Ionogel-based flexible stress and strain sensors

ABSTRACT Ionogels is a kind of hybrid materials composed of ionic liquids (ILs) and solid polymer network matrix, has been extensively investigated in the most recent decade. Due to the excellent mechanical properties and ionic conductivity, their promising applications in flexible stress and strain sensors have been proposed and explosively developed. In this review, we briefly summarize research progresses on ionogel based flexible stress and strain sensors (IFSSs) from five aspects, including material synthesis, device fabrication, working principles, characteristics and performances, and potential applications. Some outlooks and perspectives are also proposed at the end of review. The review is expected to provide reference and new insights into the research of IFSS. Graphical abstract


Introduction
The demand for real-time social communication, internet information and health monitoring has promoted the rise and development of wearable electronic devices. For wearable devices, the constant pursuit of lightweight and portability always stimulate the research interests to develop flexible electronic components. In recent years, flexible electronics are accelerating into the commercial stage and beginning to enter people's daily lives. For instance, foldable or rollable screens enable the smart phone with large display area portable, some of which even could be worn as a wristlet. Among different portable and wearable electronics, sensor is an important constituent part for converting physical quantities into electrical signals. Typically, flexible sensors that can sense stress and strain have been a research hotspot for their promising applications in soft robots [1], virtual reality interaction [2], human motion detection and healthy monitoring [3,4] and medical prosthesis [5]. As the name suggests, stress sensors can convert force exert on itself into an electrical signal. Normally, when an object is subjected to pressure, deformation also occurs, and deformation degree is depended on mechanical properties such as hardness and elastic modulus of the material. As stress sensors, their deformation degree is far less than the magnitude of the force exerted, i.e. their electrical characteristics (resistance, capacitance, etc.) are more sensitive to stress or pressure instead of strain or deformation [6][7][8]. Conversely, strain sensors are more responsive to their deformation, and are flexible devices with excellent deformation ability and elastic recovery ability [9][10][11][12]. Some flexible sensors sensitive to both stress and strain can detect pressure and strain, respectively [13][14][15][16].
As main parts of flexible stress/strain sensors, the active materials and/or conductive materials should have both flexibility and stress/strain responsiveness. Incorporation of conducting micro/nano fillers with various structures (i.e. particle, tube, fiber, sheet, and layer) [17][18][19] into elastomer or polymer matrix is a traditional method to prepare stress/strain-responsive material. Conductive passages among the conductive fillers will change when elastomer composites are deformed under an external stress and strain, which will be reflected in the overall resistance change of the composites. The sensitivity and hysteresis of these sensors are usually related to mechanical properties of the elastomers themselves. Furthermore, the opaque fillers seldom meet the need of transparency for sensors. Conductive hydrogels, as a kind of flexible composites made from a three-dimensional (3D) network of cross-linked hydrophilic polymers, conductive polymer/fillers and water, show their unique advantages in bioelectronics, drug delivery and tissue engineering because of their high transparency, conductivity, stretchability, and biocompatibility [20]. Thanks to the high stretchability and mechanically toughness of hydrogel composites, some flexible sensors possess strain sensing which could be used for human motion detection [21]. Adopting special polymer network structure and fillers, hydrogel based flexible strain sensors even obtain recyclability [22]. However, there has some disadvantages that cannot be overcome at present, such as easiness of evaporation, narrow work temperature range, etc.
The emergence of ionogel opens up another pathway for intelligent flexible electronic sensors. It is a type of hybrid material, whose polymer network is formed in ionic liquid (IL). ILs are compounds completely composed of ions with melting point below 100°C, and most of them have the nonvolatility under ambient conditions, and air and water stability [23,24], which aqueous ionic solution in hydrogels do not possess. The chemical structures of common used ILs in ionogel based flexible stress and strain sensor (IFSS) are listed in Table 1. The term 'ionic gel' normally covers another gel materials which contain aqueous salt solution as ionic conductor [25][26][27]. Here, 'ionogels' involve only those contain ILs as conductors in this review. Ionogel entraps IL into its polymer network matrix, so it inherits the excellent properties of ILs (including ionic conductivity, high thermal stability, wide electrochemical window, etc.) and solid state network structure (including elasticity, flexibility, stretchability and transparency) at the These merits contribute to their potential applications in the field of flexible supercapacitors [28], electrochromic devices [29], catalysis [30,31], and sensing [32][33][34]. This review paper mainly summarizes the progresses of ionogel based flexible stress/ strain sensors (IFSSs) in recent years. The preparation of ionogel, the manufacturing process, the characteristics and properties, the working principles of ionogel based flexible sensors and their applications are presented. In the last part of the review, some outlooks and problems in the practical application of ionogel based sensors are also described, which might promote their performance improvement and practical applications.

Synthesis of Ionogel in IFSS
Ionogel is the main constituent material of IFSS. There have been a lot of review literatures discuss the synthesis routes of ionogels and their classification [35][36][37][38]. For high porosity and specific surface area, ionogel electrolytes used in batteries have porous matrices, which are often based on nanoparticles. These ionogel electrolytes are mainly prepared by sol-gel, impregnation and covalent grafting, where IL is entrapped into the pores of matrix through IL hydrolysis and condensation reaction, impregnated into the nanoporous matrix or covalently tethering cation to a variety of inorganic nanoparticles [39]. While ionogels used in sensors usually adopt polymer matrix out of condition for flexible, robust and integrated molding devices. The synthesis of ionogel in IFSS and their typical features are summarized in Table 2. Although there are various methods for synthesizing ionogel, photopolymerization is the most popular synthetic method, attributed to their costeffectiveness, simplicity, controllability and potential in 3D printing technologies. In 2019, Nie's group used butyl acrylate (BA) as the polymer monomer, 2-hydroxy-2-methylpropiophenone (1173) as a photoinitiator, 1,6-bis(acryloyloxy)hexane (HDDA) as a crosslinking agent, and bis(trifluoromethylsulfonyl imide ([BMIM]TFSI) as an IL to synthesize a transparent, stretchable, stable, and self-adhesive ionogel through one-pot photopolymerization (Figure 1(a)) [40]. By means of this simple method, they also polymerized acrylic acid (AA) units in the ILs by UV irradiation with the introduction of a poly (ethylene glycol) diacrylate (PEGDA) cross-linker [41]. Chen et al.  [DCA])) through in situ one-step photopolymerization of 3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) and AA [43]. Thermal polymerization is another frequently used method, which is commonly achieved by thermal initiator ammonium persulfate (APS). Wang's group in situ synthesized an air-stable, highly conductive, and semitransparent ionogel by dissolving N,N'-methylene bis(acrylamide) (NNMBA) and APS in 1-vinyl-3-ethylimidazolium dicynamide ([VEIM] [DCA]) and heating the solution at 80°C (Figure 1(c)) [44]. They also thermally polymerized the mixture solution of [VEIM][DCA] ILs/ APS/ silicon dioxide (SiO 2 ) on flexible substrate to prepare an ionogel for flexible highsensitive pressure sensor [45,46]. One-pot thermal polymerization method can also be adopted to synthesize poly(methyl methacrylate-co-butylmethacrylate) /poly(vinylidene Hydroxyethyl acrylate (HEA) and agarose; PEDGA [59] (Continued)  [74] fluoride-co-hexafluoropropylene) (P(MMA-co-BMA)/PVDF-HFP) double-network (DN) ionogel ( Figure 1(d)) [47]. Zhang (Figure 1(e)). Except free radical polymerization, some ionogels for sensors are prepared through electrostatic interactions, hydrogen bond interactions, π-π bond interactions and a thiolene click reaction. Figure 2  In terms of material selection, [EMIM]BF 4 is the most used IL for its affordable price, relative high ionic conductivity (4.09 mS/cm), low melting point (−81°C) and high decomposition temperature (403°C). To obtain high conductivity of finished ionogel device, [EMIm][DCA] is often adopted due to its very high ionic conductivity of 2.8 S/m at 25°C. For polymer network design and synthesis, covalent bonded networks in situ formed in ILs, such as photopolymerization, thermal polymerization usually endow mechanical robust ionogel. Physical cross-linking polymer networks through hydrogen bonding, ππ interactions, etc. could achieve self-healable ionogel. In addition, the compatibility between block monomers and ILs and the ILs content also should be considered, which influence the mechanical properties and conductivity.  [57]. (b) Forming polyimide ionogels by solution displacement attributed to hydrogen bonding [54]. (c) Locking ILs into the elastomer network through hydrogen bonding [61]. (d) The thiolene click reaction for preparing DN click ionogel [63].

Resistive
Tensile strain Stretchable and mechanical robust 3D printing Ionogel film or tube [49] Ionogel crosslinked network [50] Integrally-formed and encapsulated with elastomer [53] Solution cast Ionogel film [40,47,48,54,56,57,60,61] Sandwiched with flexible shell and copper foil [45] Tensile strain and temperature Stretchable, mechanical robust and thermosensitive Interfacial iontronic Pressure High unit area capacitance (UAC) and signal-to-noise ratio (SNR) Spin coating Patterned ionogel film sandwiched with flexible electrodes and shell [64] Irregular bump structured conductive elastomer sandwiched with ionogel film and PDMS shell [66] Micropatterned ionogel film sandwiched with ITO/PET films [75] Leaf surface patterned ionogel from sandwiched conductive elastomer [65] Tactile controller Sandwiched electric skin and integrated with circuit; sensor array [69] Pressure Solution cast Sandwiched membrane with chamber and sensor array [71] Tactile and pressure Printing or writhing All-paper based sensor array [73] Pressure Paper-based and packaged with complex shaped shell [72] All-paper based, sandwiched with printing electrodes and has different origami structures [74] Electro-spinning All-fabrics based sensor sandwiched with conductive fabrics [67] Sandwiched with Al foil and insulator shell [76]

Manufacturing and Structure of IFSS
The forming process of ionogel and device structure design of present IFSS have summarized in Table 3. As seen from the table, solution cast is the easiest forming process to synthesis ionogel material (Figure 3(a)). Hence, most of IFSS are thin film structure originated from the solution-casted synthesis process of ionogel [40,41,44,[46][47][48]54,56,57,60,61]. As a stretchable strain sensor, simple film is the optimum device structure. But for special purpose of structural design and application, different processing or forming methods need to be applied. 3D printing has been a popular prototyping and manufacturing technology because of its advanced precision, high efficiency and high controllability. Especially it is the most effective way for forming complex architectures. Typically, the precursor solution of ionogel is put into dedicated 3D printer with UV exposure device, and the ionogel sensors with different shapes will be obtained after chemical cross-linking under the UV radiation [49]. The circular tube-like IFSS can be worn on finger to detect its bending, which is more convenient than those pasted on finger by adhesive tape (Figure 3(b)). In Figure 3(c), Wong et al. 3D-printed an IFSS with hourglass auxetic structure, which had more robust mechanical strength, lighter mass, more sensitive response to strain and larger elongation [50]. It is impossible to achieve integrated forming with encapsulating material and volume production of IFSS by solution cast, and artificial tailoring and assembling. By means of 3D printing, Crump et al. manufactured a batch of stretchable strain sensors encapsulated by (polydimethylsiloxane) PDMS through one process (Figure 3(d)) [53].
Another common structure of IFSS is sandwich-like structure, which needs handassembling. Such structure is often designed for three purposes. One is to protect ionogel itself from external interference. The sensor is often encapsulated by flexible material, such as Eco-flex or silicone rubber elastomer to be protected from the electronic signal interference of human skin, which is usually applied as wearable sensor (Figure 4(a)) [45]. If conductive ionogel is used as flexible electrode of sensor, it should be covered with a protecting layer. This structure is commonly used in two type IFSS. One is capacitive-type sensor, which has an elastomer spacer sandwiched between two ionogel electrodes ( Figure 4(b)) [51,55]. Another is interfacial iontronic type sensor, which has a conductive layer to contact with ionogel electroyte layer to form elastic electrolytic-electronic interface. To obtain the high unit area capacitance (UAC), coarse or patterned surface of ionogel layer or conductive layer need to be manufactured. These micro-nano texture prepared by spin coating ionogel or conductive elastomer precursor on template with surface texture, such as leaf [65], sandpaper [66] and lithography mold [75]. Ionogel fiber mats inherently have large specific surface area prepared through electrospinning, which is also an effective means to manufacture interfacial iontronic type sensor [67,76]. Ionogel film can also be used as soft electrodes of triboelectric principlebased stress sensors, where the ionogel layer is often stacked with a dielectric layer (Figure 4 (c)) [43,63] or contacted directly with a dielectric layer to generate charges (Figure 4(d)) [48].
Paper-based devices have advantages of low-cost, simple manufacturing and environmentally-friendly, and attract a lot of research interests in recent years. Liu et al. directly write the ionogel precursor solution as an conductive path on the paper (Figure 4(e)) [52]. Through the novel design, they made two pressure sensors with different sensing principles, piezoresistive and triboelectric. All-paper based IFSS has another major merit: various  [41]. (b) 3D printing manufactured complex architectures [49]. (c) 3D printed hourglass auxetic structure [50]. (d) Volume production of IFSS by 3D printing [53].  [45]. (b) Ionogel as the electrode of capacitive type sensor [55]. (c) Single-electrode mode TENG IFSS prepared by stacking ionogel and PDMS dielectric layer [43]. (d) Contact-separation mode TENG IFSS working by contacting ionogel with patterned PDMS dielectric layer [58]. (e) Paper-based IFSS prepared by direct-writing ionogel precursor solution on paper [52]. structure designs. They could be tailored into different kirigami structures or folded into fancy origami structures [72,73,74].
Actually, it can be found in the Table 3 that the forming process of ionogel is highly relevant to device structure. For example, simple stretchable strain sensor could be mainly made by solution casting ionogel film; Manufacturing process of paper-based IFSS need to write the liquid ionogel precursor on paper followed by curing treatment, then be sandwiched with other components; Spin coating ionogel on template facilitate the micro structure of IFSS. Furthermore, the device structure is depended on sensing principles of IFSS, while the choice of principle is closely related to sensing function to be achieved.

Working principles of IFSS
In the consideration of correlation, the principles of IFSS are also summarized in the Table  3. The sensing principles of IFSS could be classified into 5 types: resistive, capacitive, interfacial iontronic, piezoelectric and triboelectric. They are suitable for different sensing functions as shown in Table 3, and discussed separately.

Resistive type
Resistive-type sensors mainly depend on the deformation of their active materials that can response to external stress or strain. With simple structure and assembly, most of IFSS adopt piezoresistive principle ( Figure 5(a)). Only by connecting leads with ionogel film and amperemeter, the current flow through ionogel can response to its strain according to the Equations (1) and (2): The resistance R of active material is related to its shape (length L, cross-sectional area A) and intrinsic characteristics, resistivity ρ. Equation (2) is derived through substitution of Ohm's law, where the current I will change with the deformation (L and A) of ionogel when constant voltage is applied on its ends. Resistive principle could be adopted to achieve both stress/ pressure and strain sensing, and as far as we know, tensile strain sensors are almost based on resistive principle. To obtain large elongation ratio, the polymer matrix and chemical composition of ionogel in tensile strain sensors should be considered and designed for robust mechanical properties.

Capacitive type
As mentioned above, conductive ionogel film can be used as the electrode of sensors. When a dielectric layer is sandwiched between two ionogel films, a simple parallel-plate capacitor will be obtained (Figure 4(b)) [51,55]. According to the Equation (3): when the sensor deforms under an external force, the distance d between two ionogel parallel plates will be changed, thus generating the variation of capacitance C. As a stress sensor, its active area S between two plates usually remains unchanged. k is dielectric constant of interlayer material. The flexible sensor based on capacitive principle involve two opposite electrodes, so cannot suffer large deformation, and only applies to pressure sensing.

Interfacial iontronic type
Interfacial iontronic sensor is a kind of new emerging type of IFSS in recent years, which mainly realize pressure and tactile sensing. They are different from traditional flexible electrostatic capacitive sensors mentioned in the above section. Interfacial iontronic sensors mainly utilize the supercapacitive nature of the electrical double layer (EDL) that occurs at the electrolytic-electronic interface [77]. The EDL capacitance of the sensor is dependent on the interfacial contact area between the ionogel layer and the counter electrode. When the ionogel contacts with the electrode, ions in the asperities of the ionogel layer and electrons on the electrode attract each other and accumulate, which (d) Single-electrode TENG principle [63]. (e) Contact-separation TENG principle [58].
forms an ultrahigh specific capacitance at micro/nano scale. With the load increases, the number of the asperities contacted with electrode increases, i.e. the EDL area increases. So the capacitance of the sensor can be regarded as the sum of numerous EDL capacitance [65,78] (Figure 5(c)). EDL effect based sensors usually have ultrahigh pressure sensitivity, high noise immunity, high pressure resolution, high spatial definition, and optical transparency. Similarly, interfacial iontronic sensor needs contact electrode to form electrolytic-electronic interface, so has layered assembly structure which cannot support stretchable deformation, and normally, strong mechanical properties like stretchable strain sensors are not required for its flexible components.

Piezoelectric type
Up to now, very few study has fabricated IFSS with the piezoelectric principle. The incorporation of piezoelectric components into ionogel matrix forms piezoelectric composites. After polarization, the dipole moment vector of piezoelectric components can be orientated. When the composite is subjected to an impulse compress stress, the dipole moments of deformed piezoelectric nanoparticles change and generate piezoelectric potential to realize a sensitive sensing. Villa et al. prepared a novel ionogel/BaTiO 3 nanocomposite by UV photo-cross-linking reaction [62]. The nanocomposite had a polymer interpenetrating network (IPN) composed of polyvinylpyrolidone (PVP), a random copolymer of (hydroxyethyl) methacrylate (HEMA) and acrylonitrile (AN) (poly-HEMA-co-AN) and piezoelectric BaTiO 3 nanoparticles with chemically modified hydrophilic surface. The polarized sensor can achieve a low frequency and directional discriminative pressure sensing by combining piezoionic and piezoelectric activity, generating electric charge due to a redistribution of the mobile ions across the polymer matrix and to the presence of the polarized BaTiO 3 nanoparticles, respectively ( Figure 5(b)). For piezoelectric type sensors, the piezoelectric particles in composites play an important role in sensing pressure. However, ionogel as conductive matrix, its corporation with piezoelectric particles will be a challenge for polarization of the composites. Besides, the piezoelectric effect in conductive ionogel matrix is still not entirely clear. The separate structure design of ionogel and flexible piezoelectric component in sensor may be better than ionogel/piezoelectric particles composites to achieve piezoelectric type ionogel sensor.

Triboelectric type
Triboelectric nanogenerator (TENG) is proposed by Wang's group at 2012 [79,80,81]. After nearly a decade of research and development, TENG is penetrating into different fields, such as energy harvesting, health monitoring, biomedical sensing and wearable electronics. TENG based flexible sensors allure a lot of attention by virtue of two main advantages: 1. Wide materials selection and flexibility achieved easily by polymer structure; 2. Self-powering without external energy supply [82,83]. IFSS based on TENG mechanism usually adopt two typical structural types: single-electrode and contact-separation. Single-electrode TENG mode IFSS has simple structure, and it generates current signals by contacting its dielectric layer with external object such as human finger [43,63]. When the external object approaching the dielectric surface material of the sensor, due to their different electronegativity, electrostatic charges will be induced in dielectric layer, and meanwhile, electrons flow between the ionogel electrode and earth terminal for reaching a potential balance ( Figure 5(c)). For contact-separation TENG mode IFSS, the contactseparation process occurs inside the sensor, in which two different dielectric layer and spacer between them are needed [43,52,58]. It has the similar principle with singleelectrode mode, but electrons flow between two ionogel electrodes ( Figure 5(d)). Dielectric and conductive layers are two necessary components of triboelectric type sensor. The sensor works through triboelectrification between two dielectric objects in contact-separation motion, which is structurally difficult to perform large deformation. Those stretchable TENG mode IFSS only perform normal pressure sensing at tensile state or detect bending strain through contact-separation at the bend [43,63].

Properties and performances
By contrasting and summarizing the principles, structures and functions of ionogel based sensors, it can be found that flexible ionogel based sensors just performs two sensing functions: tensile strain and stress (pressure), according to five mechanisms. Although they have some similar performance characteristics, such as gauge factor (GF, i.e. sensitivity) and response time, they cannot be compared together because of the difference of two types of ionogel based sensors on function and working modes. Bsesides, they both have their own performance characteristics. Therefore, their main performances are summarized separately in Tables 4 and 5.

Stretchablility
Generally, stretchable strain sensors can be characterized by the properties including the properties of ionogel materials and the sensing performances. For stretchable strain sensors, the elongation of ionogel is used to characterize tensile property of the sensor, which is the ratio of the maximum length in tension state to the natural length, while the tensile strength is the tensile stress at the maximum tensile length. Low tensile strength is not conducive to resist large defamation. The elongation and tensile strength are mainly related to the polymer network, IL content and their compatibility. Figure 6(a) shows an ionogel with ultrahigh stretchability of higher than 5000% at a tensile strength of 1.75 MPa. The polymer double network was synthesized by two-step photopolymerization of ethyl acrylate (EA) monomer, and then the ionogel was yielded by incorporating [C 2 mim][NTf 2 ] IL into the network through hydrogen bonding [61].

Conductivity
Conductivity is an important parameter of ionogel to judge its electrical property. It depends heavily on the conductivity of IL locked in the polymer network. Li's group and Liu's group immobilized [EMIm][DCA] in poly(vinyl alcohol)-poly(vinylpyrrolidone) (PVA-PVP) complexes and PAMPS-based double networks, respectively. Owing to the high conductivity (28 mS/cm) of [EMIm][DCA] at room temperature, the corresponding ionolgels reached a high conductivity of 19.7 and 19 mS/cm, respectively [56,57]. However, the ionic conductivity of IL, as well ionogel, was affected by ambient temperature. The Vogel-Fulcher-Tammann (VFT) Equation (4) descripts the conductive behavior of the ionogel [84,85]: where A is a pre-exponential factor related to the number of ion carriers, B is the activation energy of ion transport, and T 0 is the idealized glass transition temperature of the ionogel, which is lower than the glass transition temperature of the ionogel by 20-50 K.
With the conductivity-temperature dependence relationship, the ionogel can realize sensing of temperature. The bimodal sensor based on ionogel prepared by Sun et al. can detect temperature changes over the range of 10 to 90°C by a 95.4% resistance variation (Figure 6(c)) [59]. Without influence by volatilization, thermal stability of ionogel materials is usually better than hydrogel and they also possess a wider operating temperature.

Sensitivity and linearity
Performance metrics of traditional sensors can also be applied to IFSS as sensing devices. Sensitivity is the key parameter to judge their response to external stimulations. For piezoresistive IFSS, the gauge factor (GF) is the slope of the fitted ΔR/R 0 to the external strain or stress, where R 0 is the resistance of IFSS at its original state, ΔR is the variation of resistance when IFSS is subjected to strain or stress, and strain is the elongation ratio ( Figure 6(d)). Similarly, GF of capacitive and interfacial iontronic IFSS is the ratio of the fitted ΔC/C 0 to stress (Figure 6(e)). Iontronic pressure sensors emerged in the last decade have much larger sensitivity than traditional capacitive sensors, and have become a promising pressure sensing solution [77]. It is worth noting that GF may not be a constant in the whole working range. In another words, the slopes of the fitted ΔR/R 0 to strain or stress are different in different strain /stress scope (Figure 6(d)). For triboelectric IFSS, their GF is the ratio of triboelectric outputs (voltage or current) to stress (Figure 6(f)).
Linearity represents the linear relationship of ΔR/R 0 to strain /stress (Figure 6(g)). The good linear response can significantly facilitate the calibration process, and improve signal accuracy and reliability, which is essential in practical applications of strain sensors. Generally, capacitive sensors have good linearity, while resistive sensors sometime have nonlinearity due to non-uniform morphology during deformation.

Response time
Response time is the time required from exerting /release external stimulation to generate / reset electric signals, which reflects the response speed of the sensor. The shorter is the response time, the faster sensor gives electrical feedback. For stretchable strain ionogel based sensor, 80 ms interval can be observed from 1.0 to 1.4 of ΔR/R 0 for the agarose / PHEA /[EMIM]Cl DN IFSS (Figure 6(h)) [59]. Comparing Table 1 with 2, it can be found that the response time of most pressure sensors are lower than tensile strain sensors, which could be attributed to the tiny elastic deformation during sensing external pressure. Furthermore, owing to the high UAC and EDL effect, iontronic pressure sensors with subtle structure design can reach ultralow response time [67,71], which is significantly lower than traditional piezoresistive pressure sensor's [44,46]

Hysteresis
Through conducting loading and unloading pressure cycles, the hysteresis behavior of the sensor can be investigated, which reflected the ability of IFSS to detect strain quantitatively and the independence of strain history. The hysteresis phenomenon is mainly attributed to viscoelastic nature of ionogel [86,87]. The degree of hysteresis (DH) is calculated as the Equation (5) [88]: where A Loading and A Unloading are the area of loading and unloading curves, respectively. The lower DH value, the lesser hysteresis in the response to strain [88].
In addition, durability and temperature tolerance determine the service life and operating temperature range of IFSS, respectively. These parameters are mainly related to properties of ionogel materials. The ionogel based ionic skins produced by Li et al. demonstrated ultra-durable sensing properties over 10,000 uninterrupted strain cycles and retained its original sensing properties even after 200 days of open storage ( Figure 6 (i)) [60].
Self-healing ionogel has the ability to repair mechanical damage independently, and may be an option to increase the service life of IFSS ( Figure 6(j)). Generally, their self-healing ability is benefited from the reversible hydrogen bonds in the polymeric network [56,60] or the reversible coordination bonds and ionic bonds among incorporated nanoparticles and polymer chain [48].

Temperature tolerance
As mentioned above, although resistance is affected by ambient temperature, IFSS have wider operating temperature window than hydrogel-based sensors because of the nonvolatility of ionogel. For instance, Wang et al. utilized poly(ionic liquid) (PIL) as the ionic conductive network and hyper branched polymer as macro-cross-linkers to 3D print ionogel, which could work at an extreme low temperature (−60°C) and a high temperature (250°C) (Figure 6(k)) [49]. Owing to this merit, IFSS can work at high temperature environment where hydrogel-based sensors are complete failure, and low temperature environment where hydrogel-based sensors become brittle because of the frozen water inside.

Working range
The working range decide the top and bottom sensing limitation of a sensor. Tensile strain sensors normally can work in their maximum elongation ratio, though have different sensitivity values at different tensile ratio [40,55,56,60]. For pressure sensors, working range is an important characteristic. The top limitation of working range represents how  [61]. (b) The maximum elongation of ionogel [61]. (c) The temperature sensibility of IFSS [59]. (d) IFSS may have different GF in different elongation ranges [48]. (e) The GF of capacitive IFSS [55]. (f) The GF of triboelectric-type IFSS [52]. (g) The resistance change of IFSS has a good linear relationship with strain [41]. (h) An resistance IFSS with 80 ms responsive time [59]. (i) High stability with 10,000 cycles of duration [60]. (j) Self-healable ionogel [56]. (k) An IFSS that can work in the temperature range of −60 ~ 250 °C [49]. much load can be responded, while the bottom limitation of working range reflects the minimum load can be detected by the sensor. Owing to sensitively changing EDL capacitances at the interfacial contact areas, ionogel based iontronic sensors have low limitation of detection. Hence, micro-nano structure design and construction is key to iontronic pressure sensors.
Because of the differences on sensing modes between the ionogel based tensile strain sensors and pressure sensors, the development progress of their main performance indictors cannot be discussed generally together. Ionogel based tensile strain sensors are emerging flexible sensors in the last five years. Maximum strain as the indictor of their stretchability has been improved from 157% (Ref. [57]) at 2017 to 5000% (Ref. [61]) at 2020 (Figure 7(a)). With a few exceptions, most of ionogel based tensile train sensors have a maximum sensitivity of less than 3. Altogether, comprehensive performances (maximum strain and sensitivity) of ionogel based tensile strain sensors are basically improving (Figure 7(a)) in recent years. One of the major advantages of ionogel based sensors over aqueous ionic gel based sensors is wider working temperature window. After several years of development, ionoge based tensile strain sensors already can operating well from the low temperature of −60°C to high temperature of 250°C (Figure 7(b)). Interfacial Figure 7. The development progress of (a) maximum strain, sensitivity (sensitivity is not provided in Ref. [50,57]) and (b) temperature tolerance on ionogel based tensile strain sensors; and the development progress of (c) sensitivity, response time (response time is not provided in Ref. [51,69]) and (d) working range on ionogel based pressure sensors in recent years.
iontronic pressure sensors were developed for the last decade. Their sensing components are ionic materials, and solid ionogels are just one of them. Although their sensitivities can be measured in different ways, in the same measure method, the sensitivity still presents a trend of constant improvement (Figure 7(c)). The sensitivity increased from 9.55 kPa −1 in 2017 (Ref. [66]) to 131.5 kPa −1 in 2019 (Ref. [64]) and increased from 3.1 nF/kPa in 2015 (Ref. [71]) to around 100 nF/kPa in 2017 (Ref. [67]) and 2018 (Ref. [51]). The response time of ionogel based iontronic pressure sensors have reduced to millisecond level, while the response time of piezoresistive pressure sensor is still higher than iontronic pressure sensor's [44,46] (Figure 7(c)). Their lower limitation of detection are as low as a tenth of Pascal and upper limitation can reach to 750 kPa (Figure 7(d)). Wide detection range is conducive to their practical applications.

Motion moniroring
Possessing stress and strain detecting function, IFSS can be applied in various fields. Human motion monitoring plays an important role in patient's motor function restoration. 3D printed tube-like IFSS can be put on human finger to detect its bending movements (Figure 8(a)) [49]. It also can be printed into simple stripe shape and fixed on  different body parts, such as wrist and elbow, to detect their movements (Figure 8(b-c))) [49]. Sensitive IFSS even can precisely monitor the bending angle of wrist, which is a promising application in evaluating patient's recovery situation (Figure 8(d)) [45].

Human-machine interface
If IFSS is integrated with signal acquisition and analysis processing system to fabricate a wearable smart glove, it will be a potential product in motion capture and virtual reality interaction. For example, Wang et al. prepared an IFSS array connected with signal processing system, which can wirelessly control an unmanned aerial vehicle (UAV) (Figure 9(a)) [45]. Combining IFSS array with commercially available integrated circuit components consolidated on a flexible printed circuit board (PCB), V. Amoli et al. fabricate a wearable aerial drone microcontroller (WADM), capable of controlling a drone's number of rotations and even its direction simultaneously and/or selectively during its flight (Figure 9(b)) [69]. Pan's group design and fabricate a smart glove with a numbers of all-fabric ionogel based sensing units, which can detect the pressure on the palm when grabbing an object (Figure 9(c)) [67].

Real-time health monitoring
With the advent of the Internet of Things, considering more concern on human health in daily life, real-time health monitoring is becoming a great demand. IFSS can meet such requirements by virtue of their sensitive detection of breathing [55], swallowing [47], voice [45], human pulse [52,59], and eye movements [45,64] (Figure 10). Compared with traditional sensors, IFSS have high sensitivity and fit the human skin better, and much more convenient to use than traditional non-flexible sensing device. Besides, compared with complicated circuit design and components integration of traditional sensors, manufacturing process of IFSS is relatively simple. Figure 10. IFSS can real-time sensing physiological characteristics, such as (a) breathing [55], (b) swallowing [47], (c) speaking [45], (d) pulse beating [52] and (e) eye movements [64], for monitoring human health situation.

E-skins and soft robotics
E-skins and soft robotics are also potential application fields of IFSS. E-skins can help robots have similar tactile sensing function as human skin [52]. IFSS also can be integrated into soft robotic fingers for somatosensitive manipulation, which can perform the grasp action while sensing the grasping force [89]. Ionogel nanocomposites with the properties of low frequency and directional discriminative pressure sensing have a huge potential in development of smart soft control panel [62].

Summary and outlook
In this review, we present the recent progresses in the field of employing ionogels as active components in flexible stress and strain sensors. The current development of IFSSs is discussed, from the aspects of material synthesis, device fabrication, characteristics and performances, working principles and potential applications, which can be summarized as follows: (1) photoradical polymerization and thermal polymerization are the most commonly used synthetic methods for ionogels in IFSS, while hydrogenbonding interactions are usually adopted to prepare self-healing ionogel. (2) Most of IFSS are ionogel films themselves, formed by solution casting. But 3D printing is being favored by more and more researchers. (3) Lots of applications demonstrates huge potential of IFSS in the fields of wearable electronics, motion monitoring, biosensing, soft robotics and intelligent interaction. However, there are some aspects that need further research and development: (1) Improvement of material properties. Ionogels vary considerably in properties and characteristics, and it needs further improvement and standardized synthesis for real product applications. Ionogel is still in a very early stage and its material properties involve many indictors. Utilizing the high-throughput experimentation and materials genome, it may accelerate the constitution design and formula optimizing. (2) Structure design of devices. The performance of device is a comprehensive result of material properties and structural design. The monotonous film structure design of IFSS will be improved in the future. There have been some reports on subtle structure designed IFSS through electrospinning, direct-writing and micro-nano machining technologies. Various manufacturing methods will provide the possibility for the design of various complex structures. (3) Applications in micro-nano devices. IFSS also need to adapt to the trend toward miniaturization, which is conducive to be integrated with other electric components. The standardized synthesis of materials and precision machining of devices is a prerequisites. The promising standardized manufacture of ionogel based sensing devices might be 3D printing, which is also suitable for manufacturing tiny devices. By virtue of photopolymerization, some ionogels might be able to be prepared and formed during photolithography manufacturing process so that can be integrated into micro-nano devices directly. Furthermore, it's worth noting that the research about the application of interfacial iontronic principle in micro-nano electronics have been conducted. (4) System integration. Current applications of IFSS are just demonstrated as single sensing components, i.e. there are lack of closed-loop application of smart system from sensing to feedback. As a crossed research field, electronic circuit design, data acquisition and processing, computer programming and software development will be of great benefit to the application of IFSS in commercial product.
In summary, as a new research direction in recent years, IFSS are attracting increasing attentions and extensive efforts are devoted from researchers. Although there are some aspects to be improved and intensively studied, we have reason to believe that IFSS will be an important part of future wearable and flexible electronics.