The cation of ILs determines key properties such as electrical conductivity, side-chain functionalization, and electrochemical stability. The types of IL cations commonly used in the preparation of ionic gels can be mainly categorized into imidazolium, pyridinium, quaternary ammonium, and phosphonium types.

Imidazolium-based ILs have been the most extensively studied and, due to their high ionic conductivity (>10 mS·cm^-1) and wide electrochemical window (~4.5 V), have become the preferred materials for preparing ionic gels. However, their biotoxicity needs to be optimized by adjusting the type of anion^[15]. Imidazolium-based ILs generally exhibit relatively high cytotoxicity, which typically increases with the length of the alkyl chain. The longer the alkyl chain, the stronger the hydrophobicity, leading to more pronounced membrane insertion and disruptive effects, and thus greater toxicity. Moreover, the metabolic products of long-chain ILs are more complex, and they tend to remain in the body for a longer period. Overall, their biocompatibility is rated as low; they show toxicity against various cell lines, potentially causing cell membrane damage and metabolic inhibition, and require careful consideration when applied in human-contact scenarios^[16].

Pyridinium-based ILs, on the other hand, exhibit relatively moderate electrical conductivity (~1–10 mS·cm^-1) and an electrochemical window (~3.5–4.0 V), making them highly promising for electrochemical sensing and solid-state electrolyte applications. However, their biotoxicity is generally higher than that of imidazolium-based ILs with comparable chain lengths, and their biocompatibility is more limited^[17]. Improving biocompatibility requires optimizing the anion type and alkyl chain length^[18].

In comparison, quaternary ammonium ILs and phosphonium ILs, with their outstanding biocompatibility and exceptionally high thermal stability (>400 ℃) respectively, occupy an important position in biomedical materials and extreme environment applications^[19].Quaternary ammonium ILs are regarded as a class with relatively good biocompatibility and low toxicity, particularly choline-based and short-chain symmetric quaternary ammonium salts, which are among the most promising candidates for biomedical applications. Research on phosphonium ILs is relatively limited; their toxicity varies widely and is highly dependent on the specific structure. Long-chain alkyl substitution may increase toxicity, and their biocompatibility potential still requires further research to confirm. It is worth noting that the toxicity trend of the cation itself can be significantly influenced by the counterion with which it is paired.

The anion of ILs determines key performance characteristics such as viscosity, thermal stability, electrochemical window, and functional properties. The main types of IL anions commonly used in the preparation of ionic gels include inorganic anions such as bis(trifluoromethanesulfonyl)imide ([TFSI]^-), hexafluorophosphate ([PF₆]^-), and halide anions (Cl^-, Br^-, I^->), as well as organic anions such as acetate ([Ac]^-), amino acid anions, lactate, and azobenzene ([Azo]^-).

ILs based on inorganic anions such as [TFSI]^- and [PF₆]^- offer advantages such as low viscosity (<50 cP) and high thermal stability (>300 ℃), but they carry a potential risk of hydrolysis to form HF^[20]. In humid environments, [TFSI]^- and [PF₆]^- anions may hydrolyze to produce corrosive and toxic HF and its derivatives, and the fluorine-containing groups pose potential risks of bioaccumulation and environmental persistence, often exhibiting moderate to high toxicity to aquatic organisms^[21]. Among these, the hydrolytic stability of [PF₆]^- is generally lower than that of [TFSI]^-. Although halogen anions are relatively low in cost, they typically exhibit high cytotoxicity and ecotoxicity, and their use should be minimized in ion gel designs that prioritize high biocompatibility or environmental friendliness^[22].

In contrast, ILs based on functionalized anions such as [Ac]^-, lactate, amino acid anions, and light-responsive azobenzene ([Azo]^-) can impart enhanced or intelligent properties to ionic gel interfaces through hydrogen bonding or the action of responsive groups. Acid anions such as [Ac]^-and lactate, as well as amino acid anions, not only modulate the physicochemical properties of ILs (e.g., by lowering the melting point and increasing hydrophilicity), but also—owing to their origin from biomass or metabolic pathways—typically exhibit significantly improved biocompatibility and lower cytotoxicity. Their combination with biocompatible cations (such as choline) makes them an ideal choice for developing green, biodegradable ionic gels, particularly for applications involving human contact. However, their electrochemical stability is generally inferior to that of fluorinated anions, which limits their use in high-voltage electrochemical devices. The functionalized anion [Azo]^-can endow ionic gels with light-controlled properties (such as photoinduced deformation and light-triggered release), offering great potential in smart devices. Nevertheless, research on its biosafety is severely lacking. Light-sensitive groups may generate reactive oxygen species (ROS) that damage cells, and their metabolic pathways and long-term toxicity remain unknown. Any application in biocontact scenarios must therefore be preceded by rigorous and comprehensive toxicological assessments.

Overall, the cation type of ILs is the dominant factor influencing their bio-environmental toxicity. The length of the alkyl chain in ILs is a key regulatory factor: toxicity increases with longer alkyl chains. Similarly, the role of the anion is also crucial; ILs paired with different anions exhibit vastly different bio-environmental toxicities. This provides clear guidance for material selection and design of ionic gels for various applications. For bio-contact scenarios such as human–machine interaction, electronic skin, and wearable health monitoring, ILs based on choline or other short-chain quaternary ammonium cations paired with biocompatible anions (such as [Ac]^-,amino acid residues, and lactate) should be prioritized. Highly toxic halogen anions should be strictly avoided, and fluorine-containing anions should be used with caution. For flexible electronic devices that do not involve direct biological contact, while meeting core performance requirements in electrochemistry and mechanics, environmental risks should also be considered, and components that are biodegradable or have low ecotoxicity should be preferred.

Physical blending is one of the simplest methods for preparing ionic gels. This method utilizes the swelling process of the polymer matrix to form a network structure, effectively restricting the mobility of ILs and thereby yielding an ionic gel. The method is easy to perform, exhibits good processability, and is widely used in the preparation of ionic gels^[28].

Li et al.^[29]employed a physical blending method, using hydrophobic ILs as a solvent to uniformly disperse methyl methacrylate and a proton-conducting polymer (PCM) that possesses both hydrophobic and hydrophilic groups. Subsequently, through covalent cross-linking, an ion gel with high ionic conductivity was successfully prepared. In this process, the hydrophobic groups on the PCM (—COOCH₃)enable their uniform dispersion within the hydrophobic matrix, while the hydrophilic charged groups (—SO₃ ^-, —N(CH₃)₃ ⁺) enhance the ionic conductivity of the gel, ultimately forming a composite gel system that combines a stable interface with efficient ion transport channels. This method achieves uniform dispersion of hydrophobic and hydrophilic components through simple physical blending, featuring a straightforward process, mild reaction conditions, and effective preservation of the functional properties of each component. However, ion gels prepared by physical blending may experience phase separation or reduced mechanical stability over long-term use, necessitating optimization through the introduction of a cross-linked structure. Qu et al.^[30]used dimethylformamide (DMF) as a solvent to disperse thermoplastic polyurethane (TPU), SiO₂, and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP][TFSI]), then employed physical blending to prepare TPU/SiO₂-ILs ion gels, as shown in Fig. 2A. The use of DMF as a solvent facilitates the thorough mixing of TPU and ILs, allowing for cross-linking between the two via high-density hydrogen bonding.

In addition, by introducing polymer matrices with specific functions, ionogels can be endowed with new functional properties. Choi et al.^[31]used poly(styrene-n-butyl methacrylate) (PS-r-PnBMA) as the polymer and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMI][TFSI]) as the ILs, and prepared a multimodal ion gel P(S-r-nBMA)/ILs via physical blending (Fig. 2B). This gel not only combines the temperature-sensitive properties of PnBMA with the conductivity of [BMI][TFSI], but also exhibits excellent dual-signal output capabilities, with its transmittance and resistance changing in response to variations in ambient temperature and degree of deformation, respectively.

In-situ polymerization is another important technique for preparing ionic gels, involving the dissolution of monomers in ILs followed by free-radical polymerization to form the ionic gel^[32]. This method is characterized by rapid synthesis, strong controllability, and high versatility, making it highly favored in the field of ionic gel preparation^[33]. Depending on the initiation method, in-situ polymerization can be classified into photoinitiated, thermally initiated, radiation-initiated, redox-initiated, enzyme-initiated, and plasma-initiated polymerization^[11,34-35].

In photopolymerization systems, common initiators include benzoin methyl ether, 2-hydroxy-2-methylpropiophenone, and benzophenone—photosensitive radical initiators. Under ultraviolet light (typically at a wavelength of 365 nm), these initiators absorb photon energy and undergo homolytic cleavage, generating radicals with initiation activity that trigger the polymerization of monomers. The low reaction temperature of this process makes it particularly suitable for heat-sensitive systems. For example, Poh et al.^[36]used trimethylbenzoyl diphenyl phosphine oxide as an initiator and achieved the rapid preparation of an ion gel within 1 minute via a free-radical polymerization reaction between vinyl-functionalized 1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide and a PEGDA crosslinker terminated with acrylate groups. Compared to conventional thermal polymerization, which may lead to IL decomposition or non-uniform gel networks, photopolymerization not only proceeds rapidly at low temperatures but also effectively safeguards the chemical stability of the electrolyte. Furthermore, Ming et al.^[37]employed hydroxy cyclohexanone benzophenone (UV-184) as an initiator, blended fluorinated butyl acrylate with butyl acrylate as a flexible polymer matrix, and combined this with 1,3-dimethylimidazolium bis(trifluoromethanesulfonylmethane imide), which possesses phase-transition properties, successfully preparing a phase-change ion gel. In this system, the flexible polymer matrix imparts excellent tensile properties to the gel, while the phase-changing ILs enable the gel to reversibly switch between an ionic conductor and an insulator. Notably, the conductor-insulator transition of the phase-changing ILs is temperature-controlled, and the photopolymerization process avoids high temperatures that could interfere with this phase-transition behavior.

For thermally initiated polymerization systems, common initiators include thermally decomposable agents such as ammonium persulfate (APS), azobisisobutyronitrile, and benzoyl peroxide. The underlying principle is that heating raises the initiator to its decomposition temperature, generating primary free radicals that initiate the chain polymerization reaction. In this process, the reaction temperature is a key factor in regulating the decomposition rate of the initiator and the concentration of free radicals, thereby directly influencing the polymerization reaction kinetics and the formation of the network structure. For example, Chen et al.³⁸used APS as the initiator and acrylic acid (AA) and 1-vinyl-3-hexadecylimidazolium bromide ([C₁₆VIm]Br) as monomers to prepare a supramolecular ionic gel, PAA/C₁₆VImBr, whose preparation principle is illustrated in Figure 3.By introducing hydrophobically functionalized IL structures, the supramolecular interactions within the gel network are effectively enhanced, endowing the gel with excellent underwater sensing performance. Compared with photo-initiated polymerization, thermally initiated polymerization is particularly suitable for systems that require penetration through thicker samples or where light shielding effects are present. However, in certain cases, ILs may interfere with the polymerization or gelation process, thereby limiting their scope of application.

For radiation-initiated polymerization systems, the underlying principle involves using high-energy radiation (such as gamma rays, electron beams, or X-rays) to directly interact with monomer or solvent molecules. Through ionization and excitation processes, free radicals are generated, thereby initiating the polymerization reaction. This approach eliminates the need for additional chemical initiators, resulting in products free from initiator residues, and allows the polymerization process to be carried out at ambient or even low temperatures. Due to the strong penetrating power of high-energy radiation, this technique is particularly suitable for polymerization reactions in thick samples or within encapsulated systems. For example, Shi et al.^[39]used gamma-ray irradiation to successfully prepare a novel star-shaped ionic gel based on 1-vinyl-3-butylimidazolium hexafluorophosphate, octahedral vinyl oligosilsesquioxane, and lithium hexafluorophosphate solution. However, radiation-initiated polymerization technology requires expensive dedicated radiation sources (such as cobalt sources or electron accelerators), and the associated equipment is complex with high maintenance costs. The operation also necessitates strict professional safety measures to ensure operator safety. Moreover, high-energy radiation may cause structural damage to certain sensitive materials, and the reaction mechanisms are relatively complex, making process control challenging.

In addition to the three conventional initiation methods mentioned above, other methods such as redox initiation, enzymatic initiation, and plasma initiation are also widely used in the preparation of ionic gels. Among these, the redox initiation system relies on electron transfer reactions between oxidants and reductants, enabling rapid generation of free radicals and initiation of polymerization at relatively low temperatures. However, this system requires precise control over the ratios and mixing order of multiple components, and residual oxidizing/reducing species may affect the performance and stability of the ionic gel. Enzymatic initiation technology utilizes biological enzymes (such as horseradish peroxidase and glucose oxidase) to catalyze redox reactions in substrates, generating free radicals under mild conditions—such as room temperature and neutral pH—to initiate monomer polymerization. Its advantages include mild reaction conditions and high biocompatibility. However, enzymes are relatively expensive, and certain ILs can inactivate enzymes due to their toxicity, resulting in insufficient stability of enzymes in the preparation of ionic gels. Plasma initiation involves the generation of highly reactive species (free radicals, ions, excited-state molecules, etc.) by plasma, which bombard the monomer surface to initiate polymerization. This method enables efficient polymerization without the need for chemical initiators; however, it requires a vacuum system and a plasma generation device, and its applicability to nonpolar monomers is limited.

When the solubility of monomer molecules in ILs is limited, the solvent exchange method becomes an effective strategy for preparing ionic gels^[40].This method primarily involves two steps: First, a polymer network is constructed in an aqueous or organic solvent medium through polymerization or gelation. Subsequently, the resulting hydrogel or organogel is immersed in ILs, where solvent exchange is used to remove the original solvent, ultimately yielding an ionic gel. Kim et al.^[41] used DMF and dimethyl sulfoxide as a binary solvent to construct a poly(vinyl alcohol) (PVA)-based polymer network, and employed the solvent exchange method to introduce 1-ethyl-3-methylimidazolium dicyanamide ([EMI][DCA]) into the network, successfully preparing a high-performance ionic gel and effectively addressing the issue of limited PVA solubility in [EMI][DCA]. In this process, the PVA microcrystals formed through solvent exchange-induced crystallization serve as physical cross-linking points, significantly enhancing the compactness and stability of the network structure. The resulting ionic gel exhibits excellent toughness, fracture strain, and thermal stability. Compared with physical blending, this method overcomes the compatibility limitations between polymer networks and ILs, enabling the universal incorporation of various ILs.

In addition, for ionic gels in which gelation cannot be directly carried out in ILs, the solvent exchange method can also be used for preparation. For example, Ren et al.^[42]used methanol as the initial solvent, pentaerythritol tetraacrylate as the crosslinker for the covalent network, and the anion 1,2,4,5-benzenetetracarboxylic acid as the crosslinker for the ionic bond network. Under the catalysis of triethylamine, a polymer backbone of the gel was constructed via a thiol-ene click chemistry reaction with 1,2-ethanedithiol; subsequently, solvent exchange was performed under vacuum at 80℃, introducing poly(1-butyl-3-vinylimidazolium hexafluorophosphate) (PIL-BF₄) into the system, thereby obtaining the ionic gel CIGL, as shown in Figure 4. During the solvent evaporation process, the chain rearrangement effect of PIL-BF₄enabled the loading of PIL-BF₄to reach 80 wt%, and by forming a high-density ionic crosslinking network, the process ultimately achieved a synergistic enhancement of CIGL’s toughness and ionic conductivity.

In summary, the principles and advantages/disadvantages of preparing ionic gels using physical blending, in-situ polymerization, and solvent exchange methods have been summarized, as shown in Table 2. In practical applications, these methods are not used in isolation; combinations of different methods or the integration of these methods with other external conditions may be more conducive to preparing ionic gels with specific properties.

Traditionally, ion gels have primarily adopted a single-network (SN) structure, which refers to a polymeric gel system composed of a single polymer chain network. In such gels, all polymer chains are interconnected within the same network, with no additional cross-linking points or hierarchical network layers. This type of gel exhibits a highly ordered and ideal network structure; however, due to the lack of an effective energy dissipation mechanism, stress tends to concentrate during stretching, thereby limiting its mechanical performance. Consequently, current research focuses on enhancing the mechanical performance of ion gels by designing their network structures.

Multi-network ionic gels are gel materials formed by integrating multiple independent polymer networks with different properties, primarily including double-network (DN) ionic gels and triple-network (TN) ionic gels. Specifically, DN ionic gels consist of a rigid network and a flexible network^[53];while TN ionic gels further introduce a dynamic, reversible network on the basis of the DN structure. Compared with traditional single-network (SN) ionic gels, the multiple polymer network structures in DN and TN ionic gels, through synergistic interactions, endow them with superior tensile strength and reversible deformability, enabling them to recover their original shape after being subjected to stress, and their mechanical properties outperform those of any individual SN ionic gel formed by a single network. For example, Zhao et al.^[54]prepared a DN ionic gel in which polyacrylamide (PAM) serves as the rigid network, and aminolyzed agarose cross-linked with 1,3,5-benzyltriformaldehyde forms the flexible sacrificial network. During deformation, the rigid PAM network effectively dissipates energy and significantly increases the apparent work required to break the ionic gel; meanwhile, the flexible sacrificial network dissipates energy through chain segment slippage or bond rupture, further enhancing the gel's mechanical performance. As a result, this gel exhibits excellent tensile properties (>800%), elasticity, and durability. However, under substantial loading, fracture of the rigid network can cause irreversible damage to the structure of the DN ionic gel, thereby affecting its overall performance. In light of this, Xu et al.^[55]designed and prepared a cellulose-based TN ionic gel (BPC) composed of an ionic liquid/poly(N,N-dimethylacrylamide-methacrylic acid-acrylamide)/cellulose system, featuring a multi-level energy dissipation mechanism, with a tensile strength of 0.48 MPa and an elongation at break of 899.69% (Figure 5). First, cellulose forms a long linear primary rigid network, providing initial mechanical support and enhancing chain entanglement; on this basis, poly(N,N-dimethylacrylamide-methacrylic acid-acrylamide) forms a dynamic hydrogen-bonding network as the second flexible network, dissipating energy through chain segment slippage and hydrogen bond rupture; finally, ionic bonds and electrostatic interactions between 1-butyl-3-methylimidazolium chloride and the cellulose network form a third dynamic, reversible network, substantially increasing the energy dissipation density and ultimately establishing a synergistic system in BPC that combines cellulose skeleton support, sacrificial bond energy dissipation, and dynamic network reversibility.

In contrast, interpenetrating polymer networks (IPNs) are three-dimensional network structures formed by two or more independently cross-linked polymer networks that interpenetrate each other. In such systems, if only one polymer exhibits a cross-linked structure while the other remains in a linear, uncross-linked form, the resulting structure is defined as a semi-interpenetrating polymer network (SIPN). Compared with SN ionic gels, IPN/SIPN ionic gels, through structural integration, are better able to leverage the advantageous properties of each component and can significantly enhance the structural stability and mechanical performance of the gel. For example, Wu et al.^[56]developed a SIPN ionic gel based on poly(ionic liquid) (PIL) and xanthan gum (XG). In this system, PIL serves as the cross-linked network, forming physical entanglements with the linear, uncross-linked XG to create an interlocked structure. Meanwhile, the hydrogen bonds and ionic bonds formed between PIL and XG can reversibly break, dissipating energy during deformation and inhibiting crack propagation, thereby enhancing the gel's mechanical performance and resulting in compressive and tensile strengths of 0.76 and 0.48 MPa, respectively.

In summary, by designing the polymer network structure (such as multi-networks, interpenetrating networks, etc.), the mechanical properties of ionic gels can be effectively regulated. For example, DN ionic gels enhance their tensile performance and reversible deformation capability through the synergistic interaction between rigid and flexible networks. This structure holds great potential for flexible sensors, which require materials that maintain high tensile properties while also exhibiting excellent elasticity and durability to withstand complex deformation conditions and long-term use. However, DN ionic gels also suffer from the irreversible fracture of the rigid network, and poor compatibility between different networks may lead to uneven stress distribution^[57].TN ionic gels further introduce a dynamic, reversible network on the basis of the DN structure, enhancing fatigue resistance through a multi-level energy dissipation mechanism. However, the complex cross-linking structure may reduce ion transport efficiency and controllability in fabrication. Therefore, TN ionic gels are more suitable for applications with extremely high demands on mechanical performance but relatively relaxed requirements for ionic conductivity, such as joint structures in soft robots, wearable protective devices, or biomimetic load-bearing materials. In addition, IPN/SIPN ionic gels achieve a balance between high toughness and conductivity through physically interpenetrating networks, avoiding potential biocompatibility issues arising from chemical cross-linking. As such, they are more suitable for biomedical materials and other applications with stringent biocompatibility requirements; however, the absence of chemical cross-linking also results in slightly reduced stability.

In addition to regulating the polymer network structure, introducing non-covalent interactions with dynamic and reversible properties as sacrificial bonds within ionic gels can also effectively enhance their mechanical performance^[58-59]. The core of this strategy lies in leveraging the reversible breaking and reformation of non-covalent interactions under external loading to achieve effective energy dissipation, thereby enhancing the toughness of ionic gels^[60].

Hong et al.^[61]successfully constructed an ion gel PBAEM/ILs with outstanding mechanical and conductive properties through a dynamic metal–ligand coordination-induced gelation strategy. First, a copolymer network was formed using poly(butyl acrylate)-stat-poly[(2-acetoacetoxy)ethyl methacrylate] (PBA-stat-PAAEM) containing ligand sites, with AAEM segments forming physical cross-linking points in ILs. Subsequently, Ni²⁺was introduced to selectively construct a metal–ligand coordination network within the insoluble domains of AAEM, thereby forming the gel (Fig. 6). This strategy selectively enhances the mechanical properties of the gel (tensile strength > 0.6 MPa) without requiring additional thermal or photochemical treatment steps. In addition, Ren et al.^[62]used acryloyl chitosan (AcCS), a polysaccharide derivative, as a cross-linker and synthesized a poly(acrylic acid-co-1-vinyl-3-butylimidazolium chloride) ion gel (AA-IL/AcCS) via a one-pot method. Compared with the commonly used N,N-methylenebisacrylamide cross-linker, the abundant hydroxyl and amino groups along the AcCS molecular chain provide numerous physical and chemical cross-linking sites, enabling AA-IL/AcCS to maintain high tensile elongation (600%) and tensile strength (137 kPa) while also exhibiting a high ionic conductivity (0.1 mS·cm^-1)).

In summary, the construction of non-covalent interactions effectively dissipates energy through a reversible breakage-reassembly mechanism, significantly enhancing the mechanical performance of ionic gels while maintaining the continuity of ion transport channels. Their dynamic reversibility endows ionic gels with exceptional self-healing capabilities, superior toughness, and stimulus responsiveness. These properties make this strategy inherently well-suited for applications demanding high durability and environmental adaptability, such as wearable electronic devices, skin-adhering flexible biosensors, and biomimetic soft materials requiring adaptive functionalities^[63]. Wearable devices are prone to repeated deformation and accidental damage; their efficient self-healing ability can greatly extend device lifespan and reduce maintenance costs. Energy dissipation via dynamic bond breakage is a key mechanism for enhancing material toughness and preventing small cracks from propagating into catastrophic failure, thereby ensuring device reliability in complex operating environments. However, the strength and stability of dynamic bonds often exhibit an inverse relationship: strong bonds (such as certain coordination bonds) heal slowly or require external stimuli, while weak bonds (such as some hydrogen bonds) offer limited mechanical contributions and are highly susceptible to environmental influences. Furthermore, the environment-sensitive nature of dynamic bonds limits their reliable application in extreme or fluctuating conditions—such as high temperatures, high humidity, or strong acidic/alkaline environments—as seen in scenarios like implantable medical devices for real-time sweat monitoring, deep-sea flexible sensors, and outdoor wearable electronics for cold regions.

Based on their formation mechanisms and functional characteristics, the microphase-separated structures of ionic gels can be classified into two types: phase-transition ionic gels and phase-separated ionic gels.

Phase-transition ionic gels are a new class of gel materials that undergo reversible microstructural phase transitions triggered by external stimuli such as temperature, humidity, or electric fields^[64-65].Their toughening mechanism is primarily based on phase-transition-induced effects, such as heat-triggered polymer chain rearrangement or solvent-induced phase transition processes that form nanoscale double-continuous structures within the gel. For example, Park et al.^[66]used 1-ethyl-3-methylimidazolium nitrate ([EMIM][NO₃]) as a solvent and prepared an ion gel with dynamic phase transition and shape-memory properties via in-situ polymerization of AM (Figure 7). When the temperature rises above the phase-transition temperature (~44 ℃), the gel transforms from a crystalline phase to an amorphous phase. At room temperature, the crystalline ion gel exhibits pronounced rigidity, with a Young’s modulus of 40.8 MPa and an elongation at break of 15%. After the temperature-induced phase transition, the amorphous ion gel displays exceptional flexibility, with a Young’s modulus reduced to 26.9 kPa and an elongation at break increased to 272%, showing almost no physical deformation after 500 cycles of tensile testing. Similarly, Mirzaei-Saatlo et al.^[67]proposed a phase-transition ionic gel, [Ch]Fo/HEC, prepared from 2-hydroxyethyl cellulose (HEC) and the ionic conductor choline formate ([Ch]Fo). The [Ch]Fo/HEC exhibits rubber-like behavior because HEC, acting as a polymer matrix, possesses elastomeric properties, with a tensile strength of 1.69 MPa and an elongation at break of 180%.

Phase-separated ionic gels are obtained by controlling the compatibility between polymers and ILs to induce microscopic phase separation, resulting in gel materials composed of a polymer-rich rigid phase and a solvent-rich flexible phase^[68-70]. When the compatibility between polymers and ILs is low, hydrophobic polymer chains spontaneously aggregate to form a rigid phase, providing mechanical support through dense non-covalent interactions; hydrophilic polymers, on the other hand, form a homogeneous network with ILs, and the solvent-rich phase maintains the gel’s ionic conductivity through continuous ion channels. Under external force, the non-covalent bonds in the rigid phase break preferentially to dissipate energy, while the flexible phase accommodates large deformations through chain segment sliding, thereby synergistically enhancing the strength and toughness of phase-separated ionic gels^[71]. For example, Wang et al.^[72]dissolved AM and AA monomers in 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) and then carried out in-situ polymerization under UV light to obtain P(AM-co-AA) ionic gels. In these gels, PAA chains exhibit good compatibility with EMIES, forming a homogeneous and elastic gel network, while PAM chains repel EMIES, forming a phase-separated structure that gives the ionic gel a high tensile strength of 12.6 MPa and a high elongation at break of 600%. Moreover, this method is also applicable to other similar monomer–IL combinations. In addition, Liu et al.^[73]found that phase separation of poly(N-isopropylacrylamide) in ILs is relatively common. Inspired by this, Xie et al.^[74]controlled the cooling rate to regulate the phase-separation behavior of PAM, thereby effectively tuning the mechanical properties of the ionic gels, as shown in Figure 8. The gels prepared exhibit a tensile strength of up to 31.1 MPa and a compressive strength of 122 MPa.

In summary, both phase-transition ionic gels and phase-separated ionic gels are effective approaches for toughening ionic gels. Although both utilize microphase-separated structures, phase-transition ionic gels focus on dynamic regulation and intelligent responsiveness, while phase-separated ionic gels concentrate on static performance optimization. The hard microregions generated by microphase separation—such as crystalline domains, glassy blocks, and ion-enriched regions—act as physical cross-linking points and reinforcing phases, significantly enhancing the material’s modulus, strength, creep resistance, and dimensional stability. This strategy is particularly suitable for applications with stringent requirements for mechanical stability and long-term shape retention. For example, in the field of solid-state electrolytes, ionic gels with well-developed microphase-separated structures can effectively resist lithium dendrite penetration, maintaining the stability of the electrode/electrolyte interface; their excellent creep resistance is crucial for battery cycle life^[75].In flexible substrates or microfluidic chips that require precise shape and dimensional stability, microphase-separated structures can also provide reliable support. However, phase-separated interfaces may act as barriers to ion migration, significantly reducing ionic conductivity. This poses limitations for all applications that rely on efficient ion transport, such as sensor response speed and battery rate performance.

Inorganic nanoparticles (INPs) possess unique surface and size effects, and have garnered significant attention from researchers as inorganic toughening phases for enhancing the performance of ionic gels. During deformation of ionic gels, INPs uniformly distributed within the gel can serve as cross-linking points, effectively alleviating stress concentration and thereby improving gel stability. The toughening mechanisms of INPs can be categorized into three types: crack bridging effect, energy dissipation mechanism, and regulation of the gel network structure.

When an ionic gel is subjected to external forces and cracks form, INPs can span across both sides of the crack, acting as a “bridge” to effectively impede further crack propagation. This is because crack propagation requires overcoming the interaction forces between the INPs and the gel matrix. The INPs are tightly bonded to the gel matrix through chemical bonding, physical adsorption, and other mechanisms, thereby increasing the required force for crack propagation. For example, Watanabe et al.^[76]prepared a DN ionic gel composed of clustered silica nanoparticles (SiO₂ NPs) and PIL, which exhibits superior mechanical properties compared to PIL SN ionic gels. This is because the silanol groups (―Si―OH) on the surface of SiO₂ NPs can form hydrogen bonds or coordination bonds with ionic groups in the gel. When a crack propagates near the SiO₂ NPs, these bonding forces enable the SiO₂ NPs to bear part of the tensile stress, thereby increasing the energy required for crack propagation and achieving a toughening effect.

INPs can trigger multiple energy dissipation processes within ionic gels. First, the addition of INPs alters the microstructure of the ionic gel, introducing heterogeneity. Under external forces, this heterogeneous structure gives rise to stress concentration zones, which in turn initiate local plastic deformation, interfacial friction, and other energy dissipation processes. For example, Kim et al.^[77]used gold nanoparticles (AuNPs@BACA) coated with N, N′-bis(acryloyl)cystamine as a chemical cross-linker and self-healing site to fabricate an ionic gel with high stretchability (~750%) (Fig. 9A). Under external loading, the flexible gel matrix undergoes plastic flow around the highly rigid AuNPs@BACA, dissipating substantial amounts of energy. At the same time, interfacial friction between the INPs and the matrix also converts mechanical energy into heat and other forms of energy, effectively dissipating externally applied energy and thereby enhancing the mechanical properties of the ionic gel. Similarly, Lu et al.^[78]prepared a Gr‑PAM/PVA ionic gel reinforced with graphene (Gr) nanosheets (Fig. 9B), which exhibits a high tensile modulus of 3.64 MPa, a fracture elongation of 217%, and an ionic conductivity of 0.186 mS·cm^-1. The toughening mechanism of the Gr‑PAM/PVA ionic gel stems from the "nano-pullout effect": under external loading, the interfacial bonding between the Gr nanosheets and the matrix is gradually disrupted, and the pullout process requires overcoming friction and adhesion, thereby converting mechanical energy into interfacial energy and dissipating energy. Moreover, the microporous structure formed after the Gr nanosheets are pulled out induces local plastic deformation in the matrix, further increasing crack propagation resistance and secondary energy dissipation efficiency. This multiplicative energy dissipation mechanism synergistically enhances the fracture toughness of the ionic gel.

In addition to the mechanisms mentioned above, INPs can also achieve synergistic toughening by regulating the crosslinking density and topological structure of ionogel polymer networks. Their mechanisms of action mainly involve two aspects: first, serving as physical crosslinking points to enhance network crosslinking; second, restricting polymer chain movement and inducing their ordered arrangement through adsorption^[79].This structural optimization enables the gel to effectively disperse stress under load, preventing fracture caused by localized stress concentration, thereby synergistically enhancing the toughness of the ionogel. For example, Li et al.^[80-81]incorporated attapulgite (ATP), a naturally occurring anisotropic nanoclay with a surface rich in hydroxyl groups, into the gel's crosslinked network to prepare anisotropic nanocomposite ionogels, as shown in Fig. 9C. Under the dual effects of its anisotropic structure and strain-induced phase separation, this gel exhibits a high tensile elongation of 747.1% and a tensile strength of 6.42 MPa.

By introducing INPs, ion gels can be effectively toughened, with the key lying in their uniform dispersion within the polymer network. An excessively high INP content can lead to particle agglomeration, reducing the homogeneity of the ion gel network and thereby weakening its mechanical properties; conversely, an insufficient filler content makes it difficult to enhance gel performance. This strategy demonstrates unique value in applications requiring extreme mechanical performance, multifunctional integration, or stability under special environmental conditions. For example, ion gels containing heat-resistant INPs (such as SiO₂ NPs, clays, etc.) can significantly enhance their mechanical stability and anti-dehydration capabilities in high-temperature environments, making them suitable for sensing or sealing applications in extreme operating conditions such as automotive electronics and aerospace. However, for applications requiring high transparency (such as optical devices) or extreme flexibility (such as biomimetic skin), the introduction of INPs often has the opposite effect. In fact, the toughening of ion gels by INPs is typically not achieved through a single type of INP alone, but rather through the synergistic interplay of multiple factors, including polymer network structure modulation, non-covalent interactions, and microphase separation structures.

In summary, the strategies for enhancing the mechanical properties of ionic gels mainly fall into four categories: (1) regulating polymer networks through multi-network and interpenetrating network structures, leveraging multi-network synergy to improve mechanical performance while addressing poor network compatibility; (2) constructing dynamic non-covalent interactions as sacrificial bonds, dissipating energy via reversible bond-breaking and reformation mechanisms, though constrained by complex fabrication processes; (3) designing microphase-separated structures, such as stimulus-responsive phase-transition gels for dynamic toughening, or forming rigid–flexible biphasic synergistic energy dissipation through polymer/solvent phase separation; (4) introducing inorganic nanoparticles to achieve multiple energy dissipation pathways via crack bridging, nanoparticle pull-out effects, and network crosslinking regulation, with the effectiveness depending on the uniform dispersion and optimized content of INPs. Table 3summarizes the enhancement strategies for the mechanical properties of ionic gels, along with their advantages, disadvantages, and applicable scenarios.