Ionogel can be mainly formed between the IL and the polymer chain by the physical crosslinking method through hydrogen bonds, electrostatic interaction, and host–guest interaction. The hydrogen bond was formed by the combination of H atoms and high-electronegative atoms in molecules, endowing ionogel with good reversibility and controllable crosslinked strength. For instance, Liu Y. et al. (2021) prepared conductive hydrogel by a simple physical crosslinking method. As shown in Figure 5A, PVA/EMImAc/H₂O hydrogels exhibit the advantages of robustness, high flexibility, and elasticity under the effect of hydrogen bonds, but it is prone to failure in extreme environments. In order to solve this problem, the electrostatic interaction based on chemical bonds between anions and cations can be further introduced. For instance, Ding et al. (2017) used the electrostatic interaction to lock 1-ethyl-3-methylimidazolium dicyandiamide ([EMIm][DCA]) into poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (PAMPS) in Figure 5B. Moreover, its durability may be solved by the host–guest interaction, which is the self-assembly of monomer molecules through the host–guest recognition characteristics. For instance, Zhang et al. (2015) prepared ionogel in Figure 5C by taking advantage of the host–guest interaction between b-cyclodextrin (b-CD) and bisimidazolium ILs (Bis-C₁₂(mim)Br). Under the effect of the host–guest interaction, the ionogel can be used as a quasi-solid electrolyte for dye-sensitized solar cells with excellent long-term stability.

The polymer chain entanglement of physically crosslinked ionogel is instantaneously reversible (Sprugis et al., 2019), but the heat-resistance and solvent-resistance are poor, and the permanent deformation is also serious.

The chemical crosslinking method mainly includes photopolymerization crosslinking and thermal polymerization crosslinking. Photopolymerization is a typical free-radical polymerization method by activating monomer molecules with light to conduct chain polymerization. This method has the strength of controllability, low polymerization temperature, and high reaction selectivity. For instance, Xu et al. (2021) prepared a multifunctional ionogel through a one-pot photopolymerization, as shown in Figure 5D. This method requires the participation of ultraviolet light, which is difficult to apply in scenarios without light. Fortunately, thermal polymerization makes up for the defects of photopolymerization. Chain polymerization can be conducted by activating monomer molecules in the way of thermal initiation. As illustrated in Figure 5E, Lan et al. (2020) developed a stretchable transparent dual-network ionogel by a simple one-pot thermal polymerization method. However, both photo-initiated polymerization and thermal-initiated polymerization require the input of external energy, making it difficult to synthesize the desired ionogel in a mild environment. Accordingly, Ran et al. prepared IL-click ionogel under mild conditions based on the ionic crosslinking and covalent crosslinking methods in Figure 5F. Click chemistry is a reaction in which a pair of click-functional groups rapidly and selectively reacts with each other to form stable conjugates under mild conditions. Since the click reaction is highly efficient under mild conditions, ionogels can be prepared by simply mixing PEGDA and poly(1-butyl-3vinyl imidazolium fluoborite) (PIL-BF₄) methanol solutions without additional oxygen, humidity, or heating condition (Ren et al., 2019).

The chemically crosslinked ionogel contains permanent connections formed by covalent bonds, offering ionogel high strength and controllable crosslinked degree.

The physical–chemical crosslinking method combines the reversibility of physical crosslinking with the stability of chemical crosslinking. In Figure 5G, the ionogel was prepared through hydrogen bonds and thermal polymerization, which possess transparent, mechanically robust, and high ionic conductivity (Cao et al., 2020). Sun et al. prepared a stimuli-responsive ionogel (Figure 5H), comprising chemically crosslinked polymer poly(N-isopropylacrylamide) (PNIPAM), physically crosslinked iron ions, and a carboxyl group. Physical crosslinking formed by ion coordination can improve the mechanical strength of ionogel, making it controllable between the strong coordination of Fe³⁺ and the weak coordination of Fe²⁺. In addition, covalent crosslinked PNIPAM and conductive ILs are used as thermal switches to obtain thermal/redox dual stimuli-responsive ionogel (Sun et al., 2018).

Physically and chemically crosslinked ionogels can make up for the shortcomings of a single physically crosslinked or chemically crosslinked ionogel, but the challenge is the rational design of experiments to determine the key performance of ionogels.

In brief, ILs can be physically trapped into the polymer matrix (Mantravadi et al., 2016; Cao et al., 2017; Singh et al., 2017), the weak interfacial force result in poor mechanical properties, but it has good physical properties such as stretchability and self-healing. In addition, ILs can also react with the polymer matrix and be fixed in the polymer matrix through polymerization (Muldoon and Gordon, 2004; Ohno, 2007; Zgrzeba et al., 2015), which strengthens the mechanical properties of ionogel. Meanwhile, the strong crosslinking prevents losing ILs from the surface to the environment. However, ionogel is usually prepared by free radical polymerization, which is limited by the reaction conditions and the type of polymer matrix and ILs. To design required ionogels, the physical–chemical crosslinking method can endow ionogels with excellent physical properties by controlling the crosslinked degree. In addition, ionogels can reversibly expand and contract under external stimuli (e.g., light and heat), which has great potential in the intelligent control system.

In 2016, the World Health Organization published a set of alarming data. Approximately 700,000 deaths are attributable to “superbug” infection every year, and the annual number of deaths may increase to 10 million by 2050. The traditional antibacterial materials have developed drug resistance so the need for an alternative and sufficiently powerful antibacterial material so that bacteria never develop resistance is urgent (Wei et al., 2020). Due to the electrostatic effect on disturbing bacterial membranes, IL-based antibacterial materials have attracted extensive research interest (Zhao et al., 2014). Recently, the antibacterial mechanism and intelligent IL-based antibacterial materials have developed rapidly (Zhao et al., 2015).

As shown in Figure 6A, the anionic sulfonated hyperbranched polyglycerol (hbSPG) and cationic quaternized polyethylenimine (QPEI) were introduced onto the poly(ether sulfone) (PES) membrane (Li et al., 2019). The membrane showed anti-protein adsorption through the surface hydration network, preventing the adsorption of hydrophobic impurities and antibacterial performance because of its hierarchical architecture and the free quaternary ammonium base after being immersed in wastewater for 2 months, showing great potential in the application of anti-protein pollution.

Moreover, the antibacterial mechanism of BILIMs and the effect of chain length on antibacterial ability were further studied. In Figure 6B, Qin et al. (2017) showed that the antibacterial efficiency of PILs against Gram-positive bacteria increased with the increase of the alkyl chain length of substituents, the reason may be that hydrophobic segments are more easily inserted into bacterial membranes, leading to bacterial death. Furthermore, the antibacterial mechanism was systematically explored at the molecular level (Zheng et al., 2020). As shown in Figure 6C, ILs were readily embedded into the bacterial membrane as the molecular size increases, resulting in the instability of the lipid bilayer and further promoting antibacterial performance. This work demonstrated the electrostatic interactions between molecular sizes of IL-based materials and Gram-negative bacteria with the effect of an antibacterial ability. Moreover, the responsive self-replenishing coating offers an idea strategy for the renewable anti-biofouling surface. In Figure 6D, Ye et al. (2019) mixed the solid/liquid binary mixture of ILs with a semicrystalline polymer. After heating, the surface of the ionogel changes from a solid to a liquid-infused state, which promotes the removal of biofilm/bacteria. After the surface is damaged, ILs released from the inside to the surface and crystallized on the surface to realize the self-replenishment of ILs on the surface. This work provides an opportunity for ionogels to be used as a functional coating with renewable anti-biofouling properties.

In addition to killing bacteria, algae pollution can be resisted by preparing a multi-scale IL polymer brush-modified antifouling surface (He et al., 2021). 1-(1H-Benzotriazolyl) methyl-3norbornene methyl-1H-imidazolium bromide ([BNIm][Br]) was grafted onto the bio-inspired sharklet substrate prepared by 3D printing of acrylic resin (Figure 6E). Compared to the bare sharklet substrate, the removal rate of Porphyridium increased from 40.7% to 60.1%. Therefore, ILs can be grafted onto the substrate surface or infused into a porous surface to obtain antibacterial function (Ye et al., 2012), which can be adjusted by the length of the ILs alkyl chain, hydrophobicity, and molecular size.

Some organic substances such as protein and grease contained in wastewater will cause deposition on the surface and corrosion of water treatment equipment (Wei et al., 2018), which poses a real threat to the environment (Wei et al., 2022). Many physical and chemical methods (such as distillation and electromagnetic radiation) have been developed for organic pollutants separation in wastewater but have the problems of low efficiency and high energy consumption (Zhang X. et al., 2020). To overcome the defects of the aforementioned methods, the antifouling coating inspired by nature can reduce the deposition of these organic substances and maintain the initial function of the equipment surface.

Deng et al. prepared a series of PIL-based oil/water separation membranes with adjustable surface wettability using N-vinylimidazolium IL and divinylbenzene (Figure 7A). The hydrophilic poly(1-vinyl-3-butylimidazolium acrylate)-based membrane (PILM-1) transports water and retains oil, while the hydrophobic poly(1-vinyl-3-octylimidazolium hexafluorophosphate)-based membrane (PILM-5) removes oil and retains water. Two types of membranes with opposite wettability show excellent oil/water separation efficiency (over 99%) during the treatment process and realize continuous oil/water separation of 46 L/12 h (Deng et al., 2020). Additionally, Liu et al. (2022) prepared an IL-modified membrane by grafting 1-butyl-3-vinylimidazole bromide IL onto PVDF for improving the wettability and antifouling performance of the membrane (Figure 7B), demonstrating excellent separation performance for cationic dyes in wastewater.

The development of bio-inspired coatings is a promising strategy to solve the increasingly serious problem of organic fouling, which has the advantages of reliable service life, effective cost reduction, and broad application prospects in water treatment equipment.

Some adhesion of solids (e.g., mineral scale and ice) can pose serious problems to humans in daily production and life. Undesired scale adhesion in pipes will cause fuel waste due to decreased heat transfer efficiency. Furthermore, once ice forms on a surface (e.g., aircraft), it will seriously endanger the normal operation of the equipment and put humans in danger. Therefore, dealing with the solid adhesion problem is of great significance to ensure normal operation. BILIM has achieved success in antifouling and anti-icing applications due to the existence of the liquid barrier layer on the surface.

Compared to the Fluorinert FC-70-infused surface, the IL-infused surface is more stable on the coating surface because of its moderate surface energy than with fluorinated lubricating oil. Using IL-BMIm as a liquid layer, Charpentier et al. (2015) developed a polypyrrole coating on a stainless-steel substrate using a potentiostatic deposition method in Figure 8A. Due to the slippery and low adhesion properties of the liquid-infused surface, calcium carbonate (CaCO₃) was significantly reduced by 18 times, offering a novel solution for anti-scaling. In addition, Saul et al. (2021) infused IL-BMIm into the FeCO₃ layer on X65 carbon steel to prepare a novel kind of BILIM (Figure 8B). There is no need to use a functional layer between the substrate and the lubricant interface and shows enhanced anti-scaling ability against CaCO₃.

In addition to the adhesion of scales, the adhesion of ice may also cause surface failure or even serious accidents. Therefore, many efforts have been made to reduce the formation and adhesion of ice on the surface. As shown in Figure 8C, the smooth surface obtained by infusing [HMIm]Cl into the polymer has excellent anti-icing performance compared to the bare glass and the polymer surface without an IL infusion. The surface extends the icing time to 3605s and reduces the adhesion of ice (Rao et al., 2019). In addition, Zhuo et al. (2020) developed a novel anti-icing ionogel to inhibit the growth of ice to mitigate the harm caused by accretion ice, as illustrated in Figure 8D. The ionogel demonstrated an outstanding ability to inhibit the growth of ice and prevent the formation of frost in a humidified environment. It is observed that the positive and negative ions of [BMIm]Br separate at the front interface of ice growth, creating an electric field that prevents ice growth due to the formed interface liquid layer with poor adhesion to the substrate.

Although the research on ILs in anti-scaling and anti-icing is small, the existing literature has shown great potential in removing undesired adhesion. It is expected to lead the scientific research wave in anti-scaling and anti-icing by utilizing the lubricating properties and freezing tolerance properties of ILs.

The rapid rise of the internet has popularized the use of electronic products and improved the life quality of humans. However, traditional electronic devices suffer from limited toughness and poor self-healing properties, while IL-based electronic devices have excellent performances such as flexibility, self-healing property, and reconfigurable property caused by charge effect and high conductivity. Therefore, many research studies have paid attention to building new multi-functional electronic devices based on ILs. The unique mechanical properties of ILs make them promising for various biological sensors.

To obtain photonic ionogels (PIGs) with good stability and synergistic sensitivity, Lyu et al. (2021) developed an ion skin by locking 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) into poly(ethylene glycol) phenyl ether acrylate (PEGPEA) polymer elastomer as shown in Figure 9A. Under a slight tensile strain, the PIGs immediately changed from orange to green. When the strain reached 40%, the PIGs changed to blue, showing excellent mechanochromic performance. They can be used for ion skin sensors and photoelectric interactive devices.

The deformable electronic skin device was designed to convert local stress into a spatially resolved optical signal (Lee et al., 2021). The electrochemical performance of the composite material is controlled by the electrochemiluminescence (ECL)-active material (ECL: Ru(bpy)₃)[PF₆]₂, IL: [EMIm][TFSI]), and the mechanical performance is controlled by polyurethane matrix (Figure 9B). When pressure is applied to the film from the top, cations leave and anions are retained due to the difference in the mobility rate of anions and cations, thereby appearing negative charge layer and positive charge layer on the film. When the pressure was removed, the electrical potential will slowly return to its original value until a uniform ion distribution is obtained. For the aforementioned reasons, the ECL skin device platform can convert mechanical stimuli into visual readings, laying the foundation for the design of tactile sensors for human–machine interaction with electronic skin. In addition, the structural changes of porous ionogels can also be used to visualize electronic signals. As described in Figure 9C, Kwon et al. (2021) prepared porous ionogel by in situ crosslinking polymerization of PEA-g-PS-g-PDVB and [EMIm][TFSI]. After applying pressure, the elastic porous ionogel collapsed and the pores closed. Therefore, the contact area between the gel and the electrode increases, resulting in a higher ECL capacitance. The applied pressure can be directly displayed by the brightness of the emitted light, and it is expected to become an important part of high-performance, functional ion electronics. Additionally, molding soft BILIMs into specific structures with strong adhesion and high conductivity by utilizing 3D printing has prospects for intelligent devices such as biosensors, wearable electronic devices, and smart robotics (Chen et al., 2022).

Electronic devices based on ILs have made innovations in the development and application of flexible wearable devices (Wang et al., 2022). IL-based electronic devices are mainly in the form of ionogel, which confines the ILs in an elastic polymer matrix. ILs have excellent properties such as conductivity and electrochemical stability, thereby improving the electrochemical performance of electronic devices. In addition, conductive additives such as carbon-based materials and metal nanoparticles were introduced into the polymer network to improve high conductivity and mechanical properties. However, most of the research is still in the theoretical research or basic research stage and generally has problems with stability and durability. The future research direction is to continue to optimize the performance of biological devices to obtain high-performance devices.

Underwater adhesion generally has problems such as poor adhesion and instability, which seriously affect the efficacy and service life of adhesive materials in practical applications. IL-based adhesive tape has broad application prospects due to its underwater stability and adhesion, the adhesive strength of which can be adjusted by changing the types of ILs. In addition, physical interaction can be introduced into adhesive tape based on ionogel to further enhance the adhesion strength.

As a common physical interaction, hydrogen bonds can be used to enhance the adhesion of the tape. As shown in Figure 10A, Guo et al.2020 developed supramolecular hydrogel containing poly(urethane-urea) (PUU) and ILs (Chen 2021 Guo, 2021). The hydrogen bond endows the ionogel with excellent adhesion, which can be adhered to any substrate. It can be attached to a human’s skin as a sensor to monitor various movements. Additionally, an efficient adhesive tape was prepared by simply introducing flexible alkoxy into the cationic skeleton of PILs containing TFSI⁻ anion (Figure 10B). The strong hydrogen bond and electrostatic interaction between flexible alkoxy chains and PILs both contribute to high cohesive energy and interfacial adhesion energy (Zhang et al., 2021). Alkoxy PILs can adhere to various substrates such as glass, ceramic, and stainless steel.

In addition to the hydrogen bond, the effect of a charge also enhances adhesion. The transparent ionogel adhesive tape was composed of hydrophobic ILs and poly(hexafluorobutyl acrylate-co-methyl methacrylate) (Huang et al., 2021). The ionogel adhesive tape achieved high adhesive strength through an ion-dipole interaction, which can be reversibly adhered to various substrates, such as glass, steel, and PTFE, and the adhesive strength of ionogel was higher than that of most commercial adhesive tapes (Figure 10C). Cations of the ionogel can repel salt ions from the substrate surface, and quickly and firmly adhere to various substrates in salt solution (Yu and Wu, 2021) (Figure 10D). The ionogel possesses the performance of high ionic conductivity, underwater adhesion, self-healing, and stretchability, which can lift an object up to 5 kg underwater and repeatedly adhere to the substrate for sensors.

IL-based adhesive tapes take full advantage of the characteristics of ILs and polymer networks to obtain high adhesive strength through physical interaction including hydrogen bonds and charge interaction. The adhesive strength, self-healing ability, stretchability, optical transparency, electrical conductivity, and other properties of IL-based adhesive tape can be controlled by reasonably designing the structure of ILs.

Many efforts have been made to apply adhesive materials to wound dressings; however, little attention has been paid to combining high adhesion with antibacterial/antiinflammatory functions in existing solutions. The incorporation of ILs brings high adhesion to antibacterial wound dressings, which makes up for the aforementioned problem. As shown in Figure 11A, Yu et al. (2020) designed an antibacterial hydrogel containing pyrrole ILs, which showed effective antibacterial activity against Gram-negative and Gram-positive bacteria. The antibacterial and adhesive properties of ionogel give it potential in wound dressing applications. Inspired by the bio-adhesion of mussel dopamine, Yang et al. (2021) fabricated an adhesive hydrogel dressing containing PDA. As shown in Figure 11B, PDA components are rich in catechol groups, so they can adhere to various surfaces, especially in special areas requiring large movements, which can be used as a wound dressing. Inspired by trees, Zhang Y. et al. (2020) synthesized a novel type of lignin/PILs composite hydrogel dressing. As shown in Figure 11C, self-healing hydrogel obtained by the supramolecular interaction between lignin and PILs can promote wound healing in rats. The introduction of lignin with a three-dimensional network structure can significantly improve the mechanical property and antioxidant activity of hydrogel dressing. The IL-based hydrogel dressing with excellent antibacterial performance can promote skin wound healing wonderfully. As depicted in Figure 11D, (Li et al. (2021) grafted positively charged IL 1-vinyl-3-butylimidazolium ([VBIm]Br) and Al³⁺ onto the main chain of hydrogel through covalent interaction. On the one hand, ILs can promote the hydrophobic interaction between hydrophobic segments of hydrogel, which can show the good mechanical properties of hydrogel after absorbing water. On the other hand, charged ILs can promote the migration of Al³⁺, improving the self-healing performance of hydrogel and accelerating the self-healing of rat wounds. The synergistic effect of strong adhesion and antibacterial properties of the IL-based wound dressing will play a huge role in wound healing.

Considering the biosafety of BILIMs in bio-adhesion and wound dressing, numerous studies have shown that choline, some ammonium ions, and glycine betaine can be utilized as cations to obtain biological ILs with biodegradable and low toxicity (Gomes et al., 2019). IL-based wound dressing possesses a good bactericidal effect on common bacilli, cocci, and fungi, which has good biocompatibility without destroying the normal cell structure. In addition, the introduction of charged ILs endows IL-based wound dressing with good mechanical properties and adhesion, which is conducive to promoting the self-healing of the wound. Therefore, BILIM provides a broad application prospect in wound dressing.