Elastomeric polymers for conductive layers of flexible sensors: Materials, fabrication, performance, and applications

In the wave of the Internet era created by computer and communication technology, flexible sensors play an important role in accurately collecting information owing to their excellent flexibility, ductility, freeform bending or folding, and versatile structural shapes. By endowing elastomeric polymers with conductivity, researchers have recently devoted extensive efforts toward developing high‐performance flexible sensors based on elastomeric conductive layers and exploring their potential applications in diverse fields ranging from project manufacturing to daily life. This review reports the recent advancements in elastomeric polymers used to make conductive layers, as well as the relationships between elastomeric polymers and the performance and application of flexible sensors are comprehensively summarized. First, the principles and methods for using elastomeric polymers to construct conductive layers are provided. Then, the fundamental design, unique properties, and underlying mechanisms in different flexible sensors (pressure/strain, temperature, humidity) and their related applications are revealed. Finally, this review concludes with a perspective on the challenges and future directions of high‐performance flexible sensors.


INTRODUCTION
With the development of Internet of things (IoT) and the prevalence of intelligent terminals, flexible electronic devices present a huge market potential. As early as 2004, the team of Takao Someya, an electrical engineer from the University of Tokyo, developed a flexible robot skin consisting of multilayer high-performance pressure sensing polyimide (PI), pentacene organic semiconductor, and multilayer gold and copper electrodes. [1] Even though it was wrapped around a 4-mm-thick cylindrical rod, it allowed an uninterrupted flow of electric current. Subsequently, Takao Someya, John A. Rogers, and Zhenan Bao et al. did many pioneering studies in the field of flexible electronics, which promoted the development of flexible electronics. [2][3][4][5][6][7] As one of the core components in flexible electronics, sensors influence the functional design and future development of wearable devices. Conventional sensor units mostly comprise singlecrystal silicon. Polycrystalline metal or metal oxide thin films exhibit high stability and sensitivity but are mismatched with the wearable device concept owing to the shortcomings of rigidity and excessive mass. Owing to advances in materials science and micro-/nanotechnology, flexible sensors have rapidly developed because of their unique features. Compared to traditional rigid sensors, flexible sensors cannot only sense surrounding stimuli and provide corresponding information, but also have good flexibility, ductility, bendability, as well as be arranged according to the application scene. [8][9][10] Flexible sensors can be classified into two main categories based on the way in which flexibility is achieved: structural and material. The structural flexibility of sensors is achieved by designing traditional material geometries such as fractal structures, [11] wave geometries, [12] serpentine, [13] and kirigami [14] shapes. Since novel materials and conduction mechanisms have been developed, the realization of flexible sensing from a material perspective is gradually attracting research attention. The materials used to construct flexible sensors include cellulose, [15] paper, [16] PI, [17] polyethylene terephthalate (PET), [18] and elastomeric polymers. [19] Elastomeric polymers have started to emerge in the field of Aggregate. 2023;4:e319.
wileyonlinelibrary.com/journal/agt2 1 of 34 https://doi.org/10.1002/agt2. 319 flexible sensors in recent years due to their excellent stretchability and durability. With continuous in-depth research, researchers have achieved breakthroughs in this field, achieving key features such as light weight, ultrathinness, superior flexibility, high sensitivity, and rapid response of flexible sensors based on elastomeric polymers. [20][21][22] Emulating working principle of biological skin, the basic theoretical underpinning of flexible sensor research is the use of different conduction methods to convert external stimuli into electrical signals, thus enabling the visual transmission and recording of external signals. So, the conductive layer is the crucial component of the flexible sensor, which quantifies the environmental stimuli (pressure/strain, temperature, humidity) in the form of electrical signals, giving the flexible sensor ability to sense the environment. Therefore, achieving superior quality elastomeric conductive layers is the core of manufacturing flexible sensors. There are three main categories of elastomeric polymers for making conductive layers: traditional elastomers, hydrogels, and ionogels. Owing to their inherent insulating capabilities, traditional elastomers require the addition of conductive fillers such as nanometals (e.g., silver nanowires/nanoparticles [AgNWs/AgNPs], copper nanowires (CuNWs), gold nanosheets), [23][24][25] carbonaceous fillers (e.g., carbon nanotube [CNT], carbon black [CB], graphene [GR], reduced graphene oxide [rGO]), [26][27][28][29] MXene, [30] and intrinsically conductive polymers (e.g., poly (3,4-ethylenedioxythiophene) :poly(styrenesulfonate) [PEDOT:PSS], polypyrrole [PPy], and polyaniline [PANI]) [31][32][33] to achieve conductivity. Apart from adding conductive fillers, [34] hydrogels can also achieve ionic conductivity by introducing a compositing hydrophilic matrix with ionic pendant groups or electrolytes. [35] Ionogels exhibit good conductive properties because they are inherently rich with ionic liquids (ILs). [36][37][38] Elastomeric polymers can form good interaction with conductive fillers/electrolytes (covalent/noncovalent bonds). More importantly, they facilitate the design and optimization of three-dimensional (3D) networks of polymers on an extremely large scale, thereby tuning the electrochemical performances, mechanical properties, and biofunctionalities of elastomeric polymers. Therefore, the conductive layer made using these elastomeric polymers can be designed and fabricated to obtain high-performance flexible sensors with various functions. The interaction of elastomeric polymers with the conductive fillers/electrolytes and the relevant properties of the flexible sensors are shown in Table 1.
Although other reviews have described flexible sensors based on elastomeric polymers, [39][40][41][42] they lack a systematic generalization and summarization. Unlike other reviews, this review highlights how elastomeric polymers (traditional elastomers, hydrogels, and ionogels) can be made into conductive layers with excellent mechanical and electrical properties. Further, it reveals the role of elastomeric conductive layers in providing sensors with high flexibility and performance. In Chapter 2, we provide an overview of the preparation of conductive layers based on elastomeric polymers, particularly discussing the unique advantages of each elastomeric polymer and the excellent performance of the corresponding conductive layer. In Chapter 3, we discuss the use of elastomeric conductive layers as active layers or electrodes, which are designed and fabricated to achieve various types of high-performance flexible sensors. Chapter 4 outlines the significant potential of flexible sensors for applications in fields such as intelligent robotics, human-machine interfaces (HMIs), motion detection, and health care. Finally, personal perspectives are provided on the challenges and future developments in this field. The main structure of our review is shown in Figure 1.

ELASTOMERIC POLYMERS FOR CONDUCTIVE LAYERS
The conductive layer is the core component of the sensor, which converts and transmits signals. Therefore, forming conductive layers using an elastomeric polymer-based system is an important step in achieving flexible sensing. Elastomeric polymers generally need to be cross-linked to obtain elasticity, and the elastic modulus can be adjusted by controlling the degree of cross-linking. The cross-linked molecular chains of elastomeric polymers make them recoverable after tension, torsion, or compression. Elastomeric polymers for making conductive layers include traditional elastomers, hydrogels, and ionogels. [43][44][45]

Traditional elastomer composites
Traditional elastomers such as poly(dimethylsiloxane) (PDMS), polyurethane (PU), and styrene ethylene butylene styrene (SEBS) are widely used in flexible sensors. However, traditional elastomers are insulators that must be composited with conductive fillers to be used as the conductive layer for flexible sensors. The methods of preparing the conductive layer for traditional elastomers include two main types: homogeneous dispersion and heterogeneous assembly methods. [46][47][48] 2.1.1 PDMS PDMS is a silicon elastomer with good elasticity, high transparency, chemical and thermal stability, simple processing steps, and biocompatibility with human tissues and cells. Therefore, it can used in sensors as matrix material and provides sensors with flexible property. When introducing PDMS, a mixture of PDMS prepolymer and curing agent is generally used to satisfy the requirements of different shapes and conditions. [49][50][51] The heterogeneous assembly method of conductive layers involves transferring or coating conductive materials onto PDMS. An important advantage of the heterogeneous assembly method is that the conductive materials and elastomers are deposited separately, and the selection of conductive material is not limited by miscibility. The conductive materials are usually dissolved in organic solvents to form a homogeneous solution, which is uniformly deposited on the PDMS surface, then the solvent is evaporated to obtain a flexible conductor. There is a variety of solution-based deposition methods, including spray-coating, [7,52,53] drop-casting, [54] spin-coating, [55] and dip-coating. [56][57][58] Spray-coating is widely used because of its simple and convenient characteristics. Cheng et al. [52] applied a thermal spray-coating method Acid-treated CNTs/AgNP are assembled on the TPU nanofiber surface by H-bonding and ultrasonication.
-Sensitivity of strain is 3.42 (0-50%), 4.77 (50-100%), 5.82 (100-180%) -Stability of 250 cycles [394] CNT/CNF/borax /PVA hydrogel -Ideal electroconductivity (10.0 S/m) -Fast self-healing (<20 s) at room temperature -Strain sensitivity (∼3) [203] PPy/Fe 3+ /PVA hydrogel PPy polymerized on the Fe 3+ area embedded in the PVA aggresome to form the 3D network.  [350] to uniformly deposit the monolayer MXene nanosheets on microstructured PDMS using van der Waals forces ( Figure 2A). As pressure increases, the bionic microspines effectively increase the contact area of the conductive channels, thus causing an increase in the current. For the drop-casting method, Xu et al. [59] dropped the solution of MXene particles and MXene nanosheets on the prepared rGO/PDMS substrate ( Figure 2B). After evaporation of the droplets, the hybrid nanomaterials were uniformly deposited on the PDMS substrate. Flexible sensors assembled based on this conductive layer exhibiting robust mechanical stability in the presence of external stimuli. The advantage of spin-coating is that it produces uniform layers, and the thickness can be controlled by the rotation rate. Ramendra et al. [60] spin-coated the photoreactive conductive ink comprises the PEDOT:PSS within a carrier protein matrix on the methacrylate-functionalized PDMS ( Figure 2C), and the ink was then exposed to ultraviolet (UV) light through a photomask to cross-link the exposed areas. Consequently, this covalent attachment enables the realization of mechanically robust, flexible sensors that can function in liquid environments. Besides, PDMS can be directly immersed in the solution of conductive material, which can omit the spraying step and simplify the experimental process. Kim et al. [56] obtained CNT network-coated thin porous PDMS sponges (CCPPS) by dip-coating porous PDMS sponges using the sugar template in CNT-isopropanol dispersion and repeatedly squeezing for CNT coating on the surfaces of the PDMS backbone ( Figure 2D). Electrical percolation paths along the 3D-interconnected CNT networks formed upon CCPPS and the resistance was ∼3 kΩ. In a high-resolution array configuration, CCPPS exhibits a negligible barrel-like phenomenon, which prevents electrical and physical interference among adjacent sensors.
Although deposition of the conductive material solution on the PDMS surface is simple and efficient, the interface adhesion is sometimes weak. To resolve the interface problem between the conductive material and the PDMS film, the conductive material solution was spray-coated onto the surface of the mold, not the PDMS film. The flexible conductive layer can be obtained by spraying the conductive material on the microstructured mold, casting the PDMS precursor mixture, thermally curing, and carefully peeling off from the mold. [61][62][63] To fabricate a spider web-like stretchable electrode (SET), Zhao et al. [64] spray-coated AgNW/CNT nanocomposite conductive ink onto a spider web-like template, which was placed on a glass plate. The PDMS precursor was then spin-coated onto the ink-coated glass and cured. Finally, the PDMS film containing the spider web-like SET was stripped ( Figure 2E). The linewidth of the spider web-like SET was 500 μm, and the total weight of the Ag NWs/CNTs was 15 mg. The resistance of the SET was approximately 42 Ω when the weight ratio of Ag NWs:CNTs was 3:1.
A uniform mixture solution of conductive material and PDMS precursor was prepared using the homogeneous dispersion method. The conductive material is completely mixed with PDMS precursor and maintains a good binding force with the PDMS matrix after curing, thus protecting the network from strain-/pressure-induced disconnection. [70][71][72][73] Lee et al. [74] dissolved CB NPs and PDMS precursors in toluene, and then mixed and cured them to obtain CB/PDMS (C-PDMS) composites. The CB NPs are uniformly distributed in the PDMS matrix to enable the electric conductivity of C-PDMS to reach 7.54 S/m ( Figure 2H). The resistance of C-PDMS responds easily to a variety of pressure/strain and thermal stimuli, which demonstrates its potential for applications in multifunctional flexible sensors. In addition, the rGO/PDMS, [73] multiwalled carbon nanotube (MWCNT)/PDMS, [46] and MXene/PDMS [75] elastomeric composites produced by homogeneous dispersion method were investigated, also exhibiting high stretchability and electrical conductivity.  [52] Copyright 2020, American Chemical Society. (B) Drop-casting MXene particles and nanosheets on rGO/PDMS. Reproduced with permission. [59] Copyright 2022, American Chemical Society. (C) Spin-coating photoreactive conductive ink on PDMS. [60] Copyright 2018, Elsevier. (D) Dip-coating fabrication procedures of the CNTcoated porous PDMS sponges. Reproduced with permission. [56] Copyright 2019, American Chemical Society. (E) Spray-coated AgNW/CNT nanocomposite conductive ink on glass and spin-coated PDMS precursor. Reproduced with permission. [64] Copyright 2021, American Chemical Society. (F) 3D interconnected GNP-w-CNT infiltrated with PDMS. Reproduced with permission. [66] Copyright 2021, John Wiley and Sons. (G) The anGPF aerogel filled with PDMS. Reproduced with permission. [69] Copyright 2021, Elsevier. (H) CB uniformly distributed in the PDMS matrix using the homogeneous dispersion method. Reproduced with permission. [74] Copyright 2020, John Wiley and Sons 2.1.2 PU PU elastomers are a class of elastomers with repeating carbamate (-NHCOO-) groups on the main chain obtained by reacting a dimeric or polyol with a dimeric or poly(isocyanate). It can used to make conductive layers for flexible sensors because of its outstanding mechanical, physical, and chemical properties as well as its excellent biocompatibility. [76][77][78] Liu et al. [77] prepared an elastomeric conductive sponge via the dilute chemical polymerization of aniline to grow conductive PANI nanowires (PANINWs) on the surface of the PU sponge.
Owing to the conductivity of PANINWs, this conductive PU sponge can function as both the triboelectric layer and electrode. Furthermore, PU has a wide range of structures and properties owing to the extensive chemical components that can be used in their synthesis, such as waterborne polyurethane (WPU), [79,80] polysiloxane-dimethylglyoximebased polyurethane (PDPU), [81] polybutadiene-based poly(urea-urethane) (PBPUU). [82] Thermoplastic polyurethane (TPU) is a very important branch of PU elastomers that is widely used in making conductive layers of flexible sensors. [83] From the microstructure perspective, TPU has a unique microphase structure of hard segment (formed by the aggregation of urethane groups) and soft segment (formed by the aggregation of nonpolar ether and ester groups), which makes it not only possess the flexibility of elastomers, but also the rigidity of plastic, exhibiting excellent macroscopic physical and processing properties. It is worth noting that the flexible sensor made by combining TPU and conductive material incorporates the advantages of these two elements, exhibiting excellent stretchability, high sensitivity, and wide sensing range. [84,85] Recently, electrospinning has become an effective, simple, and scalable technique for preparing micro-/nanofibers. TPU is easily spun electrostatically into fiber films, which have the advantages of good flexibility, good tensile properties, high porosity, and large specific surface area. To prepare TPU fibers, TPU microspheres are first dispersed into a mixture of organic solvent to produce a homogeneous TPU solution; the homogeneous solution is then transferred into a plastic syringe with a metal needle connected to a high-voltage power supply unit. A layer of aluminum foil is wrapped around a high-speed rotating cylinder to collect the TPU fiber/mat. Finally, TPU fibers are overlapped with each other by joint points, developing an excellent 3D porous network. [86] Also, customized patterning of electrospun fibers can be achieved by using a specially designed collector comprising patterned electrodes. [87] However, the original TPU fibers are not conductive and cannot be individually used as sensing materials. To tightly bind the conductive filler and TPU fiber, the method of dip-coating followed by ultrasonication is preferred. [88,89] The ultrasound produces a strong energy, which can anchor the conductive filler to the TPU fiber, thus forming a well-bound force. [90] Wang et al. [91] reported a 3D conductive network by using rGO to decorate the flexible TPU electrospun mat through ultrasonication. Benefiting from the special 3D network structure and excellent flexibility of the TPU fibrous network, the rGO/TPU mat exhibits high specific elongation (670 ± 20%) and electrical resistivity of 5.0 ± 0.5 Ω m. In addition, vacuum filtered [92,93] and in-situ polymerization/reduction [94,95] methods can also endow TPU fibers with electrical conductivity. Dong et al. [92] constructed a MXene/MWCNT/TPU composite conductive network by utilizing the porous TPU fiber mat (fabricated by electrospinning) as a skeleton to assemble the conductive fillers through a step-by-step vacuum filtration method ( Figure 3A). The MXene or CNT suspension was pumped into the TPU fiber mat until almost all the conductive fillers reached the bottom, adequately and uniformly covering the bottom surface of the TPU fiber mat. Zhai et al. [95] constructed a layer of conductive network by anchoring PANI nanoparticles (PANINPs) on the surface of electrospun TPU films through in situ polymerization; the formation of fibrous conductive channels endow the PANINP/TPU film with good conductivity.
Compared with electrostatic spinning, wet spinning can be spun by mixing TPU and conductive materials into a homogeneous solution, but the wet spinning stretch ratio is smaller, and the refinement is primarily achieved through small spinning pinholes and solvent removal from the solidification bath. [96][97][98] Qu et al. [96] fabricated a highly stretchable coresheath fiber with TPU as the core and CNT/TPU as the sheath using a coaxial wet spinning technique ( Figure 3B). The interpenetration between the TPU and CNT/TPU formed an interfacial conductive network, which enables the fiber to maintain a conductive path, even at a high elongation of 500%. In addition, owing to the existence of the interfacial conductive network, the core-sheath structure formed through wet spinning technology has stronger adhesion between the core and sheath layer, which ensures that the fiber has high durability.
To further improve the performance of TPU, it can be modified with PDMS [99,100] or cellulose nanocrystals (CNCs). [101,102] Gao et al. [100] used PDMS-decorated TPU nanofibers after conductive materials formed on the surface; the stretchable conductor became superhydrophobic and anticorrosive. When conductive TPU fibers are used in strain sensors, the surface of the fibers is coated with CNC to induce microcrack structures during the stretching process, thereby greatly increasing the sensitivity of the sensor. Based on this principle, Liu et al. [102] successfully developed an adjustable wearable strain sensor by coating CNC and GR on TPU fibers, then dip-coating in Hf-SiO 2 /ethanol dispersion, which enabled the sensor to obtain superhydrophobicity, high sensitivity, and wide sensing range.
PU has unique characteristics such as diverse chemical structures and easy modification over against PDMS. Hence, PU can be more easily chemically modified to achieve selfhealing properties. The self-healing mechanism of PU can be classified as reversible covalent binding (e.g., disulfide bonds and Diels-Alder [DA] bonds) [103][104][105][106][107] and noncovalent binding (e.g., hydrogen bonds and metal coordination bonds). [108][109][110][111][112][113] To enrich the self-healing function of PU, Yeh et al. [106] designed and synthesized a dual-mode selfhealing PU-based elastomer by combining the advantages of stronger reversible DA bonds and weaker disulfide bonds with faster dynamic exchange. The elastomer backbone introduced polypropylene glycol (PPG) into the PU segment and used cystamine (CYS) as a cross-linker to form PU-CYS. To verify the healing performance of the PU-CYS film, cracks were induced on a glass substrate, and cracks <30 μm wide in the PU-CYS film could heal within 5 min at 60 • C due to the presence of DA and disulfide bonds; the healing mechanism is shown in Figure 3C. And Zhang et al. [112] constructed a covalently cross-linked PU elastomer (Cu-DOU-CPU) with triple dynamic bonds (DOU covalent bond, Cu-DOU ligand bond, and hydrogen bond) by forming Cudimethylglyoxime-urethane (Cu-DOU) complexes in PU. During mechanical deformation, the dissociation and recombination of the weaker bonds (hydrogen and Cu-DOU) can significantly dissipate energy, resulting in higher toughness. The stronger covalent bonds ensure the structural integrity and stable mechanical properties. Meanwhile, the Cu coordination facilitates the exchange reaction of DOU groups, which facilitates self-healing after damage and eventually mechanical repair, as shown in Figure 3D. A self-healing and stretchable conductor is constructed by coating Cu-DOU-CPU with gallium-indium-tin eutectic, which shows considerable potential for the next generation of flexible sensors.

SEBS
The SEBS copolymer was obtained by hydrogenation of styrene-butadiene-styrene (SBS), which converts unsaturated butadiene into saturated carbon-carbon bonds. The biocompatibility, heat resistance, and UV radiation resistance of the SEBS copolymer were greatly improved compared to SBS. [114,115] Owing to the ultralarge stretching range, good mechanical performance, excellent antioxidative activity, and high thermal stability, SEBS shows unique superiority in elastomeric conductive layers. [116,117] Similar to TPU, SEBS is also an inherently stretchable thermoplastic elastomer, so SEBS-based elastomeric conductive layers can be fabricated by electrospinning or wet-spinning. [118,119] For example, Zeng et al. [118] obtained a CNT/SEBS hybrid fiber by the scalable wet-spinning method. Owing to the strong interaction and good interfacial compatibility between CNTs and SEBS, CNTs were uniformly dispersed in the SEBS matrix and no significant aggregation was observed. Under the action of the shear flow field, majority of the CNTs in the fibers are oriented along the fiber axis, which is beneficial to the fiber strength (36 MPa) and electrical conductivity (30.2 S/m).
In particular, there are strong interfacial interactions between carbon nanomaterials (rGO, GR, CNT) [120][121][122][123][124][125] and SEBS due to π-π interactions among the carbon nanomaterials and phenyl group of SEBS. Xu et al. [126] prepared the MWCNT-reinforced SEBS substrates via the electrostatic spinning technique, and subsequently cut the SEBS/MWCNT membranes into squares and anchored the octadecyltrichlorosilane (OTS)-modified superhydrophobic MWCNTs on the SEBS membranes by sonication ( Figure 3E). The synergistic and strong π-π interactions between OTS-MWCNTs and SEBS membranes form a highly conductive hierarchical structure on porous substrates, which results in high F I G U R E 3 Fabrication and microstructure of conductive layers made by PU, SEBS, and other elastomers. (A) Electrospinning and vacuum filtration method for fabricating the MXene/MWCNT/TPU composite film. Reproduced with permission. [92] Copyright 2022, American Chemical Society. (B) Wet spinning process for MWCNT/TPU fibers. Reproduced with permission. [98] Copyright 2019, Royal Society of Chemistry. (C) Illustration of the healing concept relating to the disulfide and DA reversible dynamic bonds in PU-CYS. Reproduced with permission. [106] Copyright 2022, Royal Society of Chemistry. (D) Cu-DOU-CPU elastomer structure, self-healing DOU/Cu coordination and hydrogel bonds. Reproduced with permission. [112] Copyright 2019, John Wiley and Sons. (E) π-π interactions between OTS-MWCNTs and SEBS. Reproduced with permission. [126] Copyright 2022, American Chemical Society. (F) Combing modified AgNPs/SEBS superhydrophobic coatings on the NR substrate. Reproduced with permission. [127] Copyright 2018, American Chemical Society. (G) Schematic diagram of the internal structure of flexible XSBR/SSCNT layer. Reproduced with permission. [140] Copyright 2022, John Wiley and Sons. (H) Structural diagram of the PEDOT:PSS/NR films through tensile and release. Reproduced with permission. [136] Copyright 2021, American Chemical Society conductivity and stability of the MWCNT/SEBS conductive layer.
As SEBS comprises C/H elements and no polar groups, the choice of SEBS can endow the conductive layer with highly hydrophobicity. [127,128] Su et al. [127] combined the modified AgNPs with SEBS on a prestretched natural rubber (NR) substrate and then acquired a highly stretchable and conductive superhydrophobic coating ( Figure 3F). Modified AgNPs embedded in SEBS can construct charge-conducting pathways in the coating layer, forming a rough structure on the coating surface. Hence, the obtained coating exhibits high conductivity with a resistance of ∼10 Ω and superhydrophobicity with a water contact angle greater than 160 • .

Others
Other traditional elastomers such as carboxyl styrenebutadiene rubber (XSBR), [129][130][131] ecoflex, [132][133][134] nature rubber (NR), [135][136][137] and carboxyl nitrile-butadiene rubber (XNBR) [138,139] can also be used for the fabrication of elastomeric conductive layers and show potential in flexible sensors. Lin et al. [140] designed a hydrogen bond crosslinked network based on XSBR and hydrophilic sericin noncovalently modified CNT (SSCNT). Given the small amount of SSCNTs forming a hydrogen-bonded crosslinked network with the polar carboxyl groups of XSBR ( Figure 3G), the XSBR/SSCNT composite exhibits high stretchability (up to 217%), superior strength (12.58 MPa), high conductivity (0.071 S/m), and lower percolation threshold (0.504 wt%). Yang et al. [136] obtained a free-standing and stretchable film by blending PEDOT:PSS with NR latex ( Figure 3H). Polar substances such as aliphatic acids and proteins can adsorbed on the surface of NR particles, which enables interaction with polar PSS. The segregated structure established by NR contributes to the formation of the PEDOT:PSS conductive network. Therefore, with proper secondary doping treatment, PEDOT:PSS/NR composite films exhibit good electrical conductivity (87 S/cm) and tensile properties (490%). In the future, more traditional elastomers will be discovered for new flexible sensors, so that both the function and performance of the sensor can be improved.

Conductive hydrogels
Conductive hydrogels excel among various types of conductive polymer materials due to their high electrical conductivity, ease of synthesis, high specific surface area, and excellent flexibility. Unlike traditional elastomers, conductive hydrogels can achieve conductivity not only by adding conductive fillers, but also by adding free moving ions to the elastic 3D hydrogel networks to achieve ionic conductivity. [141][142][143] Due to the large number of reversible physical cross-linked networks are formed in hydrogels through hydrogen bonding and electrostatic interactions, the conductive hydrogels can also possess a very good self-healing property. [144][145][146] This subsection mainly introduces the conductive hydrogels formed by polyacrylamide (PAM), PVA, and polyacrylic acid (PAA).

PAM
PAM is a water-soluble polymer with many amide groups in its structure, which is widely used to construct the hydrogel matrix due to its good biocompatibility, hydrophilicity, and nontoxicity. [147,148] However, the conventional PAM has low strength, poor temperature resistance, and low conductivity. Thus, researchers have prepared high-performance conductive PAM hydrogels by introducing the double network (DN) structure, free ions, and nanofillers, which can be applied in flexible sensors.
Owing to the interaction between two polymer networks, DN hydrogels comprising two polymer networks with different physical properties exhibit enhanced mechanical strength and self-healing ability compared with their single network counterparts. The DN hydrogel consists of a first network that dissipates energy by breaking reversible noncovalent bonds and a second network that maintains network resilience. Gelatin [149,150] exhibits excellent biocompatibility and increases PAM flexibility. Zhu et al. [149] introduced a gelatin/PAM-clay composite hydrogel network penetrated with Zn 2+ and glycerol (Gly) ( Figure 4A). In this gel network, polymer chains and Gly molecules are coupled to water molecules through hydrogen bonds, and Zn 2+ ions are attached to water molecules and polymer chains through electrostatic interactions. Thus, the resulting gel exhibited high conductivity (0.60 S/m) and stretchability (>500%). Alginate [151][152][153] is a biodegradable and biocompatible natural polyelectrolyte that not only improves the mechanical properties of PAM hydrogels, but also provides large amounts of conductive ions. Qiao et al. [153] incorporated sodium alginate (SA) and tannic acid (TA) into the PAM hydrogel network, and the electrical conductivity of the hydrogel is significantly increased by the presence of SA, making it an ideal ionic conductor. TA, SA, and PAM can form multiple reversible weak hydrogen bonds with each other, resulting in greater energy dissipation during the stretching process, thus imparting high ductility, low stiffness, and high elasticity to the hydrogel. Cellulose nanofibril (CNF) [154,155] possesses excellent biocompatibility, renewability, and outstanding elasticity, which attracts increasing attention as a reinforcing filler. Yu et al. [154] incorporated CNF, which functions as a dynamic connected bridge, to endow the PAM with a hierarchical honeycomb-like cellular structure, leading to significant mechanical strengthening ( Figure 4B). CNF can also enhance the ionic conductivity of the PAM hydrogel due to negative carboxylate groups on the surface. Carboxymethyl chitosan (CMC) is an amphoteric polyelectrolyte with nontoxic and biocompatible properties. [156] However, under the combined effect of hydrophobicity, hydrogen bonding, and electrostatic repulsion, CMC tends to aggregate in acidic aqueous solutions. Thus, Ding et al. [157] transformed swollen and soft PAM/CMC hydrogels into strong and tough gels by the hydrolyzed process in acid aqueous solution with additional heating. The hydrolyzed process makes PAM/CMC hydrogels more compact and robust by increasing the physical interactions and chemical cross-links inside the gels. In addition to introducing the natural second network structure in PAM hydrogels, it is also advisable to introduce polyethyleneimine, [158] silica nanofibers, [159] or ultrahigh molecular weight polyethylene, [160] which serves to enhance the hydrogel matrix. Gly and Zn 2+ . Reproduced with permission. [149] Copyright 2020, American Chemical Society. (B) Formation of hydrogen bonds between water and CNF/PAM. Reproduced with permission. [154] Copyright 2022, American Chemical Society. (C) Ionic interaction between c-MWCNT and poly(acrylamideco-2-aminoethyl acrylamide hydrochloride) (P(AM-co-AEMA)) hydrogels. Reproduced with permission. [166] Copyright 2020, Royal Society of Chemistry. (D) Undeformed and deformed state of 30 wt% PAM hydrogel at −15 • C and the conductivity as a function of temperature for the 0 wt%, 10 wt%, and 30 wt% CaCl 2 hydrogels. Reproduced with permission. [174] Copyright 2018, John Wiley and Sons. (E) Scheme showing the preparation of conductive poly(NIPAM-coβ-CD)/CNT/PPy hydrogels and temperature-dependent swelling ratios of hydrogels. Reproduced with permission. [185] Copyright 2018, American Chemical Society. (F) PA/PVA hydrogel prepared by FT cycle and its flexible, transparence properties. Reproduced with permission. [191] Copyright 2022, Elsevier. (G) Preparation of PANI/PVA-based conducting hydrogels by ITLP. Reproduced with permission. [198] Copyright 2020, Cell Press. (H) Multicomplexation between PVA and CNT-CNF nanohybrids dynamically cross-linked in the presence of borax. Reproduced with permission. [203] Copyright 2019, Elsevier Conductive fillers such as rGO, [161] MXene, [162] or conductive polymers [163,164] can also be incorporated into PAM hydrogels as both conductive and reinforcing roles. However, due to their own strong van der Waals forces and inherent hydrophobicity, conductive fillers tend to agglomerate into bundles or entangle in the hydrogel network, which greatly diminishes their functionality. One way to address this problem is to modify the filler, such as carboxyl-functionalized MWCNT (c-MWCNT). Because of the abundant carboxyl functional groups, c-MWCNT can be well dispersed in PAM-based hydrogels through ionic interaction ( Figure 4C). [165,166] Another method is to introduce a third substance. Sun et al. [167] employed montmorillonite (MMT) as a green dispersant and stabilizer of CNTs in the aqueous solution, which can integrally retain high conductivity of CNTs. Lu et al. [168] revealed that the incorporation of 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers (TOCNs) could toughen the PAM hydrogel and effectively facilitate the dispersion of CNTs in the gel network. Polydopamine (PDA) is considered an exceptional material for improving the hydrophilicity of hydrophobic substrates, providing ideas for functionalization studies to overcome the poor dispersion of hydrophobic conducting nanofillers/polymers in water. [169,170] Hence, Xie et al. [171] obtained PDA-rGO by polymerizing the DA monomer and reducing the GO in an alkaline environment. Owing to the good dispersing effect of PDA on rGO, there are many sheet-like structures firmly attached on the walls of pores in PAM/SA hydrogel, and more microfibrils both on the pore walls and inside the pores bridging the adjacent pore walls, which may prove favorable for the mechanical strength (143.2 kPa) and conductivity (19.39 mS/cm) of the hydrogel.
Regardless of the attractive merits of hydrogels, conventional hydrogels are prone to dehydration in arid environments and freezing at low temperatures, severely limiting the scope of application of the flexible sensors. To overcome these challenges, hygroscopic salts, phytic acid (PA), or polyols are incorporated in hydrogels to enhance the resistance to dryness and depress the freezing point of hydrogels. [171][172][173] Hygroscopic salts can affect the formation of hydrogen bonds among water molecules to reduce the freezing point and impart ionic conductivity to hydrogels. Morelle et al. [174] synthesized a series of PAM-alginate DN hydrogels and immersed them in CaCl 2 solutions with different concentrations. In addition to exhibiting high ductility and toughness at low temperatures, the hydrogels containing 30 wt% CaCl 2 maintained optimum ionic conductivity below 0 • C ( Figure 4D). PA with six phosphate groups has abundant hydrogen bonding to the acceptor site, which increases the possibility of PA binding to water through hydrogen bonding, thus preventing the crystallization and evaporation of water in the hydrogels. [175] Zhang et al. [176] constructed a conductive PAM-based hydrogel using H 2 O/PA binary solvent. Benefiting from the strong hydrogen bonding between PA and H 2 O molecules, the hydrogel exhibited a mass retention of 82% after 15 days in ambient environments and maintained large stretchability (1266%) and excellent conductivity (0.041 S/cm), even at −20 • C. Polyols have low freezing point and can form strong hydrogen bonding between water molecules, so as to improve the frost resistance of hydrogels. Wu et al. [177][178][179] introduced Gly and ethylene glycol (EG) into the hydrogel network to prepare the antifreezing and antidrying PAM/carrageenan DN hydrogel. Owing to the strong hydrogen bonding between water and Gly and EG molecules, the organic hydrogels can maintain their deformability, self-healing ability, and electrical conductivity at low temperature.
As a branch of PAM, poly(N-isopropyl acrylamide) (PNIPAM) [180][181][182][183][184] is a type of thermosensitive material. PNIPAM has a lower critical solution temperature (LCST) feature, where the intermolecular forces between PNIPAM chains and water molecules become sufficiently strong to increase the volume of the hydrogel (swelling) by water absorption when the temperature drops below the LCST. However, when the temperature increases above the LCST, the interaction between the polymer and the solvent weakens, resulting in a decrease in the hydrogel volume (deswelling) due to the desorption of water. Deng et al. [185] designed conductive hydrogels with superior mechanical and selfhealing properties using acryloyl-β-cyclodextrin (AC-β-CD), NIPAM, CNT, and PPy ( Figure 4E). This poly(NIPAM-coβ-CD) hydrogel displayed a high conductivity of 34.93 S/m due to the macroporous structure with an interconnected network of CNT and PPy nanoaggregates in the matrix. Based on the properties of PNIPAM, the swelling ratio of the hybrid hydrogels at 25-50 • C is 14.9-23.0, which demonstrates the possibility of its application in temperature sensors.

PVA
The PVA hydrogel contains numerous hydroxyl groups in the molecular chain; hence, it is a hydrophilic polymer with good biocompatibility and excellent biodegradability. And PVA can be induced by intensive hydrogen bonding interactions to form crystalline structural domains; these crystalline regions can provide matrices with abundant physical cross-linking points, thus making PVA a good polymer backbone material for the preparation of high-strength hydrogels. [186][187][188] According to this feature, the preparation of PVA hydrogels can be cross-linked by freeze-thaw (FT) cycles to form physical crystals, which is simpler than chemical and radiation cross-linking to improve the mechanical properties and expand the application of PVA hydrogels in flexible sensors. [189][190][191] Yang et al. [191] designed a PVA/PA conductive hydrogel by pouring the solution into a mold and freezing at −20 • C for 12 h, followed by thawing at room temperature 3 h. During the FT cycle, the mixed solution is transformed into a PVA/PA hydrogel with stretchable and transparent performance. Molecular chain entanglement and the phase separation of PVA form physical crosslinkages, which can enhance the role of the hydrogel matrix ( Figure 4F). Conversely, Liu et al. [192] revealed that the physically cross-linked PVA hydrogels in the IL/H 2 O binary solvent system exhibited better mechanical properties and transparency than the traditional PVA hydrogel prepared by the FT method due to the formation of homogeneous and small PVA microcrystals. Similar to PAM, when introduced nanofillers such as CNT, [193] GR [194] or PEDOT:PSS, [195] PANI, [196] and PPy [197] can be introduced into PVA hydrogel to reinforce the matrix and enhance conductivity, the phase instability and poor miscibility of nanofillers in PVA hydrogel matrix tend to hinder the overall performance, which affects the formation of conductive percolative networks. Zhao et al. [198] addressed this challenge with an ice-templated, low-temperature polymerization (ITLP) strategy and created stretchable conducting PANI/PVA hydrogels. The ice-templated gel (ItG) PANI/PVA exhibits a hierarchical network by assembling interconnected, uniform nanofibrils to microsheets with a dendritic structure ( Figure 4G). Ultralow-temperature templating of the high-hydrophilic solution created a unique continuous dendrite micronetwork, whereas low-temperature polymerization effectively suppressed undesirable aggregation and yielded uniform nanofibers. Compared with the hydrogel obtained by conventional liquid-phase polymerization, the toughness is enhanced by 29 times and the electrical conductivity is improved by 83 times due to the hierarchical dendritic microstructure with mitigated nanoaggregation of the hydrogel.
Since PVA contains substantial hydroxyl groups, it is facile to form reversible borate ester bonds with borax (Na 2 B 4 O 7 ⋅10H 2 O) to strengthen the matrix and endow the hydrogel with self-healing ability. Borax dissolves rapidly in water and decomposes into trigonal planar boric acid (B(OH) 3 ) and tetrahedral borate ions (B(OH) 4 − ). The bonding force between B(OH) 4 − and PVA is relatively stronger than the those of the individual single components, thus forming dynamic cross-linking bonds in PVA. [193,199,200] While B(OH) 4 − ions interact with PVA chains, they can also form reversible complexes with additives such as MXene, [201,202] CNF-CNT, [203] chitosan, [204] and silk fiber, [205] which act as bridged PVA and additives, thus improving their dispersion in PVA. Han et al. [203] reported an electroconductive hydrogel based on a PVA-borax hydrogel and CNT-CNF nanohybrids that combine the conductivity of CNTs and template function of CNFs ( Figure 4H

PAA
Compared with PAM and PVA, the PAA hydrogel is rich in carboxyl groups, which can form strong hydrogen bonding interactions with water molecules; the gel material has a high swelling rate in water. [206][207][208] Feig et al. [209] fabricated conductive interpenetrating networks by infiltrating loosely cross-linked PEDOT:PSS into PAA hydrogels to form a secondary polymer network. The low concentration of PEDOT:PSS enables the secondary polymer network to have excellent conductivity (>10 S/m) and stretchability (>100%). Meanwhile, the introduction of the second network by PAA is often accompanied by the addition of some divalent or trivalent cations such as Fe 3+ , [210][211][212] Al 3+ , [213][214][215] or Ca 2+ . [216,217] These cations cannot only form a dynamic cross-linked network with the COO − ions by PAA, but also act as ionic conductors. For example, Lei et al. [218] slowly injected a mixture of Na 2 CO 3 and SA into a solution of CaCl 2 and PAA and stirred it to form a white viscous hydrogel. Among them, amorphous CaCO 3 particles with particle sizes less than 3 nm were physically cross-linked by PAA and alginate chains due to the chelation of carboxyl groups with Ca 2+ . Moreover, because the slightly dissolved free Ca 2+ in CaCO 3 can function as an ion carrier, it can be used as the flexible electrode of the capacitive stretchable sensor without adding any other salt solute. Shao et al. [219] prepared hydrogels by constructing synergistic interfacial dynamic coordination bonds of TA-coated CNCs (TA-CNCs) and PAA chains with metal ions Al 3+ in a covalent polymer network ( Figure 5A). Possible coordination modes among TA-CNC, PAA, and Al 3+ are (I) metal-phenolic coordination between TA-CNCs, (II) metal-carboxylate coordination between PAA chains, and (III) hybrid bridging between the TA-CNCs and PAA chains. These multiple reversible coordination bonds in the PAA-based hydrogel contribute to the remarkable mechanical properties involving ultrastretchability (2952%), high compression performance (95% strain without fracture), toughness (5.60 MJ/m 3 ), and good adhesive properties. Jing et al. [220] incorporated dopamine-rGO and Fe 3+ into the PAA network to form PAA/PDA/rGO/Fe 3+ hydrogels. In this hydrogel, the covalent bonds in PAA provided a strong and stable network, and the reversible ionic interactions between the carboxylic groups of PAA and Fe 3+ , the strong coordination bonds between PDA attached to rGO and Fe 3+ , as well as the hydrogen bonding between rGO and PAA, contributed to its self-healing properties. In the hydrogel solution, the dopamine-modified rGO exhibited good dispersion, which not only reinforced the hydrogel network, but also formed an effective conductive pathway for the hydrogel system. Recently, liquid metal Ga was observed to activate the rapid gelation of the PAA hydrogel, and the Ga 3+ from liquid metal under acidic conditions also functioned as cross-linking sites with the PAA chain, which provides a new idea for PAA cross-linking systems. [221][222][223] Poly(acrylic acid-co-acrylamide) (poly(AA-co-AM)) hydrogels formed by the copolymerization of AA and AM are also extensively researched. This hydrogel combines the characteristics of PAA and PAM and contains numerous carboxyl and amide groups for superior performance modulation. [224][225][226] Li et al. [227] reported a poly(AAco-AM)/chitosan/MXene hydrogel using an extremely simple one-pot free radical polymerization approach. The hydroxyl/fluorine group on MXene can form hydrogen bonds with the carboxyl/amide group on poly(AA-co-AM) and chitosan, which enhances the stress transfer and energy consumption of the polymer network and lays the foundation for a stable conductive path. Therefore, the optimized hydrogel possesses excellent mechanical performance (1000%) and improved electrical conductivity (1.34 S/m).

2.2.4
Other hydrogels Zwitterions are dipole ions containing both positive and negative charges on the same molecule, exhibiting an integral neutral charge. [228] PAA has a unique polycarboxylic acid hydrogen-bonded network chain, in which the introduction of entropy-driven supramolecular amphiphilic competing zwitterion networks causes a series of self-healing, highly elastic, transparent, and strain-hardening conductive hydrogel. Zhang et al. [229] prepared the PAA/betaine hydrogel by mixing the reaction precursor of AA, betaine, and H 2 O with Irgacure 2959 as the photoinitiator. In particular, the initial Reproduced with permission. [229] Copyright 2021, Springer Nature. (C) Chemical structure of the polyzwitterions (PAA-co-DMAPS), dynamic self-healing process, and reshaping to adapt to the irregular surfaces of a prosthetic hand. Reproduced with permission. [236] Copyright 2018, Springer Nature. (D) Structure of conductive cellulose hydrogel in BzMe 3 NOH aqueous solution and conductive performance of CCH at 10 • C and −40 • C. Reproduced with permission. [239] Copyright 2019, American Chemical Society softness of the ionic hydrogel network is attributed to the weak complexation formed by the zwitterion, which subsequently fragments during stretching to form an immensely stiffened the hydrogen-bonded polycarboxylic acid network ( Figure 5B). This successive debonding of two competing dynamic networks and the rapid reorganization of the zwitterions driven by entropy results in ultrahigh stretchability (1600%), complete self-healing (∼100% efficiency), apparent strain-stiffening (24 times enhancement of differential modulus), and excellent elastic recovery (97.9 ± 1.1% recovery ratio, <14% hysteresis). Apart from acting as conductive media in hydrogels, zwitterions such as 1-vinyl-3-(carboxymethyl)-imidazole (VCMI), [230,231] sulfobetaine methacrylate (SBMA), [232,233] and propylsulfonate dimethylammonium propylmethacrylamide (PDP) [234] can also participate in polymerization to form polyzwitterion hydrogels. Therefore, polyzwitterions, which are charged ampholytic polymers, are gradually attracting significant research interest. [235] They have distinct combined advantages in improving the water retention capacity, adhesion, and conductivity. In addition to the good conductivity of the polyzwitterion hydrogel, the conductivity can be further improved by adding electrolyte solutions such as KCl, [234] NaCl, [236] and ZnCl 2 . [233] This ion movement could be enhanced by the weak interactions between the mobile ions and the zwitterionic polymers. [237] Lei and Wu [236] presented a supramolecular polyzwitterion hydrogel prepared by the random copolymerization of AA and 3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) without chemical cross-links and further being swollen in NaCl aqueous solutions ( Figure 5C). Dynamically cross-linked networks of the supramolecular polyzwitterion hydrogel enable strong elasticity, great stretchability, autonomous self-healing, recyclability, and even flexible reconfiguration. Moreover, ion transport within the zwitterionic networks enables flexible sensors derived from it to sense external stimuli, including strain, stress, and temperature.

Ionogels
Ionogels, comprising polymer networks swollen with ILs, have recently attracted considerable attention for elastomeric conductive layers. Polymer chains in ionogels can form hydrogen bonds and electrostatic interaction with ILs, giving them unique characteristics such as high ionic conductivity, high thermal and chemical stabilities, a wide electrochemical stability window, stretchability, and self-healing ability. [247,248] The formation of this hydrogen bonding network can provide good compatibility between the polymer matrix and the ILs, reducing the risk of leakage of the IL under large deformation. [249,250] Compared with hydrogels, owing to the negligible vapor pressure of ILs, ionogel-based flexible sensors can work stably in an open atmosphere and even in a vacuum for a prolonged period without performance decrements. [251][252][253] PAA undergoes charge interactions and hydrogen bonding with ILs due to carboxyl ionization, thereby making it highly compatible with ILs. [254][255][256] Lai et al. [257] obtained PAA ionogels by immersing the purified PAA hydrogel (F127 as cross-linker) into excess 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) and the remaining water was further removed by vacuum drying ( Figure 6A). The PAA ionogel is conductive (1.9 S/m) due to the existence of considerable amounts of ions as charge carriers in their networks. Owing to the chemical and physical double cross-linking structure of PAA and [EMIM][DCA], the PAA ionogel is highly stretchable (>850%), tough, and fatigue-resistant. Sun et al. [258] also synthesized a PAA ionogel by polymerizing AA in the IL 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl). This one-step synthesis method is simpler, more efficient, and environmentally friendly. In contrast, PAM and PVA have poor compatibility with most of ILs, so they are usually selected as copolymers to introduce phase separation to form physical cross-linking sites to adjust the mechanical properties of the ionogels. [259,260] However, Li et al. [261] found that the use of halometallate ILs as ionic solvents can prepare supertough PAM ionogels. Because of the metal coordination sites provided by the anions in halometallate ILs can form dynamic and reversible physical cross-links with the PAM chains, the PAM ionogel not only has favorable mechanical properties, but also can be green recycled through water.
Polyzwitterions [262][263][264] and poly(ILs) (PILs) [265][266][267] have received considerable attention for preparing highperformance ionogels because of their unique ion-dipole and dipole-dipole interactions with ILs and higher ionic conductivity compared with hydrogels. Zhao et al. [264] creatively prepared a zwitterionic ionogel fiber by a simple mold method using [EMIM][DCA] as the only dispersion medium instead of other organic solvents or water. Physically and chemically cross-linked networks were constructed using AM and SBMA by in-situ photopolymerization. Therefore, dynamic interactions including interchain hydrogen bonds, ion-dipole interactions, and dipole-dipole interactions were formed in the ionogel fiber, simultaneously improving the strength and endowing the ionogel fiber with unique selfadhesiveness, high tolerance under vacuum (1.325 kPa), and increased conducting temperature range (−80 • C to 150 • C) ( Figure 6B).
To improve the stability of ionogels in humid environments or water, fluorinated polymers are ideal matrices for the preparation of ionogels due to their low surface energy, high humidity insensitivity, and excellent thermal/chemical stability. Polyvinylidene fluoride (PVDF) and its copolymers are used for their electroactivity and easy processing. [268][269][270] However, the rigid crystalline regions between PVDFs lead to poor stretchability and self-healing properties, which severely reduce the durability and reliability of the synthesized ionogels. Therefore, the use of copolymers formed by acrylate monomers containing fluorine groups as ionogel matrix can adequately balance this limitation and maintain good stretchable and conductive properties while maintaining good water resistance. [271] Moreover, 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) and 2,2,2-trifluoroethyl acrylate (TFEA) are widely used because the -CF 3 group can interact with the ionic dipole in ILs to enhance the compatibility of polymer matrix with ILs and improve the stability of the ionogels. [272,273] Xu et al. [274] prepared ionogels via the one-step photoinitiated polymerization of TFEA and AM in hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI]). The abundant noncovalent interactions including hydrogen bonding and ion-dipole (imidazolium-CF 3 ) interactions endow the ionogels with excellent stretchability (>1000%), resilience, and conductivity (2.92 mS/m) ( Figure 6C). The fluorine-rich polymeric matrix provides high tolerance against water and various organic solvents, as well as tough underwater adhesion on different substrates. Meanwhile, the use of PILs containing fluorine functional groups can also provide an excellent waterproof effect. Yu et al. [275,276]   [DCA] and zwitterionic polymers, endowing the ionogel with high resistance to harsh environments. Reproduced with permission. [264] Copyright 2022, Elsevier. (C) Physically cross-linked ionogels formed through polar -CF 3 groups in P(TFEA-co-AM) copolymer network interacting with mobile [EMIM] [TFSI]. Reproduced with permission. [274] Copyright 2021, John Wiley and Sons. (D) Digital image of the PU-IL ionogel and its corresponding schematic structure. Reproduced with permission. [280] Copyright 2020, John Wiley and Sons Traditional elastomers such as PDMS, [277] SEBS, [278] and TPU [279] can also be used as matrix to form ionogels, broadening their applicability. Li et al. [280] fabricated an ionogel by impregnating ILs into a mechanically robust TPU network comprising crystallized poly(ε-caprolactone) (PCL) and flexible poly(ethylene glycol) (PEG) that are dynamically cross-linked with hindered urea bonds and hydrogen bonds ( Figure 6D). This design enables the synthesized ionogel to have outstanding mechanical strength (0.4 MPa), stretchability (300%), high conductivity (0.12 S/m), and excellent self-healing ability.

DESIGN, FABRICATION, AND PERFORMANCE OF FLEXIBLE SENSORS
Elastomeric conductive layers play a crucial part in flexible sensors. The resistivity of most elastomeric conductive layers change with external stimuli (pressure/strain, temperature, humidity); thus, they can be used as the active layer of flexible sensors to sense the stimuli. [154,[281][282][283] Elastomeric conductive layers that are insensitive to stimulation can be used as flexible electrodes to output stable electrical signals. Once the elastomeric conductive layer is obtained, the methods used to design and fabricate the flexible sensor will directly determine the function and performance of the sensor. In this chapter, we classify flexible sensors according to their functions and explain the influence of the performance of flexible sensors from the perspective of design principles and fabrication methods.
To evaluate the performance of a flexible sensor, several related parameters are necessary as evaluation criteria. Sensitivity is a parameter that reflects the measuring effect and accuracy of the sensor. In general, the sensitivity of flexible sensors is defined as S = dX/dP, where S is the sensitivity, X is the quantitative output signals, and P is the imposed stimuli (When P is the applied pressure, the unit of sensitivity is kPa −1 ). Detection limit refers to the limit of the minimum stimuli that the sensor can detect, which is extremely useful for monitoring ultralow and weak stimulus. Linearity determines the practicality and application range of the device. Response time is equally important, more dynamic stimulus still exist in daily life; therefore, the response speed must reach a certain critical value to accurately measure these signals. And stability displays the durability and reliability of flexible sensors.

Pressure/strain sensors
Pressure and strain sensors are devices capable of detecting external mechanical stimulus and transmitting electrical signals. This signal not only shows the relationship between the stimulus and the device, but also provides data from the size, shape, location, and distribution of the applied force. For flexible pressure and strain sensors based on elastomeric conductive layers, common types for converting mechanical stimulus information into electrical signals include piezoresistive, capacitive, triboelectric, and potentiometric.

Resistive
The basic operating principle of resistive sensors is that the resistance change is due to variation in the electrically conductive path in the active layer or changes in the contact area with the electrodes; this change can be induced by a corresponding electrical signal response. [284] In brief, an elastomeric active layer is sandwiched between two electrode layers to fabricate flexible resistive sensors. [285][286][287] Because of simple structure and straightforward measurement method, resistive pressure/strain sensors are the most commonly used. Tan et al. [288] fabricated a strain sensor based on TPU-boron nitride nanosheet (TPU-BNNS) membranes, which can be tightly bonded with the TPU fiber membranes deposited with graphene nanoribbon (GNR) nanonetworks, and then attached two copper foils to each of the exposed ends of the fiber membranes ( Figure 7A). Owning to the excellent stretchable and conductive properties of the GNR-TPU layer, the sensitivity of the strain sensor can reach 35.7 when the applied strain is ≥60%. In particular, the enhanced thermal conductivity of the TPU-BNNS film facilitates the rapid transfer of operational heat caused by changes in resistance to the environment. However, the sensitivity and detection limit are restricted due to the inherent viscoelasticity and modulus of elastomeric polymers. One method of resistive sensors to improve performance is to introduce micropores or cracks. [289][290][291] To reduce the modulus of the conductive layer and improve the sensitivity of the sensor, micropores such as sponges [292] and aerogels [293] have been designed. Among them, aerogels have extremely high porosity, low density, and high specific area, which are more significant in improving the sensitivity of flexible sensors. Shi at al. [294] used MXene nanosheets and 3-glycidyloxypropyldimethoxymethylsilane (GPDMS) to form aerogel composites, and then the aerogel was integrated on a flexible PET substrate coated with interdigital electrodes to fabricate a resistive pressure sensor ( Figure 7B). The hierarchical multilevel structure of aerogel enables the detection of external forces by shrinking (or expanding) the space between the nanochannels, which, in turn, leads to considerable resistance variations. Thus, this flexible sensor exhibits an ultralow detection limit (0.0063 Pa), high-pressure sensitivity (1900 kPa −1 ), and extraordinarily sensing robustness. Cracks are novel sensing structures that can also improve sensitivity. When cracks are generated and propagated, part of the conductive path is cut off and the resistance changes dramatically, resulting in increased sensitivity. [295,296] Li et al. [297] designed and fabricated a highly stretchable and ultrasensitive fiber strain sensor based on a strain-sensing bilayer (rGO/AgNPs on PDMS) design associated with a novel surface wrinkleguided differential microcracking mechanism ( Figure 7C). During stretching, the robust as-wrinkled rGO interlayer directs the susceptible conformal AgNP surface layer to preferentially crack at the wrinkling troughs, followed by sliding of the packed rGO nanosheets in the rGO layer. Hence, the obtained strain sensor achieves unprecedented sensitivity, both in subtle and large strain ranges (sensitivity: 420 (0-2%), sensitivity: 1.1 × 10 9 (110-125%)), ultralow strain detection limit (0.01%), and ultrafast response time (0.13 ms).
An alternative method to improve sensitivity is micropatterning, which involves the controlled introduction of micronsized patterns in the active layer with a variety of potential shapes, such as microdome, [298,299] micropillar, [300] and pyramid. [301,302] The geometry of the micropatterned active material is susceptible to deformation under external forces, resulting in significant changes in the area between contact points or facing sensing elements, leading to high sensitivity or a large sensing range. However, these structures require a combination of traditional photolithography and silicon etching processes for fabrication, and the approaches are complex, costly, time-consuming, and highly contaminated. Therefore, another popular way to achieve microstructures is to replicate the hierarchical structures of biological and commercially available molds such as lotus leaf, [303] rose petals, [304] sand paper, [305] and silk scarf. [306] Shi et al. [303] fabricated hierarchical structures on GR/PDMS layers by replicating the surface of lotus leaf, which endows the GR/PDMS layers with both micro-and nanoscale patterns. Two patterned GR/PDMS layers are stacked together faceto-face to construct a simple pressure sensor. Finite element analysis (FEA) shows that the rapid and steady increase in contact area and load is due to the hierarchical structure controlling the deformation behavior and pressure distribution at the contact interface ( Figure 7D). As a result, the prepared pressure sensor exhibits a wide linearity range (0-25 kPa), high sensitivity (1.2 kPa −1 ), low detection limit (5 Pa), and great stability (>1000 cycles). Meanwhile, skin-like folds with thickness gradients obtained by prestretching are relatively simple to produce and possess excellent performance. [307] Using these hierarchical structure objects, it is possible to partially simplify the manufacturing process to fabricate active layers of micropatterned structures. However, due to the predetermined shape and size of the biological and commercial template, it is not possible to adjust the geometric parameters of the microstructures. Other strategies without the use of templates such as forming wrinkled structures using the prestretching method [308,309] or modulating the surface morphology of microdome-patterned composites films by adjusting thermal foaming time [310] open a new avenue to fabricate micropatterned active layers to design high-performance resistive sensors.
Physically, quantum tunneling effects describe the pattern of this type of signal on the quantum scale. In particular, Fowler-Nordheim (F-N) tunneling allows a large tunneling distance, and thus enables the development of a sensing mechanism. Thus, inspired by the F-N tunneling, Shi et al. [70] fabricated a pressure sensor by spin-coating extremely low concentration urchin-like hollow carbon spheres (UHCSs, <1.5 wt.%) loaded in a PDMS dispersion, which is far below the percolation threshold ( Figure 7E). The sensors reach an ultrahigh sensitivity of 260.3 kPa −1 at 1 Pa and exhibits a vertical-direction conduction and horizontal-direction insulation phenomenon under pressure.

Capacitive
Capacitive sensors are usually a parallel-plate capacitor comprising two parallel electrodes and a dielectric layer that works by converting pressure into a change in capacitance. Reproduced with permission. [288] Copyright 2020, Springer Nature. (B) Resistive pressure sensor fabricated by integrating the GPDMS aerogel on top of an interdigital electrode-coated flexible PET substrate. Reproduced with permission. [294] Copyright 2022, Springer Nature (C) Strain sensors based on wrinkledirected microcracking bilayer configurations. Reproduced with permission. [297] Copyright 2022, Elsevier. (D) Deformation process for hierarchical structure determined by FEA. Reproduced with permission. [303] Copyright 2018, John Wiley and Sons. (E) 3D scheme of the UHCS-PDMS pressure sensor. Reproduced with permission. [70] Copyright 2020, Springer Nature. (F) Illustration of the ionic pressure sensing mechanism of the PVA/H 3 PO 4 hydrogel sensor and FEA modeling of the contact stress distribution under 0 kPa, 1 kPa, 4 kPa and 16 kPa. Reproduced with permission. [316] Copyright 2022, John Wiley and Sons. (G) Diagram of the micro-nanostructured capacitive sensor structure using artificial PDMS reed leaf. Reproduced with permission. [317] Copyright 2019, American Chemical Society. (H) Triboelectric mechanism of the contact separation mode and the SVP mode corresponding to the external electrical output. Reproduced with permission. [323] Copyright 2021, John Wiley and Sons. (I) Flexible pressure sensor based on the potentiometric mechanotransduction mechanism. Reproduced with permission. [324] Copyright 2020, Science The capacitance (C) is calculated as C = ε 0 ε r A/d, where ε 0 is the dielectric constant of the vacuum medium, which is a constant value, ε r is the relative dielectric constant of the dielectric between the two electrodes, and A and d are the relative area and distance between the two electrodes, respectively. These three variables (ε r , A, and d) are all sensitive to changes in pressure or strain. Capacitive sensors have the advantages of high sensitivity, low power consumption, and large-area integration; the elastomeric conductive layer can be used as the electrode. [198,232,235,311] For instance, Du et al. [312] developed a capacitive sensor by utilizing electrospun sandwichstructured elastic films (ESEFs). The ESEF-based capacitive sensor comprises a TPU mat dielectric layer sandwiched between two nanocomposite (TPU/MXene/AgNW) mat electrode layers. In electrode layers, TPU is used as the backbone to allow ESEFs to possess high-tensile strength and elongation at break, and AgNW and MXene are added to form a robust conductive network to endow ESEFs with good electrical conductivity. To achieve accurate monitoring of various mechanical stimuli in daily life, the sensor achieved wide response range (strain: 0-150%; pressure: 0-70 kPa), relatively high sensitivity (strain: 1.21; pressure: 0.029 kPa −1 ), low response time, and outstanding stability (2000 cycles).
Meanwhile, hydrogel/ionogel can also function as a deformable and reliable separator in the capacitor, which can detect pressure and strain changes. Here, the hydrogel/ionogel film provides free ions to form an electrical double layer (EDL) between the hydrogel/ionogel and electrodes, in which the negative and positive charges are separated by nanometers; the EDL capacitance mainly depends on the interfacial contact area between the hydrogel/ionogel and electrodes. Prior to loading, the initial contact area of the interface is very small; thus, the initial capacitance is small. As the load increases, the interfacial contact area between the hydrogel/ionogel and the electrodes increases as a function of the pressure, resulting in an enhanced interfacial EDL capacitance. [313][314][315] For example, Guo et al. [316] designed a capacitive sensor using microstructured PVA/H 3 PO 4 hydrogels and electrodes of indium tin oxide/PET. In this sensor, the cations (H + ) in the hydrogel layer attracted electrons from the electrode and aggregate at a nanometer distance to form EDL ( Figure 7F), resulting in ultrahigh capacitance and greatly improving the sensitivity (3200 kPa −1 ) of the pressure sensor.
Instead of using the sandwiched structure to form capacitive sensors, other novel structures have also been designed. Liu et al. [317] obtained an artificial PDMS reed leaf by replicating the natural reed leaf by soft lithography twice; then Au electrodes were coated on both the front and back side to fabricated the capacitive-type sensor ( Figure 7G). The capacitive sensor in the low-pressure region exhibited high sensitivity (0.6 kPa −1 ) and fast response/recovery time (180/120 ms).

Triboelectric
Recently, triboelectric nanogenerators (TENGs) have demonstrated excellent performance in energy harvesting and signal generation. Thus, TENGs can be used as flexible sensors to detect pressure or strain information and are capable of achieving extremely high sensitivity, without relying on an external power source for the entire process. [318] TENG sensors in the single-electrode mode are widely used and can be successfully prepared by sandwiching an elastomeric conductive layer as an electrode between two insulating films. [319][320][321] TENG sensors primarily through frictional electrification and electrostatic induction effects to influence the flow of charge with high transient power; however, the electrostatic induction generated by the external environment can cause signal interference. [173,322] To enhance the signal output of the TENG sensor, Luo et al. [323] prepared flexible single-electrode mode TENG sensors by connecting MXene/PVA hydrogels with external conductive copper wires and then encapsulating it in ecoflex. More importantly, the MXene/PVA hydrogel itself can also output a triboelectric signal under force due to the streaming vibration potential (SVP) mode. The contact separation mode and the SVP mode correspond to the external electrical output in the same direction ( Figure 7H), which can synergistically enhance the entire electrical output of the TENG. When the TENG sensor was stretched to 200%, it could produce an open-circuit voltage, a short-circuit current, and a transferred charge amount of 7 V, 4 nA, and 1.18 nC, respectively, revealing great application potentials in wearable self-powered body movement monitoring.

Potentiometric
In the skin sensory system, cutaneous mechanoreceptors can perceive external mechanical stimuli via the variation in membrane potential. Inspired by the skin sensory behavior, Wu et al. [324] first reported a potentiometric mechanotransduction mechanism based on the mechanically regulated potential difference measured between two electrodes. When two electrode materials (Prussian blue-modified graphitic carbon [PB/C] and Ag/AgCl) are in contact with a PVA hydrogel containing an electrolyte of NaCl, a potential difference can be generated between the two electrodes ( Figure 7I). By manipulating the electrolyte composition in the hydrogel and forming microstructures on the surface of the hydrogel, the electrolyte/electrode interface is modulated by external mechanical stimulation, resulting in a change in the measured potential difference between the two electrodes. Using this mechanism, the mechanical stimulus can be encoded as a change in potential difference, thus yielding a potentiometric pressure sensor. [188,325] Wu [324] fabricated a potentiometric sensor with sensitivities of 48.6 mV/N and 3.6 mV/N at forces of 0-3 N and 3-10 N; the fabricated sensor exhibited rapid response (71 ms) and recovery (106 ms) behaviors. rGO/GO/PVA/MXene [326] and PVA/ZnCl 2 /NH 4 Cl [327] conductive hydrogel systems have been subsequently developed, confirming the potential of potentiometric sensors.

Temperature sensors
Temperature is closely related to human life and is an essential parameter for evaluating human health and monitoring the surrounding environment. Most physical or physiological changes in the human body cause changes in the body temperature; hence, the use of temperature sensors for the real-time monitoring of body surface temperature changes is one way to understand the physical condition of humans.
Based on the principle that the electrical signal of the elastomeric conductive layer changes with temperature, it can be classified into four categories.

Electron conduction efficiency
The most widely studied method involves using the difference in the electron conduction efficiency of conductive fillers at different temperatures such as rGO [328] and PANI. [210] The sensing mechanism of the temperature sensor can be illustrated by the transport of interfillers including carrier hopping and tunneling conduction at the boundaries between adjacent fillers. When temperature increases, transport by means of hopping across barriers at the filler junctions was improved, thereby increasing the conductance of the conductive layer channel ( Figure 8A). [329][330][331] Trung et al. [331] developed a stretchable temperature sensor by encapsulating the serpentine structure rGO/PU fiber in PDMS, which can eliminate the strain induced interference ( Figure 8B). Apart from embedding into the PDMS substrate, this free-standing stretchable fiber (FSSF) can also be sewn onto stretchable fabric, on a bandage, or directly attached to human skin. Due to the conductive performance of rGO at different temperatures, the FSSF sensor exhibits high sensitivity (0.8%/ • C), increased stretchability (stretching up to 90%), and excellent strain insensitivity (0.37 • C of inaccuracy for strains at 0-50%). However, because of the poor thermal conductivity of the elastomer material, its response/recover times (7/70 s) are much longer compared to that of the pressure/strain sensor.

Coefficient of thermal expansion
Another principle is to use elastomeric matrices with high coefficients of thermal expansion such as PDMS and PNI-PAM. When the PDMS-based conductive layer expands with increasing temperature, the conductive sensing network undergoes mechanical deformation, leading to the formation and opening of microcracks. These microcracks result in the noteworthy destruction of conducting paths and rapid increases in the resistance. [332,333] However, this sensor is susceptible to other external factors and can only be used as a noncontact sensor. For PNIPAM hydrogels, the previous section mentions that the increase in temperature desorbs the PNIPAM network, resulting in a reduced volume. [180,184] The reduced volume of PNIPAM allows for a denser percolation network of fillers and decreases the resistance. This feature can work synergistically with the temperaturesensitive characteristics of the filler to improve the sensitivity of the temperature sensor. Oh et al. [334] synthesized PNI-PAM/PEDOT:PSS/CNT hydrogel temperature sensors with extremely high thermal sensitivity (2.6%/ • C at 25-40 • C) that can accurately detect changes in the skin temperature of 0.5 • C ( Figure 8C). However, owing to the high specific heat capacity of water, response/recovery times (167/605 s) are slower.

Ionic conductivity
As the temperature increases, the ionic mobility of the ionic conductive hydrogel and ionogel increases, thus increasing the ionic conductivity. In addition, the enhanced dissociation of ions at high temperatures also causes an increase in the carrier concentration. Both effects contribute to reduced resistance at elevated temperatures. [251,335,336] However, when conductive hydrogels or ionogels are used as resistive temperature sensors, their response and recovery times are still long. For instance, the PAM/carrageenan/KCl hydrogel temperature sensor exbibits a response time of 13 s, and spends up to 120 s to achieve full signal recovery with natural cooling. [337] In addition to resistive sensors, conductive hydrogels and ionogels can be made into capacitive temperature sensors. [338,339] Wu et al. [338] fabricated a temperature sensor by pasting two silver electrodes at both ends of the PAM/carrageenan/KCl/LiBr hydrogel layer ( Figure 8D). Importantly, the temperature sensor exhibited a considerably higher thermal sensitivity in capacitance mode (18.83%/ • C) than that in resistance mode (0.94%/ • C) above room temperature. Further, the response/recovery times (0.19/0.08 s) are much faster. The excellent performance of capacitive temperature sensors is attributed to the rapid response of the hydrogel geometry, phase transition effects, and ion accumulation at the hydrogel electrode interface to changes in temperature, which act cooperatively to cause changes in capacitance. The high sensitivity and short response/recovery times demonstrate the great advantages and application potential of conductive hydrogel and ionogel capacitive temperature sensors.

Thermoelectric effect
Thermoelectric materials can generate a thermoelectric voltage by converting the temperature difference between the two ends of the material into a potential difference. Since the temperature gradients are ubiquitous in nature, thermoelectric sensors have been considered exceptional candidates as self-powered temperature sensors. [340,341] Chen et al. [342] fabricated a self-powered flexible temperature sensor using PAM/calcium-alginate/Li 2 SO 4 ion-conductive hydrogel; its working mechanism is shown in Figure 8E. When the temperature difference occurs on both sides of the ion-conductive hydrogel, Li + and SO 4 2− electronic charge carriers will migrate from the hot side to the cold side. In the cross-linking PAM/calcium-alginate hydrogel matrix, the Li + have lower resistance to migration and migrate faster than the SO 4 2− because they are smaller in size than the SO 4 2− . When the distance between the Li + and SO 4 2− is greater than the Debye length, the Li + will gather on the cold side and the SO 4 2− stay on the hot side, resulting in a thermoelectric voltage. Therefore, this flexible temperature sensor can sense the temperature difference (ΔT) well and the sensitivity can reach 11.5 mV/K.

Humidity sensors
A humidity sensor is one that senses changes in relative humidity (RH) and is of great importance for industrial, agricultural, medical, and domestic applications. When the elastomeric conductive layer of carbonaceous fillers/elastomer composites is used in a flexible humidity sensor, the swelling effect of the system caused by the absorption of water molecules into the composite, combined with the F I G U R E 8 Structure and working mechanism of flexible temperature and humidity sensors. (A) Temperature sensing mechanism of the difference in electron conduction efficiency of conductive fillers. Reproduced with permission. [329] Copyright 2022, Springer Nature. (B) Illustration of the FSSF temperature sensor, which can be embedded into a PDMS substrate, sewn on stretchable fabric, placed on a bandage, or directly attached to human skin. Reproduced with permission. [331] Copyright 2018, American Chemical Society. (C) Flexible temperature sensor comprising the PNIPAM/PEDOT:PSS/CNT hydrogel. Reproduced with permission. [334] Copyright 2018, American Chemical Society. (D) Structure of temperature sensor in which the PAM hydrogel is encapsulated by PDMS. Reproduced with permission. [338] Copyright 2021, American Chemical Society. (E) Working mechanism of ion-conductive hydrogel-based flexible thermoelectric sensor when temperature is different on the two sides of the electrodes. Reproduced with permission. [342] Copyright 2021, Elsevier. (F) Stretchable humidity sensor using rGO/PU as a humidity sensing layer, and resistance change as a linear function of humidity. Reproduced with permission. [345] Copyright 2017, Springer Nature. (G) Sensing mechanisms of the k-carrageenan/PAM/KCl hydrogel under humidity and the formation of hydrogen bonds between water and the k-carrageenan or PAM. Reproduced with permission. [351] Copyright 2019, Royal Society of Chemistry. (H) Illustration of the dual-sensing ionic skin and the relative resistance changes under different RH values (insets show the color change of ionic skin under different humidity conditions). Reproduced with permission. [352] Copyright 2021, Royal Society of Chemistry conductivity of the carbon material, which is also affected by water molecules, causes a change in resistance. [343,344] Trung et al. [345] proposed a transparent and stretchable humidity sensor that employs rGO/PU as a humidity sensing layer ( Figure 8F). The reduced hole concentration in rGO/PU due to water molecules cooperated with the swelling absorption effect to increase the resistance. The humidity sensor exhibits a sensitivity of 0.1188/RH% and response/recovery times of 3.5/7 s. Another type of humidity sensor uses the ionic conductivity mechanism. Electrolytes are added to the elastomer to make a stretchable conductive layer, which causes a decrease in resistance as the number of dissociated ions and ion mobility increase with water content and RH. [346,347] Some researchers have investigated XSBR/sodium lignosulfonate (SL) [129] and XSBR/citric acid (CA) [130] systems as humidity sensors; although SL and CA have more water-absorbing groups, the XSBR matrix does not absorb water well, resulting in a very slow response time, which limits its application. Therefore, conductive hydrogels or ionogels with abundant hydrophilic groups exhibit tremendous advantages as flexible humidity sensors. [154,[348][349][350] To further improve the humidity sensing of the conductive hydrogel, Wu et al. [351] introduced Gly into the k-carrageenan/PAM/KCl hydrogel. A room-temperature humidity sensor with a two-electrode chemoresistive structure was fabricated using the prepared conductive hydrogel as the transducing material ( Figure 8G). In conclusion, the response/recovery times of the humidity sensor are 0.27/0.3 s and the sensitivity is 10/RH%. The high response/recovery times and sensitivity of the humidity sensors originated from the ready formation of hydrogen bonds between the water molecules and abundant hydrophilic groups in the k-carrageenan, PAM, and Gly molecules, and physically adsorbed water molecules function as free water, which transports in free volume. Conductive hydrogels have a very high potential as conductive layers for flexible humidity sensors due to the presence of hydrophilic groups. Interestingly, Bai et al. [352] obtained a dual-sensing ionic skin by introducing Al 3+ to offer ionic conductivity and highly substituted hydroxypropyl cellulose to form cholesteric liquid-crystal structures in a poly(2-amino-4pentenoic acid sodium-co-acrylamide) (PASCA) hydrogel. In addition to changes in the humidity through changes in resistance, the synthetic ionic skin can also respond to visual information by adjusting the pitch of the liquid crystal in the presence of humidity (fast response: 0.5 s; wide range: 30-90% RH) ( Figure 8H).

Multifunctional sensors
Single-function sensors cannot provide sufficient information to satisfy human requirements; thus, there is an urgent need for multifunctional sensors with different capabilities to detect and distinguish various external stimuli. Using the aforementioned pressure/strain, temperature, and humidity sensing mechanisms, many elastomeric conductive layers can make electrical signal changes in response to different types of stimuli. [114,140,328,353] Wang et al. [354] fabricated a flexible multifunctional sensor, which takes the form of a multilayer stack: a polytetrafluoroethylene (PTFE) electrification layer, AgNWs coated on Cu sheets as electrodes, and GR/PDMS sponge composites as the responsive component to thermoelectric and resistive effects ( Figure 9A). The sensor exhibited outstanding sensitivity (15.22 kPa −1 ) and high accurate temperature resolution (1 K) based on the piezoresistance and thermoelectric property of the GR/PDMS composite. In addition, it enables inferences to the properties of materials based on the triboelectric mechanism of the PTFE electrification layer. However, these sensors can only achieve a single signal input and cannot simultaneously respond to multiple stimuli. To obtain flexible sensors that can simultaneously respond to multiple stimuli, it is necessary to organically combine the material selection, mechanism integration, structural design, and manufacturing methods of sensors to prevent the interference between signals. [355][356][357][358] One effective approach is to assemble multiple sensing units inside the flexible sensor, each of which simultaneously senses different stimuli and outputs signals. [115,[359][360][361] Bae et al. [359] fabricated a bimodal sensor with stimulusdiscriminating and linearly sensitive by integrating the top part (Parylene C substrate, Ni/Ti electrode, rGO thermistor, and dielectric layer) and the bottom part (Parylene C substrate and microstructural SWCNT/PDMS array) ( Figure 9B). In the design, the rGO thermistor is a square with a sensing area of 1 cm 2 that takes the form of a line across the middle of the top part. Therefore, this sensor exhibits linear and high pressure sensitivity (0.7 kPa −1 up to 25 kPa), linear reproducible temperature sensitivity (0.83%/ • C at 22-70 • C), and rapid response time to pressure (50 ms) and temperature stimuli (100 ms). To prepare multifunctional sensors, Xu et al. [361] first developed tilt, strain, and humidity sensors individually. A tilt sensor based on confining a liquid metal droplet inside a cavity can track at least 18 slanting orientations. A strain sensor based on embedding laser-induced graphene (LIG) in PDMS can induce the increase in resistance under stretching. A humidity sensor was fabricated by depositing ZnIn 2 S 4 nanosheets on the porous LIG interdigital electrodes. By rationally integrating the LIG-based tilt, strain, and humidity sensors in the wearable body condition sensor system, multiple channel signals can wirelessly transmit to a smartphone interface via Bluetooth ( Figure 9C).
Another strategy is to develop a new elastomeric conductive layer and measure the different electrical signals of the conductive layer for simultaneously sensing multiple stimuli. When the conductive hydrogels or ionogels are used as electrodes, it can sense pressure by capacitive mechanism. Its own ionic conductivity increases with temperature, so it can also be used as a resistive temperature sensor. [362,363] However, due to the accelerated ion mobility, the capacitance and resistance are sensitive to both pressure and temperature, causing crosstalk in the signal. To solve this problem, Ren et al. [364] prepared a gradient ionogel using the electric field induction method and performed resistance tests with two electrodes on the cathode side of the ionogel ( Figure 9D). Owing to the special structure of the gradient ionogel, the anode side (low density) of the ionogel expands significantly under pressure, whereas the cathode side (high density) deforms less. Therefore, the resistance of the cathode is not sensitive to pressure (0.0026% kPa −1 ) and can be corrected for the pressure measured by the capacitor with the temperature corresponding to the resistance. Also, You et al. [365] revealed that the relaxation time (τ) of the ionogel can be used as a strain-insensitive intrinsic variable for detecting temperature without any geometrical information of the sensor. The temperature effect on the capacitance can be eliminated by normalizing to the reference capacitance (C 0 ) at the measured temperature. Thus, the multimodal ion-electronic sensor with a simple sandwich structure ( Figure 9E) can simultaneously detect temperature and various tactile motions (shear, pinch, spread, torsion, etc.) by measuring the variables at only two measurement frequencies (τ and C/C 0 ). Therefore, to make the flexible sensor simultaneously respond to multiple stimuli, the problem of multiple signal crosstalk can be solved from the perspective of the structure of the sensor or the material of the flexible conductive layer so that the signal can be output accurately.

ADVANCED APPLICATIONS OF FLEXIBLE SENSOR
The value of the resulting high-performance flexible sensors can be demonstrated when they are used in real-life applications. With the advancement of data acquisition, processing, and transmission technologies, flexible sensors exhibit the F I G U R E 9 Structure and working mechanism of flexible multifunctional sensors. (A) Exploded-view schematic diagram showing the multilayer stack structure of the multifunctional sensor. Reproduced with permission. [354] Copyright 2020, Science. (B) Diagrams of stimulus-discriminating and linearly sensitive bimodal sensor. Reproduced with permission. [359] Copyright 2018, John Wiley and Sons. (C) Multifunctional flexible sensor system integrated with tilt, humidity, and strain sensors. Reproduced with permission. [361] Copyright 2021, John Wiley and Sons. (D) Structure of the gradient ionogel and the flexible bimodal sensor with the sandwich structure, which is made from gradient ionogels. Reproduced with permission. [364] Copyright 2021, John Wiley and Sons. (E) Structure of multimodal ion-electronic sensor and response of the sensor to a shear force. Reproduced with permission. [365] Copyright 2020, Science ability of adaptive measurement, in-sensor analysis, and wireless data transmission based on a stable power supply. Elastomeric conductive-layer-based flexible sensors with different functions can be applied to various scenarios, such as intelligent robots, human-machine interfaces (HMI), motion detection, and health care.

Intelligent robotics and HMI
In recent years, the field of intelligent robotics and HMI has faced significant challenges, with high sensitivity, accuracy, reproducibility, mechanical flexibility, and low cost emerging as new requirements. The development of flexible sensors F I G U R E 1 0 Applications of flexible sensors for intelligent robots & HMI. (A) Smart manipulator grabbing and transporting delicate objects on the assembly line. Reproduced with permission. [370] Copyright 2020, Elsevier. (B) Motion of the robotic arm, which can be real-time and accurately monitored by flexible pressure sensors. Reproduced with permission. [371] Copyright 2021, American Chemical Society. (C) Comparison of the pressure response of the sensory prosthetic hand without pressure feedback and with single/mutilevel feedback when manipulating cake. Reproduced with permission. [375] Copyright 2021, John Wiley and Sons. (D) Schematic diagram of the vehicle control system; the vehicle accepts and completes forward, backward, turn right, and turn left instructions. Reproduced with permission. [78] Copyright 2021, Royal Society of Chemistry. (E) Potential of the multifunctional sensor as a flexible game controller. Reproduced with permission. [330] Copyright 2022, John Wiley and Sons. (F) Multilayered electronic transfer tattoo, which can remotely control the robotic hand. Reproduced with permission. [382] Copyright 2021, Science offers a unique opportunity for use in advanced intelligent robotics and HMI design. [366][367][368] Intelligent robots are designed to help humans to accurately and rapidly perform a variety of operations, as well as to perceive and provide feedback on objects they encounter. [369] For example, biomimetic flexible dual-mode pressure sensors based on the interlocked P(VDF-TrFE)/rGO and PDMS/rGO films can possess synergic effects and enable the sensor to detect over broad pressure and frequency ranges. As a proof-of-concept, this dual-mode pressure sensor was successfully integrated with manipulators to decode the complex and delicate picking processes ( Figure 10A). [370] Mimicking the ability of humans to grasp and carry objects is a common function of intelligent robots. This function comprises several motions, such as moving, grasping, and lifting of the robotic arm ( Figure 10B). [371] Flexible sensors in intelligent robots convert the state of the contacted object into an electrical signal to enable the robots to perceive the hardness, [311] weight, [364] and grasping state of the object. [372] The flexible sensors can also be integrated into the bionic hand to perceive object characterizations and finger bending. [373,374] Figure 10C depicts a highly sensitive and robust soft robot hand prosthesis with ultracapacitive sensing that combines PAM/NaCl hydrogels with commercially available conductive fabrics. [375] The prosthesis was integrated within a soft bionic hand to provide industrial robots and amputees with robust pressure feedback when handling delicate objects. If the electrical signal exceeds the threshold, the bionic hand with pressure feedback system will receive a signal to stop pressing the delicate object.
HMI is principally a process that relies on flexible sensors to capture the motion state of the body as well as real-time bioelectrical and peripheral signals. Subsequently, after the system processes and transmits the signals, the machine performs specific functions. [376][377][378] In the output section, the machine can broadly refer to a mechanical actuator, [78] robot, [379] or computer. [380] Figure 10D shows that the motion of the vehicle is controlled by different finger flexions, and the stability of the PVA/PU sensor under large strain enables the vehicle to complete specific movement modes by the bending of different fingers. Using the pressure response characteristics of the flexible sensor, the sensor array can be used as a keypad and thus output on the computer. [173,270,330] As shown in Figure 10E, owing to the good bending stability, rapid response time, and long-term durability of the CNF/PDMS sensor, the array of three flexible sensors was successfully used to play a more complex game, Red Runner. Also, by wearing a flexible sensor on the hand to establish a connection with the robot, remote control of the robot can be achieved. [381,382] Multilayered integration of the electronic tattoo can enable the crease amplification effect, which can triple the output signal of integrated strain sensors. The tattoo can be transferred to different surfaces and forms a firm attachment, where no solvent or heat is required. Figure 10F depicts the multilayered electronic transfer tattoo that remotely controls a robotic hand by simulating human finger gestures. Therefore, flexible sensors with stable performance exhibit considerable potential for applications in intelligent robotics and HMI.

Motion detection
Monitoring human movement signals is an effective method for assessing and monitoring human movement status. Detecting and recognizing the postures of human movement enables the analysis and understanding of the information conveyed by the human exerciser, which can be substantially used in social activities. Therefore, the detection, recognition, and analysis of human motion postures are becoming an important multidisciplinary research direction. [383][384][385] Based on the excellent electromechanical properties of the flexible sensor, it can be attached to various parts of the body joints such as fingers, [190] wrist, [386] knee, [387,388] and elbow [389] to monitor the movement ( Figure 11A). Moreover, when the flexible sensor is placed on the sole of shoes, it is possible to obtain the real-time status of walking. [255,390] Monitoring of the plantar pressure can promote the understanding of the mechanisms governing human gait. Figure 11B shows a foot pressure measuring system built by locating the lattice structure pressure sensor (LPS) on five critical positions of an adult's insole. [391] The designable LPS consisting of a CNT-embedded TPU active layer was integrated with the bottom interdigital electrode, which can measure the foot plantar pressure distribution in real time under stationary, standing, and walking conditions (initial contact, foot flat, and forefoot push off). The results obtained are well matched with common gait models and validate the accuracy of the foot pressure measurement system.
In addition to vigorous sports motions, delicate facial expression motions (frowning, eye blinking, and cheek bulging) [171,392,393] and ultradelicate motions (phonating and coughing) [88,170,394] can also be detected. The face sensorbased speech recognition system enables private, undisturbed communication, regardless of environmental noise. As shown in Figure 11C, a strain mapping of facial skin deformation during speech was evaluated using nanomesh sensors. [395] This device was synthesized from reinforced PU/PDMS nanomeshes and exhibited excellent sustainability, linearity, and durability with low hysteresis. Its thinness, geometry, and softness provide minimum mechanical interference on natural skin deformations. The results indicate that nanomesh devices reflect the actual skin deformations with minimal mechanical constraints; the skin with nanomeshes can still be strained and compressed freely during speech.
Since fibers are essential one-dimensional building blocks for making various fabrics, it is possible to seamlessly embed fiber-like flexible sensors into arbitrarily designed fabric structures to obtain wearable devices. [396,397] Figure 11D illustrates a full-body tactile textile for the study of human activities. This textile is based on coaxial resistive fibers (comprising PDMS as the matrix and graphite/Cu NPs as the conductive filler) produced using an automated coating technique. [398] By using digital machine knitting technology, these functional fibers can be turned into large-scale sensing textiles that conform to arbitrary 3D geometries. An AIbased computational workflow used tactile textiles to assist in the calibration, recording, and analysis of the human body's interaction with the external environment.

Health care
Human health is the most important aspect as the aging of society and the change in lifestyle lead to the normalization of chronic diseases. Flexible sensors are receiving increasing attention for the measurement of human vital signs and physiological information; further, they have promising applications in the fields of disease diagnosis, rehabilitation treatment, and daily health assessment. [399][400][401] Integration of flexible sensors in the dust mask enables the monitoring of normal oral breathing status. [402,403] As shown in Figure 12A, based on the high-response sensitivity to humidity, the PAM humidity sensor can recognize light mouth breath, deep mouth breath, and nose breath by monitoring the varying humidity between exhaled moisture and the environment. The pulse signals of humans originate from the pressure return of the artery and usually have three clearly distinguishable peaks: systolic peak (P1), diastolic peak (P2), and reflective peak (P3). The flexible sensor not only detects the pulse signal, but also clearly depicts these three peaks ( Figure 12B). [140,404] Also, blood pressure (BP) is an important physiological parameter that reflects the vascular condition of the body, particularly the pressure in blood vessels. [116,405] Figure 12C illustrates a single flexible and scalable device integrated on a SEBS substrate, which is capable of monitoring BP and heart rate (HR) in parallel F I G U R E 1 1 Applications of flexible sensors for motion detection. (A) Flexible sensor used as sensors in detecting human motions. Reproduced with permission. [388] Copyright 2020, American Chemical Society. (B) Schematic demonstration of the foot pressure measuring insole and its corresponding mapping of pressure distribution. Reproduced with permission. [391] Copyright 2021, American Chemical Society. (C) Facial skin strain mapping during speech of "a", and "u", and "o" with nanomesh sensors on the face. Reproduced with permission. [395] Copyright 2020, Science. (D) Textile-based tactile learning platform for the study of human activities. Reproduced with permission. [398] Copyright 2021, Springer Nature while ensuring mechanical performance and avoiding signal crosstalk.
Continuous monitoring of electrocardiography (ECG) signals plays an integral role in the prevention and treatment of cardiovascular diseases. Flexible sensors can be effectively used in ECG because of their good mechanical properties and high sensitivity. [406][407][408] Figure 12D depicts an overview of the full system functionality where data from self-healable sensors (embedding the elastic conductive CNT network with PDMS-MPU 0.4 -IU 0.6 ) were collected from the surface of the human body, subsequently processed, and then transmitted wirelessly to the self-healable light-emitting capacitor (LEC) display for visual interpretation. In addition, ionogels containing fluorine groups demonstrate better adhesion ability, conductivity, and stability underwater. Thus, ionogel-based flexible sensors can effectively detect ECG signals, even after 14 days of immersion in water due to its excellent resistance to water; the data can be used to warn users of the potential risk of a heart attack. [276]

CONCLUSION AND PERSPECTIVE
In summary, flexible sensors have recently attracted considerable research interest because of their excellent flexibility and ductility, high sensitivity, rapid response speed, and functional diversification. In this review, we present an up-to-date review of the recent advancements in elastomeric F I G U R E 1 2 Applications of flexible sensors for health care. (A) Schematic diagram of breath monitoring by humidity sensor. Reproduced with permission. [154] Copyright 2022, American Chemical Society. (B) Real-time current-time curve of the sensor attached to the wrist of a test person and typical signals measured in one period. Reproduced with permission. [404] Copyright 2019, American Chemical Society. (C) Schematics showing the inputs are transduced and outputted as BP and HR signals by the device and the layer-by-layer layout of the integrated sensor. Reproduced with permission. [116] Copyright 2021, Springer Nature. (D) Overview of the system with sensors wirelessly communicating values to the display. Reproduced with permission. [407] Copyright 2020, Springer Nature polymer-based flexible sensors, focusing on the research progress of elastomeric conductive layers and corresponding diverse flexible sensors (pressure/strain temperature, humidity). The elastomeric conductive layer is a major factor influencing the performance of wearable flexible sensors. A well conducting layer enables the sensor to make close, conformal contact with the target while exhibiting high sensitivity and resolution. Elastomeric polymers can form good interaction with conductive fillers/electrolytes (hydrogen bond, π-π interaction, electrostatic force, etc.); thus, the obtained elastomeric conductive layer exhibits excellent mechanical and electrical properties, which can lay the foundation for the manufacture of high-performance flexible sensors. Flexible sensors based on traditional elastomers have the advantages of high strength, excellent resilience, and good environmental stability. However, the transparency is poor, and it is difficult to achieve multifunctionality. Flexible sensors based on hydrogels and ionogels possess high tensile rate, favorable transparency, and biocompatibility; however, they exhibit low strength, poor environmental stability, and liquid leakage. Because the ionic conductivity of hydrogels and ionogels are influenced by many factors such as pressure, temperature, and humidity, the realization of multifunctionality is simple; nevertheless, signal crosstalk readily occurs. Traditional elastomers, hydrogels, and ionogels have their own advantages and disadvantages, the choice of materials is a key factor in determining the performance and properties of flexible sensors, and their combination may have complementary effects. For example, use of traditional elastomers to encapsulate hydrogels or ionogels, fabricate elastomer-hydrogel/ionogelelastomer EDL structures, and make ionogels by using traditional elastomers as matrices. Notably, flexible sensors have been extensively and commercially incorporated in practical applications, such as intelligent robots, HMI, motion detection, and health care, owing to their low cost, low toxicity, special flexibility, superior sensitivity, and tunable performance. Although extensive advancements have been achieved in the design and fabrication of elastomeric polymer-derived flexible sensors, there remain considerable strides to be made and several challenges must be addressed for practical applications. First, unlike metal conductors, elastomeric polymers rely on composite conductive fillers or electrolytes to achieve conductivity, and there are uneven changes in the conductivity during stretching. And interactions between the long-chain molecules within the elastomeric polymer scan cause severe hysteresis in the conductive layers under strain, resulting in nonuniform signals during stretching and recovering, which impact the sensitivity, linearity, detection limit, response time, and stability of flexible sensors. These properties are inversely related; the optimization of one performance may cause a decrease in another property. It is an important consideration for designers to balance and optimize the performance of flexible sensors according to different conditions and requirements. Second, the developed flexible sensors are expected to be more multifunctional in the future. The main challenge is solving the problem of signal crosstalk from the material and structure. Therefore, the integration of flexible sensors and device units is of great significance. The future trends will be to effectively combine diverse units with different sensing types and calibrate the sensor signal to improve the credibility of the output signal. Third, transitioning from lab scale to industrial production is a problem faced by flexible sensors. At present, most flexible sensors are still manufactured manually; hence, the production efficiency is low. The existence of artificial uncertainties results in low reproducibility of the sensor performance. Therefore, the large-scale manufacture of flexible sensors with uniform quality is a crucial step in the industrialization of flexible sensors. Finally, 3D printing technology has increased in recent years, requiring both a rapid flow of elastomeric polymers in the pipe and fast curing after printing. The existing method is to modulate the rheological behavior of polymers by doping with various particulate components. However, the ratio requirements are extremely critical and the configuration process is complex. Simplifying this procedure is the key to achieving fine precision, high efficiency, and low energy consumption for 3D printing flexible sensors. The development of flexible sensors combines different disciplines, such as chemistry, materials science, mechanical engineering, electronic engineering, biology, and computer engineering, which require researchers from various fields to collaborate and work together to fabricate sensors for human benefits.

A C K N O W L E D G M E N T S
This work was financially supported by the State Key Program of National Natural Science Foundation of China (No. 52130303) and the National Natural Science Foundation of China (No. 51773147).

C O N F L I C T O F I N T E R E S T S
The authors declare no conflict of interests.