Recent Progress in Mechanically Robust and Conductive‐Hydrogel‐Based Sensors

Flexible electronic technology has developed rapidly in recent years, showing broad application prospects in motion monitoring, wearable devices, and personalized medicine. Consequently, the demand for high sensitivity and wide sensing range has gradually increased. Conductive hydrogels have high flexibility, excellent conductivity, and good biocompatibility, making them ideal candidates for fabricating flexible sensors. However, conductive hydrogels exhibit weak mechanical stability, which limits their applications. Therefore, sufficient mechanical properties and fatigue resistance are usually needed to fulfill their application requirements. Herein, the research frontiers of sensors based on mechanically robust conductive hydrogels are reviewed. While published papers always focus on the configuration design and application of sensors and the improvement of sensing performance, research on the network design of hydrogels and their effects on mechanical properties and sensing performance are limited. It is attempted in this review to fill this gap by focusing on the design principles of mechanically enhanced conductive hydrogels and their applications in flexible electronic devices. Herein, hydrogels’ structural designs, toughening mechanisms, mechanical properties, and sensing applications are discussed. The different working mechanisms of flexible sensors composed of tough conductive hydrogels and their applications are also reviewed. Finally, the future development directions and challenges in this field are highlighted.

Conductive hydrogels can be prepared by adding conductive polymers, fillers, and free ions. [37]43] Hydrogel conductivity is a key factor in determining its sensing performance.46] In practical applications, flexible sensors often face complex stress environments; therefore, hydrogels usually need to have sufficient mechanical properties and fatigue resistance to cyclic loads to meet application requirements. [47]The distribution of conductive networks throughout the matrix affects their conductivity, mechanical strength, toughness, and sensing properties.Therefore, designing a fine network structure is crucial for producing hydrogels with excellent electrical and mechanical properties. [48,49]Common strategies for enhancing hydrogels include using double-network, [50] topological, [51] tetra-polyethylene glycol (tetra-PEG), [52] and composite hydrogels. [53]Combining the conductive mechanism with the strategy for high-strength and high-toughness hydrogels, a high-strength and high-toughness conductive hydrogel can be prepared, which promotes the development of hydrogel sensors.
While many published papers have focused on the configuration design and application of flexible sensors and the improvement of sensing performance, research on the network design of hydrogels and their effects on mechanical properties and sensing performance have been limited.This review attempts to fill this gap by focusing on the design principles of strong and tough conductive hydrogels and their application in flexible electronic devices (Figure 1).This paper discusses the structural design, toughening mechanism, mechanical properties, and sensing applications of these hydrogels and summarizes the different working mechanisms of flexible sensors from those of tough conductive hydrogels.Finally, the review highlights the future development directions and challenges in this field.

Different Types of Tough Conductive Hydrogel
Naturally, conductivity originates from the transmission of electrons or ions. [54]For example, inorganic materials such as metals often conduct electric currents through electrons, whereas transmitting conductive signals through free-charged ions in organisms is universal and extremely important. [38,55]The cross-linked network structure of the hydrogel contains a large amount of water, the water molecules can freely flow across the polymer network, and the high-frequency flow of water molecules provides a feasible method for preparing conductive hydrogels. [56]59][60][61][62][63] In practical applications, flexible sensors frequently encounter challenging stress environments and usually have to bear periodic loads, which require hydrogels to have robust mechanical properties to maintain structural stability and reduce the attenuation of sensing performance in the recycling process. [64,65]owever, conventional hydrogels exhibit low toughness owing to their uneven cross-linked network structure and lack of effective energy-dissipation mechanisms to prevent crack propagation and stress concentration. [66,67]Hence, when hydrogels are subjected to external forces, stress transfer in the nonuniform network focuses on structural defects or weak crosslinking points, resulting in rapid crack propagation and hydrogel rupture, [68] severely limiting their practical application.Thus, designing high-toughness hydrogels with excellent ductility, compressive resistance, and self-recovery ability is key to solving these problems. [69,70]From the relationship between the structure and properties of hydrogels, the density and dynamic characteristics of cross-linking points, uniformity of the network structure, and density and flexibility of the polymer chain affect the mechanical strength of hydrogels.The design of a hydrogel network is important for improving mechanical properties and toughness.This section briefly introduces the main types and preparation methods of conductive hydrogels and describes the relationship between their network structures, mechanical properties, and conductive properties.

Electronic Conductive Hydrogel
Electronic conductive hydrogels generally comprise a conductive network and a hydrogel matrix.The conductive network comprises conductive fillers or polymers formed by in situ polymerization, while hydrogels act as stretchable and deformable matrices. [49,71]79] Introducing electronic conductive materials provides conductivity to hydrogels and improves their tensile and mechanical properties.The direct addition of metal materials to hydrogels affects their elasticity and tensile properties.Therefore, metal fillers are typically introduced as NPs or nanowires. [2]In 2001, Willner et al. [61] successfully introduced Au NPs (AuNPs) into polyacrylamide (PAAm) hydrogels through the "breathing" mechanism, which endowed the hydrogels with good electrical conductivity.Kim et al. [80] successfully constructed a gelatin methacrylate (GelMA) hydrogel for biomedical applications by incorporating Au/SiO 2 hybrid NPs into a GelMA matrix (Figure 3a).Introducing hybrid NPs improves the mechanical properties of hydrogels and endows them with excellent electrical conductivity.The stretchability of hydrogels is important for the realization of flexible sensors.To facilitate high tensile properties, Hyun et al. [81] used the polymer polydimethylsiloxane (PDMS) buckling pattern as a template and added a PEG/ AgNP precursor solution to the trench.Through simple UV cross-linking, they successfully obtained a highly tensile conductive composite hydrogel (2.2 Â 10 7 S m À1 ) with an ordered zigzag structure.The hydrogel exhibited high stretchability owing to the interpenetrating network between the polymer gel and AgNPs and an ordered zigzag morphology (Figure 3b).Chen et al. [82] proposed a new method for preparing copper NPs by reducing CuCl 2 in a continuous medium and developed nanoconducting PAAm-g-polyvinyl alcohol/Cu NP (PAAm-g-PVA/CuNP) thin films with selective vapor-induced sensing properties through a simple self-assembly process (Figure 3c).The hydrogel film exhibits high electrical conductivity, good vapor responsiveness, and different resistance responses to different gases and has significant potential in gas-sensing applications.By introducing the metal NPs and nanowires, the mechanical properties and conductivity of the hydrogel were enhanced.However, there are always low interfacial interactions and high incompatibility between metals and polymers, which may lead to agglomeration of metals and phase separation of polymer networks from metals. [2,42]Therefore, great efforts still should be made for fabricating conductive hydrogels based on metals.
Carbon-based nanomaterials, such as carbon nanotubes (CNT), graphene, and carbon fibers, are considered conductive materials with significant development prospects because of their unique properties, such as high conductivity, good environmental stability, and good biocompatibility. [83,84]The excellent stability of carbon-based nanomaterials in humid environments Reproduced with permission. [141]Copyright 2018, Wiley-VCH.Fingers and knees bending.Reproduced with permission. [115]Copyright 2019, The Royal Society of Chemistry.Touch panel: Reproduced with permission. [168]Copyright 2016, AAAS.Healthcare: heart rate monitoring.Reproduced with permission. [134]Copyright 2019, The Royal Society of Chemistry.Electrocardiogram (ECG) and electromyography (EMG).Reproduced with permission. [170]Copyright 2019, Elsevier.Artificial skin: working mechanism and application of artificial skin.Reproduced with permission. [171]Copyright 2014, Wiley-VCH.Ionic skin with multiple responses properties.Reproduced with permission. [172]Copyright 2020, American Chemical Society.Ionic skin for distinguish different finger-bending signals.Reproduced with permission. [173]Copyright 2014, Wiley-VCH.
has significantly promoted their application as conductive nanocomposite hydrogels, making them good alternatives to metal nanomaterials. [85]Cai et al. [86] successfully introduced single-walled carbon nanotubes (SWCNTs) into PVA/borax hydrogels, endowing them with excellent electrical conductivity (Figure 3d).In addition, because of the dynamic borate-ester bonds between PVA and borax, the hydrogel exhibited excellent self-healing properties, taking only 3.2 s to restore the electrical conductivity to its original state.The rapid self-healing property ensures the rapid recovery of the sensor's electrical properties and prevents degradation of the sensing performance during the large deformation process.As a new 2D nanomaterial, graphene has lightweight and high electrical and mechanical properties expected to meet the requirements of portable electronic product applications and has attracted much attention.
Liu et al. [87] prepared solvent-resistant graphene-assisted conductive hydrogels by copolymerizing a hydrophobic monomer, 2-methoxyethyl acrylate, a hydrophilic monomer, and acrylic acid (AA) in a mixed solvent of water and dimethyl sulfoxide (DMSO) (Figure 3e).Graphene endows hydrogels with excellent electromechanical response properties, demonstrating great application prospects in wearable sensors.Han et al. [80] obtained a graphene-based conductive hydrogel by converting GO into conductive graphene via polydopamine (PDA) reduction (Figure 3f ).PDA participates in constructing the hydrogel network and endows the hydrogel with good mechanical and self-healing properties while also reducing GO content, thereby effectively improving the conductivity of the hydrogel.
[90] Owing to their tunable electrical conductivity, they have attracted considerable research interest in the past few years. [33,44]Therefore, introducing conducting polymers into hydrogels is an effective strategy for imparting electrical conductivity.PANI is a polymer produced by the in situ polymerization of aniline and has excellent electrical conductivity and good biocompatibility.Gazit et al. [91] successfully constructed a two-component all-organic conductive hydrogel by combining it with dipeptide N-fluorenyl methoxycarbonyl diphenylalanine (Fmoc-FF) (Figure 3g).Owing to its all-organic composition, the hydrogel demonstrates excellent biocompatibility and can be used as a soft substrate to support the growth of cardiomyocytes in response to electrical stimulation.Shi et al. [92] synthesized a thermoresponsive and conductive hybrid hydrogel via the in situ formation of continuous networks of conductive polymers (PPy) cross-linked by phytic acid in poly-(N-isopropyl acrylamide) (PNIPAM) matrices (Figure 3h).The hydrogel showed fast thermal response conductivity owing to the thermally responsive polymer PNIPAM, indicating wide application prospects in stimulation-responsive electronic devices.Rong et al. [93] introduced PEDOT:PSS into a PVA hydrogel system and obtained a conductive organic hydrogel using a water/ethylene glycol (EG) binary solvent (Figure 3i).Owing to the multiple hydrogen bonds between the EG and PVA molecules, conductive hydrogels have excellent mechanical properties and maintain good elasticity and electrical conductivity at low temperatures.
MXenes are a class of 2D inorganic compounds composed of transition metal carbides, metal nitrides, or metal carbonitrides with several atomic layers thickness. [94,95]The MXene has a large specific surface area, excellent conductivity, and outstanding hydrophilicity.These properties make MXenes become an ideal candidate to improve the conductivity of the hydrogel. [63,96]Yu et al. [97] successfully fabricated a MXene nanocomposite organohydrogel (MNOH) with frost resistance, self-healing properties, and high conductivity by immersing the hydrogel, which consist of PAAm, PVA, and MXene into the EG solution (Figure 3j).The MNOH exhibits excellent mechanical properties, high conductivity, and good self-healing properties.In addition, due to the presence of organic solvents, MNOH also has excellent anti-drying and anti-freezing properties.By introducing MXene and dimethyloctadecyl-(3-trimethoxysilylpropyl) ammonium chloride in sodium alginate (SA)-PAAm hydrogel substrate, Liu et al. [98] successfully developed a MXene-based conductive hydrogel (Figure 3k).The hydrogel exhibits high conductivity, good antibacterial properties, great mechanical properties, and self-healing properties, which further promote the application of conductive hydrogel in the biomedical-related flexible electronics.Wu et al. [99] obtained a conductive hydrogel by introducing the multifunctional multilayer MXene (m-MXene) into the polymer hydrogel substrate (Figure 3L).Here, the m-MXene cannot only serve as conductive filler to improve the conductivity of the hydrogel, but also can be served as initiator and cross-linker to initiate the polymerization of the cationic monomers (2-(acryloyloxy)ethyl) trimethylammonium chloride (DMAEA-Q) and cross-linking of the polymer.The resultant hydrogel possesses outstanding conductivity (3.8 S m À1 ), high stretchability (strain over 2000%) and elasticity, and good self-healing abilities.There are a large number of hydrophilic groups (ÀOH, ÀO, etc.) on the surface of MXene, which makes it have good hydrophilicity and easy to disperse in water.Conductive hydrogels with MXene as conductive fillers have excellent mechanical properties and high electrical conductivity, making them ideal candidates for flexible sensors.

Ionic-Conductive Hydrogel
The conductivity of ionic-conducting hydrogels is caused by the directional movement of free-charged ions embedded in the hydrogel network. [100]Ionic-conductive hydrogels have flexibility similar to biological tissues, making them ideal candidates as soft substrates for flexible wearable electronic devices. [101,102]he acid can be dissolved in the hydrogel to ensure ionic conductivity.The ionically conductive hydrogels obtained by this method are typically used to prepare flexible supercapacitors.Zhi et al. [103] used vinyl hybrid silica NPs (VSNPs) to cross-link PAAm to form a hydrogel, which was then soaked in phosphoric Figure 4. Representative ionically conductive hydrogels based on different conductive fillers.a) Schematic of the preparation process of ionic-conductive hydrogel based on phosphoric acid and its application as supercapacitor.Reproduced with permission. [103]Copyright 2017, Wiley-VCH.b) Fabrication process of the iron-ion-based conductive hydrogel.Reproduced with permission. [106]Copyright 2018, Elsevier.c) Formation mechanism of the poly-ionic-liquid-based conductive hydrogel.Reproduced with permission. [107]Copyright 2018, Wiley-VCH.d) Schematic of the synthesis of polyelectrolyte conductive hydrogels.Reproduced with permission. [108]Copyright 2019, American Chemical Society.
acid (H 3 PO 4 ) to obtain an ionically conductive hydrogel (Figure 4a).The hydrogel has high tensile properties and good ionic conductivity and can be used as an electrolyte to prepare stretchable and compressible supercapacitors.The most common method for preparing ionic-conducting hydrogels involves dissolving metal salts in the hydrogels.Metal ions can provide conductivity, enhance the network, and improve mechanical properties. [104,105]Fu et al. [106] successfully constructed a mechanically reinforced conducting hydrogel by introducing a hydrophobic unit, poly(-caprolactone)-poly(ethylene glycol)poly(-caprolactone) (PCEC), into a Fe 3þ -cross-linked polyacrylic acid (PAA) hydrogel (Figure 4b).The hydrogel exhibited ultrahigh tensile (up to 3800%) and excellent recoverability because of the synergistic effect of metal coordination and hydrophobic interactions.Simultaneously, owing to the presence of ions, the hydrogel also exhibits good conductivity (0.025 S m À1 ), which is expected to be applied to flexible and stretchable electronic devices.Sun et al. [107] obtained an ionicconducting hydrogel through the one-step polymerization of a positively charged imidazolium ionic liquid monomer containing a urea group and negatively charged 3-sulfopropyl methacrylate potassium salt monomers (Figure 4c).Owing to the electrostatic interaction between ionic liquids and the hydrogen bonding between urea groups, the hydrogel exhibits excellent mechanical properties (tensile strength %1.3 MPa, breaking strain %720%) and good self-healing properties (healing efficiency is about 91%).Fu et al. [108] developed a zwitterionic nanocomposite hydrogel physically cross-linked with exfoliated laponite (Laponite XLG) nanosheets (Figure 4d).The hydrogel was composed of zwitterionic [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide and 2-hydroxyethyl methacrylate (HEMA) copolymers.The incorporated zwitterionic polymers can form interchain dipole-dipole associations, effectively strengthening the hydrogel.Moreover, the reversible physical interactions endowed the hydrogel with rapid self-healing ability without any stimulation.

Conductive Hydrogel with Enhanced Mechanical Properties
Designing and controlling the structure of the conductive networks in hydrogels can lead to the development of highperformance conductive hydrogels with excellent strength, toughness, and conductivity. [2,49]These hydrogels meet the requirements for flexible sensors subjected to cyclic loads in practical applications.Advanced strategies for preparing strong and tough hydrogels, such as double networks, composites, and topological hydrogels, have laid the foundation for creating novel high-performance conductive hydrogels. [35,36,109]The conductive network can act as an energy dissipation mechanism, thereby improving the hydrogel's overall mechanical and electrical properties, including strength, toughness, conductivity, and sensing capabilities. [15]onstructing a double-network structure is important for fabricating conductive hydrogels with good mechanical properties. [33,110]Using a strong or flexible network as a matrix, a conductive polymer network is constructed in it, with the two networks connected through non-covalent interactions to obtain excellent strength and toughness. [44,85]Fu et al. [111] reported a double-network conducting hydrogel with an interpenetrating network structure comprising PANI and poly(acrylamide-co-hydroxyethyl methacrylate) (P [AAm-co-HEMA]) (Figure 5a).Owing to the formation of a double-network structure and multiple hydrogen bonds between the networks, the hydrogel exhibited excellent mechanical properties, with a tensile strength of up to 7.27 MPa.In addition, owing to the presence of conductive PANI, the hydrogel exhibited excellent conductivity of up to 8.24 S m À1 .Double-network hydrogels usually consist of two networks with different chemical structures and mechanical properties. [112]The first network acts as a rigid and brittle skeleton in the hydrogel, providing "sacrificial bonds" for the double-network hydrogel to disperse external stress and dominate the elastic modulus of the material.The second network is soft and stretchable network, which provides a substance for the double-network hydrogel and maintains the shape of the hydrogel. [113]Recently, Xu et al. [114] constructed a dual-cross-linked hydrogel composed of a cross-linked PVA interwoven aramid nanofiber (ANF) framework, in which ions and AgNPs were dispersed in a water-rich matrix.Highly branched ANF provides a robust 3D framework and is interwoven with PVA and cross-linked through non-covalent and covalent interactions, endowing the hydrogel with high strength (%7.5 MPa), high ductility (%407%), and good selfrecovery (%99.5%)(Figure 5b).The presence of conductive ions endows the hydrogel with a conductivity of 2 S m À1 , making it useful as a resistive sensor for monitoring human motion.Different from the conventional double-network hydrogel, the dual-cross-linked hydrogel is composed of a unique nanofiber network.ANFs provide a robust 3D framework for the hydrogel network, and PVA can be regarded as the shell of the nanofibers.Due to the existence of multiple hydrogen bonds between ANF and PVA, PVA is stably coated on the surface of ANF, forming a unique nanofiber network structure.
Combining NPs, nanorods, or nanosheets with a conductive hydrogel matrix can yield conductive nanocomposite hydrogels, and introducing nanomaterial pairs can improve their strength and toughness. [115]Wan et al. [116] successfully developed mechanically enhanced and ionically conductive hydrogels by introducing tannic acid (TA)-coated cellulose nanocrystals (TA@CNCs) into PAA networks (Figure 5c).The synergistic effect of nano reinforcement and metal coordination bonds effectively improves the toughness of the hydrogel (5.60 MJ m À3 ).The hydrogel-based resistive strain sensor exhibited a wide strain response (strains up to 2000% could be measured) and the sensitivity increased significantly with an increase in strain.
Topological structures exist widely in nature.Various biological macromolecules, such as DNA, RNA, and proteins, have topological structures, and the topological structures significantly affect the stability and physiological functions of these molecules. [117]Topological hydrogels are a class of polymer materials that combine the dynamic reversibility of non-covalent bonds and the structural characteristics of covalent topological polymers. [118,119]Due to their excellent mechanical properties, they have attracted more and more attention from researchers in recent years.As shown in Figure 5d, Hu et al. [51] proposed a hydrogen bond topology network regulation strategy and obtained a dynamic gel material consisting of cellulose macromolecules, ionic liquid, and H 2 O. H 2 O plays a crucial role in regulating the hydrogen bond topology between components.As the water content increases, the microstructure transforms into a dense and compact Turing pattern network, endowing the gel with good stretchability, toughness, and high ionic conductivity.
In this section, we discuss some advances in tough conductive hydrogels.Different strategies for strengthening hydrogels have different mechanisms.For double-network hydrogels, the introduction of a brittle sacrificial network with high cross-link density provides an efficient energy dissipation mechanism for hydrogels, effectively improving the mechanical properties of hydrogels. [120]Different from double-network hydrogels, the improvement of mechanical properties of nanocomposite hydrogels mainly comes from the synergy effect between nanofillers (NPs, nanofibers) and polymer networks and the mechanical reinforcement of nanofillers themselves. [121]Topological hydrogels are cross-linked by reversible non-covalent bonds, which being able to enhance mechanical properties of the traditional hydrogel.In addition, due to the higher regular mechanically interlocked and macrocycle-based microstructure, topological hydrogels exhibit better stretchability and reversibility than conventional hydrogels. [70]s mentioned earlier, through the introduction of conductive fillers, researchers successfully constructed a series of conductive hydrogels with enhanced mechanical properties.Electronic conductive polymers and ionic-conductive polyelectrolytes can be used to construct conductive networks, which can not only improve the conductivity of the hydrogels, but also form a dual-network structure with the polymer network to improve the mechanical properties of the hydrogels. [108,111]Metal/ carbon-based nanomaterials and MXene are usually used as nanofillers, which work synergistically with polymer networks to construct nanocomposite reinforced conductive hydrogels. [81,87]The distribution of conductive fillers throughout the matrix is extremely important for the electrical conductivity, mechanical strength and toughness, and sensing performance  [111] Copyright 2018, Wiley-VCH.b) Dual-cross-linked aramid nanofiber (ANF) hydrogel with high tensile strength and good strain-sensing properties.Reproduced with permission. [114]Copyright 2021, American Chemical Society.c) Nanocomposite conductive hydrogel based on tannic acid (TA)-coated cellulose nanocrystals (TA@CNCs) and poly(acrylic acid) (PAA). [116]Reproduced with permission.Copyright 2018, American Chemical Society.d) Topological network conductive hydrogel based on cellulose and ionic liquid.Reproduced with permission. [51]Copyright 2020, Elsevier.
of the hydrogel. [35]Conductive fillers need to be evenly dispersed throughout the polymer network to ensure a continuous conductive path, which in turn guarantees stable sensing performance.When the conductive filler is unevenly distributed or aggregated, the material conductivity changes, which hinders the overall sensing performance.Therefore, the introduction of conductive fillers and the construction of conductive network are crucial for conductive hydrogels, which not only affect their conductive and mechanical properties, but also affect their sensing properties.

Tough Conductive-Hydrogel-Based Sensors
Owing to its high-stress threshold and good conductivity, a mechanically robust and stretchable conductive hydrogel is an ideal candidate material for wearable, flexible sensors. [122]lexible sensors can detect and quantify external stimuli and convert them into electrical signals. [25,123]Common methods for converting external information into electrical signals are to detect resistance, capacitance, piezoelectricity, and triboelectricity changes. [21,124]he working principle of resistive sensors is based on the piezoresistive effect (Figure 6a).The piezoresistive effect occurs when a sensor deforms in response to external stimuli, changing the material's resistance.Therefore, the output signal can intuitively reflect the magnitude and frequency of external stimuli. [125]he capacitive sensors typically comprise parallel capacitors with a sandwich structure (a dielectric layer sandwiched between two electrodes) (Figure 6b).When external stimuli are applied, the contact area and thickness of the dielectric layer change, causing a change in the capacitance. [126]Like capacitive sensors, piezoelectric sensors present a typical sandwich structure (Figure 6c), usually constructed using piezoelectric hydrogels sandwiched between two electrodes.Piezoelectric sensors are based on piezoelectric effects. [127]Piezoelectric materials are deformed when subjected to external stimuli, and the internal charge of the state is polarized, thus outputting a current or voltage.Unlike resistive and capacitive sensors, piezoelectric sensors do not require an external energy supply.The working mechanism of the triboelectric sensor was based on the triboelectric effect, a type of contact-induced electrification (Figure 6d).When two materials are in contact, opposite static charges are produced on the surface.After the strain/pressure was released, the two surfaces with opposite charges automatically separated and generated compensation charges, thus forming a potential difference.This mechanism enables a triboelectric device to generate electrical signals in response to various mechanical stimuli; thus, it can be used as a self-powered tactile sensor. [128,129]The sensing performance of a sensor is related to the sensor's type, structure, and configuration and to the properties and structure of the hydrogel itself.Table 2 summarizes and compares the main structures and properties of representative stress-strain sensors based on highly conductive hydrogels.
Table 3 presents a comparison of flexible sensors based on different working mechanisms.Resistive and capacitive flexible sensors have been widely studied owing to their simple working principles and easy preparation.This section introduces the basic design of flexible resistive and capacitive sensors from the viewpoint of working mechanisms and strategies to achieve high sensitivity and a wide working range.

Resistive Sensor
Resistive sensors have been extensively studied because of their easy fabrication, simple structure, and easy signal acquisition. [21,127]Resistive hydrogel sensors are based on the principle of resistance changes.During the straining process or under loading, the conductive network in the hydrogel is deformed, and the conductivity or resistivity changes accordingly. [130,131]The conductive hydrogel sensor can convert an external force or deformation into an electrical signal for sensing. [132]In general, the electrical resistance of a hydrogel can be calculated by the formula R = ρL/A, where ρ is the resistivity of the hydrogel, and L and A represent the length and cross-sectional area of the hydrogel, respectively. [2,133]To represent the sensitivity of the resistive hydrogel sensor, the gauge factor (GF) is usually used, calculated by the formula GF = (ΔR/R 0 )/ε, where ΔR represents the change in resistance, R 0 is the initial resistance, and ε represents strain or pressure. [6,134]The GF is an important index for evaluating the sensing performance of sensors.In general, the higher the GF value, the better the sensing performance of the device. [135,136]Conventional conductive hydrogel sensors usually suffer from low sensitivity; therefore, various material selection and structural optimization strategies have been proposed to address this issue.
Generally, the shape deformation of conductive hydrogels used in resistive sensors can be categorized into two main types: tensile and pressing. [35]Both electronic and ionic-conductivehydrogel-based resistive sensors measure changes in resistance under applied deformation; however, their sensing mechanisms differ owing to the nature of conductivity. [16,36]lectronic conductive-hydrogel-based resistive sensors rely on changes in the distance between the conductive materials within the hydrogel caused by deformation, which alters the resistance of the hydrogel. [15,37]As the hydrogel is deformed, the distance between the conductive materials changes, leading to a change in the measurable resistance.Guo et al. [137] prepared an electronconductive nanocomposite hydrogel by introducing laponite and CNT into a PNIPAM substrate as a nano-reinforcement (Figure 7a).The introduction of rigid conductive CNT significantly increases the cross-link density of the polymer chains and hydrogen bonds in the hydrogel, thereby effectively improving its mechanical properties.In addition, the hydrogel-based  resistive sensor exhibited good sensing performance and could be used for human motion detection, including joint flexion (such as finger, wrist, and knee) and pulse detection.Yao et al. [135] developed a nanocomposite conductive hydrogel using chemically cross-linked PAAm as the backbone and oxidized multi-walled CNTs (oxCNTs) as the reinforcing agent (Figure 7b).The multiple hydrogen bonds between the gelatin and oxCNTs help the oxCNTs to be uniformly dispersed in the system.The physical interaction between the oxCNTs, gelatin, and PAAm chains effectively improves the performance of the hydrogel.The hydrogel exhibited excellent mechanical properties (fracture strain exceeding 1000% and the fracture stress reaching 0.71 MPa) and good electrical conductivity (0.067 S m À1 ).Owing to tunnel resistance and contact resistance effects, the hydrogel sensor exhibited higher sensitivity under high strain (GF = 3.37, at 250%-700%).Furthermore, the sensor exhibited a fast response (300 ms) and excellent durability (over 300 cycles) and can be used for human motion monitoring.Recently, researchers have developed a series of conductive hydrogels with anisotropic network structure and applied them to the field of resistive sensors.These hydrogels exhibited high mechanical strength and good sensing properties. [138,139]As Figure 7. Representative resistive sensors based on electronic conductive hydrogel.a) Conductive nanocomposite hydrogel with 3D printability for human motion monitoring.Reproduced with permission. [137]Copyright 2019, American Chemical Society.b) CNT-based conductive hydrogel with high stretchability and toughness for resistive strain sensor.Reproduced with permission. [135]Copyright 2020, Elsevier.c) Polydopamine (PDA)-based conductive hydrogel with anisotropic structure for resistive sensor.Reproduced with permission. [138]Copyright 2022, Elsevier.
shown in Figure 7c, Lu et al. [138] constructed an anisotropic conductive hydrogel based on dialdehyde cellulose-PDA (DAC-PDA) by magnetic field induction.Due to the presence of anisotropic structure, the hydrogel exhibits improved anisotropic mechanical strength, the tensile and compression stress in parallel direction is much higher than the perpendicular direction.The anisotropic structure also endows the hydrogel with anisotropic conductivity, and the conductivity in the parallel direction is as high as 7 S m À1 .The anisotropic conductivity further affects the sensing performance of the hydrogel, and different directions of stretching and compression have different sensitivities.
Ionically conductive hydrogels are highly stretchable and exhibit excellent transparency and compatibility, making them ideal for strain sensors designed to detect and monitor human motion. [31,130]The ionic compounds present in the hydrogel help toughen the material and maintain its structural integrity even under large tensile deformations.The stretch-induced reduction in the cross-sectional area and increase in length can affect the resistance of the hydrogel, which can be measured to detect changes in strain or deformation. [35,42]These sensors have potential applications in wearable technologies, health monitoring, robotics, and other fields. [28,47]Li et al. [130] developed an ion-conducting double-network hydrogel based on gelatin and PAAm using a simple solvent-immersion method (Figure 8a).During the Na 3 Cit soaking process, the ions diffused into the PAAm/gelatin pre-hydrogel enhanced the gelatin chains' hydrophobic and chain bundling effect; ionic interactions and hydrogen bonding were also observed during this process.Subsequently, a double-network hydrogel was formed, exhibiting high tensile strength (1.66 MPa) and break elongation (849%), as well as rapid self-recovery and good fatigue resistance.In addition, the ion-conducting hydrogel can be used as a strain sensor, monitors pressure, and exhibits high sensitivity.
Conventional conductive hydrogels are limited by their inability to maintain their conductivity and stretchability under extreme environmental conditions. [65,140]For example, at subzero temperatures, hydrogels can freeze and lose their conductivity and stretchability, rendering them unsuitable.Similarly, at ambient temperatures, hydrogel networks can lose moisture via evaporation, which limits their long-term usability. [141,142]There is a growing need to develop conductive hydrogels with superior anti-freezing and long-lasting moisturizing properties to overcome these limitations. [143]Sun et al. [144] successfully prepared a nano-reinforced PVA organogel by self-assembling lignin NPs in a DMSO/H 2 O binary solvent, followed by a simple freeze-thaw process (Figure 8b).The strong hydrogen bond interaction between DMSO and water molecules effectively prevents the formation of ice crystals (À80 °C) and effectively reduces the evaporation of water.Moreover, the gel exhibited excellent sensitivity and good cycling stability.Qi et al. [145] developed a novel ion conductor based on natural bacterial cellulose (BC) and a polymerizable deep eutectic solvent (PDES) that effectively solved the dehydration and deliquescence problems of ion conductors (Figure 8c).The deep eutectic solvent comprised choline chloride (ChCl) and AA.Ionic conductors were prepared by a simple photoinitiation method.AA monomers were first polymerized to form a linear covalent backbone, cross-linked to form a polymer network, and hydrogen bonds were formed between PAA and BC to establish an interpenetrating network.Owing to the unique multilevel and interpenetrating polymer-nanofiber network structure, the ionic conductor exhibits excellent mechanical properties (fracture tensile stress 0.8 MPa and fracture compression stress 6.8 MPa) and good conductivity (0.18 S m À1 ).Moreover, this ionic conductor exhibits multiple sensitivities to external stimuli such as strain, pressure, bending, and temperature.Therefore, they can easily identify the different amplitudes of human motion with high sensitivity.
Introducing microstructures onto the surface of conductive hydrogels can enhance their sensitivity and responsiveness to external stimuli. [146,147]Microstructures can be designed to increase the surface area of a hydrogel, which can result in larger variations in the strain and contact area. [126]This can improve sensing accuracy and enable hydrogels to detect smaller environmental changes. [21,127]The shape and size of the microstructures can be tailored to suit specific applications and requirements.Jiang et al. [146] successfully constructed an electronically conductive hydrogel with a wrinkled Janus surface structure by pouring AgNWs onto the surface of a PAA/chitosan substrate (Figure 8d).The hydrogel-based resistive sensor exhibited ultrahigh sensitivity owing to its unique surface wrinkle structure (GF = 18 720, at 60-100% strain).Wrinkling hydrogel sensors can accurately monitor finger-bending motions at different angles, showing wide application possibilities in electronic skins and soft robotics.

Capacitive Sensor
Capacitive sensors are prepared based on the principle of capacitors.Capacitive sensors typically employ a parallel-plate structure in which a dielectric layer is sandwiched between a pair of electrodes. [148,149]Capacitance is a physical quantity that measures the ability of a capacitor to hold charges, which can be calculated by the formula C = εA/D, where ε, A, and d denote the permittivity, contact area, and thickness of the dielectric layer, respectively. [8,64]When the external force is applied to the capacitor, it will cause changes in the plate area and distance between the induction and driving electrodes.In addition, it also leads to certain changes in the dielectric properties and, eventually, to changes in the sensor capacitance. [126,127]Then, the measured change was collected and output to detect the loaded stress or strain.The sensitivity (S) of the capacitive sensor can be calculated by the formula S = (ΔC/C 0 )/P, here, ΔC represents the change in capacitance; C 0 is the initial capacitance, and P represents strain or pressure. [148]Owing to its light weight, high sensitivity, low noise, low-temperature bleaching, and low power consumption, it is widely used in soft robotics and bionic skin research. [126,148]nspired by natural skin, Lu et al. [150] developed a cellulose nanocrystal (CNC)-based nanohybrid-reinforced (decorated with TA and AgNPs) PVA hydrogel (Figure 9a).The unique dynamic borate bond cross-linking structure and multiple hydrogen bonds endowed the hydrogel with ultrahigh stretchability (>4000%) and good self-healing properties.Many catechol groups in the hydrogel endow it with strong and durable self-adhesion.Owing to the excellent conductivity of the AgNPs, the hydrogel exhibited a high electrical conductivity of 4.61 S m À1 .A capacitive sensor was successfully constructed using a conductive hydrogel as the electrode and very high bond (VHB) tape as the dielectric.The sensor demonstrated excellent sensing performance and can track human motion over a relatively wide perceivable strain range (up to 400%).
Conventional capacitive pressure sensors with parallel-plate structures based on electronic conductive electrodes and dielectrics suffer from low sensitivity owing to the almost unchanged volume of the polymer matrix. [21,151]In addition, the limited permittivity of the dielectric layer always results in a sensor with a low capacitance, which is highly influenced by parasitic charges and environmental magnetic noise. [149]In 2011, Pan et al. proposed a new iontronic sensor that used a glycerol/water solution of NaCl as the ionic-conductive electrolyte with good recovery properties for strain and pressure sensing.Reproduced with permission. [130]Copyright 2020, Wiley-VCH.b) Anti-freezing and antidehydration hydrogel-based flexible sensor fabricated by binary solvent.Reproduced with permission. [144]Copyright 2022, Elsevier.c) Ionic conductor based on cellulose nanofiber and polymerizable deep eutectic solvent with long-lasting moisture-maintaining properties for multifunction sensors.Reproduced with permission. [145]Copyright 2020, American Chemical Society.d) Flexible resistive sensors based on wrinkling-surface-structured chitosan hydrogels with enhanced sensitivity.Reproduced with permission. [146]Copyright 2022, Springer Nature.and PET/indium tin oxide (ITO) as the electrode. [152]Unlike conventional capacitive pressure sensors, this iontronic sensor is based on the electrical double layer (EDL) capacitance formed at the interface of the electrolyte and the electrode with an ultrahigh unit area capacitance (UAC) of about several μF cm À2, which makes it effectively overcome the effect of parasitic charges and environmental magnetic noise.An ionic liquid ([EMIM][TCM])-based ionic-conductive hydrogel was fabricated by Nie et al. [153] using a simple photo-cross-linking approach (Figure 9b).A capacitive sensor was constructed using an ion-conducting gel as the electrolyte and PET/ITO as the electrode.When an external load causes sensor deformation, an EDL is formed between the ionic conductor and the electrode.The UAC is up to 5.4 μF cm À2 , 1000 times larger than the conventional capacitive sensor, which effectively inhibits the interference of parasitic charge and environmental magnetic field and improves the sensor's sensitivity.
Conventional capacitive sensors typically comprise a bulk dielectric material sandwiched between two electrodes in a parallel-plate configuration. [64,126]The sensing performance of Reproduced with permission. [150]Copyright 2019, The Royal Society of Chemistry.b) Ionic-conductive hydrogel for capacitive sensor based on electric double layer.Reproduced with permission. [153]Copyright 2015, Wiley-VCH.c) Ionic skin based on wrinkled surface structure hydrogel with enhanced sensitivity.Reproduced with permission. [155]Copyright 2018, The Royal Society of Chemistry.d) Graded intrafillable architecture hydrogel-based supercapacitive sensor with broad sensing range.Reproduced with permission. [156]Copyright 2020, Springer Nature.
such sensors often depends on changes in the thickness of the dielectric owing to external stimuli; however, most hydrogels are inherently viscoelastic, which increases hysteresis and reduces the response speed of the device, limiting improvements in sensitivity. [148,154]Therefore, to solve this problem, researchers have made great efforts to improve the sensing range by designing the microstructure of the surface of the dielectric material. [8]nspired by skin wrinkles, Zhou et al. [155] obtained an organogel-coated hydrogel (OHGel) ionic skin with a superficial wrinkled microstructure via simple hydrophobic solvent immersion (Figure 9c).The ionic skin based on OHGel with a surface wrinkle structure had significantly superior sensitivity to the ionic skin based on OHGel without a wrinkle structure.Guo et al. [156] developed an ionic pressure sensor with a graded intrafillable architecture (GIA) comprising a PVA/H 3 PO 4 electrolyte with a surface microstructure and a PI/Au electrode (Figure 9d).Owing to the presence of different heights and unstable GIA structures in the electrolyte, the compressibility of the sensor is effectively improved, and the sensing range (360 kPa) and sensitivity (220-3300 kPa À1 ) of the sensor are greatly enhanced.Furthermore, the sensor exhibited excellent mechanical stability and could withstand more than 5000 compression/release cycles at a high pressure of 300 kPa without significant fatigue or signal decay.In addition to the structures mentioned earlier, researchers have also designed various microstructures, such as micropyramid, microdome, serpentine, and mesh structures, which effectively improved the sensing performance of capacitive sensors. [154]

Piezoelectric Sensor
Piezoelectric sensor makes use of piezoelectric effect.The piezoelectric effect occurs when the piezoelectric dielectric material deforms in response to an external force acting in a certain direction. [157]Due to the polarization of the internal charges, opposite bound charges will be generated on the opposite surface of the piezoelectric material, and electric dipoles will be formed, resulting in the formation of a piezoelectric potential. [21,25]When the applied forces separate, the material returns to its original state.Different from capacitive and resistive flexible sensors, piezoelectric flexible sensors do not need external power or batteries, can be selfpowered, and has high sensitivity and fast response time.However, static responses are unreliable.The piezoelectric coefficient (d 33 ) is an important parameter for evaluating the energy conversion efficiency of piezoelectric materials, and generally, the larger the piezoelectric coefficient, the higher sensitive the sensor. [158]s a piezoelectric material, the conductive polymer polyvinylidene fluoride (PVDF) and its copolymers have attracted extensive attention of researchers in recent years because of their good flexibility, low impedance, and high piezoelectric coefficient.Fu et al. [159] obtained a tough self-powered piezoelectric hydrogel by incorporating piezoelectric PVDF into a tough in situ synthesized polyacrylonitrile (PAN) hydrogel matrix (Figure 10a).When the applied stress causes the piezoelectric material to deform, dipole interactions occur between PVDF and PAN chains, which is conducive to the formation of the optimal electroactive β-phase PVDF in the composite, inducing the Reproduced with permission. [159]Copyright 2019, American Chemical Society.b) PEDOT:PSS conductive-hydrogel-based piezoelectric sensor with enhancing strain-sensing properties.Reproduced with permission. [160]Copyright 2022, American Chemical Society.
polarization effect and producing high piezoelectric coefficient (d 33 = 30 pC N À1 ), high power output (%30 mV and 2.8 μA), and fast response (%31 ms).The hydrogel also exhibits high toughness (1.23 MJ m À2 ) and skin-like elastic modulus (1.33-4.24MPa) and stretchability (90-175%).The hydrogelbased pressure sensor enables precise detection of physiological movements such as finger flexion, pulse, and vocal cord vibration.Li et al. [160] synthesized a piezoresistive and piezoelectric composite hydrogel for flexible strain sensors using PEDOT:PSS as conductive filler, PVDF-co-trifluoroethylene (PVDF-TrFE) as piezoelectric filler and cross-linked chitosan quaternary ammonium salt (CHACC) as hydrogel matrix by one-pot hot molding and solution exchange method (Figure 10b).The hydrogel exhibits high mechanical properties (stress, 0.33 MPa; strain, 293%) and good mechanical endurance.Due to the synergistic effect of PVDF-TrFE and PEDOT:PSS, the strain sensor based on this hydrogel has very high sensitivity (GF: 19.3), and fast response (response time: 63.2 ms).In addition, sensors based on the hydrogel can also be used for object recognition, and can also be composed of sensor arrays as flexible keyboards or wearable flexible input devices.
Although PVDF and its copolymers have been widely developed in the field of piezoelectric sensing, the low piezoelectric strain constant (d 33 ) of PVDF itself greatly limits its further application.Unlike piezoelectric polymers, inorganic materials with non-centrosymmetry, such as ZnO, ZnS, GaN, CdS, and MoS 2 , mostly have high d 33 and are promising candidates for piezoelectric pressure sensors. [161]However, these inorganic materials often suffer from low flexibility. [127]The sensing performance to convert external stimuli into electrical signals requires high sensitivity, yet flexibility, and stretchability are also crucial for the assembly of electronic skin devices.Therefore, it is necessary to develop a flexible sensor with both flexibility and high sensitivity.

Triboelectric Sensor
Similar to piezoelectric sensors, triboelectric sensors utilizing the triboelectric effect are also able to harvest energy from the surrounding environment and operate without an external power source. [162,163]The triboelectric effect is a kind of universal contact induction electrogenesis phenomenon. [25]When two different materials are in contact with each other, the surface will produce opposite electrostatic charge.When two materials with opposite charges are separated, a compensating charge is generated on the surface, creating an electric potential difference (EPD). [6,8]The EPD can be calculated by the equation EPD = σd 0 /ε 0 , here σ is the triboelectric charge density, d 0 is the separated distance of triboelectric films at a given state, and ε 0 is the vacuum permittivity. [2]The magnitude of the EPD depends on σ, which depends on the triboelectric polarity difference between the two contact materials.
In 2012, Wang et al. [163] constructed the first triboelectric nanogenerator (TENG) and applied it to the field of pressure sensing.As a flexible sensor, the TENG's flexibility, stretchability, and transparency are ideal.Conductive hydrogels possess excellent electrical conductivity, mechanical flexibility, and high stretchability, making them ideal candidates for fabrication of triboelectric sensors.Wang et al. [129] fabricated a transparent and electrically conductive contact-separated TENG using doublenetwork ionic conducting gels as electrodes and friction layer, and patterned PDMS as another friction layer (Figure 11a).The sensor has high transparency (83%) and good tensile  [129] Copyright 2019, Elsevier.b) Triboelectric nanogenerator as electronic skin based on PAAm-LiCl hydrogel.Reproduced with permission. [128]Copyright 2017, AAAS.
property (121%).The self-powered sensor showed good pressure sensitivity (0.39 V N À1 at 0 strain; 1.76 V N À1 at 50% strain), and because of the good tensile property of the sensor, the frictional electrical signal maintains a good linear relationship with the impact force at different tensile ratios.Pu et al. [128] reported a TENG with PAAm hydrogel containing lithium chloride (LiCl) (PAAm-LiCl hydrogel) as electrode and elastomer PDMS or 3M VHB as electrification layer (Figure 11b).The hydrogel-based sensor simultaneously achieved ultrahigh stretchability (strain, 1160%) and high transparency (up to 96.2%).The TENG-based electronic skin can sense strain up to 800% and pressure as low as 1.3 kPa.It has shown great application potential in intelligent Figure 12.Applications of conductive-hydrogel-based sensor.a) Dual physical cross-linked ionic-conductive-hydrogel-based sensor for human motion sensing.Reproduced with permission. [136]Copyright 2019, The Royal Society of Chemistry.b) Application of polymerized conductive polypyrrole hydrogelbased sensor as bioelectrode for physiological signal monitoring.Reproduced with permission. [164]Copyright 2020, American Chemical Society.c) Supercapacitive-sensor-based wearable health monitoring system.Reproduced with permission. [167]Copyright 2016, Springer Nature.d) MXene-hydrogelbased sensor applicable as touch panel.Reproduced with permission. [169]Copyright 2018, AAAS.
artificial skin, soft machines, and wearable electronic products.Triboelectric sensors generally show relatively low sensitivity compared to resistive sensors.Moreover, the structures of triboelectric sensor devices are complex, and they not only need conductive hydrogels, but also other materials to form triboelectric pairs.In addition, the operation of the sensor inevitably requires a complete contact and separation process to recognize external stimulus signals.Therefore, they are better suited to detecting dynamic stimuli than static forces.

Applications
As a new type of conductive material, conductive hydrogels exhibit conductivity and various characteristics, such as good flexibility and biocompatibility. [36,42]These advantages have made conductive hydrogels ideal candidates for flexible sensors, resulting in extensive research attention.Recently, owing to their excellent foldability and stretchability, excellent electrochemical performance, and good working stability, flexible electronic devices based on conductive hydrogels have attracted increasing attention and have gradually become a research hotspot; they are widely used in various artificial intelligence and biomedical fields. [38,49]wing to its excellent stretchability (tensile strain 300-10 000%) and flexibility, the hydrogel-based sensor showed a wider detection range than conventional elastomeric substratebased sensors. [14,17]In addition, the elastic modulus of hydrogels is similar to human skin, making them more compatible when the sensors are attached to the skin.This similarity in the modulus also helps improve sensor readings' accuracy, making them ideal for detecting human movement. [6,16]Inspired by cartilage, Xu et al. [136] prepared a fully physically cross-linked ionicconducting dual network hydrogel (Figure 12a).The hydrogel comprised a hydrophobic PAAm-PAA network and an ioncross-linked chitosan network.The presence of ions (Fe 3þ , Na þ , Cl À ) gives the hydrogel excellent conductivity, making it useful as a strain sensor for monitoring human motion.Hydrogels can be integrated into human joints, such as the neck, elbow, wrist, and knee, to monitor human motion.The hydrogel strain sensor exhibited different resistance signal changes in response to the bending of different joints, and the corresponding resistance signal changes remained almost unchanged after multiple joint movements.
Conductive-hydrogel-based sensors have become increasingly popular in biomedical applications.They can monitor and record various physiological signals from the human body, such as heart rate and blood pressure. [16,44]Chen et al. [164] designed a biocomposite electrode based on conductive polypyrrole and silk fibroin protein hydrogel to realize long-term monitoring of dynamic electrophysiological signals (Figure 12b).Electrodes can be used to collect dynamic electrophysiological signals such as ECG, electroencephalogram, and electrooculogram, which can provide useful data support for health monitoring and disease diagnosis.
Wearable personal health monitoring systems are the core solution for next-generation health-monitoring technology. [165,166]Monitoring equipment can continuously perceive, acquire, analyze, and store a large amount of physiological data in the daily activities of the human body and provide necessary, accurate, and long-term assessment and feedback on the health status of the human body.Pan et al. [167] developed a pressuresensing platform based on ionogel droplets (Figure 12c).The platform comprises an ion-conducting gel electrolyte and PET/ITO film electrodes, adopts the principle of interface capacitive sensing, has a simple structure, and is highly sensitive to pressure changes.By combining the sensor with leggings, a wearable device that can measure the interface pressure of a venous lower-extremity ulcer (VLU) was successfully constructed, effectively overcoming the limitations of conventional VLU interface pressure measurement devices.
The human-machine interaction interface is the interface for information and command exchange between humans and machines, which can sense external stimuli and provide interactive responses for users. [132]Flexible touch panels provide an intuitive and simple interface for human-computer interaction and are considered future electronic components. [16]Touch panels are used in various applications, including smartphones, tablets, and televisions. [49,168]Currently, the most commonly used touch panel technologies include resistive and capacitive touch.Alshareef et al. [169] developed an MXene-based PVA conductive hydrogel and assembled it as a capacitive sensor to monitor pressure changes (Figure 12d).The hydrogel-based sensor can be used as a sensing film for signature recognition, realizing stable and continuous monitoring of the writing force, speed, and sequence, showing great application potential in touch panels.
With the deepening of research, more and more conductive hydrogels have been applied in the field of flexible sensors.Different application scenarios have different performance requirements for conductive hydrogels.Taking motion monitoring as an example, hydrogel materials are often faced with complex stress environments, so mechanical strength and high stretchability are often required.For bioelectrodes, the biocompatibility of hydrogels is crucial.When applied to wearable devices, we need to consider the compliance and adhesiveness of hydrogel materials to avoid interface separation and loss of sensing signals.For touch panels, the transparency of the hydrogel is essential.Therefore, for different application scenarios, we need to comprehensively consider the properties of hydrogels to meet the application conditions and obtain the best sensing performance.

Conclusions and Outlooks
In this review, we summarize and discuss the recent research progress in flexible sensors based on conductive hydrogels.First, we introduce the design and fabrication of different types of conductive hydrogels, and their structure-property relationships.Second, we discuss the working mechanism and sensing performance of different types of flexible sensors based on conductive hydrogels.Owing to excellent mechanical properties and excellent electrical conductivity, as well as good flexibility and stretchability, flexible sensors based on conductive hydrogels have shown promising application prospects in fields such as motion monitoring, wearable devices, and human-machine interfaces.
With the development of biomaterials, pressure/strain sensors based on conductive hydrogels have made considerable progress in mechanical properties, conductivity, and sensing properties.However, the development of conductive hydrogels still faces the following challenges.1) Maintaining stable conductivity under working conditions: conductive hydrogels contain a large amount of water, which inevitably causes them to inevitably freeze at subzero temperatures and dry in hot environments, resulting in a decrease in electrical conductivity, and thus affecting their sensing performance, which severely limits their practical application at extreme temperatures.Therefore, designing hydrogels with high conductivity and resistance to extreme temperatures remains critical for the development of flexible electronic devices; 2) Whether to work stably for a long time in a complex environment: in practical applications, conductive hydrogels are often faced with complex stress environments, and the long-term cycling process will lead to hydrogel fatigue, resulting in a decrease in mechanical properties, which in turn affects the sensing performance.In addition, the irreversible damage of the conductive network of the conductive hydrogel will also lead to a decrease in the conductivity of the hydrogel, thereby affecting the sensing performance.The construction of flexible sensing devices often requires the integration of conductive hydrogels with other functional units.Compared with traditional electronic materials, conductive hydrogels have natural flexibility and low Young's modulus, which often leads to interface separation during use.Therefore, developing multifunctional hydrogel with high conductivity, high mechanical properties, good conformability, reliable adhesion, and selfhealing property is necessary; and 3) Whether conductive hydrogels can avoid damage to biological tissues or cause an inflammatory reaction when forming conformal contact with biological tissue: when applied in biomedical fields such as smart wearable devices, artificial skin, and bioelectrodes, conductive hydrogels must have good biocompatibility to ensure that they do not cause immune reactions and have no toxic side effects.
The basic performance requirements of conductive hydrogels for preparing flexible, wearable electronic devices are high conductivity, adjustable mechanical properties, and high stretchability.In the practical application of current flexible wearable electronic devices, conductive hydrogels are often required to have various additional functions.Under the condition of satisfying high conductivity, how to simultaneously achieve high-efficiency antibacterial properties, good biocompatibility, high mechanical strength, excellent self-adhesion, and good self-healing properties is still a challenge.In addition, the high water retention of conductive hydrogels and packaging technology related to hydrogel sensors and other device units should be considered.Finally, the preparation of hydrogels with high mechanical strength and high conductivity remains a challenge, which is very important for their application in flexible sensing.Overall, in our opinion, the development of flexible electronic devices based on conductive hydrogels will have an indispensable impact on human life.

Figure 2 .
Figure 2. Representative strategies for synthesis of conductive hydrogels.

Figure 6 .
Figure 6.Schematic of structure and working mechanism of conductive-hydrogel-based strain/pressure sensors operated under a) resistive, b) capacitive, c) piezoelectric, and d) triboelectric mode.

Figure 8 .
Figure 8. Representative resistive sensors based on ionic-conductive hydrogel.a) Ionic-conductive double-network hydrogel based on PAAm and gelatin with good recovery properties for strain and pressure sensing.Reproduced with permission.[130]Copyright 2020, Wiley-VCH.b) Anti-freezing and antidehydration hydrogel-based flexible sensor fabricated by binary solvent.Reproduced with permission.[144]Copyright 2022, Elsevier.c) Ionic conductor based on cellulose nanofiber and polymerizable deep eutectic solvent with long-lasting moisture-maintaining properties for multifunction sensors.Reproduced with permission.[145]Copyright 2020, American Chemical Society.d) Flexible resistive sensors based on wrinkling-surface-structured chitosan hydrogels with enhanced sensitivity.Reproduced with permission.[146]Copyright 2022, Springer Nature.

Figure 9 .
Figure 9. Representative capacitive sensors.a) Electronic conductive hydrogel based on CNCs and polyvinyl alcohol (PVA) for capacitive sensor.Reproduced with permission.[150]Copyright 2019, The Royal Society of Chemistry.b) Ionic-conductive hydrogel for capacitive sensor based on electric double layer.Reproduced with permission.[153]Copyright 2015, Wiley-VCH.c) Ionic skin based on wrinkled surface structure hydrogel with enhanced sensitivity.Reproduced with permission.[155]Copyright 2018, The Royal Society of Chemistry.d) Graded intrafillable architecture hydrogel-based supercapacitive sensor with broad sensing range.Reproduced with permission.[156]Copyright 2020, Springer Nature.

Table 1 .
Advantages and disadvantages of flexible sensor and traditional sensor.

Table 2 .
Summary of conductive hydrogels under different sensing modes.
a)N/A: not applicable.

Table 3 .
Comparison of different mechanisms used for flexible sensors.