Polymer ionogels and their application in flexible ionic devices

Polymer ionogel (PIG) is a new type of flexible, stretchable, and ion‐conductive material, which generally consists of two components (polymer matrix materials and ionic liquids/deep eutectic solvents). More and more attention has been received owing to its excellent properties, such as nonvolatility, good ionic conductivity, excellent thermal stability, high electrochemical stability, and transparency. In this review, the latest research and developments of PIGs are comprehensively reviewed according to different polymer matrices. Particularly, the development of novel structural designs, preparation methods, basic properties, and their advantages are respectively summarized. Furthermore, the typical applications of PIGs in flexible ionic skin, flexible electrochromic devices, flexible actuators, and flexible power supplies are reviewed. The novel working mechanism, device structure design strategies, and the unique functions of the PIG‐based flexible ionic devices are briefly introduced. Finally, the perspectives on the current challenges and future directions of PIGs and their application are discussed.


| INTRODUCTION
Nowadays, a wide range of flexible devices has been developed, including flexible organic light-emitting diodes, 1,2 flexible transistors, [3][4][5] electronic/ionic skin, [6][7][8] flexible electrochromic devices (ECDs), [9][10][11][12] flexible actuators, 13,14 and flexible power supplies, 15 and so forth.These diverse, flexible devices have revolutionized user experience by providing a new level of convenience, portability, and versatility, enabling individuals to explore technology in innovative and exciting ways.The rapid development of flexible devices has created a significant demand for advanced and multifunctional flexible conductive materials.Based on different conduction principles, conductive materials used in flexible devices can be classified into two categories: electronic conductors (such as composites of elastomers and metal nanomaterials) and ion conductors (including ionogels and hydrogels doped with salts). 16olymer ionogels (PIGs), composed of a polymer matrix and ionic liquids (ILs)/deep eutectic solvents (DESs), are widely recognized as one of the most significant flexible ionic conductors.The polymer chains interlock or entangle to form a three-dimensional (3D) network within the polymer matrix, with the voids of the network being filled with ILs such as imidazole cation derivatives or quaternary ammonium salt derivatives. 17,18he presence of this 3D ion channel network within the polymer matrix facilitates ions to directionally migrate and redistribute under external electric fields.The combination of solid-state polymer networks and ILs/ DESs offers many possible properties, including high ionic conductivity, [19][20][21] nonvolatility, high thermal and electrochemical stability, excellent mechanical properties, [22][23][24] transparency, and self-healing capabilities. 25ompared to conventional ionic hydrogels, PIGs address the issue of water evaporation and exhibit an expanded operating temperature range (from below 0 °C to above 100 °C).][28] Due to their intrinsic ionic conductivity, PIGs possess some unique properties and applications that are entirely distinct from those of flexible electronic conductors. 180][31] Some flexible PIGs contain the IL with oxidation-reduction properties, which can simplify the structure of flexible ECDs and endow more designable solutions in materials and functions. 32The high transparency of flexible PIGs allows the transport of ions without hindering optical signals, expanding their use in flexible electric actuators or light-driven actuators. 33The easily formed electronic double layer (EDL) and the excellent stretchability in flexible PIGs make them well-suited for applications in flexible power supplies with large deformation as well as in ionic skin. 15,16,34As depicted in Figure 1, various novel flexible ionic devices have been well developed based on these unique flexible PIGs.
This review classifies PIGs into three categories based on their polymer matrix: carbon chain PIGs, heterochain PIGs, and elemental organic PIGs.In accordance with this classification, we summarize recent advances in structural design strategies, preparation methods, basic properties, and advantages of novel PIGs.Furthermore, we introduce the latest research developments in PIG-based flexible ionic devices, which include flexible ionic skin, flexible electrochromic devices, flexible actuators, and flexible power supplies.In this section, we provide a brief overview of the unique functions of these devices, their novel working mechanisms, and the strategies employed in designing their device structures.Lastly, we present the potential applications of PIGs in flexible devices and discuss their prospects.

| RESEARCH PROGRESS OF PIGS BASED ON DIFFERENT POLYMER MATRIX 2.1 | PIGs based on carbon chain polymer
Carbon chain polymer is a kind of the most common polymers whose backbone is exclusively composed of carbon atoms.These polymers, such as polyacrylic acid (PAA), polyacrylamide (PAAm), and their derivatives, have garnered extensive attention in the field of PIGs owing to their abundant monomer sources and simple preparation processes.Here, according to the different topologies of the PIGs' network, we review them in the order of single network (SN), double network (DN) and multinetwork (MN), respectively.Table 1 summarizes some basic properties of carbon chain polymer-based PIGs as discussed in this section.

| PIGs with SN topology based on carbon chain polymer
PIGs with a SN topology typically consist of a sole polymer matrix that forms a 3D network filled with ILs.The SN topology is achieved through various interactions between macromolecules, such as covalent cross-linking, hydrogen bond, van der Waals forces, ionic bond, and ligand bond, and so forth. 52I G U R E 1 Polymer ionogels based on different polymer matrix and their application to flexible electronic skin and flexible applications on optoelectronic devices.
Acrylic acid (AA) and its ester derivatives are considered to be among the most important and ideal monomers for constructing SN PIGs due to their unique advantages.These advantages include the presence of multiple functional groups, ease of polymerization (either thermal-or photo-polymerization), and high transparency.Yuan et al. 35 prepared PAA ionogels by in-situ photopolymerization in the IL 1-butyl-3methylimidazolium trifluoromethanesulfonate ([BMIm]-TfO) (Figure 2A).The polymerization reaction could be completed within 120 s, achieving a conversion rate of approximately 80%.This photopolymerization method demonstrated remarkable speed and efficiency.The resulting ionogels exhibited good stretchability (1200%) and excellent self-adhesive property due to strong hydrogen bonding interactions.Moreover, the PAA ionogels showed high transparency (>90%) due to the good compatibility between AA and [BMIm]TfO.These remarkable properties make PAA ionogels highly suitable for strain sensing applications, offering fast response times, high sensitivity, and low electrical hysteresis, making them excellent for monitoring various human motions.Similarly, Sun et al. 36 synthesized a novel PIG based on polybutyl acrylate (PBA) through in-situ photopolymerization of butyl acrylate (BA) in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm]TFSI).The PBA ionogel exhibited superior environmental stability and underwater adhesion due to the hydrophobic nature of BA and [BMIm]TFSI, as well as the high boiling point of [BMIm]TFSI.However, the mechanical properties of the PBA ionogel were significantly reduced compared to the PAA/[BMIm]TfO system.Especially for the elongation at break, it is because the lack of hydrogen bonding between BA and [BMIm]TFSI, resulting in an elongation at break of approximately 700%.
As depicted in Table 1, homopolymers can form PIGs with favorable elongation at break and conductivity; however, their tensile strength tends to be relatively low.On the other hands, the copolymerization of multiple monomers can introduce more interactions between macromolecules, thereby enhancing the mechanical properties of PIGs.An efficient strategy in copolymer ionic gels is to use monomers with different solubilities in ILs, resulting in phase separation and physical cross-linking.Kim and Moon 37 developed a nonvolatile and highly   transparent PIGs by combining a copolymer of poly-(methyl methacrylate-ran-butyl acrylate) (PMMA-r-PBA) with the ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ([EMIm]TFSI) (Figure 2B).By adjusting the copolymer ratio, this ionogel exhibited an elongation at break of 850% and a high tensile strength of approximately 200 kPa.The copolymerization of PMMA with the IL-insoluble and low-Tg polymer of PBA resulted in physically cross-linked ionic conductors, making the ionogel stronger. 37Acrylamide (AAm) also serves as an essential structural monomer for the matrix of acrylic copolymers.Wang et al. 38 reported a simple in-situ photopolymerization technique to create an ultra-tough and stretchable ionogel by the random copolymerization of two common monomers (AAm and AA).This P(AAm-co-AA) ionogel consisted of polymer constituents with varying solubilities in 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIm]ES).PAAm is poorly soluble in [EMIm]ES and PAA is highly soluble in [EMIm]ES, leading to the phase separation within the same network (Figure 2C).The PAAm-rich phase, with hydrogen bond, contributed to energy dissipation and toughening of the ionogel, while the elastic solvent-rich phase (PAA) facilitated large strain, synergistically enhancing the toughness of the ionogels.These ionogels exhibited excellent tensile strength (12.6 MPa), high stretchability (600% strain), and were capable of easily lifting 1 kg of weight in comparison to single PAAm or PAA gels.
Although ILs improve the stability of PIGs compared with hydrogels, they still face potential leakage problem either to large deformation or at high/low temperatures, which is because conductive ILs usually fail to be effectively bonded by inert polymer networks.Polyzwitterion can also provide some intermolecular dynamic interactions between polymer matrix and IL, which is benefit to keep a high and stable content of IL and enhance mechanical properties.In the meanwhile, polyzwitterion can provide a compatible dynamic conductive zwitterionic nanochannels for ILs.Therefore, the incorporation of polyzwitterion into PIGs demonstrates a good strategy to guarantee a high and stable ionic conductivity.Lei and Wu 39 reported a highly transparent and ultrastretchable conductor with stable conductivity during large deformation.The zwitter ionic monomer 3-dimethyl (methacryloxyethyl) propanesulfonate (DMAPS) provided zwitter ionic channels and hydrogen bond acceptors, and the carboxyl group on acrylic acid monomer can be used as a hydrogen bond donor (Figure 2D).After copolymerizing the two monomers, a PDMAPS-PAA PIG was prepared by adding different ratios of IL of [EMIm]ES.PDMAPS provides conductive zwitter ionic nanochannels for the IL of [EMIm]ES, and the ionogel can maintain stable conductivity during large deformation and at different temperatures.The dynamic hydrogen bond networks lead to an ultra-stretchable strain (>10,000%), high-tensile strength (~9 MPa), and self-healing ability.What's more, the ionogel is highly transparent (>90% transmittance).
As research progresses on PIGs, challenges such as toxicity and high manufacturing costs associated with ILbased PIGs have been identified.Therefore, an ideal alternative solvent and ion conductor for PIGs would be of good environmental and chemical stability, as well as low cost and low toxicity.DESs are a novel solvent with low melting point, and the melting temperature of DESs is far below that of either individual component.More importantly, DESs are low cost and toxicity, which make DESs a charming substitute for traditional ILs.DESs are typically formed by combining hydrogen bond donors (HBDs) with hydrogen bond acceptors (HBAs) in specific molar ratios to achieve eutectic mixtures with low melting points. 53In addition, DESs also behave some similar physicochemical properties with ILs, including low volatility, suitable ionic conductivity, wide liquid range, and highly tunable.
Recently, DESs have been utilized in the preparation of PIGs.For instance, Zhang et al. 54 reported a PAAmbased eutectic conductive PIGs synthesized in DESs, [ChCl, HBA + urea, HBD], or [ChCl, HBA + EG, HBD], using a α-helical peptide structure as a cross-linker.The cross-linker of the α-helical "molecular spring" structure increased the intramolecular hydrogen bonding in the αhelical peptide chains (Figure 3A), significantly improving the mechanical strength of the gels while maintaining high resilience simultaneously.Moreover, it inherits the benefits of the DES, effectively addressing issues related to solvent volatilization and freezing.Consequently, this novel eutectic gel demonstrates unprecedentedly prolonged and stable capabilities for sensing human motion or mechanical movement.Remarkably, the electrical signal exhibits minimal drift even after undergoing 10,000 deformations over a period of 29 h or within the wide temperature range of −20 °C to 80 °C.Similarly, Wang et al. 55,56 prepared robust and highly conductive DES-based PIGs as electrolytes for flexible quasi-solid-state supercapacitors and strain sensors at room temperature.These properties demonstrate that DES-based PIGs are a novel highly conductive, antifreezing, low cost, and environmentally friendly materials for the development of future flexible ionic devices.
Furthermore, when utilizing DESs as the solvent for PIGs, the risk of leakage still persists due to the liquid nature of DESs.To address this issue, zwitterionic polymer could also be used to manipulate the mechanical properties and ionic conductivity of DES-based PIGs through additional ionic associations.Bu and colleagues conducted the synthesis F I G U R E 3 (A) Alpha-helical "molecular spring" novel peptide cross-linked eutectic gel structure.Reproduced with permission: Copyright 2022, Springer Nature. 54(B) Schematic diagram of LM-PDES elastomer initiated and cross-linked by LM.Reproduced with permission: Copyright 2021, John Wiley and Sons. 57f zwitterion-containing DES gels using P(AA-cosulfobetaine vinylimidazole) (P(AA-co-VIPS)).This involved in situ copolymerization of AA and the VIPS zwitterionic monomer, facilitated by the addition of a small amount of poly(ethylene glycol) diacrylate (PEGDA) chemical crosslinker within the DES (ChCl:EG = 1:2, molar ratio).P(AAco-VIPS) DES gels exhibited high mechanical properties (a tensile strength of 176 kPa and a fracture strain of 1370%) and excellent ionic conductivity (4.1 × 10 −3 S/cm).The cycling stability of the supercapacitors was evaluated at 1 A/g for 2000 cycles. 40In addition, there has been a recent trend toward developing polymerizable deep eutectic solvent (PDES) to obtain liquid-free ionic elastomers.Wang et al. 57 have successfully created a highly transparent, superstretchable, and self-healing liquid-free SN ion-conductive elastomer.AA and ChCl were selected as HBD and HBA to get the polymerizable DES.Liquid metal (LM) nanodroplets were employed to initiate the polymerization of the DES, which were simultaneously used as crosslinker to establish a liquid-free polymer network (Figure 3B).The resulting liquid-free ion-conductive elastomer demonstrates remarkable characteristics, including high transparency (94.1%), exceptional stretchability (2600%), self-healing ability, and an impressive ionic conductivity of 1.3 × 10 −3 S/cm.Notably, this liquid-free ion-conductive elastomer inherently withstands freezing and drying and remains functional even under harsh conditions.Similarly, by using AAm and AA as HBDs and ChCl as HBA, Li et al. 41 designed and prepared conductive and self-healing liquid-free ionic elastomers based on a PDES copolymer.By adjusting the molar ratio of AA to AAm, the mechanical properties of the ionic gels can be tailored.The self-healing capability of the liquid-free ionic elastomers is attributed to the presence of multiple hydrogen bonds, while their mechanical properties can be finely tuned by modifying the molar ratio of AA to AAm.It is found that several other impressive self-healing and conductive liquid-free ionic elastomers have been synthesized using copolymers based on similar PDESs (poly(AAm/ ChCl-co-MA/ChCl) or poly(AAm/ChCl)+ poly(AA/ ChCl)). 58,59Recently, LiTFSI has become a unique HBA in DESs.In addition to hydrogen bonding interaction between HBD and TFSI-, Li-O, or Li-N coordination bond interactions can also be found in this DES system which will affect the mechanical properties and Li ion conductivity of the corresponding PIG.Yao et al. 42 designed and prepared a PDES by simply mixing N-cyanomethacrylamide (NCMA, HBD) with lithium bis(trifluoromethyl) sulfonimide (LiTFSI, HBA) on the same side chain, and obtained a supramolecular eutectic poly NCMA/LiTFSI gel after polymerization.These PDESs exhibited excellent mechanical properties (tensile strength is 12.9 MPa, fracture energy is 45.3 kJ/m 2 ), strong adhesion and 3D printing adaptability due to the reversible reconstruction ability of amide hydrogen bonds and cyanide-cyanide dipole-dipole interaction in the gel network.In addition, the gel also showed high-temperature responsiveness owing to the high dependence of the Li ions conductivity of the poly NCMA/LiTFSI gel.

| PIGs with DN topology based on carbon chain polymer
DN PIGs consist of two interconnected polymer networks that are permeable or semipermeable to each other, along with the inclusion of ILs.Since the concept of DN gels was proposed, 60,61 the preparation and modification of DN PIGs have become popular research topics.In general, the first network in DN PIGs is usually brittle, rigid, and fully crosslinked, providing sacrificial bonds during deformation, dissipating large amounts of energy, and providing mechanical strength and rigidity to the DN PIGs.On the other hand, the second network is usually ductile, flexible, and weakly cross-linked, filling inside the first type of network, absorbing external stresses, and providing flexibility to the ionogel.Compared to SN PIGs, DN PIGs offer greater flexibility in designing and exhibiting higher mechanical strength due to a wider range of component selection and the presence of two crosslinked networks.The material compositions and their interaction in the DN structure are the key factors to obtain excellent electrical and mechanical properties of DN PIGs. 62In addition, the preparation process of DN PIGs is more complex than that of SN PIGs, which will also affect the final properties of DN PIGs.
As a very important carbon chain polymer, PAA derivatives also play a crucial role in DN PIGs and they are very easy to form a covalent connection network in DN system.Lan et al. 43 synthesized a DN PIG by combining a physically cross-linked poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) with a chemically cross-linked poly(methyl methacrylate-cobutyl methacrylate) (P(MMA-co-BMA)) in an IL of [EMIm]TFSI via a simple in situ thermal polymerization method.The chemically cross-linked P(MMA-co-BMA) elastomer network possessed good compatibility with the ILs conferred a significant proportion of IL (70 wt%) in the DN PIG, which facilitated an impressive ionic conductivity exceeding 1 × 10 −3 S/cm at room temperature, while the crystallizable P(VDF-co-HFP) formed a physically cross-linked and rigid network in the PIG (Figure 4A), resulting in excellent mechanical properties with a tensile strength of 2.31 MPa and an elongation at break of 307%.Moreover, the DN ionogel demonstrated remarkable toughness, elasticity, and transparency across a wide temperature range, ranging from −40 °C to 80 °C.Similarly, Tang et al. 44  and physically cross-linked poly(vinylidene fluoridehexafluoropropylene) (P(VDF-co-HFP) networks with ILs via a in situ polymerization method (Figure 4B).This resulted in a high-strength gel with a breaking tensile stress of 660 kPa and strain of 268%. 44Li et al. 45 reported an ionogel incorporating 4-hydroxybutyl acrylate (HBA) cross-linked network and poly(vinylidene fluoride) (PVDF).The DN PIG exhibits threedimensional (3D) printed property, amazing stretchability (1500%), high conductivity (0.36 S/m at 105 Hz), and an extremely low glass transition temperature (−84 °C).Most importantly, the PVDF endowed the PIG with a self-powered capacity.
Recently, polyzwitterionic polymer and polyionic liquid (PIL) have been introduced into PIGs owing to their good compatibility with IL, which is benefit to optimize the conductive pathways along the ionic networks and improve ionic interaction in the DN system.Wang et al. 46 incorporated a water-soluble IL (1-ethyl-3methylimidazolium tetrafluoroborate ([EMIm]BF 4 ) into a DN system comprising of the polyzwitterionic polymer PAS (P(AAm-VBIPS)) and PVA.In this system, the hydrogen bonding between PAS and PVA, as well as the ionic bonding between PAS and ILs, provided sacrificial networks (Figure 4C), while the PVA chain segments formed a unique dense network that provided rigid mechanical support.The PIGs exhibit an excellent tensile strength of 1.9 MPa and an ultrahigh ionic conductivity of 7.24 × 10 −3 S/cm at room temperature, which were significantly better than that of conventional SN polyzwitterionic hydrogels or PVA hydrogels.Ren et al. 47 reported a DN PIG by combining a PIL based ionic cross-linked network with thiol-ene based covalent cross-linked network.The first physical crosslinked sacrificial network was formed through ionic interactions between poly(1-butyl-3-fluoroimidazole) (PIL-BF 4 ) and 1,2,4,5-benzene tetracarboxylic acid (BTCA).A second chemical crosslinking network was connected by F I G U R E 4 (A) Schematic of the preparation of the P(MMA-co-BMA)/P(VDFco-HFP) DN ionogel.Reproduced with permission: Copyright 2020, American Chemical Society. 43(B) Schematic of the composition of the P(MMA-co-FMA)/P (VDF-co-HFP) DN iongel.Reproduced with permission: Copyright 2018, American Chemical Society. 44(C) Schematic of the preparation of PVA/PAS-ILs DN iongel.Reproduced with permission: Copyright 2022, Elsevier. 46ovalent bonds using mild thiol-ene click chemistry.The resulting DN ionogel exhibited a tensile strength of 2.28 MPa and maintained excellent tensile strength and elasticity even after 10,000 fatigue cycles at a stress of 0.875 MPa.Moreover, by utilizing 1-propyl-3-methylimidazolium tetrafluoroborate ([PMIm]BF 4 ) IL as the solvent, the DN ionogel demonstrated a wide working temperature range (−75-340 °C) due to the IL's favorable low-temperature performance.The click ionogel retained a tensile elasticity of more than 1000% even when the temperature dropped below −40 °C.Additionally, the DN ionogel exhibited high ionic conductivity, transparency, non-combustibility and ultra-long tensile property, which held great potential for future wearable devices.
Besides the material composition, the properties of DN PIGs can also be influenced by the preparation methods and reaction conditions.Chen et al. 63 introduced a novel electrohydrodynamic (EHD) printing method for the fabrication of ionogel films in a polydimethylsiloxane (PDMS) mold.In this method, a precursor solution containing AA, 2-hydroxyethyl methacrylate (AA-HEMA), and chitosan (CS) in 1-butyl-3methylimidazolium chloride ([BMIm]Cl) was sprayed onto the PDMS mold under a high electric field, promoting enhanced conductivity in the resulting gels.Subsequently, the PDMS molds with the precursor were exposed to ultraviolet (UV) light to induce the formation of ionogel films with a CS-(AA-HEMA)-DN structure.The EHD method yielded ionogel films with high conductivity and frost-resistance.The CS-(AA-HEMA)-DN ionogel film exhibited a tensile strength of 0.28 MPa at 335% strain, an electrical conductivity of 1.231 × 10 −3 S/cm, and a rapid response time of 81 ms.
Compared to conventional DN gel systems that utilize organic rigid networks, the utilization of rigid inorganic microgel networks as sacrificial bonds enables the development of novel organic/inorganic DN ionogel systems.In reference, 48 interpenetrating organic/ inorganic DN PIGs were reported for the first time, employing an in situ polymerization method with N,Ndimethylacrylamide (DMAAm) and ethyl orthosilicate (TEOS) as raw materials.The silica nanoparticle network within the DN ionogels serves as a sacrificial bond that easily fractures under load, dissipating energy while maintaining the material's mechanical strength through the stretchable and elastic PDMAAm network.By regulating the reaction kinetics or formation sequence of the inorganic/organic components, the microstructure of the silica particle network in the PDMAAm/SiO 2 DN ionogel system can be readily controlled.This approach resulted in exceptionally high mechanical strengths, with compressive strengths exceeding 28 MPa at 80 wt% IL content. 48Furthermore, Kamio and colleagues 64,65 also successfully prepared DN PIGs with comparable high mechanical properties by directly utilizing SiO 2 nanoparticles as a raw material.
DESs have shown great promise as a viable alternative to ILs in both SN PIGs and DN PIGs.Wu et al. 49 synthesized a solid-state conductive ionogel (SCIg) by using PDES (poly(AA-ChCl)), cross-linkers, and a photoinitiator (Figure 5A).The SCIg possesses a DN F I G U R E 5 (A and B) Schematic illustration of the molecular configuration, formulation, and composition of SCIg ionogels.(C) The exothermic process of ionogel in the process of photocuring polymerization.Reproduced with permission: Copyright 2023, Elsevier. 49tructure comprising a stretchable first network of PDES and a rigid second gelatin network (Figure 5B).The ionogel ink undergoes a sol-to-gel transformation within 30 s under UV light (Figure 5C).Gelatin and glycerin were utilized as cross-linkers to enhance noncovalent interactions and improve the self-healing ability of the SCIg.The multiple hydrogen bonds formed between the two networks in the SCIg confer excellent mechanical properties and high transparency.Through the adjustment of component content, the stretchable SCIg exhibits remarkable tensile properties (λ = 26.6 elongation at break while withstanding 0.97 MPa stress), exceptional self-healing efficiency (>95%), and excellent electrical conductivity (137.8 × 10 −3 S/cm).These characteristics position the SCIg as one of the most promising materials reported to date.Moreover, the SCIg has been employed in the development of stretchable resistive sensors, compressible capacitive sensors, and electronic skins.

| PIGs with MN topology based on carbon chain polymer
PIGs with high-performance MN structures have been developed to enhance their mechanical, electrical, and selfhealing properties.However, compared to SN and DN structures, research on these MN PIGs is relatively limited due to the complexity and multiple components involved.Zhu et al. 50designed a triple-polymer-based ionomer gel consisting of PVA, PAA, and PAAm, as depicted in Figure 6A.This gel exhibited remarkable transparency, good mechanical, and self-healing properties.The intramolecular and intermolecular hydrogen bonds of the three polymers acted as reversible cross-links, resulting in a high self-healing efficiency (93.84%).To further improve the mechanical strength and self-healing properties, graphene oxide (GO) nanosheets, cellulose nanocrystals (CNC), and the IL [BMIm]HSO 4 were added to the polymer solution, forming additional reversible cross-links such as hydrogen and ionic bonds.The resulting [BMIm]HSO 4 PIGs exhibited excellent mechanical properties, including high tensile strength (up to 15.9 MPa), significant elongation (610%), and outstanding toughness (53.7 kJ/m 3 ).In this system, the IL acted not only as a solvent but also as a cross-linking agent, greatly enhancing the mechanical properties and conductivity of the gel.
DESs have also shown promise in the development of MN PIGs.Wu et al. 51 explored a DES composed of ChCl and urea to prepare PEI/PAA/CMC PIGs with exceptional performance.As depicted in Figure 6B, loosely cross-linked PAA was incorporated as the first network, dynamically cross-linked network of electropositive PEI and electronegative PAA was formed as the second one, and carboxymethyl cellulose (CMC) reinforced filler network was the third one.The resulting PIGs exhibited outstanding mechanical properties, with the tensile strength and strain effectively controlled within the range of 0.15-7.9MPa and 232%-1161%, respectively.Importantly, the PIGs maintained the desired electrical conductivity of 1 × 10 −4 S/cm.This design strategy utilizing DES demonstrated effective control over the mechanical properties while preserving the electrical conductivity of the gel.

| PIGs based on hetero-chain polymers
Hetero-chain polymers refer to polymers that contain heteroatoms (N, O, S) in their main chain, in addition to carbon atoms.Examples of hetero-chain polymers include polyethylene glycol (PEG) derivatives, polyurethane (PU) derivatives, and polyimide (PI) derivatives. 66,67These synthetic hetero-chain polymers may endow some intrinsic unique properties of these polymers to PIGs, such as easy modification (PEG), high mechanical strength (PU), high temperature resistance (PI), and so forth.Moreover, some natural hetero-chain polymers like cellulose and chitosan have also found applications in PIGs as matrices, owing to their low cost, biodegradability, renewability, and excellent biocompatibility.Additionally, the presence of hetero-atoms in these polymers facilitates the compatibility and dynamic intermolecular interactions with ILs/DESs, endowing heterochain PIGs with high ionic conductivity and self-healing properties.In this section, we will discuss synthetic heterochain matrix and natural polymer matrix separately, while maintaining the order of topology for each part.Tables 2  and 3 summarize some basic properties of PIGs based on synthetic and natural hetero-chain polymers as discussed in the respective section.

| Synthetic hetero-chain polymers for PIGs
PEG derivatives are highly versatile and can be easily modified by introducing active functional groups, which impart new properties to PEGs, such as self-healing or photocurable capabilities.Fu et al. 68 synthesized a PIG by utilizing O,O″-bis(2-aminopropyl)-poly(propylene glycol)poly(ethylene glycol)-poly(propylene glycol) (H 2 N-PPG-PEG-PPG-NH 2 ) and benzene-1,3,5-tricarboxaldehyde (BTC) through a Schiff reaction (Figure 7A).They were subsequently combined in [EMIm]TFSI to form a supramolecular polymer network consisting of imine-bonded cross-linked self-healing PEGs.The reversible nature and high bond energy of the imine bonds contribute to the ionogel's remarkable mechanical strength, resilience, wide operating temperature range, and self-healing ability at room temperature.These PIGs demonstrate exceptional solubility for CO 2 , attributed to the favorable compatibility of the glycol chain segments with [EMIm]TFSI and strong interactions with CO 2 .Consequently, they have been successfully employed as membranes for CO 2 separation, benefitting from their good CO 2 solubility.The ability to undergo F I G U R E 6 (A) Schematic illustration for the preparation of ion gels.Reproduced with permission: Copyright 2018, American Chemical Society. 50(B) Schematic diagram of components and preparation process of PIG gel related to DES.Reproduced with permission: Copyright 2022, John Wiley and Sons. 51epeated self-healing within 48 h significantly enhances their service life and reliability.These findings present a promising approach for developing self-healing PIGs based on PEG with high mechanical strength and outstanding CO 2 separation properties.Saruwatari et al. 69 synthesized a tetra-armed PEG matrix material functionalized with anthracene at the end (tetraPEG-Ant) (Figure 7B).The group of anthracene can undergo a photo-induced reversible [4 + 4] cycloaddition reaction, since anthracene dimerises upon irradiation at wavelengths >350 nm, whereas the dimer is dissociated by heating or by irradiation at wavelengths <300 nm.When combined with an IL of [EMIm]TFSI, a PEG-based ionogel with photocurable properties was created.The PEG-based PIG utilizes the photodimerization of anthracene as a dynamic covalent bond, enabling photohealing of the ionogel.The tetraPEG-Ant ionogels exhibit exceptional tensile properties, with an elongation at break of 950%.The ionogel sheets can be successfully healed through continuous heating or UV irradiation, with a healing efficiency of 66%.Zhong et al. 70 developed a novel PEG-based PIG that can be patterned using standard photolithography and soft imprinting lithography techniques.The ionogel is synthesized through in situ photopolymerization of a mixture comprising a trithiol derivative (TT), PEGDA, and monofunctional poly(ethylene glycol) methyl ether methacrylate (PEGMA) in the presence of [EMIm]TFSI IL.By adjusting the ratio of raw materials, the crosslinking degree of the ionogel can be controlled, thereby enabling control over the mechanical properties of the material.This ionogel serves as a negative photoresist for micropatterning and exhibits a certain level of conductivity, with an ionic conductivity of 2.4 × 10 −3 S/cm.These properties pave the way for the fabrication of high-performance flexible electrochemical microdevices.
PU derivatives have been utilized in PIGs due to its remarkable elasticity, wear resistance, and fatigue resistance.Li et al. 71 conducted a study where they fabricated an ultra-durable ionic skin with exceptional self-healing properties and high sensitivity by incorporating ILs into poly(urea-urethane) networks, as depicted in Figure 8A.The PU network comprises both crystalline poly(ε-caprolactone) and flexible PEG, which are dynamically cross-linked through hindered urea bonds and hydrogen bonds.This unique design leaded to PIGs that exhibited high mechanical strength, excellent healing property, and good elasticity.Furthermore, the ionic conductivity of the PIGs reached as high as 1.2 × 10 −3 S/cm.The corresponding ionic skin sensors retain their original sensing properties even after undergoing 10,000 strain cycles and being stored in air for 200 days.Polyimide (PI) is widely employed in the aerospace and electronics industries due to its notable thermal stability, exceptional mechanical properties, and chemical resistance.By combining polyimide with nonvolatile IL, it becomes possible to overcome the limitations of gel sensors regarding mechanical properties and thermal stability.Xiang et al. 72 synthesized a new polyimide ionogel using a one-step solvent exchange method, as illustrated in Figure 8B.They achieved an outstanding performance with the PI ionogel (PI-iGel) by immersing a PI organogel in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF 4 ).[BMIm]BF 4 was selected as the electrolyte and solvent due to its high ionic conductivity (4.09 × 10 −3 S/cm), low melting point (−81 °C), and high decomposition temperature (403 °C).The prepared PI-based PIGs maintained exceptional mechanical properties (tensile strength ~7.1 MPa, elongation at break ~320%) and high thermal stability, while exhibiting high ionic conductivity (5.2 × 10 −3 S/cm).Their stable conductivity and mechanical properties in the range of −60-250 °C ensured their strain sensing capability in special conditions.

| Natural hetero-chain polymer-based ionogel
Cellulose is a typical natural hetero-chain polymer used in PIGs.Some cellulose derivatives, such as methyl cellulose or carboxymethyl cellulose, are compatible with ILs.They readily dissolve in ILs, allowing for high IL content in cellulose-based PIGs and thereby resulting in high ionic conductivity. 77In addition, cellulose offers several other advantages, such as wide availability, excellent spinability, and favorable mechanical properties.Liu et al. 73 utilized a natural polymer, bacterial cellulose (BC), and an eco-friendly IL 1-ethyl-3methylimidazolium dicyandiamide ([EMIm]DCA) to fabricate transparent, flexible, and robust SN PIGs (bacterial cellulose ionogels (BCIGs)) using a modified co-solvent evaporation method (Figure 9A).BCIGs containing 95 wt% of [EMIm]DCA demonstrated notable tensile strength (3.05 MPa), skin-like mechanical stretchability (40.99%), significant adhesion properties, and high ionic conductivity (2.88 × 10 −2 S/cm).ee et al. 74 developed a DN ionogel with enhanced mechanical strength and ionic conductivity by utilizing cellulose nanocrystals (CNCs) and hyperbranched polymeric ionic liquid (PIL) as the matrix materials for PIGs, [EMIm]TFSI was used as the IL (Figure 9B).The interpenetrating network formed through multiple hydrogen bonds and electrostatic ionic interactions between CNCs, PIL, and IL immobilized a substantial amount of IL (95 wt%), while maintaining certain mechanical properties.The resulting PIGs exhibited good tensile strength (0.26 MPa), excellent high compressive elastic modulus (5.6 MPa), and an ionic conductivity of 7.8 × 10 −3 S/cm, comparable to that of pure IL.
In addition to cellulose, agarose and chitosan are also commonly used components in PIGs as natural hetero-chain polymer.The biopolymer agarose is an algal polysaccharide with great hydrogen bonds.Agarose could form an ionogel in IL of 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) through the transition of helical bundle structures between high and low temperatures.When heated to 70 °C, the agarose could be dissolved in the ionic liquid to form a solution; then, the solution formed an ionogel when cooled to 30-40 °C.As shown in Figure 9C 76 elongation at break (700%), high tensile strength (0.37 MPa), good transparency (80%), superior self-adhesion, and a wide working temperature range (−30 ~60 °C). 75The chitosan contains lots of active groups similar with other biopolymers, such as amine groups and hydroxy groups, which are benefit to the compatibility with IL and the formation of the cross-linked network in DN PIGs.As shown in Figure 9D, chitosan/poly(N-isopropyl acrylamide-co-N,N'diethylacrylamide) DN ionogels were prepared in the IL of [EMIm]TFSI.The mechanical properties could be reinforced by ionic and covalent cross-linking of the chitosan network using trimeric phosphates and glutaraldehyde, respectively.The resulting PIGs demonstrated good mechanical properties (0.235 MPa) and superior ionic conductivity (5.15 × 10 −3 S/cm).Stretchable ACEL devices fabricated from these PIGs exhibited stable operation even under ultrahigh elongation at break exceeding 1200% and experienced severe mechanical deformations such as bending, rolling, and twisting.Additionally, the developed ACEL devices displayed consistent luminescence even after 1000 stretch/release cycles or exposure to a severe temperature of 200 °C. 76wing to the abundant hydroxy groups, biopolymers is also well compatible with DES.Qin et al. 78 reported the use of a DES (choline chloride (ChCl, HBA) + ethylene glycol (EG, HBD)) as the gel electrolyte and gelatin as the polymer matrix, resulting in a PIG with high tensile properties (elongation at break >300%) and a room temperature ionic conductivity of 2.5 × 10 −3 S/cm.This makes it a promising candidate for ionic skin applications.

| PIGs based on elemental organic polymers
Elemental organic polymers are a class of polymers characterized by backbones that do not primarily consist of carbon atoms.Instead, they are predominantly composed of elements such as silicon, boron, aluminum, oxygen, nitrogen, phosphorus, or sulfur.These polymers incorporate organic groups into their side chains.Silicone rubber exhibits exceptional properties, including outstanding resistance to low temperatures, thermal stability, elasticity, and nontoxicity, making it a widely elemental organic polymer matrix in PIGs.Table 4 summarizes some basic properties of typical elemental organic polymer-based PIGs as discussed in this section.
To enhance compatibility with ILs, a polar amino group was introduced into the side chain of poly-(aminopropyl-methylsiloxane) (PAPMS), effectively increasing its polarity.A new kind of polysiloxanesupported ionogel is successfully prepared by using PAPMS grafted with [2-(methacryloyloxy)ethyl] trimethylammonium chloride (METAC) in the IL of [EMIm]TFSI.Polysiloxane-loaded PIGs formed two fully physical cross-linked polymer network.One is based on the ionic aggregates among METAC, and the other one is based on hydrogen bonds between PAPMS and the crosslinker of tannic acid (TA) (Figure 10A).The IL ([EMIm]TFSI) was incorporated into the compatible PAPMS DN system, resulting in a PIG with good mechanical properties, high ionic conductivity (1.19 × 10 −3 S/cm), and excellent self-healing ability.The damaged ionogel exhibited nearly complete self-healing (83%) at room temperature over a 12h period.Moreover, the PIG demonstrated satisfactory adhesion to various solid materials and maintained its high ionic conductivity and self-healing properties even at subzero temperatures. 79t is worth to discuss that thermal stability is a unique advantage for silicone-based PIGs. Lee et al. 80 developed a novel inorganic-organic hybrid PIGs by using ionic polyhedral oligomeric sesquisiloxanes (I-POSS) with inorganic cores.As shown in Figure 10B, cross-linkable ionic POSS was obtained by the Menshutkin reaction between vinylimidazole and triallylamine.Subsequently, ion exchange with LiTFSI was performed to achieve the TSFI-anion structure.The resulting ionic POSS gels (I-POSS-G) exhibited high ionic conductivity values across all temperatures and demonstrated exceptional cycling performance with a large coulombic efficiency approaching 99% when utilized as a polymer electrolyte for Li-ion capacitors.
In addition, PDMS is a flexible and nontoxic material that is commonly employed as a substrate or encapsulation material for PIGs in flexible ionic devices.A Sn-based perovskite mesophase ionogel (PIGel) was reported, 81  Electronic skins (e-skins) 82,83 is a flexible and stretchable sensor system designed to replicate the sensing abilities of human skin.It can detect a wide range of external stimuli, including stress, strain, temperature, chemicals, and light.The fundamental sensor components of e-skin consist of conductive elastomers and electrodes.Among these sensors, tactile perception is a crucial function that converts external mechanical stimuli into electrical signals. 84Based on the mechano-electric transformation mechanisms, current flexible tactile sensors in e-skin can be classified into two categories: electron tactile sensors (ETS) and ionic tactile sensors (ITS). 85he sensing mechanism of ETS relies on the movement of electrons, and the primary active material in these sensors is electronic conductors or piezoelectric materials, such as PDMS/carbon nanotubes, paper/silver nanoparticles, PVDF, and so forth. 86These flexible sensors demonstrate high sensitivity within a specific range of stretching or compressing.However, due to the strong chemical bonding forces within their molecular structures or the fragile electron conducting network, ETSs typically possess a high Young's modulus and a limited strain range (<10%). 87n the other hand, the sensing mechanism of ITS relies on ions, with the key flexible ion-conductive material being PIGs.In the equilibrium state, the anions and cations are evenly dispersed within the PIGs.When exposed to external stimuli, they migrate separately and redistribute within the ITS, generating an electrical signal.ITSs offer advantages such as low Young's modulus, high stretchability, good stability, and high transparency due to the properties of PIGs.Importantly, the sensing mechanism of ITSs closely resembles that of real human skin, where Na + transport into and out of cells generates electrical signals transmitted to the nervous system, enabling the sensing of external stimuli.Therefore, ITSs exhibit remarkable biocompatibility and hold great potential for emerging technologies, including wearable and bio-inspired sensor platforms. 88,89Moreover, by mimicking human skin, ITSs aim to achieve higher sensitivity and a broader pressure response range. 90In recent years, significant advancements have been made in the development of ITSs, and the resulting ionic skin can be utilized in applications such as human health monitoring, humanmachine interaction, and the internet of things, 75,[91][92][93] and so forth.7][98] In the following section, we will briefly describe various ionic transduction mechanisms, their structural designs, and the unique functions of flexible ITSs based on PIGs according to these three types.

| Resistive ionic tactile sensor
Resistive ITSs perceive external mechanical strain through changes in resistance.The sensing mechanism of resistive ITS is similar to that of conventional resistive ETS, with the difference lying in the conductive carrier.PIGs contain numerous ion channels, and when an electric field is present, ions migrate to form an ion current.When subjected to external mechanical strain, the length and cross-sectional area of the ion channels have been changed, resulting in alterations in the resistivity of the ITS. 99It is important to note that the PIG elastomer plays a crucial role in resistive ITS, providing a stretchable and conductive framework.Furthermore, PIGs are very easily modified with various novel material properties, which enables the incorporation of additional functionalities into resistive ITS, such as water durability.
The incorporation of hydrophobic F elements into PIGs has been demonstrated as an effective approach for enhancing solvent durability (especially for water).Extensive research has been conducted on ITSs based on these F element-containing PIGs.Xu et al. 100 have reported on a class of physically cross-linked multifunctional hydrophobic PIGs.The material combines a hydrophobic and solvent-resistant fluoropolymer, P(TFEA-co-AAm), with a hydrophobic IL, [EMIm]TFSI.The chemical structures of the PIG precursors and the properties of the PIG were shown in Figure 11A.The resulting PIG demonstrates excellent tolerance to aqueous solutions and various organic solvents.In addition, it exhibits self-healing property both in air and underwater and demonstrates strong adhesion to various polymer matrices even in water.Furthermore, it can be dissolved in ethanol, allowing for recovery and reshaping into arbitrary forms.Consequently, the fluoropolymer ionogel-based ITS displays sensitive and reproducible sensing signals across a wide range of tension or compression deformations, including stable strain sensing in various liquid media.Similarly, Yu and Wu 101 reported a hydrophobic fluoropolymer which endowed its ionogel with excellent stability, adhesion, and self-healing ability underwater.As a result, the corresponding F-ITSs demonstrated better adhesion ability, conductivity than the commercial one, rendering them reliable sensors for realtime monitoring of human motion and electrocardiography (ECG) in water, as illustrated in Figure 11B.

| Supercapacitive ionic tactile sensor
The conventional supercapacitive ITS typically consists of a pair of electrodes with a PIG material serving as the intermediate layer.The supercapacitive interfacial sensing is based on the interfacial effect known as the electronic double layer (EDL). 102When the PIG comes into contact with the electronic conductor (electrode), an ultrathin EDL forms at the interface with a thickness of only a few nanometers.In a supercapacitive ITS, there are two interfaces, resulting in the presence of two ultrathin EDLs.These ultrathin EDLs contribute to high F I G U R E 11 (A) Schematic of the components of the ionic gel, swelling exhibitions, underwater stress-strain curves of original and healed ionogel samples for 24 h at room temperature, underwater adhesion exhibitions of ionogels on various surfaces, such as glass, nitrile rubber, steel, and copper.Reproduced with permission: Copyright 2021, John Wiley and Sons. 100  capacitive values, particularly under a certain bias, which exhibits significant changes when subjected to even slight external mechanical forces.The capacitive values of the supercapacitive ITS are directly proportional to the contact area between the PIG and electrodes, owing to the ultrathin EDLs.Therefore, the sensitivity of the supercapacitive ITS can be greatly modulated by designing the microstructures of the PIGs or electrodes. 103emplate methods are commonly used in designing the microstructures of the PIGs or electrodes.
Zou et al. 104 presented a P(VDF-HFP)-based PIG incorporated with [EMIm]TFSI.By employing silicon wafer templates, micro structured PIG films were obtained.These films served as the foundation for the fabrication of flexible super capacitive ITSs.The dielectric layer's cross-sectional views during deformation under increasing pressure were depicted in Figure 12A.The super capacitive ITSs exhibited a sensitivity of 145.45 kPa −1 within the low-pressure range (<400 Pa) with good linearity.The detection limit was 0.4 Pa, and the dynamic response time was approximately 44 ms.Furthermore, the mechanical stability of the device exceeded 6200 cycles.This remarkable performance enabled the sensor's application in F I G U R E 12 (A) Chemical structures of the P(VDF-HFP)-based PIG and illustrations of the sensing mechanism of the patterned ITS.Reproduced with permission: Copyright 2020, Elsevier. 104(B) Schematic illustration of operating principle of the sensory system with porous iongels.Reproduced with permission: Copyright 2021, American Chemical Society. 105onembedded medical monitoring, such as human pulse and respiration.Additionally, a 7 × 7 pixel sensor array was designed and successfully employed for pressure distribution monitoring.Overall, this study presented a promising approach for the fabrication of highperformance, scalable, and cost-effective flexible pressure sensors.
Kwon et al. 105 proposed a novel sensitive and visualizing super capacitive ITS based on porous PIGs.The porous PIG is fabricated through in situ crosslinking polymerization by immersing commercial sugar cubes into a monomer mixture of styrene (Sty), ethyl acrylate (EA), and divinylbenzene (DVB).After selectively dissolving sugar cube in deionized water under sonication, porous morphology of PEA-r-PS-r-PDVB was obtained.Porous PIGs were prepared by swelling of the copolymer with [EMIm]TFSI.The ITS could be readily fabricated by inserting the porous PIG between two electrodes.Considering porous structure of the gel, the area of the EDL created at the interface would be changed according to the pressure, and the bilayer capacitance was determined directly by the EDL.As depicted in Figure 12B, the porous PIGs could undergo significant deformation even under minimal pressure by closing the pores, resulting in substantial changes in the gel-electrode contact area and bilayer capacitance.The device demonstrated a high sensitivity of approximately 152.8 kPa −1 , a wide sensory pressure ranging from 100 Pa to 400 kPa, and excellent durability exceeding 6000 cycles.Moreover, the functionality of porous PIG-based sensor systems was expanded to include electrochemiluminescence (ECL) by incorporating luminophores into the gel.The mechanical deformation of the emissive ECL ionoskin leaded to a decrease in overall gel impedance and simultaneously initiated a series of electrochemical reactions that excited luminophores.Notably, the light intensity showed a linear relationship with the applied pressure, indicating the ability of the emissive ionoskin to transduce applied pressure into electrical and optical output signals in the same time.As a proof-of-concept demonstration, successful monitoring of the pressure induced by finger bending was achieved using the ECL ionoskin, providing a visual indication.
Furthermore, the utilization of 3D printing technology has recently been investigated in the realm of super capacitive ITS, presenting a notable advantage for the advancement of next-generation ionoskin employed in biological signal sensing.Luque et al. 88 proposed the utilization of biocompatible carboxylic acid choline ILs in conjunction with polyvinyl alcohol/phenol to create a biocompatible PIG.This PIG exhibited the potential for direct 3D printing as a thermally reversible ink (Figure 13A).The resulting gel demonstrated exceptional stability and flexibility, while simultaneously maintaining a high ionic conductivity of 1.8 × 10 −2 S/cm.The Young's modulus of the gel ranged from 14 to 70 kPa.
F I G U R E 13 (A) Schematic diagram of simulation process of 3D printing.Reproduced with permission: Copyright 2021, John Wiley and Sons. 88(B) The 3D-pringting architecture, circuit schematic illustration and a photograph of an integrated transparent sensory system.Reproduced with permission: Copyright 2019, Springer Nature. 39he 3D printing pressure sensor using this PIG exhibited a sensitivity of 0.1 kPa −1 .This sensitivity highlighted its suitability for monitoring motion and physiological signals.Furthermore, the sensor displayed remarkable stability in ECG monitoring, retaining the characteristic ECG waveform even after several weeks.By 3D printing technology, the fabrication of novel multifunctional supercapacitive ITS becomes more feasible and accessible.
Lei and Wu 39 developed a novel flexible multilayer supercapacitive ITS using 3D printing technology.The device structure was depicted in Figure 13B, where PIGs were directly 3D printed onto a dielectric elastomer, forming a three-layer structure.After a minimum drying period of 1 week, the top and bottom PIGs were affixed to four metal electrodes, while the middle dielectric elastomer facilitates charge storage in the gel layer.Within the sensing system, the plane-parallel capacitance could be measured from electrodes 1 and 2 in response to external mechanical stimuli.Variations in humidity within the topmost ionogel layer induced moisture-driven ion flow, resulting in voltage changes between electrodes 1 and 3, enabling humidity monitoring.Changes in resistance between electrodes 2 and 4 in the bottommost ionogel allowed for temperature monitoring.Thus, the multilayer structure of the sensing system achieved simultaneous simulation of natural skin responses to mechanical, humidity, and temperature stimuli, providing diverse sensing capabilities for soft robots.In addition to common sensory features found in natural skin, such as strain, stress, humidity, and temperature, the researchers had also incorporated additional functionalities into the sensing system.Through 3D printing an array of ionogel electrodes on a dielectric polyethylene substrate, the novel sensing system could discriminate different liquid molecules based on their polarities and surface tensions.By assessing polarity, volatility, and wettability to the dielectric polyethylene layer, the sensing system enabled a robotic hand to differentiate various types of liquid molecules.The polarity of the liquid molecules determined the extent of capacitance increase, while volatility and wettability related to the duration of capacitance increase.This sensing system empowered the soft robot to make preliminary predictions about the physical properties of unknown liquids through real-time electrical signal comparisons.Overall, this ionogel-based capacitive sensing system is expected to play a significant role in future advancements in soft robotics.

| Piezoelectric ionic tactile sensor
Piezoelectric ITS is a self-powered sensor capable of converting mechanical stimuli into an electrical signal.
Its inherent self-powering capability renders it highly suitable for utilization in biomedical implant sensors and portable wearable sensors, owing to its compact size, lightweight nature, minimal signal interference, and enhanced sensitivity.Piezoelectric ITS can be broadly classified into three categories based on their distinct mechanisms for generating piezoelectric signals.
One type of ITS is prepared using materials exhibiting a piezoelectric effect, such as zinc oxide (ZnO), barium titanate, and PVDF-TrFE, which have been extensively investigated for their applications in piezoelectric nanogenerators (NGs).When these piezoelectric materials experience an external force in a fixed direction, it leads to electropolarization within the crystal, resulting in the generation of opposite charges on both sides of the crystal.These charges are integrated into the ITS, enabling the conversion of the stress signal into an electrical signal. 106,107Sun et al. 108 introduced PIGs as a dielectric layer in a coplanar structure of graphene transistors (GTs), moreover, the piezoelectric polymer PVDF-TrFE could be readily patterned on the extended gate region to implement the piezoelectric gate in an NG.The long-range polarization of ions in the PIGs facilitated the coplanarization of the gate electrode with the channel.The resulting device structure was equivalent to a piezoelectrically driven strain sensor array system that combines an NG with a GT.The piezoelectric potential induced by external strain was effectively coupled into the channels of the GT through the PIG gate dielectric, leading to charge accumulation in the channels.Consequently, the transistor exhibited a distinct transient pulse output signal when subjected to external strain on the NG.The fabricated strain sensor demonstrated exceptional performance characteristics, including high sensitivity (Gauge factor = 389) and a minimum detectable strain as low as 0.008%.The device also exhibited good durability after 3000 bend-release cycles.Furthermore, the strain sensor, fabricated on a rubber substrate, successfully detected continuous hand motion.Notably, the incorporation of piezoelectric NG allowed for the operation of the GT in ITS system at an ultra-low drain voltage (0.1 V).
The second type is centered on a triboelectric nanogenerator (TENG), an environmentally friendly mechanical energy harvesting device that combines the effects of frictional electricity and electrostatic induction to produce electrical energy.This device serves not only as a power source but also as a self-powered ITS, offering an efficient and eco-friendly energy solution for ITS applications.Cheng et al. 109 developed a flexible, stretchable, self-healing, environmentally stable, and robust PIG-TENG.The device's structural arrangement was illustrated in Figure 14A.Collectors comprised of carbon nanotube (CNT)-doped PIGs were employed, while a stretchable frictional electrode layer composed of electrospun poly(vinylidene fluoride)/polyurethane (PVDF/PU) nanofiber film with enhanced frictional resistance was utilized.The experimental findings demonstrated peak values of 180.72 V, 10.22 μA, and 2.44 W/m 2 for the PIG-TENG in terms of voltage, current, and power density, respectively.Notably, the energy harvesting performance of the PIG-TENG was improved when subjected to a 100% stretching condition, and the device could be restored to its initial state even after experiencing severe damage and undergoing washing processes.Furthermore, the PIG-TENG exhibited favorable temperature stability.With the utilization of this novel PIG-TENG, real-time electrical signals could be captured to detect joint motion, cardiac status, and tactile motion, eliminating the need for supplementary power sources and management modules.Thus, this PIG-TENG held immense promise for enabling human health monitoring and facilitating human-machine interaction.
The third type is unique to PIG material derived from the piezoelectric ion effect.PIGs exhibit viscoplasticity within their network when subjected to applied stress, thereby altering the distribution of ions embedded in the polymer matrix.This uneven distribution results in a nonuniform charge distribution within the PIG, leading to the generation of a voltage signal known as the piezoelectric ion effect.Lee et al. 110   The working mechanism of this visualized ITS was illustrated in Figure 14B.A thermoplastic polyurethane (TPU) and IL combination formed a viscoplastic TPU based PIG that exhibited a piezoelectric ionic effect when subjected to applied stress.By introducing an ionic transition metal complex (iTMC) luminophore into the PIG, ECL could be achieved at a specific voltage due to the charge transfer reaction between the oxidized and reduced forms of the iTMC.In this PIG-iTMC system, the enhancement of the voltage signal generated by the piezoelectric ion effect leaded to an increase in the iTMC luminescence intensity.Consequently, the luminescence intensity of the "ECL skin" varied with normal and tensile stresses.Since the piezoelectric ion effect only occured at the location where the stress was applied, visual spatial mapping of tactile stimuli could be achieved without the need for complex driving schemes.The device concept demonstrated in this example might provide novel insights for the design of low-power, mechanically sensitive, and visually stimulating ITSs.

| Flexible ECDs
][114] The integration of ECDs in the curtain walls of office buildings or aircraft portholes enables automatic regulation of infrared radiation and natural light illumination.In addition, ECDs exhibit wide applications in information displays and defense industries. 115,116The structure of ECDs prepared by conventional electrochromic materials such as metal oxides 117,118 and conducting polymers 119,120 is usually multilayer structure, which is complicated to prepare and suffers from poor stability, 121 electrolyte leakage, 122 and low ionic conductivity. 123In contrast, PIG-based ECDs exhibit high capacitance, excellent thermal and chemical stability, low vapor pressure, and a wide electrochemical window.][126] Viologens, N,N-substituted bipyridiniums, have garnered significant attention due to their notable and reversible color change from colorless to blue or violet during their initial redox process.However, the longterm stability of solution-based viologen ECDs is compromised by the formation of dimers and the poor solubility of V +• ions, leading to irreversible bleaching processes.To address these challenges, Wang et al. 127 incorporated PILs (poly(VBImBr)) as ionogels in ECDs to prevent viologen dimer formation and enhance stability.
In addition, by modifying the chemical structures of viologen derivatives with different halogenated benzene ring derivatives, Moon et al. 129 achieved multicolored ECDs using PIGs (P(VDF-co-HFP+([BMI]TFSI or [BMI]BF 4 )) as the electrolyte.It is depicted in Figure 15C, multicolored ECDs were achieved by simply blended different viologen derivatives into the PIGs with dimethyl ferrocene.Moreover, the introduction of thiophene group into viologen is also a facile way to adjust its color.Zhang et al. 130 reported a EC PIGs with excellent transmissivity, high mechanical robustness, and ultrastable reversibility by using poly(ethyl acetate), [EMIm]TFSI IL, and thienoviologen-based electrochromophores.The thienoviologen-based ECD demonstrated good EC performance in terms of contrast ratio, response time, and coloration efficiency.Even after storing in ambient atmosphere for 2 years, it still exhibited excellent cycling The viologen-based ECDs architecture.Reproduced with permission: Copyright 2020, Elsevier. 127(B) ECD configuration (left) and molecular structures of components in the EC gels (right).Reproduced with permission: Copyright 2017, Elsevier. 128C) The structure of EC gel and multicolored ECDs.Reproduced with permission: Copyright 2016, American Chemical Society. 129tabilities and maintained its contrast ratio at 88% after 500 cycles.More interestingly, the PIG-ECDs showed a fast conductivity response to strain, which makes the ECD as a novel visible strain sensor.Thus, this new type of thienoviologen-based PIG shows promising application in visible sensors to monitor mechanical motion by the changes of both color and electrical signals.
In recent years, with the growing interest in applying electrochromic technology to flexible and wearable electronics, there is a need for flexible ECDs that can operate at extreme temperatures.The utilization of PIL or DESs as electrochromic electrolytes in ECDs has emerged as a promising solution for operating under extreme temperature conditions.Lee and colleagues 131 successfully synthesized a PIG in situ via a rapid photocuring process that took less than 1 min.An ECD could be readily fabricated by sandwiching the PIG between the substrates.Li-doped PIL B has also been used in fabricating an ECD based on P-WO 3 and TiO 2 electrodes.Additional Li + ion was necessary to be inserted into the bulk WO 3 lattice, resulting in the EC phenomenon.The PIG of Li-doped PIL B exhibited exceptional properties, including high transparency, stretchability, and excellent physicochemical stability encompassing thermal, electrochemical, and air stability, thereby enabling their operation under diverse environmental conditions.Based on these advantages of this PIG, the researchers constructed a high-performance flexible ECD, achieving notable contrast ratios (56.4%), fast response times (1.7/6.4 s), and remarkable cycling durability (>90% retention over 3000 cycles).Especially, due to the thermal stability of the PIG, the ECDs effectively worked over a wide temperature range (−20 °C to 100 °C) (Figure 16A).In another study, 132 an ionogel electrolyte based on a DES was developed, exhibiting excellent optical, electrical, and mechanical properties across an extended temperature range (−40 °C to 150 °C).The DESs possessed resistance to a wide range of temperatures, low vapor pressure, favorable ionic conductivity (0.63 × 10 −3 S/cm), and a large breaking elongation (>2000%), making them highly suitable as gel electrolyte components for flexible ECDs with outstanding high/low temperature durability.Furthermore, these DES-based electrolytes exhibited excellent processability  132 owing to dynamic interactions such as lithium and hydrogen bonding between the DES and polymer matrix, which enabled direct patterning in ECDs by the 3D printing technology for the first time (Figure 16B).

| Flexible actuators
Flexible actuators employ PIGs as active driving materials, enabling them to exhibit remarkable deformability in response to diverse stimuli, such as voltage, 133 light, 134 and temperature. 135These actuators possess advantageous features such as high stability in air, flexibility, lightweight, ease of preparation, and low cost.4][145] Based on their distinct working mechanisms, PIG-based actuators can be generally classified into electrically driven and optically driven actuators.
In electrically driven actuators based on PIGs, the imbalanced motion of cations and anions occurs within the polymer network under an applied bias, resulting in volume change and achieving the desired actuation effect.Imaizumi et al. 146 presented a triple-layered structure for a PIG-based actuator.The ionogel electrolyte consisted of PS-b-PMMA-b-PS (Figure 17A) and [EMIm]TFSI, which was sandwiched between two composite carbon electrodes.By subjecting the actuator to voltammetry within a voltage range of ±2.0 V, the displacement response was observed.These observations indicated that the actuator's displacement was attributed to the charging and discharging of two EDLs formed at the interface between the activated carbon electrode and the PIG layer, with the bending displacement consistently occurring toward the anode side.The effective and rapid electromechanical conversion of PIG-based actuators relied on the different diffusivities of cations and anions.For [EMIm]TFSI in PS-b-PMMA-b-PS, the cationic diffusivity was larger than the anionic diffusivity.Such imbalanced transport between the cation and anion induced concentration polarization of [EMIm]TFSI inside the PIG layer.On the anodic side, the concentration of anion became lower than that in the bulk, while on the cathodic side, cation became larger than that in the bulk.This concentration polarization caused swelling of the cathodic electrode layer and deswelling of the anodic electrode layer, which appeared to explain such displacement.
Mechanical strength and ionic conductivity are the most important properties of PIGs that are intended for use as flexible actuators.Shioiri et al. 149  properties (an elastic modulus of 171 kPa and a fracture energy of 1890 kJ/m 3 ) and a high ionic conductivity of 6.3 × 10 −5 S/cm.The PIGs with homogenous network structures were tough enough to be applied to flexible actuators, and the bending behavior of flexible actuator could be well observed.However, the response speed of the LiTFSI actuator was slower than that of the [EMIm]-TFSI actuator.This slower response was attributed to that the ionic conductivity in the LiTFSI actuator is two orders of magnitude lower than that in the [EMIm]TFSI actuator (6.4 × 10 −3 S/cm).
In addition, the utilization of flexible electrode materials also plays a critical role in electrically driven actuators employing PIGs as a base.The interfacial adhesion between the electrodes and PIGs, the conductivity, capacitance, and mechanical properties of the electrodes, are crucial factors governing the deformation and stability of flexible actuators.Recently, Kotal et al. 147 successfully developed high-performance ionic artificial muscles using mesoporous sulfur and nitrogen co-doped reduced graphene oxide (Th-SNG) and polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) as a highly flexible and nonmetallic conducting electrode, while sulfonated polybenzimidazole (SPBI) embedded with mobile ionic liquid ([EMIm]BF 4 ) served as the PIG.As depicted in Figure 17B, the Th-SNG/PEDOT:PSS electrode in the actuator exhibited excellent adhesion to the surface of the SPBI-IL membrane, with no delamination observed.Moreover, the synergistic effects of the co-doping and the combination significantly enhanced the electro-chemo-mechanical properties of the electrodes: ultrahigh capacitance and electrical conductivity along with outstanding mechanical stability.The actuator based on Th-SNG/PEDOT:PSS electrodes displayed a remarkably improved actuation performance: large bending strain (up to 0.36% under 1 V at 0.1 Hz, 4.5 times higher than that of PEDOT:PSS electrodes) and good durability (96% of initial strain after 18000-cycle demonstration).Another approach to address electrode cracking and delamination during actuation is the combination of metal nanocomposites with PIGs.For instance, Yan et al. 148 pioneered a novel actuator design employing ionogel/metal nanocomposites (IGMNs), which comprised chemically cross-linked poly(acrylic acid)-acrylonitrile ionogel and gold nanocomposite electrodes.The fabrication of thin gold nanocomposite electrodes (approximately 100 nm in thickness) via supersonic cluster beam implantation minimally affected the mechanical properties of the PIG, while simultaneously affording controlled electrical properties and a large surface area for ion storage (Figure 17C).These IGMN-based actuators exhibited exceptional performance characteristics, including high response rates at low voltages (up to 1.04% net strain at 5 V) and remarkable durability in terms of frequency response (up to 76000 cycles at 2 V and 1 Hz).Notably, these actuators demonstrated sensitivity to electric fields even at a low voltage of 0.1 V and exhibited excellent performance in response to low-voltage electrical stimuli.Moreover, the manufacturing process for these actuators was costeffective and suitable for industrial-scale production.
Optically driven actuators based on PIGs 150 offer the potential for wireless actuation and remote control when stimulated by light. 151Incorporating photochromic molecules 152 into PIGs is a common approach for developing optically driven actuators.The motion of these actuators is triggered by changes in the size or polarity of the photochromic molecules, such as azobenzene, 153 spiropyran, 154 diaryl ethylene, 155 and so forth.By combining the advantages of polymer gels and ILs, PIGs with photochromic molecules effectively address the limitations of hydrogel actuators, 156 which tend to become inactive in open environments.These optically driven actuators exhibit superior durability, stability, and functionality, making them highly suitable for practical application.
Ma et al. 157 utilized free radical co-polymerization of butyl acrylate (BA) with azo resins 4-in an ionic liquid ([BMIm]TFSI) to synthesize a thermosensitive/photosensitive ionogel (P(AzoMA-r-BA).P(AzoMA-r-BA) copolymers exhibited a phase transition characteristic of lower critical solution temperature (LCST) when dissolved in [BMIm]TFSI due to the solvato-philic nature of cis-AzoMA and the solvato-phobic nature of trans-AzoMA.The LCST behavior was influenced by the photoisomerization state of the azobenzene moiety and the composition of AzoMA within the copolymers.At a bistable temperature, the photo-induced expansioncontraction transition of BA-AzoMA gel was reversible.Subsequently, a photoresponsive actuator was fabricated.Under UV irradiation from the left side, the ionogel exhibited bending toward the right, while visible light irradiation from the same side allowed the gel to revert to its original shape.Similarly, UV irradiation from the right side induceed bending toward the left, and subsequent visible light irradiation from the same side promotes shape recovery.The impact of radiation time on the bending angle size was investigated, revealing that the bending angle reacheed its maximum after 100 min of UV irradiation and remained relatively stable in the absence of light.In contrast, under visible light irradiation, the bending angle decreased and the bent gel gradually returned to its initial shape (Figure 18A).
Saez et al. 158 reported a p(SPNIPAAm) ionogel functionalized with spirobenzopyran moieties (SP).As shown in Figure 18B, sp could be switched upon light irradiation between two states, which were thermodynamically stable.The p(SPNIPAAm) ionogel could be swollen when exposed to the acidic solution, and the main channel could be closed.The swollen p(SPNIPAAm) ionogel actuators were yellow due to the presence of the protonated SP moiety (MC-H + ).On the other side, when the ionogel was irradiated with white light, MC-H + will change to the neutral SP and release H + and water, which caused contraction of the gel, thus opening the main channel to fluid movement.As a result, optically driven actuator based on ILs incorporated within the p(SPNIPAAm) gel was successfully developed.The optically driven actuator was reversible and triggered conformational rearrangements in the bulk p(NIPAAm) gel by acid or white light, changing it from a more hydrophilic to a hydrophobic nature, which in turn induced water loss and contraction of the gel.Furthermore, it was demonstrated that by using different ILs within the polymer matrix, the kinetics of the actuation could be controlled through IL mediation (rate of protonation/deprotonation, the movement of counter ions and water) in the ionogel matrix.Therefore, ionogels containing different ILs were found to respond at different times when exposed to a common white light LED, allowing for the selective opening of the ionogel photo-actuator at a desired time.This study demonstrated the successful operation of a reusable ionogel-based photoactuator within a microfluidic channel, without the need for complex actuator designs.

| Flexible storage power
With the rapid advancements in the field of flexible, wearable, miniaturized, and integrated electronic devices, 45 there arises an imperative to develop lightweight, highperformance, and structurally functional integrated flexible power sources and technologies. 159Among the various approaches, the utilization of TENG that harnesses electrostatic effects emerges as a promising avenue, 160 offering notable advantages such as high energy efficiency, simplicity, and scalability. 161Within a TENG, the conversion of mechanical energy derived from friction into electrical energy is achieved through the triboelectric effect. 162articularly, PIGs have gained significant attention and are extensively employed in TENG due to their distinctive properties, such as flexibility, stretchability, self-healing, freeze resistance, ion conductivity, and easy processing, and so forth.As the main functional layer, the properties of PIG materials greatly affect the performance of TENG.The flexibility, stretchability, and self-healing of PIGs render them highly suitable for seamless integration into various wearable and portable devices as flexible storage power. 163he freeze resistance of PIG-based TENGs can be influenced by the freeze resistance properties of the PIG material itself.Sun et al. 164 devised a novel PIG-based TENG with an extensive temperature range.The ionogelbased TENG operated on the principle of contact initiation and electrostatic induction in single electrode mode, as depicted in Figure 19A.The aluminum sheet/ human skin and PDMS membrane acted as positively and negatively charged frictional materials, respectively.When the two surfaces came into contact, equal amounts of positive and negative charges were generated.Upon separation, negative charges on the PDMS membrane surface caused positive charges to accumulate at the interface in the ionogel, while negative charges in the ionogel moved to the ionogel/metal electrode interface, forming an electrical double layer structure.Electrostatic equilibrium was achieved when the aluminum sheet/ human skin and PDMS membrane reached their maximum separation distance.By repeating the contact separation action, the I-TENG generated alternating currents.The authors had demonstrated that the I-TENG could efficiently harness mechanical energy, particularly the energy generated by human motion.The electrical output performance remained stable for at least a month, and the I-TENG could light up 40 green LEDs connected in series with just one hand tap.Especially, the I-TENG showed high transparency, stretchability, durability, and stable electrical performance across temperatures ranging from −20 °C to 100 °C.
Notably, one of the key advantages of incorporating PIGs in TENGs is their remarkable high ionic capacitance.This attribute becomes crucial for enhancing the output voltage and current of TENG.Under compressive forces, PIGs attract counterions, resulting in the formation of a high capacitance two-dimensional electrical double layer (2D-EDL) at the interfaces of air/PIG or PIG/friction layer.7][168] Microstructure plays a crucial role in the formation of 2D-EDL in TENGs by increasing the contact area of electrodes, enhancing friction, altering charge distribution, and suppressing surface reactions.The utilization of templates for the construction of microstructures in friction films is a prevalent approach in the fabrication of PIG-based TENGs.As shown in Figure 19B, Zhao et al. 165 employed two PIGs as the electrodes of TENG, while template-patterned PDMS served as the friction layer.When the device underwent mechanical stress, the PIG encounterd the patterned PDMS, resulting in the generation of a stress-voltage signal transition, thus operating as a TENG capable of illuminating an LED.Notably, the brightness of the LED remained unchanged even after extending the PIG, indicating the PIG film's remarkable resistance to deformation-a desirable characteristic for electrode materials in TENG applications.In addition, the flexible processing methods of PIGs facilitate to obtain diverse microstructures.Ye et al. 169 employed a triboelectric nanogel composed of P(VDF-HFP) in an IL of [EMIm]-TFSI as a functional material for fabricating a TENG.The nanogel component, in the form of nanofibers, was produced through electrospinning onto an Al foil bottom electrode, while a Kapton-coated Al film served as the top electrode.A polymer spacer (Kapton) was used to connect the nanogel nanofiber film and the top electrode.Under an external bias, the PIG established an EDL with remarkably high capacitance.When compressive force was applied, the confined charges attracted counterions.
Upon release, positive and negative ions accumulated at the interfaces between the air/PIG and PIG/electrode, forming a 2D EDL with enhanced capacitance.Consequently, the TENG achieved high-performance outcomes, with the output voltage and current reaching substantial values of 45 V and 49 μA/cm 2 , respectively.The power generated by this TENG was sufficient to independently power 15 LEDs, without requiring any additional external energy source or rectifier.Furthermore, the TENG demonstrated excellent stability during continuous operation at 5 Hz, as evidenced by a negligible decrease of approximately 5% in output voltage after 10000 operating cycles.n addition to TENG, PIGs can also be used as solid polymer electrolytes (SPEs) in the field of flexible/ stretchable energy storage devices, because ILs possess a very low vapor pressure and wide electrochemical stability window and are additionally nonflammable and highly ion conductive.These PIGs with ILs have the potential to improve the electrochemical properties of SPEs without the drawbacks of the addition of volatile or flammable plasticizers.These PIGs usually consist of ternary components: an IL, a polymer, and a lithium salt.Consequently, these materials possess a highly tunable composition with respect to the targeted properties such as mechanical stability, high ion condutivity, and electrochemical stability. 53,170hen et al. 171 recently presented a design for a flameretardant and flexible electrolyte (POSS-CPIL-n), utilizing the polymerizable IL [VIm-NH 2 ]TFSI, epoxy-grafted cage-type epoxy polyhedral oligomeric silsesquioxane (EP-POSS), BA, and PEGDA as polymeric monomers (Figure 20).The confinement of IL and Li salt ([BMIm]-TFSI and LiTFSI) within a POSS-PIL-n overcame the issue of ILs leakage and balanced the high ionic conductivity with the strong mechanical strength.In this electrolyte system, the synergistic effect between the hard segment (POSS structure) and the soft segment (PBA chains) imparted the ionogel with robust mechanical properties, including a high tensile strength of up to 2.5 MPa and a fracture elongation close to 90%.This DESs commonly used in PIGs are summarized in Figure 21.The polymer matrix serves as the supporting framework, playing the key role to influence the mechanical properties and conductivity of PIGs.Moreover, different polymer matrices can introduce novel properties to PIGs, including self-healing, thermal endurance, adhesion, and solvent-free characteristics, enabling their application in various functional areas.In this review, we classify PIGs into three categories based on their polymer matrices and provide an overview of the latest developments in constitute designs, preparation methods, related properties, and advantages.From the summary, several challenges and future trends in PIGs' development can be identified.
The current ionic conductivity of PIGs is typically around 10 −6 ~10 −3 S/cm, which is still lower than that of liquid electrolytes.Achieving high ionic conductivity can be facilitated by incorporating polymer matrix with ionic groups or heteroatoms which are compatible with ILs, such as polyzwitterionic polymer and natural polymers.Designing DN or MN topologies can help to achieve high ionic conductivity while maintaining good mechanical properties.
Different polymer matrix plays a crucial role in determining the multifunctional properties of PIGs owing to their intrinsic and unique properties of polymer matrices.PAA and PEG can be easily modified with various functional groups, which enables flexible structure designs for the corresponding polymer derivatives with various properties.They are the most common matrix for PIGs.The dynamic hindered urea bonds and hydrogen bonds endow PU derivatives with excellent self-healing and mechanical properties.PI and PDMS derivatives demonstrate excellent thermal endurance.
The toxicity and high manufacturing cost of IL-based PIGs pose challenges to their wide application.To overcome these challenges, DESs are recommended as alternatives to replace toxic and expensive ILs.
The leakage of ILs presents significant challenge in PIGs, including electrode conductivity attenuation and unstable properties.The compatible polymers with ILs can also partially mitigate these issues.However, the development of new types of PIGs, liquid-free PIGs (PDES), is more preferred to overcome the challenge.

| Applications of PIGs
The typical applications of PIGs in flexible ITS, flexible ECDs, flexible actuators, and flexible power supplies have been reviewed.The working mechanism, device structure design strategies and the unique functions of the PIG-based flexible ionic devices have been briefly introduced.
It is exciting to see that some new working mechanisms are developed based on PIGs, such as the piezoelectric ion effect in piezoelectric ITS and TENG, visualizing concept in ITS, the application of EDLs in super capacitive ITS, actuator, and TENG.
F I G U R E 21 Summary of materials used in polymer ionogels.
The performance of flexible ionic devices can be significantly enhanced through the device structure design.The morphology or constitute design of the PIG was most studied.Microstructure in the flexible ionic devices can be easily obtained by template method or some wet methods, such as, 3D printing, electrospinning, and photolithography and so forth.These methods are low cost and easy to be realized for PIGs.
The unique properties of PIGs confer novel functionalities to flexible ionic devices, such as ITSs/ECDs/ TENGs with wide temperature endurance, underwater ECG monitoring ITSs, self-powered ITSs, self-healing ITSs/TENGs, color modulated ECDs, and novel optically driven actuators and so on.Therefore, multifunctional flexible ionic devices can be achieved by the reasonable design of PIGs.
It is believed that PIG-based flexible ionic devices will definitely realize from proof-of-concept devices in the laboratory to commercial products, and the future development of PIGs and their applications will be booming!

T A B L E 1
Basic properties of carbon chain polymer-based PIGs.

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I G U R E 2 (A) Preparation process of polyacrylic acid (PAA) ionogel.Reproduced with permission: Copyright 2021, American Chemical Society.35(B) Tensile and release structural changes of PMMA-r-PBA copolymer.Reproduced with permission: Copyright 2019, John Wiley and Sons.37 (C) Structures of PAA, PAAm, P(AAm-co-AA) ionicgel.Reproduced with permission: Copyright 2022, Springer Nature.38 (D) Schematic illustration of the molecular synergistic design by zwitterionic polymer.Reproduced with permission: Copyright 2019, Springer Nature.39 T A B L E 2 Basic properties of synthetic hetero-chain polymer-based PIGs.Hetero-chain polymer ILsConductivity (S/cm)

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I G U R E 7 (A) Schematic of the fabrication process of the self-healing I-SPNm/ILn ionogel by the reversable imine formation/hydrolysis. Reproduced with permission: Copyright 2022, Royal Society of Chemistry. 68(B) The chemical structure of tetraPEG-Ant and a conceptual illustration of photohealing of the tetraPEG-Ant iongel.Reproduced with permission: Copyright 2018, Royal Society of Chemistry.
, a fully physically linked DN PIG was fabricated via interpenetrating a poly(hydroxyethyl acrylate) (PHEA) network into an agarose network in [EMIM]Cl.Hydrogen bonds were the main interaction between polymers.The DN PIG exhibited remarkable F I G U R E 8 (A) Chemical structure of the PU network.Reproduced with permission: Copyright 2020, Wiley-VCH GmbH. 71(B) Schematic of the fabrication process of PI organogels and PI ionogels.Reproduced with permission: Copyright 2019, Royal Society of Chemistry. 72F I G U R E 9 (A) Schematic diagram of bacterial cellulose ionogels (BCIGs).Reproduced with permission: Copyright 2021, John Wiley and Sons. 73(B) Synthesis of cellulose nanocrystal ionogel.Reproduced with permission: Copyright 2021, John Wiley and Sons. 74(C) Preparation of the agarose/PHEA DN ionogel.Reproduced with permission: Copyright 2020, American Chemical Society. 75(D) Illustration of the network structures of PNN/Ch and PNN/cross-linked x-Ch.Reproduced with permission: Copyright 2021, John Wiley and Sons.

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I G U R E 10 (A) Synthesis process and structural schematic diagram of self-healing polysiloxane loaded ions.Reproduced with permission: Copyright 2019, John Wiley and Sons.79 (B) Synthesis of ionic POSS and manufacture of I-POSS-G and ionic poss scaffold (I-POSS-S) series.Reproduced with permission: Copyright 2017, American Chemical Society.80exhibiting favorable ionic conductivity and excellent adhesion on various polymer matrices.By utilizing different antisolvents with the same Sn-based precursor solution, MA 2 SnI 6 perovskite single crystals, or stable perovskite mesophase PIGel could be obtained separately.When loaded onto PDMS, the Sn-based PIGel was successfully applied in ionic skin sensors to detect external mechanical stimuli such as stretching, finger flexion, voice recognition, and pulse monitoring.Furthermore, the PIGel on PDMS exhibited sensitivity to temperature and voltage changes, making them suitable for temperature sensor and ECD.The versatile characteristics of perovskite PIGs, combined with the simple solution processing techniques on PDMS, offer new possibilities for conductive polymer matrices in multifunctional flexible ionic devices.3 | APPLICATION OF PIGS IN FLEXIBLE IONIC DEVICES 3.Flexible ionic tactile sensor (B) Molecular structure of ionogel precursors and long-term stability of ECG signal detected by commercial CGE and ionogel electrode in the aquatic environment.Reproduced with permission: Copyright 2021, John Wiley and Sons.101 presented a straightforward visualized ITS that converts local stress into spatially resolved visual light signals through an electrochemical process based on piezoelectric ion effect.

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I G U R E 14 (A) Schematic of the structure and multifunctions of PIG-TENG.Reproduced with permission: Copyright 2022, Elsevier.109(B) Distribution of ionic components in an ECL TPU based PIG (left) in equilibrium and under pressure applied on the top (right).Reproduced with permission: Copyright 2021, John Wiley and Sons.110

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I G U R E 16 (A) Schematic illustration of the ECD based on ITO-PET and color switched at −20 °C (top) and 100 °C (bottom) upon bending.Reproduced with permission: Copyright 2021, John Wiley and Sons. 131(B) Schematic of the direct patterning in DES-based ECDs by the 3D printing technology.Reproduced with permission: Copyright 2023, John Wiley and Sons.
prepared PIGs comprising a homogeneous PEG network and LiTFSI.The obtained PIGs exhibited superior mechanical F I G U R E 17 (A) Schematic of ionic gel electrolyte composed of PS-b-PMMA-b-PS and IL.Reproduced with permission: Copyright 2011, American Chemical Society. 146(B) The Th-SNG/PEDOT:PSS electrode of the actuator.Reproduced with permission: Copyright 2015, John Wiley and Sons. 147(C) Novel ionogel/metal nanocomposite based actuators.Reproduced with permission: Copyright 2017, John Wiley and Sons.148

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I G U R E 19 (A) The work principle and the freeze resistant performance of the PIG-TENG.Reproduced with permission: Copyright 2019, Elsevier. 164(B) Scheme of the work mechanism of the template-patterned PIG-TENG.Reproduced with permission: Copyright 2019, Elsevier.
enabled flexible bending, twisting, and folding.The PEGDA chains established effective pathways for ion conduction, resulting in an impressive ionic conductivity of up to 2.5 × 10 −3 S/cm.The incorporation of nonflammable POSS structures and nearly nonvolatile imidazolium-based IL led to a remarkable thermal stability of the PIG-based electrolyte up to 360 °C, along with excellent flame retardancy.The LiFePO 4 ||Li cell assembled with POSS-CPIL-n exhibited a high reversible capacity of 100 mAh/g at 4 C and retained 98% of the initial discharge capacity after 200 cycles at 0.2 C. Furthermore, the flexible soft-package cell with POSS-CPIL-n can easily charge a mobile phone and continuously supplies power even when the battery was cut or exposed to air, demonstrating the potential application prospects of POSS-CPIL-n in high-performance flexible electronic devices.4| SUMMARY AND PROSPECTS4.1 | Materials of PIGsOwing to their excellent properties, such as negligible vapor pressure, good ionic conductivity, high transparency, thermal stability, and electrochemical stability, PIGs are regarded as promising composite materials to replace ionized hydrogels.The polymer matrices and ILs/ F I G U R E 20 Schematic diagram of synthesis process of POSS-CPIL-n.Reproduced with permission: Copyright 2022, Elsevier.171 Basic properties of natural polymer-based PIGs.
T A B L E 4 Basic properties of elemental organic polymer-based PIGs.