Soft Ionic Materials: Design and Applications in Functional Electrochemical Systems

Abstract

By integrating the rapid ionic transport of ionic liquids with the structural integrity of polymers, ionogels achieve high conductivity, mechanical flexibility, and environmental stability. These attributes position them as promising solid-state electrolytes for soft electronics. Recent molecular innovations have yielded ionogels with remarkable stretchability, toughness, and multifunctionality, broadening their scope of applications. This Perspective highlights molecular-level strategies, such as copolymer design and dynamic cross-linking via ionic or supramolecular interactions, that tailor polymer–ion interactions and network dynamics. We then discuss how these strategies regulate ionicity, diffusivity, and segmental mobility. These microscopic processes ultimately determine macroscopic transport properties and enable advanced devices such as strain sensors, electrochromic supercapacitors, thermoelectric generators, and triboelectric nanogenerators. Finally, by integrating molecular design with mechanistic insight, we provide a forward-looking framework for developing scalable, robust, and adaptive ionogels that underpin next-generation ionotronic systems.

1. Introduction

The rising demand for medical and healthcare electronics has amplified the need for seamless human-machine integration, driving rapid advances in soft electronics. − These systems aim to replicate the mechanical compliance of biological tissues while ensuring stable operation under continuous bending, stretching, and compression. − Realizing such capabilities demands that every component of the electronic system be deformable, including not only substrates and electrodes but also active components such as sensors, − transistors, − and energy-storage units. − Among these, the electrolyte plays a pivotal role, as it governs ion transport, preserves device stability, and ensures consistent performance under mechanical deformation.

To realize such next-generation electronics, ionogels, comprising polymer networks swollen with room-temperature ionic liquids (ILs), have emerged as promising solid-state electrolytes owing to their unique combination of high ionic conductivity, nonvolatility, and mechanical durability. − Compared with hydrogels and organogels, ionogels provide broader electrochemical windows and improved environmental stability, positioning them as versatile platforms for ionotronic applications. Nevertheless, ionogels still face persistent bottlenecks: (i) a mechanical-electrochemical trade-off, , (ii) the difficulty of cointegrating multiple functions (e.g., strong adhesion, self-healing, stimulus responsiveness). ,

Over the past several years, rational molecular-level design of the polymer matrix and ionic liquids has driven significant progress in overcoming these limitations. ,− As a result, a variety of advanced ionogel architectures have been developed, each targeting distinct performance challenges. For instance, double-network ionogels that integrate physical and chemical cross-links achieve exceptional stretchability and energy dissipation. , Nanophase-separated ionogels featuring IL-rich and polymer-rich domains establish bicontinuous ionic pathways, thereby balancing mechanical robustness with efficient ion transport. Finally, hybrid nanocomposite ionogels incorporating conductive fillers (e.g., carbon nanotubes) synergistically enhance both electrical and ionic conductivity. ,

These molecular innovations have expanded the functional landscape of ionogels beyond passive electrolytes. Recent demonstrations span diverse applications, ranging from strain and pressure sensors to solid-state energy systems such as electrochromic supercapacitors (ECSs), thermoelectric generators (TEGs), and triboelectric nanogenerators (TENGs). In this Perspective, we highlight advances in the rational design of ionogels that enable such multifunctional ionotronic platforms. We examine how polymer-IL interactions can be systematically engineered to unlock new properties and discuss integrative strategies that couple molecular design with device-level fabrication. Looking ahead, we identify future opportunities for ionogels, including the development of cost-effective, size-selective anisotropic ion transport architectures. Further directions include ensuring the bio- and eco-safety of ionogel platforms and advancing energy-efficient, adaptive ionogel systems. These directions will be essential for translating ionogels into robust, scalable technologies for next-generation soft electronics.

2. Rational Design Strategies for Functional Ionogels

The internal morphology of ionogels is a key determinant of their electrochemical and mechanical performance. They typically consist of ion-conductive domains swollen with ILs and polymer-rich networks that reinforce the structure. To achieve this balance, copolymer gelators are often designed with segments of contrasting IL affinity: IL-philic domains swell to support ion transport, while IL-phobic segments aggregate into physically cross-linked regions that strengthen the network. This self-assembled architecture enables gel formation without covalent cross-linking. Essentially, the size, distribution, and continuity of these domains must be carefully regulated to avoid macrophase separation. Such separation would disrupt ionic pathways and compromise mechanical integrity.

Block and random copolymers have emerged as versatile design platforms. Block copolymers (e.g., polystyrene-b-poly­(methyl methacrylate)-b-polystyrene (PS-b-PMMA-b-PS; SMS) and PS-b-poly­(ethylene oxide)-b-PS (PS-b-PEO-b-PS; SOS)) offer well-defined microdomains, which enable elastic recovery and structural anisotropy. However, they suffer from synthetic complexity, as multistep synthesis and precise control over block length and composition are required. − In contrast, random copolymers are synthetically more accessible via one-pot polymerizations. They also facilitate a more homogeneous distribution of IL-insoluble moieties, thereby suppressing large-scale phase separation and promoting nanoscale heterogeneity. A direct comparison by Choi et al. illustrates these distinctions. Ionogels from poly­(ethyl methacrylate)-b-polystyrene (PEMA-b-PS) at a polymer/IL ratio of 40/60 formed larger (∼10 nm) IL-insoluble styrene domains upon swelling, producing fewer and more widely spaced physical cross-links. In contrast, PEMA-r-PS generated much smaller (∼1.2 nm), densely distributed styrene-rich domains that acted as efficient cross-linking points. The ionic conductivities of PEMA-r-PS and PEMA-b-PS ionogels are comparable (0.37 vs 0.41 mS·cm^–1), while the elastic modulus of the random copolymer gel is much higher (4.01 × 10⁴ vs 2.86 × 10⁴ Pa). This representative comparison highlights that random copolymerization improves mechanical robustness without compromising ionic conductivity. These distinctions in microphase structures were elucidated by synchrotron small-angle X-ray scattering (SAXS). SAXS measurements, including Guinier and Porod regime fitting, provide quantitative estimates of domain size (R _g) and network conformation (fractal exponent). In addition, differential scanning calorimetry (DSC) detected two distinct glass transitions in block copolymer ionogels, corresponding to polymer agglomerate domains and IL-swollen ionic domains, respectively. By contrast, random copolymer ionogels exhibited a single T _g, reflecting a more homogeneous dispersion of polymer and ionic liquid components.

Recent studies have demonstrated that introducing comonomers with high affinity toward ILs can further suppress this phase separation even at higher polymer loadings, maintaining a stable nanoscale morphology. , For example, PS-r-PMMA forms ionogels with 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMI]­[TFSI]) that undergo macrophase separation when the polymer content exceeds 40 wt %, disrupting the conductive network and degrading performance. In contrast, PS-r-poly­(butyl acrylate) (PS-r-PBA) maintains a well-dispersed microstructure even at comparable or higher polymer loadings, owing to the greater compatibility of PBA segments with the IL. This compatibility not only expands the processing window but also enables higher ionic conductivity. Notably, at identical IL content, PS-r-PBA ionogels exhibit significantly higher ionic conductivity (∼0.51 mS·cm^–1) than their PS-r-PMMA counterparts (0.05 mS·cm^–1). As the IL-affinity domains serve as the primary conduits for ion conduction, their electrochemical function is governed by ion fluxes, typically described by the Nernst–Planck equation: ,

$$J_i=-D_i\nabla c_i-\frac{z_i{D}_{i}F}{RT}{c}_i\nabla \varphi$$ 1

where J _i represents the ionic flux of species i, D _i is the diffusion coefficient, c _i is the ionic concentration, z _i is the valence, F is the Faraday constant, R is the universal gas constant, T is the absolute temperature, and ϕ is the electric potential. The first term accounts for diffusion driven by concentration gradients, while the second term captures drift under electric field gradients.

However, such classical formulations often overlook the complex nature of ionic motion in polymer networks, which can involve electrostatic correlations, ion-pair formation, polymer–ion binding, and spatially constrained transport pathways. To capture these effects, the Onsager formalism within the framework of linear irreversible thermodynamics offers a more comprehensive description:

$$J_i=\sum _j{L}_{ij}(-\nabla \stackrel{\circledR }{{\mu }_j})$$ 2

where $\stackrel{\circledR }{{\mu }_j}$ is the electrochemical potential of species j and L _ij denotes the Onsager transport coefficient, reflecting both ion–ion interactions and the influence of the polymer network. Physically, L _ij describes how the motion of one ionic species is coupled to that of another and is not a fixed constant but depends on the polymer network and dynamics. In ionogels, it is primarily determined by three interrelated factors: ionicity, diffusivity, and polymer segmental dynamics. − Ionicity reflects the degree of ion-pair dissociation, dictated by cation–anion interactions, and sets the concentration of free charge carriers. Diffusivity represents the intrinsic mobility of each ionic species, influenced by their size and charge distribution. Segmental dynamics describes the local mobility of polymer chains, which controls whether ions move in concert with the network or migrate independently.

While these factors appear orthogonal, they are inherently coupled through L _ij , making it a powerful descriptor of macroscopic ion transport. Each of these factors corresponds to a characteristic time scale, ionic cluster relaxation time (τ_IC), ion diffusion time (τ_diff), and segmental relaxation time (τ_seg), providing a quantitative framework to analyze ion transport dynamics (Figure a). Accordingly, the overall ionic conductivity σ can be expressed as

$$\sigma =F^2\sum _{ij}{z}_i{z}_j{L}_{ij}$$ 3

1.

Rational design framework for ionogels based on domain-specific structure and dynamics. (a) Ionic transport within the swollen domains is governed by ionicity, diffusivity, and segmental dynamics, reflecting the transient nature of ion–ion and ion–polymer interactions. (b) Polymer-rich domains are reinforced by noncovalent interactions.

This expression integrates not only individual ion mobility but also collective transport phenomena such as clustering, fixed charge environments, and polymer–ionic interactions. Together, these factors offer a holistic view of conductivity mechanisms in ionogels. Therefore, optimizing electrochemical performance requires coordinated control of these molecular-level features.

In parallel with ionic conduction, the mechanical performance of ionogels is largely governed by polymer agglomerate domains, which are dense polymer-rich regions formed via localized microphase segregation. , These domains originate from the thermodynamically driven association of polymer segments that minimize unfavorable interactions with the IL matrix. This association establishes a supramolecular backbone that imparts elasticity, strength, and dimensional stability. Within these domains, a variety of noncovalent interactions operate, ,− including electrostatic attraction between ionic side chains, − hydrogen bonding among polar groups (e.g., −OH, -NH₂, -C = O), , and π-π stacking of aromatic segments (Figure b). , Acting cooperatively, these interactions serve as sacrificial bonds: they break under stress to dissipate energy and reform to enable elastic recovery. The efficiency of this reversible cross-linking is dictated by the chemistry, density, and spatial distribution of interacting motifs along the polymer backbone, directly influencing toughness and fatigue resistance.

Collectively, the two distinct domains, IL-swollen regions and polymer agglomerates, act in concert to define the macroscopic behavior of ionogels. Designing high-performance ionogels, therefore, requires balancing these functionalities through deliberate control of polymer architecture and phase behavior. The following section examines how molecular-level design principles govern structure–property relationships in ionogels and outlines practical strategies for tailoring polymer architectures to meet electrochemical performance demands.

3. Structure-Property Relationships in Ionogels

3.1. Engineering Ion Transport Pathways for Enhanced Electrochemical Behavior

A representative strategy to enhance ionic conductivity in ionogels is to increase the IL affinity of the polymer matrix by introducing ionic domains. ,− For example, our group designed a copolymer containing a cationic segment, poly­([(3-acryloamidopropyl)­trimethylammonium] [bis­(trifluoromethanesulfonyl)­imide])-r-poly­(2-hydroxyethyl acrylate) (P­[AA]­[TFSI]-r-PHEA), in which strong electrostatic interactions with IL anions promote the formation of percolated ionic clusters (Figure a). Compared to nonionic analog composed of poly­(N-[3-(dimethylamino)-propyl]­acrylamide)-r-poly­(2-hydroxyethyl acrylate) (PDA-r-PHEA), the cationic copolymer exhibited substantially higher IL uptake and nearly 3-fold higher ionic conductivity (σ_DC ≈ 4.5 × 10^–5 S·cm^–1 vs 1.6 × 10^–5 S·cm^–1 at 25 °C), underscoring the role of polymer–ion interactions in facilitating ion transport.

2.

Design strategies for ionogels modulating ion transport and electrochemical behavior. (a) Ion-cluster-mediated self-healing through interactions between ionic polymers and ionic liquid. Reproduced with permission from ref . Copyright 2022 Springer Nature. (b) Regulation of Li⁺ flux through ionogel electrolytes with ionic pendant. Reproduced with permission from ref . Copyright 2024 Elsevier. (c) Zwitterionic copolymer-based design for selective and healable ionic thermoelectric generators. Reproduced with permission from ref . Copyright 2023 Wiley-VCH. (d) Synthesis of low- and high-ion zwitterionic ionogels and ion gradient-driven electrochemical potential generation based artificial electrolyte. Reproduced with permission from ref . Copyright 2023 Wiley-VCH.

Cationic ionogels have been employed as electrolytes in lithium metal batteries. At moderate IL contents, strong polymer-anion interactions, arising from tethered quaternary ammonium groups, immobilize the IL anions and increase the system’s ionicity. The higher ionicity elevates τ_diff by suppressing ion–ion cross-diffusion, allowing Li⁺ to become the primary mobile species (Figure b). Consequently, the Li⁺ transference number increases substantially, from 0.20 to 0.61, compared to that of nonionic polymer-based ionogels, enabling selective Li⁺ conduction and enhancing interfacial stability during battery cycling.

Transitioning from ionic to zwitterionic copolymer systems, recent advances have shown that zwitterionic architectures offer enhanced controllability over ion transport behavior, enabling more precise directional ion selectivity. ,− Zwitterionic polymers bearing pendant groups with covalently tethered cationic and anionic moieties enable the formation of p-type and n-type ionogels through controlled spatial orientation of the charged groups (Figure c). In p-type gels, outward-facing cationic units interact preferentially with [TFSI]⁻ anions in the IL, facilitating anion-dominated conduction. In contrast, n-type gels expose anionic groups at the interface, favoring [EMI]⁺ cation transport while impeding [TFSI]⁻ mobility. This asymmetric charge distribution modulates selective ion diffusion.

Similarly, zwitterionic moieties have been incorporated into ionogels to construct bioinspired ionic power units that emulate the discharge mechanism of electric eels. In this approach, zwitterionic copolymers based on [2-(methacryloyloxy)­ethyl]­dimethyl-(3-sulfopropyl)­ammonium hydroxide (DMAPS) were synthesized via photopolymerization and swollen with [Li]­[TFSI], yielding two ionogels with distinct salt contents: a high-concentration gel (H-ZIG, 81.5 mol % of zwitterionic content) and a low-concentration gel (L-ZIG, 4.1 mol % of zwitterionic content) (Figure d). Unlike conventional hydrogels, which rely on water as a solvent, ZIGs exhibit superior thermal stability, remaining nonvolatile and nonfreezing across a broad temperature range (−20 to 100 °C). The zwitterionic framework provides fixed internal charges, which suppress neutral ion diffusion and impart intrinsic selectivity, thereby improving efficiency in membrane-based ion separation. When coupled with cation- and anion-exchange membranes (CEM and AEM), the selective ion transport inherent to the zwitterionic framework can be harnessed to direct ionic gradients and polarization. This strategy enables bioinspired power generation that mimics the discharge mechanism of electric eels, where asymmetric ion migration produces measurable voltages. Thus, ZIGs uniquely combine tunable ionic content, solid-state stability, and selective ion transport to serve as functional building blocks for thermally robust, biomimetic energy devices.

3.2. Molecular Design Strategies for Mechanical Property Enhancement

For wearable and stretchable electronics, ionogels must combine softness, deformability, and mechanical resilience. Although inherently softer than conventional electronic materials, many ionogels still exhibit moduli exceeding those of soft biological tissues, potentially compromising comfort, mechanical conformity, and signal stability in skin-contact or motion-sensitive applications. Achieving the desired mechanical profile, high stretchability, toughness, and controlled viscoelasticity, therefore, demands deliberate molecular engineering. Recent approaches have targeted polymer–ion interaction strength, domain architecture, and dynamic or heterogeneous cross-linking to create networks that maintain structural integrity under large deformations while preserving functional stability.

One effective strategy to achieve skin-like mechanical compliance in ionogels is to molecularly tune IL-insoluble domains. Introducing sterically bulky pendant groups into styrene-based copolymer gelators reduces the π-π stacking density between aromatic segments, resulting in softer and more deformable physically cross-linked domains. For example, ionogels incorporating 4-tert-butoxystyrene-r-PEMA copolymer exhibited remarkable stretchability (∼1200%) and toughness (∼3.5 MJ·m^–3), as the bulky side chains disrupted compact phenyl packing and facilitated stress dissipation through domain deformation (Figure a). This design principle demonstrates that controlling interdomain interactions and molecular packing is an effective route to combining high elasticity with mechanical resilience in deformable ionic conductors.

3.

Design strategies for ionogels modulating mechanical and thermomechanical properties. (a) Loosening of π–π interactions by bulky side chains for enhanced stretchability and toughness. Reproduced with permission from ref . Copyright 2024 Elsevier. (b) Dynamic cross-linking of polymer domains by metal–ligand coordination for improved mechanical properties. Reproduced from ref . Copyright 2024 American Chemical Society. (c) Glassy toughening via ionic liquid-mediated solvent cross-links. Adapted with permission from ref . Copyright 2024 Springer Nature. (d) Thermal modulation of reversible adhesion switching via LCST phase behavior. Adapted with permission from ref . Copyright 2022 Royal Society of Chemistry. (e) Berghmans’ point-induced vitrification for reversible stiffness switching in ionogels. Reproduced from ref . Available under a Creative Commons Attribution 4.0 International License. Copyright 2022 Springer Nature, with modifications.

Complementary to this softening approach, characteristic structural motifs, such as polymer agglomerates, have also been harnessed to reinforce ionogels via supramolecular interactions. One representative approach involves incorporating transition metal ions (Ag⁺, Zn²⁺, and Co³⁺) into acetoacetate-functionalized polymer matrices (poly­(ethyl acrylate-co-acetoacetoxyethyl methacrylate), PEA-co-AAEM), where the metal–ligand coordination serves as dynamic cross-links (Figure b). By varying the coordination number (CN = 2, 4, 6), the network elasticity and relaxation behavior were systematically tuned. Notably, Co³⁺-coordinated ionogels formed highly elastic and densely cross-linked networks, as evidenced by elevated storage modulus and sustained stress relaxation. This led to a favorable combination of mechanical strength (∼1.3 × 10⁵ Pa) and stretchability (∼243%), highlighting multivalent coordination as a robust strategy to enhance both toughness and durability in ionogels.

Beyond serving as solvents and plasticizers, ILs can function as supramolecular cross-linkers through ion-dipole and electrostatic interactions with polymer chains. For example, mixing poly­(acrylic acid) (PAA) with [tributyl­(methyl)­phosphonium] [dimethyl phosphate] induces strong ionic associations between phosphonium cations and deprotonated carboxylates, forming glassy ionogels without covalent cross-linking (Figure c). The dense ionic network suppresses polymer aggregation and drives a concentration-dependent transition from a soft viscoelastic state to a stiff glassy solid. Despite containing over 50 wt % IL, these materials exhibit thermoplastic-like mechanics, with a fracture strength of 42 MPa, toughness of 110 MJ·m^–3, and a Young’s modulus nearly 4 orders of magnitude higher than conventional hydrogels. Mechanical testing was conducted on dog-bone specimens (gauge length 12 mm, width 2 mm) at a crosshead speed of 100 mm·min^–1 at room temperature. This approach exemplifies how ILs can act simultaneously as ion-conducting media and structural cross-linkers to realize mechanically robust ionogels.

3.3. Thermomechanical Modulation of Ionogels via Phase Transitions

Ionogels can be molecularly engineered to exhibit targeted electrochemical and mechanical properties, but in practical applications their operating environments often fluctuate. Thermomechanical modulation enables reversible adjustment of strength, adhesion, transparency, and conductivity in response to temperature, thereby ensuring device reliability and adaptive functionality without additional processing. One effective route is to exploit temperature-responsive phase transitions, enabling reversible modulation of mechanical and transport properties without altering the underlying chemical composition. Such thermomechanical control is governed by the lower- or upper critical solution temperatures (LCST or UCST, respectively) of the polymer–solvent (or -IL) system. ,− In LCST-type ionogels, heating above the transition temperature induces entropy-driven phase separation into polymer-rich and solvent-rich domains. This transition is accompanied by polymer chain collapse and matrix densification, which increase stiffness and reduce adhesion. In UCST-type ionogels, cooling below the transition temperature leads to phase separation due to favorable enthalpic polymer–solvent interactions. As a result, these ionogels soften upon heating and reinforce upon cooling, thereby enabling reversible thermomechanical switching. Overall, such thermoresponsive designs allow ionogels to reversibly modulate stiffness and elasticity with subtle temperature changes, imparting mechanical adaptability. Such controllable flexibility not only provides a powerful means to withstand dynamic environments but also opens pathways for their integration into adaptive electronics, soft actuators, and biointerfacing devices where tunable mechanical performance is essential.

One practical demonstration is the use of PBA and [EMI]­[TFSI]. The network remains homogeneous below the critical temperature (T _c), with surface-exposed polymer chains enabling strong viscoelastic contact. Upon heating above T _c, phase separation induces network collapse and the extrusion of IL microdroplets, which drastically reduces friction and adhesion (Figure d). As a result, adhesion on rubber substrates decreases from 45 to 0.6 kPa in air and from 21 to 2.7 kPa underwater. Importantly, T _c can be finely tuned between 32 and 100 °C by adjusting the polymer-IL composition, allowing programmable adhesion switching across a wide range of operating environments.

Another thermomechanical tuning strategy exploits vitrification-arrested phase separation at the Berghmans’ point. This point represents a critical condition on the phase diagram where liquid–liquid phase separation occurs simultaneously with the glass transition of the polymer. Under this condition, the coupling of phase separation and vitrification can freeze transient structures and morphologies in polymer solutions. In this state, the polymer-rich domain vitrifies before complete demixing, kinetically locking in a phase-separated microstructure. This mechanism enables reversible switching between soft and rigid states in response to temperature changes. For example, poly­(N-isopropylacrylamide) ionogels swollen with [1,3-dimethylimidazolium]­[TFSI] exhibit a pronounced mechanical contrast when formulated near this point, remaining soft (∼0.6 kPa) at elevated temperatures but stiffening (∼85 MPa) upon cooling due to vitrification (Figure e). Importantly, the Berghmans’ point can be precisely tuned via molecular design of the IL. Specifically, elongation of the cation’s alkyl chain increases the phase separation temperature, while the use of a more basic anion raises the polymer’s T _g. These tunable parameters offer a practical strategy to control both the onset and magnitude of thermomechanical switching across a broad operational range.

4. Emerging Ionogel-Based Applications

4.1. Electronics Applications

Just as biological cells rely on ion transport for signal transmission, ionogels use ionic migration for mechano-gated sensing in soft electronic systems. Building on their tunable ion transport and inherent mechanical compliance, ionogels are being adapted for sensory applications, particularly neuromorphic devices requiring high sensitivity and low energy consumption. , Unlike conventional sensors that rely on rigid electronic fillers (e.g., metals, carbon nanotubes), − ionotronic devices exploit ionic migration and electrochemical modulation to transduce mechanical stimuli into electrical signals. ,− Upon mechanical deformation, changes in ionogel geometry drive the migration of cations and anions toward their respective electrodes, while the soft elastic network preserves conduction pathways under strain. , This dynamic ionic redistribution enables stable, reproducible signal outputs and underpins reliable mechano-gated sensing performance, laying the foundation for adaptive and biointerfaced neuromorphic devices (Figure a).

4.

Ionogel-based sensory platforms. (a) Schematic diagram of the sensing mechanism of the ionogel, showing ion migration as charge carriers under an external electric field. Reproduced with permission from ref . Copyright 2023 Elsevier. (b) Ultrastretchable ionoskins based on binary polymer blend and real-time monitoring of elbow motions. Reproduced from ref . Copyright 2022 American Chemical Society. Schematic illustration of the pressure-sensing mechanism with (c) micropyramid structures and (d) porous ionogels. Reproduced with permission from ref . Copyright 2021 The American Association for the Advancement of Science. Reproduced from ref . Copyright 2021 American Chemical Society. (e) Conceptual illustrations of thermo-response phase transition of the ionogel and the independent detection of thermal change and mechanical deformation using stimuli-responsive ionoskins. Reproduced with permission from ref . Copyright 2023 Wiley-VCH.

A binary polymer blend of PMMA and PBA was used to construct a microphase-separated ionogel, where soft, low-T _g PBA-rich domains accommodate large strain and hard, high-T _g PMMA-rich domains suppress strain localization, yielding a stress-delocalizing network (Figure b). This architecture enables ultrahigh stretchability (∼1500%) with stable ionic conductivity. In practical tests, the ionic conductor was stretched to ∼500% and directly attached to a large-area joint (elbow), showing an immediate ΔR/R₀ change of ∼ 663% upon stretching. It also exhibited a stable resistance response under further deformation, demonstrating its suitability for skin-conformal, large-deformation sensing.

In addition to compositional optimization, architectural engineering has played a pivotal role in enhancing sensor performance. − Microstructured designs such as pyramidal arrays and hierarchical porous frameworks have been widely employed to increase the effective contact area and modulate electric double layer (EDL) capacitance. For example, when pressure is applied to the sensor, the micropyramids are compressed vertically, increasing their contact with the top electrode (Figure c). Similarly, porous ionogels with compressible open-cell architectures deform easily even under small loads, enhancing ion-electrode interfacial area and sensitivity (∼152.8 kPa^–1), while ensuring rapid structural recovery (Figure d). This structural adaptability not only enables accurate detection of low-magnitude pressures but also supports a broad sensing range up to 400 kPa without compromising signal stability.

To meet the growing demand for real-time, on-skin monitoring of diverse physiological and environmental conditions, it is essential to address the fact that multiple stimuli such as pressure, strain, and temperature, often occur simultaneously. Such co-occurrence can lead to complex signal interference, cause ambiguity in data interpretation, and ultimately reduce the reliability of wearable sensing systems. To overcome this challenge, multimodal sensing platforms capable of independently recognizing distinct inputs without crosstalk are essential for next-generation wearable electronics. One effective approach employs LCST-type ionogels, which undergo phase transitions in response to temperature changes. In this system, temperature-dependent optical transmittance derived from LCST behavior is used to track thermal variations, while strain-sensitive electrical resistance monitors mechanical deformation (Figure e). By functionally decoupling these two sensing modes, the device achieves crosstalk-free, dual-mode operation, underscoring the potential of thermoresponsive ionogels as a versatile platform for multifunctional ionoskins.

4.2. Energy Applications

As wearable and soft electronic systems become increasingly multifunctional and intimately integrated with the human body, their energy storage units must not only deliver reliable power but also provide real-time, easily interpretable feedback on operational status, ensuring user safety, system reliability, and seamless human-device interaction. Electrochromic supercapacitors (ECSs) fulfill this need by coupling charge storage with visible color changes, allowing the device itself to act as both a power source and a charge-level display without extra circuitry. ,− In ionogel-based single-layer ECSs (SL-ECSs), chromophores and redox-active species are uniformly dispersed throughout the ionogel electrolyte, enabling faradaic reactions to occur efficiently across the entire active area (Figure a). This integration removes the need for complex multilayer fabrication and improves interfacial contact between the electrochromic medium and the electrodes. Optical modulation arises directly from the redox reactions of the embedded electrochromic molecules. In one representative system, the cathodic species ethyl viologen (EtV²⁺, colorless) is reduced to the deep-blue radical cation EtV^+•, while the anodic counterpart, 1,1’-dimethylferrocene (dmFc), is oxidized to dmFc⁺. This coupled redox process can be summarized as

$${\mathrm{EtV}}^{2+}(\mathrm{colorless})+\mathrm{dmFc}\to {{\mathrm{EtV}}^{+}}^{•}(\mathrm{deep\; blue})+{\mathrm{dmFc}}^{+}$$ 4

5.

Ionogels as energy platforms. (a) An ion diffusion-controlled electrochromic supercapacitor in which ionogels provide both ionic conductivity and dimensional stability, enabling fast coloration/bleaching with minimal residual capacitance. Adapted with permission from ref . Copyright 2022 Wiley-VCH. (b) Selective ion boosting in thermoelectric generators achieved through zwitterionic p- and n-type polymers, highlighting the design logic of combining ionic conduction with thermopower for efficient p/n leg integration. Reproduced with permission from ref . Copyright 2023 Wiley-VCH. (c) Working principle of ionic triboelectric nanogenerators (TENGs), where ionogels serve as deformable ionic conductors supporting EDL formation during contact-separation cycles, thereby sustaining stable output under repeated mechanical stress. Reproduced with permission from ref . Copyright 2024 American Chemical Society.

This color change is rapid, reversible, and directly correlated to the device’s charge state. To maximize performance, SL-ECSs are engineered to optimize both the concentration and mobility of these redox-active species. For example, incorporating EtV²⁺ and dmFc into a PMMA-r-PBA/[EMI]­[TFSI] ionogel yields a high areal capacitance (∼43.0 mF·cm^–2) and exceptional optical contrast (∼96.8% transmittance change), while sustaining stable operation over 3000 cycles.

A recent advancement in ECS design addresses the limited optical tunability of conventional systems. While ECSs uniquely enable real-time visual monitoring of energy storage via color changes, their palette is often restricted by the intrinsic redox-dependent transitions of traditional electrochromic materials such as WO₃. , To overcome this constraint, electrochemically inert rhodamine dyes (800, 6G, and B) were incorporated into ionogel electrolytes in trace amounts. Unlike transition metal oxides, whose coloration is dictated by fixed electronic transitions, rhodamine dyes possess π-conjugated molecular structures that produce intense, highly tunable colors. By varying the dye type, concentration, and combinations, ECSs achieved an expanded color gamut, including blue, magenta, yellowish, purple, navy, red, cyan, dark cyan, yellow, orange, and green, without altering the device’s electrochemical window. Crucially, the addition of these dyes did not compromise electrochemical performance: dye-modified ECSs maintained a high areal capacitance (7.12 mF·cm^–2 at 0.04 mA·cm^–2) and demonstrated long-term cycling stability over 10,000 charge–discharge cycles. This strategy thus decouples color tunability from electrochemical activity, enabling multifunctional ECSs with customizable visual feedback.

Beyond energy storage, the unique capability of ionogel systems to regulate ionic motion through tailored ion–polymer interactions opens new avenues in energy harvesting. ,,,− One emerging example is the ionic TEGs, which convert low-grade thermal gradients into electricity via directional ion diffusion. − In these ionic thermodiffusive cells (or thermo-ionic capacitors), heat drives ions to migrate through the electrolyte, a phenomenon known as the Soret effect, resulting in asymmetric ion accumulation at the electrodes and the formation of electric double layers. This process generates a thermopower (1–100 mV·K^–1) that significantly exceeds that of conventional solid-state semiconductor or semimetal thermoelectrics. The underlying mechanism can be expressed as ,

$$\mathrm{\Delta }V=S_i·\mathrm{\Delta }T$$ 5

where S _i is the ionic Seebeck coefficient determined by the mobility and concentration gradient of the cations and anions. By engineering ion–polymer interactions and optimizing ionic species, thermo-ionic generators can harvest otherwise wasted low-grade heat in a lightweight, flexible, and scalable format, making them particularly attractive for wearable and remote-power applications.

For example, zwitterionic polymers are utilized to maximize S _i by selectively boosting the diffusivity of a certain ion (Figure b). Cationic boosting p-type gel is fabricated with DMAPS and [EMI]­[dicyanamide] ([EMI]­[DCA]). The anionic moiety at the outer tail boosts the [EMI]⁺ diffusivity, while the inner cationic moiety holds the anion (i.e., [DCA]⁻). Exploiting the same strategy, an anionic boosting n-type gel is fabricated with 2-methacryloyloxyethyl phosphorylcholine (MACP) and [EMI]­[TFSI]. Hence, the diffusivity of ions coupled to outer polymeric chains can be selectively accelerated, which leads to high S _i of 12.8 mV·K^–1 (p-type) and −2.8 mV·K^–1 (n-type) at ΔT = 1 K. To fabricate a practical TEG, the individually prepared p- and n-type ionogel legs are integrated side by side between two electrodes, typically formed by simple drop casting, creating modules that are electrically in series. Furthermore, the self-healing properties of the TEG modules enable stable thermoelectric performance even after the material is cut and subsequently healed, which significantly enhances the practical feasibility of wearable TEGs. This configuration allows the voltages of p/n pairs to increase constructively. The resulting ionic TEG exhibited a high energy density of ≈10.2 mJ·m^–2 and generated an output voltage of 1069 mV with a 10-legged cell under ΔT = 5 K.

Ionic TENGs are another emerging class of energy harvesters that integrate ionic conductors as the triboelectric layer. Unlike conventional TENGs, where electron transfer occurs between solid dielectric materials such as PTFE or PDMS, ionic TENGs generate electricity through ion migration and EDL formation at the interface. This ion-mediated mechanism enables efficient electrical output even under soft mechanical input or low-frequency motion. Ionogels have become a material of choice for ionic TENGs due to their high ionic conductivity, mechanical flexibility, stretchability, and environmental stability, making them ideal for wearable electronics, soft robotics, and biointegrated systems. Within ionic TENG architectures, ionogels can serve multiple roles, including (i) triboelectric charge generation, (ii) charge trapping, and (iii) charge collection/transport (Figure c).

When used directly as the triboelectric layer, mechanical contact-separation induces surface charges on the ionogel, which in turn attract counterions from within the gel, forming high-capacitance EDLs that enhance charge storage. Performance can be further improved by incorporating plasticizers such as propylene carbonate and solid salts like [Li]­[TFSI], which increase ion mobility and EDL ion density. This approach has achieved output powers of ∼372 mW·m^–2, while maintaining ultrahigh stretchability (∼1000%).

5. Conclusion and Perspective

Ionogels have emerged as a unique class of soft materials that merge the high ionic conductivity, electrochemical stability, and wide tunability of ILs with the mechanical robustness and dynamic processability. Their dual-domain structure, comprising ion-rich conductive pathways and polymer-rich load-bearing frameworks, can be precisely tailored through polymer chemistry to regulate ionic transport, mechanical strength, and dynamic responsiveness. This tunability enables application-specific performance and provides a versatile platform for applications spanning soft electronics, biointegrated devices, and energy systems. Over the past decade, advances in material design, network architecture, and processing strategies have enabled simultaneous optimization of conductivity, toughness, and environmental stability.

Despite these advances, most research remains confined to proof-of-concept demonstrations that optimize individual properties in isolation. To transition toward real-world deployment, ionogels must evolve into multifunctional platforms that integrate material-level performance with system-level capabilities. Specifically, future designs must ensure biocompatibility, reduce cost by replacing conventional ILs with naturally abundant electrolytes, dynamically adapt to environmental stimuli, and follow sustainable life-cycle principles. These challenges crystallize into three interrelated research frontiers:

5.1. Biocompatibility and Environmental Safety

For ionogels to be used safely and responsibly in real-world applications, both biocompatibility during operation and environmental sustainability at the end of life must be ensured. This requirement is particularly critical in biomedical systems, such as implantable sensors or neural interfaces, that remain in contact with the human body for extended periods. In this context, eutectogels feature low volatility, biodegradability, high drug-loading capacity, and tunable mechanical robustness, making them attractive for biomedical use. , Ionogels should be designed to exhibit such features, which can be pursued by incorporating natural polymers (e.g., chitosan, alginate, lignin) or synthetic copolymers with hydrolyzable linkages to enable controlled degradation and recyclability. The insufficient understanding of the structure–toxicity relationships of ILs remains a key limitation to predictive safety-by-design approaches. To overcome this challenge, AI/ML-based toxicity prediction models trained on curated literature and high-throughput screening data sets can be employed to identify potentially hazardous structural motifs before synthesis, while explainable AI can elucidate molecular-level mechanisms, accelerating the development of clinically viable, low-toxicity, and environmentally benign ionogels.

5.2. Ion-Selective andCost-Effective Conduction

Most ionogels exhibit nonselective ion transport, which limits their use in neuromorphic devices, artificial membranes, and ionic logic circuits. At the same time, the high cost of conventional ILs remains a barrier to large-scale commercialization. Addressing these dual challenges requires cost-effective electrolytes combined with structural control over ion selectivity. A promising direction is to employ multi-ionic, naturally abundant formulations such as flexible electrolytes based on ionic liquid crystals that provide ion transport channels combining orderliness with dynamic adjustability. By engineering polymer junctions between cationic and anionic domains, ionogels can be endowed with diode-like rectification, , while crystallization-driven domains or nanoscale channels can introduce size-selective transport. , Such architectures would simultaneously enhance selectivity, cost-efficiency, and functional versatility. In the neuromorphic context, selective ion transport is essential for mimicking synaptic plasticity and spike-timing-dependent learning. − For instance, zwitterionic ionogels that preferentially mobilize cations have been utilized to construct ionic synaptic transistors exhibiting long-term potentiation and depression behaviors. These examples highlight how structural control over ion selectivity directly translates into neuromorphic functionalities such as memory retention, signal filtering, and adaptive learning by ionogels.

5.3. Autonomous and Energy-Efficient Ionogel

Beyond static performance, ionogels must autonomously adapt their properties to changing environments with minimal external energy input. Light- or heat-induced nanodomain alignment, field-guided anisotropic networks, or humidity-driven nanocluster reconfiguration can dynamically modulate ion mobility in real time. In principle, this could yield closed-loop systems where stimulus-induced outputs feed back to regulate internal states, enabling sensing, actuation, or energy harvesting without a continuous power supply. For practical realization, several materials-level strategies can be employed. For example, photoresponsive moieties (e.g., azobenzene, spiropyran) covalently incorporated into the polymer backbone can enable reversible nanodomain reorientation under low-intensity light. Leveraging this functionality, one could design self-regulating devices that autonomously switch between on–off ionic conductivity states in response to natural day-night light cycles. Similarly, incorporating humidity- or ion-gradient-responsive fillers (e.g., hygroscopic salts, zwitterionic polymers) into ionogels would allow dynamic control of ion transport as ambient humidity fluctuates throughout the day. Such systems could also be harnessed to develop humidity-driven energy generators, directly converting environmental variations into usable energy.

Taken together, these approaches move beyond biological analogies by outlining specific design rules-selecting molecular motifs that undergo reversible transitions, engineering hierarchical nanostructures that can store and release elastic energy, and embedding feedback elements within the ionogel matrix. Such strategies provide a realistic roadmap toward ionogels that achieve adaptive, energy-efficient performance in wearable sensors, soft robotics, and sustainable energy harvesters.

By converging advances in chemistry, materials science, and device engineering, ionogels are poised to transcend their current limitations and emerge as key enablers of adaptive, sustainable technologies. The pursuit of these frontiers will not only expand the functional landscape of ionogels but also catalyze their translation into impactful real-world applications. Achieving sustainable, adaptive, and functional ionogels will require an integrated understanding of polymer chemistry, electrochemistry, bioengineering, and computational science to translate these concepts into practical systems, positioning ionogels as intelligent, environmentally responsible building blocks for next-generation soft electronics and energy devices.

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H.W.Y. and D.S.C. contributed equally to this work. CRediT: Hyeon Woo Yang conceptualization, investigation, visualization, writing - original draft; Daniel Sanghyun Cho conceptualization, investigation, visualization, writing - original draft; Juyoung Kang investigation, methodology, writing - original draft; Ji Hye Han investigation, methodology, writing - original draft; Yong Min Kim conceptualization, investigation, supervision, validation, writing - original draft, writing - review & editing; Hong Chul Moon conceptualization, funding acquisition, project administration, supervision, writing - original draft, writing - review & editing.

The authors declare no competing financial interest.