal oxides (V2O5, V6O13, TiO2, Nb2O5, WO3, and MoO3) at low temperatures (150-200 °C), which can be applicable to a flexible polymeric substrate is introduced. As a demonstration, a flexible micro-supercapacitor is fabricated by directly synthesizing mesoporous V2O5 on an indium-tin oxide-coated colorless polyimide film. The energy storage performance is well maintained under severe bending conditions.

High-Voltage Pulse-Assisted Operation of Single-Layer Electrochromic Systems for High Performance and Reliability.

A single-layer electrochromic device (SL-ECD) based on ionic conductors containing EC chromophores provides a very simple platform that can be readily fabricated by sandwiching the EC layer between two electrodes. The operation of SL-ECDs is governed by the diffusion of redox species due to their SL structure, which causes a relatively slow dynamic response. In this study, we propose an effective high-voltage pulse injection strategy to improve the performance of SL-ECDs. Applying a programmed voltage wave composed of DC and high-voltage pulses promotes coloration/bleaching switching without degrading device stability, which is more advantageous than applying high DC voltages. We modified the input voltage profile by considering fundamental parameters, such as the amplitude and duty ratio of additional voltage pulses. The coloration and bleaching dynamic responses with the optimized voltage wave are ∼62 and ∼20% faster, respectively, compared with those with the simple DC input. Furthermore, the additionally injected pulse aids in increasing the coloration efficiency from ∼95.3 to ∼168.6 cm2 C-1. Another notable feature of this system is that the device operates stably when a programmed voltage wave is used. These results indicate that the concept of high-voltage pulse-assisted operation of SL-ECDs is a straightforward but effective method for improving device performance without changing the EC chromophore or device structure.

Dynamic Metal-Ligand Coordination-Assisted Ionogels for Deformable Alternating Current Electroluminescent Devices.

Overcoming the trade-off between the mechanical robustness and conductivity of ionic conductors is a crucial challenge for deformable ionotronics. In this work, we propose a simple but effective gelation strategy for selectively improving the mechanical robustness of ionogels without compromising their ionic conductivity. To achieve this, we introduce dynamic metal-ligand coordination chemistry into the ionic liquid (IL)-insoluble domains of a physically crosslinked ionogel network structure. As a result, the overall mechanical property is remarkably improved with the aid of additional chemical crosslinking. This strategy does not require any additional heat/light (UV) treatments to induce chemical crosslinking. The homogeneous physically/chemically dual crosslinked ionogel films can be readily obtained by simply casting a solution containing Ni2+ sources, copolymer gelators, and ILs. The effects of adjusting fundamental parameters on the ionogel properties are investigated systematically. The optimized mechanically robust and highly conductive ionogels are successfully employed as deformable ionic electrodes in alternating-current electroluminescent displays, indicating their high practicality. Overall, these results validate that exploiting metal-ligand coordination dynamic bonding is an extremely straightforward strategy for selectively improving the mechanical characteristics of conductive ionogels, which are promising platforms for deformable ionotronics.

Binary Co-Gelator Strategy: Toward Highly Deformable Ionic Conductors for Wearable Ionoskins.

Stretchable ionic conductors have been actively developed due to the increasing demand for wearable electrochemical platforms. Herein, we propose a convenient and effective strategy for tailoring the mechanical deformability of ionic conductors. The mixing of poly(methyl methacrylate) (PMMA, polymer gelator) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMI][TFSI], ionic liquid) produces mechanically stiff ionic conductors. To reduce the chain entanglement of polymer gelators and induce effective dissipation of applied stresses, flexible poly(butyl acrylate) (PBA) with a low glass-transition temperature is additionally doped into the ionic conductor. An extremely stretchable (∼1500%) homogeneous ternary ionic conductor is obtained without a notable change in electrochemical characteristics, unless the content of PBA exceeds the macrophase separation limit of 3 wt %. In addition, the mechanical elasticity (1.8 × 105 Pa) and durability (e.g., recovery ratio of ∼86.3% after 1000 stretching/releasing cycles) of the conductor further support its suitability as a strain sensory platform. In contrast to conventional ionoskins that have to fit the area of target body parts, even a small piece of the ternary ionic conductor successfully monitors human motion over large areas by taking advantage of its superior deformability.

Recent progress in electrochromic energy storage materials and devices: a minireview.

Integration of several functionalities into one isolated electrochemical body is necessary to realize compact and tiny smart electronics. Recently, two different technologies, electrochromic (EC) materials and energy storage, were combined to create a single system that supports and drives both functions simultaneously. In EC energy storage devices, the characteristic feature of EC materials, their optical modulation depending on the applied voltage, is used to visually identify the stored energy level in real time. Moreover, combining energy-harvesting and EC storage systems by sharing one electrode facilitates the realization of further compact multifunction systems. In this minireview, we highlight recent groundbreaking achievements in EC multifunction systems where the stored energy levels can be visualized using the color of the device.

Ion-cluster-mediated ultrafast self-healable ionoconductors for reconfigurable electronics.

Implementing self-healing capabilities in a deformable platform is one of the critical challenges for achieving future wearable electronics with high durability and reliability. Conventional systems are mostly based on polymeric materials, so their self-healing usually proceeds at elevated temperatures to promote chain flexibility and reduce healing time. Here, we propose an ion-cluster-driven self-healable ionoconductor composed of rationally designed copolymers and ionic liquids. After complete cleavage, the ionoconductor can be repaired with high efficiency (∼90.3%) within 1 min even at 25 °C, which is mainly attributed to the dynamic formation of ion clusters between the charged moieties in copolymers and ionic liquids. By taking advantages of the superior self-healing performance, stretchability (∼1130%), non-volatility (over 6 months), and ability to be easily shaped as desired through cutting and re-assembly protocol, reconfigurable, deformable light-emitting electroluminescent displays are successfully demonstrated as promising electronic platforms for future applications.

DNA Optoelectronics: Versatile Systems for On-Demand Functional Electrochemical Applications.

Herein, we propose innovative deoxyribonucleic acid (DNA)-based gels and their applications in diverse optoelectronics. We prepared the optoelectronic DNA-based gels (OpDNA Gel) through molecular complexation, that is, groove binding and ionic interactions of DNA and 1,1'-diheptyl-4,4'-bipyridinium (DHV). This process is feasible even with sequence-nonspecific DNA extracted from nature (e.g., salmon testes), resulting in the expansion of the application scope of DNA-based gels. OpDNA Gel possessed good mechanical characteristics (e.g., high compressibility, thermoplasticity, and outstanding viscoelastic properties) that have not been observed in typical DNA hydrogels. Moreover, the electrochromic (EC) characteristics of DHV were not lost when combined with OpDNA Gel. By taking advantage of the facile moldability, voltage-tunable EC behavior, and biocompatibility/biodegradability of OpDNA Gel, we successfully demonstrated its applicability in a variety of functional electrochemical systems, including on-demand information coding systems, user-customized EC displays, and microorganism monitoring systems. The OpDNA Gel is a promising platform for the application of DNA-based biomaterials in electrochemical optoelectronics.

Soft Template-Assisted Fabrication of Mesoporous Graphenes for High-Performance Energy Storage Systems.

Graphene is a promising active material for electric double layer supercapacitors (EDLCs) due to its high electric conductivity and lightweight nature. However, for practical uses as a power source of electronic devices, a porous structure is advantageous to maximize specific energy density. Here, we propose a facile fabrication approach of mesoporous graphene (m-G), in which self-assembled mesoporous structures of poly(styrene)-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) are exploited as both mesostructured catalytic template and a carbon source. Notably, the mesostructured catalytic template is sufficient to act as a rigid support without structural collapse, while PS-b-P2VP converts to graphene, generating m-G with a pore diameter of ca. 3.5 nm and high specific surface area of 186 m2/g. When the EDLCs were prepared using the obtained m-G and ionic liquids, excellent electrochemical behaviors were achieved even at high operation voltages (0 ∼ 3.5 V), including a large specific capacitance (130.2 F/g at 0.2 A/g), high-energy density of 55.4 W h/kg at power density of 350 W/kg, and excellent cycle stability (>10,000 cycles). This study demonstrates that m-G is a promising material for high-performance energy storage devices.

A biomimetic ocular prosthesis system: emulating autonomic pupil and corneal reflections.

The human light modulation response allows humans to perceive objects clearly by receiving the appropriate amount of light from the environment. This paper proposes a biomimetic ocular prosthesis system that mimics the human light modulation response capable of pupil and corneal reflections. First, photoinduced synaptic properties of the quantum dot embedded photonic synapse and its biosimilar signal transmission is confirmed. Subsequently, the pupillary light reflex is emulated by incorporating the quantum dot embedded photonic synapse, electrochromic device, and CMOS components. Moreover, a solenoid-based eyelid is connected to the pupillary light reflex system to emulate the corneal reflex. The proposed ocular prosthesis system represents a platform for biomimetic prosthesis that can accommodate an appropriate amount of stimulus by self-regulating the intensity of external stimuli.

Porous Ion Gel: A Versatile Ionotronic Sensory Platform for High-Performance, Wearable Ionoskins with Electrical and Optical Dual Output.

The development of elastic ionic conductors offers opportunities to fabricate key wearable ionic components such as ionoskins that can perceive mechanical deformation. However, there is still plenty of room to overcome the trade-off between sensitivity and detectable range of previous systems and impart additional functionality. Here, we propose porous ion gels for high-performance, functional ionic sensory platforms. The porous ion gels can be effectively deformed by closing pores even with a small pressure, and a large change in the contact area of the gel and the electrode is induced, leading to a significant difference in electrical double-layer capacitance. The porous ion gels are applied to ionoskins after optimizing mechanical characteristics by adjusting gel parameters. The device indicates a high sensitivity of ∼152.8 kPa-1, a broad sensory pressure range (up to 400 kPa), and excellent durability (>6000 cycles). Successful monitoring of various human motions that induce different magnitudes of pressure is demonstrated with high precision. More interestingly, the functionality of the porous ion gel is extended to include electrochemiluminescence (ECL), resulting in the production of emissive ECL ionoskins. The ECL intensity from the emissive ionoskin is linearly correlated with the applied pressure, which can even be inferred even by the naked eye. The porous ion gel-based functional ionoskins are expected to be key components in future sensory ionotronics.

Reliable, High-Performance Electrochromic Supercapacitors Based on Metal-Doped Nickel Oxide.

Herein, high-performance, reliable electrochromic supercapacitors (ECSs) are proposed based on tungsten trioxide (WO3) and nickel oxide (NiO) films. To maximize device performance and stability, the stoichiometric balance between anode and cathode materials is controlled by carefully adjusting the thickness of the anodic NiO film while fixing the thickness of WO3 to ∼660 nm. Then, a small amount (≤10 mol %) of metal (e.g., copper) is doped into the NiO film, improving the electrical conductivity and electrochemical activity. At a Cu doping level of 7 mol %, the resulting ECS exhibited the highest performance, including a high areal capacitance (∼14.9 mF/cm2), excellent coulombic efficiency (∼99%), wide operating temperature range (0-80 °C), reliable operation with high charging/discharging cyclic stability (>10,000 cycles), and good self-discharging durability. Simultaneously, the change in transmittance of the device is well synchronized with the galvanostatic charging/discharging curve by which the real-time energy storage status is visually indicated. Furthermore, the practical feasibility of the device is successfully demonstrated. These results imply that the ECS fabricated in this work is a promising potential energy storage platform and an attractive component for future electronics.

Voltage-Tunable Dual Image of Electrostatic Force-Assisted Dispensing Printed, Tungsten Trioxide-Based Electrochromic Devices with a Symmetric Configuration.

Electrostatic force-assisted dispensing (EFAD)-printed tungsten trioxide (WO3)-based electrochromic devices (ECDs) displaying a dual image depending on the applied voltage are demonstrated. We first print WO3 via EFAD printing, in which the width of the printed lines can be tuned by adjusting the printing speed. The performance of the ECDs is characterized while varying the thickness of the printed WO3 film. It is determined that ∼550 nm thick WO3 is the optimal film considering maximum transmittance contrast (ΔTmax), device dynamic responses, efficiency, and long-term coloration/bleaching cyclic stability. More significantly, the coloration of the devices in this work can alternatively appear due to the use of electrolyte-soluble anodic species (here, dimethyl ferrocene, dmFc), for which WO3 films should be deposited on both electrodes and a part of the electrodes should be exposed to the electrolyte for the oxidation of dmFc. To take advantage of such features of the devices, we successfully demonstrate EFAD-printed, flexible WO3 ECDs alternately displaying a dual image, which is expected to have high potential as a functional component of printed electronics.

Various Coating Methodologies of WO3 According to the Purpose for Electrochromic Devices.

Solution-processable electrochromic (EC) materials have been investigated widely for various applications, such as smart windows, reflective displays, and sensors. Among them, tungsten trioxide (WO3) is an attractive material because it can form a film via a solution process and relative low temperature treatment, which is suitable for a range of substrates. This paper introduces the slot-die and electrostatic force-assisted dispensing (EFAD) printing for solution-processable methods of WO3 film fabrication. The resulting films were compared with WO3 films prepared by spin coating. Both films exhibited a similar morphology and crystalline structure. Furthermore, three different processed WO3 film-based electrochromic devices (ECDs) were prepared and exhibited similar device behaviors. In addition, large area (100 cm2) and patterned ECDs were fabricated using slot-die and EFAD printing. Consequently, slot-die and EFAD printing can be used to commercialize WO3 based-ECDs applications, such as smart windows and reflective displays.

Ultra-Low Power Electrochromic Heat Shutters Through Tailoring Diffusion-Controlled Behaviors.

In this study, we propose low power consumption, all-in-one type electrochromic devices (ECDs) for effective heat shutters. Considering diffusion-controlled device operation, polymeric viologens (poly-viologens) are synthesized to lower the diffusivity of EC chromophores and to minimize self-bleaching. In comparison with devices based on mono-viologens corresponding to the monomer of poly-viologens, poly-viologen-containing ECDs exhibit advantages of lower coloration voltage (ca, -0.55 V) and higher coloration/bleaching cyclic stability (>1500 cycles). In particular, poly-viologen ECDs show remarkably reduced self-bleaching as designed, resulting in extremely low power consumption (∼8.3 µW/cm2) to maintain the colored state. Moreover, we successfully demonstrate solar heat shutters that suppress the increment of indoor temperature by taking the advantage of low-power operation and near-IR absorption of the colored poly-viologen-based ECDs. Overall, these results imply that the control of the diffusivity of EC chromophores is an effective methodology for achieving single-layered, low-power electrochemical heat shutters that can save indoor cooling energy when applied as smart windows for buildings or vehicles.

3D Printed, Customizable, and Multifunctional Smart Electronic Eyeglasses for Wearable Healthcare Systems and Human-Machine Interfaces.

Personal accessories such as glasses and watches that we usually carry in our daily life can yield useful information from the human body, yet most of them are limited to exercise-related parameters or simple heart rates. Since these restricted characteristics might arise from interfaces between the body and items as one of the main reasons, an interface design considering such a factor can provide us with biologically meaningful data. Here, we describe three-dimensional-printed, personalized, multifunctional electronic eyeglasses (E-glasses), not only to monitor various biological phenomena but also to propose a strategy to coordinate the recorded data for active commands and game operations for human-machine interaction (HMI) applications. Soft, highly conductive composite electrodes embedded in the E-glasses enable us to achieve reliable, continuous recordings of physiological activities. UV-responsive, color-tunable lenses using an electrochromic ionic gel offer the functionality of both eyeglass and sunglass modes, and accelerometers provide the capability of tracking precise human postures and behaviors. Detailed studies of electrophysiological signals including electroencephalogram and electrooculogram demonstrate the feasibility of smart electronic glasses for practical use as a platform for future HMI systems.

Asymmetric molecular modification of viologens for highly stable electrochromic devices.

Viologens are one of the most well-known electrochromic (EC) chromophores. In particular, symmetric dialkyl viologens have been widely used in EC devices (ECDs), but suffer from the formation of viologen radical cation dimers that deteriorate device performance. In this work, we propose an effective route to suppress dimer formation through molecularly altering one of the N-substituents. We prepare 1-benzyl-1'-heptyl viologens and find that such asymmetric molecular structures attribute to the suppression of dimer production when used as EC chromophores. The suppression of dimer formation allows us to drive the device at relatively higher voltages, so that we could achieve viologen-based ECDs showing large transmittance changes between colored and bleached states, efficient and fast coloration, and stable coloration/bleaching cyclic operation. The results indicate that high-performance ECDs can be realized by utilizing viologens containing asymmetric molecular structures.