Ionic Flexible Mechanical Sensors: Mechanisms, Structural Engineering, Applications, and Challenges

This review covers the evolution of flexible mechanical sensors from an “electronic” language to an “ionic” language, which provides key reference information for the development of next‐generation bio‐intelligent sensing devices. Unlike sensors that rely solely on electron modulation, the core feature of the ionic flexible mechanical sensor (IFMS) is the ability of mechanical deformation to induce ion transport and compensation, thus demonstrating their conceptual similarity to biomechanical strain systems. Here, the basic design principles of flexible mechanical sensors modulated by ion transport are highlighted. This review provides a detailed description of the mechanotransduction mechanisms of IFMS devices based on ion transport modulation. First, although with similar mechanisms to conventional flexible mechanical sensors via piezoresistive, piezoelectric, and triboelectric transduction mechanisms, the core driver for IFMS devices is ions (not electrons). In addition, the transduction models and principles of action of novel transduction mechanisms that have been explored in the last decade are described in detail, which include interfacial iontronic sensing and potentiometric and electrokinetic energy conversion. According to the characteristics of the device, the relevant structural engineering is further highlighted. A comprehensive review of important IFMS application approaches (human–machine interfaces, life and health applications) is presented. Importantly, future challenges and possible solutions for IFMS devices are presented based on the existing research.


Introduction
Nowadays, we are in the era of intelligence, represented by the 5th generation of mobile communication technology (5G), DOI: 10.1002/adsr.202200099 Internet of Things (IoT), and artificial intelligence, and the development of intelligent technology has become an inevitable trend.[3][4][5] Wearable sensors based on artificial intelligence technology are profoundly changing the way people live.12][13] Flexible mechanical sensors are one of the core components of soft smart devices.With the rapid popularization of smart devices, flexible strain sensors that convert mechanical deformation into electrical signals have been put into commercial production. [14]This is a classic example of how communication of information and energy is handled through the language of "electronic."Learning from nature is an eternal theme for human beings.In nature, signal transmission and processing, energy conversion, and storage are often carried out through the "ionic" language as a medium. Recently, PIEZO homologues in animals have also been found in plants, which enable plants to sense stress and regulate the growth of apical cells. [17,18]As the largest sensory organ of human beings, skin tissue mainly relies on ions as signal carriers to realize the change of membrane potential to realize the sensing ability. [19]The formation of skin touch mainly depends on the gated ion channels (sodium ion channels and potassium ion channels) on the skin mechanoreceptors to induce ion transmembrane movement (PIEZO piezoelectric protein in the open state), forming an ion current and transmitting it to the brain through neurons. [20]Therefore, the receptors of the human skin somatosensory system composed of ionic conductors operate on the basis of ion dynamics. [21]Inspired by the phenomenon of ion mechanotransduction in human skin, research on the design of IFMS has been intensively growing in the last decade. [22]In 2012, Pan's group reported an interfacial capacitive pressure sensor, which uses the electrode interface loaded with electrolyte droplets to build a nanoscale ion-electron capacitive electric double layer (EDL). [23]In 2014, Suo's group developed a new sensory sheet based on ionic conductors, and named it "ionic skin" for the first time. [22]Subsequently, ion transport layers using ions as signal carriers, including ionic liquids, [24][25][26][27] electrolyte solution-nanofluidic membranes, [28][29][30] ionic gels, [19,[31][32][33] polyionic liquids, [34][35][36] and hydrogels, [37][38][39] are widely used in the development of IFMS.
Unlike flexible mechanical sensors that rely only on electronic mediation, the core feature of IFMS devices is that mechanical deformation can cause ion transmission and compensation.Therefore, the design of IFMS not only needs to have the characteristics of flexible biosensing devices, such as flexibility, ductility, and biocompatibility, but also needs to ensure that ions can move freely in its ion transport layer. [40]Under the action of external force, these ions can be transported freely to cause changes in electrical signals induced by the electrodes.As shown in Figure 1, we reviewed recent advances in IFMS devices and structured this review through mechanotransduction mechanisms, carriers of the "ionic" language, structural engineering, and applications.In this review, we provide the reader with a detailed description of the mechanotransduction mechanism of the IFMS device based on ion transport modulation in Section 2. First, although with similar mechanisms to conventional flexible mechanical sensors via piezoresistive, piezoelectric, and triboelectric transduction mechanisms, the core driver for IFMS devices is ions (not electrons).In addition, the transduction models and principles of action of novel transduction mechanisms that have been explored in the last decade are described in detail, which include interfacial iontronic sensing and potentiometric and electrokinetic energy conversion.Furthermore, the relevant structural engineering is highlighted according to the device characteristics.Here, special attention is paid to the design and fabrication of the ion transport layer.In this review, we summarize the design strategies of IFMS device architecture (bionic structure, self-healing, and environmental adaptability) and provide a comprehensive review of its important application modalities (human-machine interaction, life and health applications).Importantly, future challenges and possible solutions for IFMS devices are presented based on the existing research.

Mechanotransduction Mechanisms
IFMS typically convert mechanical strain into a recognizable electrical signal by causing internal ion migration through external stimuli.This review covers several types of transduction mechanisms, both active and passive.As shown in Table 1, we summarize their basic transduction models and key influencing factors under different mechanisms.

Active Transduction Mechanism (Non-Self-Powered)
In IFMS based on the active conduction mechanism, when an electric field is applied, the cations or anions in the sensing layer will migrate under the stimulation of external mechanical stress, thus forming an ionic current in the ionic material.Therefore, these kinds of IFMS generally have high energy expenditure and respond to static and slowly changing stimuli.

Piezoresistive
The conduction mechanism of piezoresistive IFMS refers to the resistance change of the device under external mechanical stress.][43] Different from conventional electron-mediated piezoresistive sensors, piezoresistive IFMS are usually composed of elastic matrix and ionic electrolyte. [44]As shown in Figure 2a, the conductive liquid (such as ionic liquid, electrolyte, and eutectic gallium indium) is filled inside the elastic matrix (such as polydimethylsiloxane [PDMS], Ecoflex, silica gel, and rubber). [32,45]When external mechanical stress is applied, the device structure deforms, and the length and cross-sectional area of the ion-conducting channel change.
Therefore, the conductive path of the conductive channel in the sensing layer changes, thereby affecting the element impedance.For example, in 2012, Park et al. demonstrated that the output response to pressure of a strain sensor composed of microchannels filled with a conductive liquid showed a linear trend over a wide strain range, in line with the theoretical model. [46]or strain sensing, when the sensing layer is strained in the axial direction, the length of the conductive channel increases and the cross-sectional area decreases, thereby increasing the overall impedance of the channel.The theoretical relationship between resistance change (ΔR) and strain () is as follows: where  is the resistivity of the ionic material, L is the length of the conduction channel, w and h are the width and height of the conduction channel cross-section, respectively,  is the applied strain, and  is the Poisson's ratio of the elastic material.For pressure sensing, pressing on the surface of the elastomeric skin reduces the cross-sectional area of the conductive channels and increases their resistance.Assuming a channel with a square cross-section, the resistance change is: where E is the Young's modulus of the material and p is the applied pressure.The performance of piezoresistive IFMS is determined by the change in resistance relative to the applied stress, which can be quantified by the gauge factor.
In this formula, ΔR/R 0 is the relative change in resistance, and R 0 is the initial resistance value when no external load is applied.Since the distribution of Poisson's ratio and resistivity of the heterogeneous structure is not uniform throughout the volume, according to Ohm's law, the fractional change in the average resistance can be determined by the change in voltage or current.In addition to the above methods, many researchers have directly fabricated piezoresistive IFMS using ionic hydrogels, whose electrical resistance is mainly affected by deformation.This type of piezoresistive IFMS requires a unique structural design, such as a zigzag structure. [47]When subjected to external mechanical strain, the sensor can monitor the resistivity to determine the strain value.

Interfacial Iontronic Sensing
In recent years, a new mode of pressure and tactile sensing utilizing ionic conductor supercapacitive interfaces has emerged, which differs from the conventional dielectrics of parallel plate capacitive sensors. [23]This interfacial ion sensor exploits the supercapacitive properties of the electric double layer (EDL) that occurs at the ionic media-electrode interface, resulting in ultrahigh device sensitivity, high noise immunity, responsiveness to static and dynamic stimuli, and thin and flexible device architecture.
][50][51][52] Specifically, this physical model based on interfacial ion sensing is established based on the tuning of the electrostatic interaction and thermal motion of charge carriers, which is called the Gouy-Chapman-Stern model (Figure 2b). [53]Typically, excess electrons at the electrode surface lead to strong electrostatic interactions with counterions at the ionic media-electrode interface.As a result, counterions are adsorbed to the electrode surface, while uniform ionic charges are repelled by the interface, leading to the formation of EDLs at the ionic media-electrode interface. Interfacial ion sensing mainly stores energy through fast reversible redox reactions on or near the electrode surface.So according to the classical EDL model, [40] its equivalent model can be expressed as the following formula: where (d, , c, Φ, T) is a combined parameter that depends on various factors, such as the dielectric constant and Helmholtz layer thickness of the electrode material, the type and concentration of the ionic medium, the ambient temperature and humidity, and the surface potential.Furthermore,  A is defined as the roughness ratio between actual and ideal smooth surface areas.It is worth noting that the product of  A and  represents the EDL capacitance per unit surface area, that is, UAC, which needs to be determined experimentally.Therefore, the model is difficult to get an accurate estimate and usually should be calibrated before use.At present, a variety of pressure sensors based on the interface ion sensing mechanism have been reported.For example, Park et al. used a randomly distributed microstructure in an ion capacitance sensor to achieve a lower detection limit of 1.12 Pa. [32]n 2017, Kim et al. reported an artificial viscoelastic mechanical sensor with ion nanochannels, achieving capability beyond Merkel cell detection. [19]Recently, Guo and co-workers reported hierarchical fillable structures in polyvinyl alcohol (PVA)-H 3 PO 4 ionogel systems.The increase of the mechanical load further increases the contact area of the ionic conductor, thus obtaining an ultra-high sensitivity of 3302.9 kPa −1 . [56]

Passive Transduction Mechanism (Self-Powered)
Currently, a large number of wearable IFMS devices based on active conduction mechanisms require extensive wiring and external power sources, limiting the further development of smart electronic devices.IFMS based on a passive transduction mechanism (self-powered) do not rely on external power sources and often feature ultra-low power consumption.It is worth noting that the self-powered IFMS itself is neither an energy storage device nor an energy conversion device, which greatly simplifies the number of units of the integrated device and reduces the complexity of the circuit.

Piezoelectric
The main working mechanism of piezoelectric sensors comes from piezoelectric materials.[59] When the piezoelectric material deforms under the action of an external force, the dipoles inside the material separate to generate polarization, and positive and negative compensation charges appear on the surface of the material, thereby generating an output voltage signal.Therefore, the direction in which stress is applied can be identified by detecting positive and negative potentials.
However, few piezoelectric IFMS based on ion transport regulation have been reported so far. Currently, in the simplest case, the open circuit voltage gradient is described as ∇V sense =  ∇P, where  is the piezoelectric coefficient.The value of  ranges from 0.01 to 100 nV Pa −1 , depending on the ion species and matrix properties.To explore the molecular origin of this effect and to investigate its application in sensing, Madden et al. presented a mechanistic picture of the piezoionic electromechanical response of a hydrogel with two mobile ions, where the cation and anion do not move at the same rate as the solvent, leading to the generation of currents and voltages (Figure 2c), [37] and further proposed an expression for the piezoelectric coefficient proportional to the difference between the diffusion coefficients of two mobile ions: where e is the electron charge, N is the concentration,  is the matrix permeability,  is the fluid viscosity, D + and D − are the diffusion coefficients of cations and anions, and D 0+ and D 0− are the effective mobility without the additional drag due to the polymer.This relationship indicates that when the diffusion coefficients of the two ions are approximately equal, the voltage generation will be zero.It also shows that by maximizing the difference in ion size, a larger sensing voltage will be obtained.Although the authors still cannot easily measure the hydrodynamic composition of dragged ions within the pores, derived expressions for the piezoelectric coefficients still provide useful predictions about the effects of permeability, viscosity, ion size, concentration, and conductivity.

Triboelectric
As an energy harvesting technology, the triboelectric nanogenerator (TENG) based on the coupling effect of triboelectrification and electrostatic induction has attracted much attention due to its inherent advantages such as diverse working modes, freedom of material manipulation, and high conversion efficiency. [62,63]t present, it has been widely used in wearable power supply, tactile sensor, and robot skin.However, most TENGs for power sources and tactile sensors use metallic materials as current collectors, resulting in non-scalability of devices.In recent years, hydrogels, ionogels/ionopolymers have been developed as deformable and effective current collectors. [33,64,65]IFMS based on the TENG mechanism usually adopt two typical structure types: single-electrode and contact-separated.The working mechanism of the single-electrode triboelectric IFMS can be described as follows.When the dielectric is in contact with the friction layer, a triboelectric effect occurs.Negative charges accumulate on the friction layer, while positive charges accumulate on the dielectric.At this time, the mobile positive and negative ions are randomly distributed in the flexible current collector.When the two friction layers are separated from each other, the unshielded negative charges will lead to the accumulation of positive ions at the interface of the current collector and the friction layer.Subsequently, an electric double layer with the same number of negative ions forms at the current collector/ground wire interface.In order to maintain electrostatic balance, free electrons flow from the ground wire to the earth through the external circuit to generate electrical output.Once the dielectric is close to the friction layer again, the electrons will return in the opposite way (Figure 2d).As a result, a continuous alternating electrical signal is generated by repeated contact-separation movements. [66]For contact-separation triboelectric IFMS, the principle is similar to the single-electrode mode, but electrons flow between two ion gel electrodes. [33]

Potentiometric
Most of the progress of existing IFMS focuses on structural engineering and material innovation, and there are few principle innovations that are different from the above traditional sensing mechanisms.With the development trend of miniaturized electronic skin in the future, the researchers proposed an all-inone self-powered pressure sensing mechanism, named potentiometric mechanical conduction mechanism. [39]This mechanism is inspired by the skin perception system.Skin mechanoreceptors enable humans to sense external mechanical stimuli through changes in membrane potential.Skin sensory cells typically carry more negative charges inside their membranes than outside when they are at rest.When skin sensory cells are exposed to external stimuli, mechanically gated ion channels open and allow transport of ions across the cell membrane, resulting in a large increase in membrane potential (depolarization).Following the release of mechanical stimuli, the membrane potential will return to the initial level (repolarization) through the regulation of specific ion pumps (Figure 2e). [67,68]To mimic this natural mechanism, Arias et al. utilized two electrode materials with different activities in contact with an appropriate electrolyte, and a potential difference would develop between the two electrodes due to different redox equilibrium reactions at the electrode-electrolyte interface.The advantage of this potentiometric-based IFMS is that it can satisfy both self-power and output a stable high fre-quency/low frequency electrical signal.This results in an efficient and energy-saving way to sense environmental stimuli.Graphene oxide (GO), with its flexibility and abundant functional groups, can adsorb various ions, and it can also be used as a solid-state electrolyte in terms of potential mechanism.The new integrated zinc-ion battery pressure (ZIB-P) sensing technology proposed by Liu et al. is to design the rechargeable solidstate battery itself as a flexible pressure sensor. [30]The insulating PVA nanofiber isolation layer renders the solid-state ZIB-P sensor in an open-circuit state when there is no external mechanical stimulus.Correspondingly, after the pressure was applied, the resistance of the separator weakened, the zinc electrode came into contact with the solid electrolyte PVDF-HFP-GO and began to discharge, and the zinc ions began to be removed from the negative electrode and intercalated into the positive electrode, allowing the battery sensor to output electrical signals.The specific reaction formula of discharge is as follows: In particular, the ZIB-P sensor has extremely high stability and will not degrade under 100 000 pressure stimulations.It also has a high sensitivity of 320.0 mV kPa −1 and a wide pressure response range of 2-368 kPa.This one-piece ZIB-P design brings a refreshing design to IFMS.

Electrokinetic Energy Conversion
As early as 2006, Dekker et al. reported experimental studies on the flow current and output power generated by a single rectangular silica nanofluidic channel under a pressure gradient, and theoretically evaluated the use of electrokinetic phenomena to convert hydrostatic energy into electrical energy. [69]The phenomenon of electrokinetic energy conversion (EKEC) arises from a combination of two microscopic causes: i) viscous forces coupling the motion of dissolved ions to that of the surrounding fluid, and ii) charged channel surfaces induce the accumulation of counterions into a screening layer of net opposite charge, called the double layer. [70,71]To sum up, the ion-selective permeable membrane is the core component of EKEC technology.For pressure sensing based on this technology, the process can be described as: due to the electrostatic repulsion behavior of the membrane surface charges, only counter ions (relative to the surface charge) can pass through the selectively permeable membrane, resulting in directional motion of net charge that promotes the generation of electrical signal under pressure (Figure 2f).Generally, in nanofluidic channels with nanometer and subnanometer confinement, the transport of ions tends to occur within the Debye length of finite thickness.Therefore, when the radius of the nanofluidic channel is reduced to the Debye length, the ion transport of the confined nanochannel is controlled by the surface charge.According to the Helmholtz-Smoluchowski equation, the electric energy generated by ion selectivity under pressure can basically reach a linear relationship: [72][73] Among them,  is the dielectric constant of the solution,  is the zeta potential of the channel surface,  is the viscosity of the solution, A is the cross-sectional area of the channel, and L is the channel length in a certain electrolyte solution and ion pore channel.Therefore, by measuring the current, the value of the differential pressure Δp corresponding to the ion-selective transport mechanism can be calculated.
At present, pressure sensing based on EKEC technology is rarely reported.A plasma-membrane-inspired IFMS comprising a novel partially reduced graphene oxide (prGO) membrane was proposed by Liu et al. [28] In this Serosa-Mimetic structure, the "Trail" structure containing -COOH groups is considered as an ion filter and ion conductor, which promotes the directional migration of cations under external mechanical stimuli, resulting in a directional flow of net charges and corresponding electric signal.As a result, pressure sensing with self-power supply, linear output and orientation awareness is realized

Structural Engineering
In 1975, Kinoshita et al. made the first tactile touch sensor. [74]hey used piezoelectric elements as strain haptics to create a visual-tactile symbiosis system and assembled it on a manipulator to recognize the shape of an object.In the following decades, with the continuous development of wearable technology, the structural design and material production of IFMS have gradually diversified.Nevertheless, the basic structural engineering of IFMS based on ion transport regulation is still mostly derived from the following three parts: substrate, electrode, and ion transport layer.In this section, we review the key properties of IFMS in structural engineering regulation in recent years, aiming to discuss considerations and development trends for future device design.

Substrate
The flexibility, long-term stability, and even comfort of IFMS are directly related to the substrate.The overall characteristics of the sensing system often depend on the reliability of the flexible substrate.The core requirements of IFMS for substrates have the following characteristics: biocompatibility, chemical inertness, stretchable/compressible, and easy processing.Since the flexible substrate is often the carrier and protector of electrodes or flexible electronic devices, or directly participates in the mechanical sensing of the device, it is particularly important to select the flexible substrate according to its function when manufacturing IFMS.
[77][78][79][80] Considering the characteristics of the polymer plastic substrate, it is more used as a support layer or an encapsulation layer in mechanically flexible devices.For example, PI is often used in situations involving high temperatures and harsh conditions due to its high resistance to heat and external mechanical forces. [81]The extraordinary resistance and thermoplasticity of PP make it often used in the encapsulation layer.Liu et al. introduced an ion-complexed GO [Ca] solid electrolyte between copper and zinc electrodes and then used the property of PP to heat shrink at 60 °C to seal the device, ensuring contact between the electrode and the electrolyte while protecting the device from interference from the external environment. [67]Bao et al. developed novel piezoresistive sensors based on elastic microstructured conductive polymers, using flexible PET as the encapsulation housing.However, polymer plastic substrates suffer from poor adhesion and sensitivity to high temperatures, making it difficult for inkjet-based printed conductive materials to adhere. [82]Therefore, it is necessary to consider the utilization of its characteristics and the improvement of its limitations when using it.
Stretchable elastomers are an excellent choice for IFMS.Elastomers can not only be used as the electrode of the sensor and the carrier of the sensing layer or the package of the device but also often directly participate in the function process of the sensor.Among various elastomeric materials, PDMS is often the first choice for the manufacture of mechanical sensors due to its excellent stretchability, commercialization, biocompatibility, and ease of processing.Therefore, it has been widely used in electronic skin and wearable sensors.Zhang et al. reported the fabrication of a high-performance flexible pressure sensor using aligned carbon nanotube/graphene hybrid films as the active material and microstructured PDMS (m-PDMS) films made directly from leaf templates as the flexible substrate. [83]Among them, simple and efficient microstructure processing technology is the key to improve sensitivity.The sharp microstructure of the PDMS surface gives the sensing layer sufficient contact points for high sensitivity even to small pressures.Besides PDMS, polyurethane (PU) and Ecoflex are also frequently used as elastomeric substrates for sensors.Dai et al. developed a stretchable hydrogel (PEDOT:PSS) as a conductive component, using two layers of polyurethane (PU) tape sandwiched together to make a transparent and wearable strain sensor with a 0-2850% ultra-wide strain range. [84]coflex silicone may have better biocompatibility than the former two.Wang et al. report a flexible and stretchable TENG using MXene/PVA as a stretchable electrode and utilizing Ecoflex silica gel as a triboelectric layer and prevent composite hydrogel dehydration. [85]The fabricated TENG is highly sensitive to local precise pressure and has great application potential in handwriting recognition.
In addition, smart fabrics are also one of the substrate candidates based on future manufacturing.[88] In particular, it is an ideal material for use as IFMS due to the hierarchical microstructure of the fabric derived from the woven design.Kim et al. reported a highly sensitive and flexible resistive tactile sensor composed of carbon nanotubes (CNTs) and nickel-plated fabric. [89]ased on the fabric's hierarchical structure and multilayer aggregate shape, the all-fabric sensor exhibits excellent pressure sensing performance and high sensitivity (26.13 kPa −1 ) over a wide pressure range (0.2-982 kPa).However, the limitations of smart fabrics are the difficulty in ensuring the firm attachment of electrode materials to the textile and the loss of mechanical strength caused by high temperatures on the fabric, as well as the deterioration of the conductive pathway due to the lack of uniformity and thus the loss of sensitivity.To improve the quality, the use of gas-phase organic chemistry to fabricate conductive materials on porous fabrics is an alternative option to be considered. [90]he choice of flexible substrates is the key to fabricate IFMS devices, because the external contact methods and characteristics depend on the bulk properties of the substrates.Therefore, the final mechanical sensing form of the IFMS device has a high correlation with the form of its substrate, which is also the core element of the flexible sensor.Researchers should fully consider their applicability and disadvantages when choosing flexible materials as substrates.It is worth noting that the above discussion mainly focuses on the potential or characteristics of flexible materials used as substrates in IFMS, and other usage methods will be discussed in the following sections.

Electrodes
As one of the core components of IFMS, the key lies in the ability to withstand large deformations or maintain excellent electrical properties when in contact or exhibit regular conductivity changes, so as to obtain electrical signals that can be easily detected.Electrode geometry, fabrication process, and construction type have a significant impact on the design and performance of IFMS devices.Electrodes have different properties and can be modified to achieve specific functions.[93][94][95] In order to maintain the versatility of IFMS, electrode preparation methods include coating, electrospinning, vacuum filtration, templated assembly, and 3D printing.][99][100][101]

Carbon Nanomaterials
Various carbon nanomaterials with 1D and 2D structures, due to their unique properties, such as high electrical conductivity (≈10 4 S cm −1 ), excellent intrinsic carrier mobility (10 6 cm 2 V −1 S −1 ), and excellent mechanical properties, have become a strong candidate for fabricating electrodes for flexible IFMS devices.
1D carbon nanomaterials (CNTs) have proven to be a promising electrode material because of their efficient charge transport, large specific surface area, and electrochemical activity.Furthermore, their outstanding flexibility extends the potential of traditional rigid devices to variable ones. [102]As opposed to simple uniaxial bending or folding, the mechanical strains induced by IFMS are typically complex patterns.Frequently involves mechanical twisting, stretching, compression, and combinations thereof.To address this issue, Hata et al. designed an ultralong vertical CNT forest to be used as a stretchable electrode, deforming in a manner similar to the structural deformation when peeling a cheese stick. [103]Key factors for ideal stretchable electrodes in wearable IFMS devices include deformation of human joints, and high electrical conductivity comparable to metals for efficient charge collection/transport. Kim et al. further proposed a novel stretchable electrode using laterally carded nanostructures of vertically grown CNT networks. [104]Due to the special comb and meander design, individual carbon nanotubes can still be well connected even under ≈100% strain.However, pristine CNTs exhibit low Young's modulus (600 MPa-30 GPa), which is difficult to meet the needs of tailoring electrodes.Zhang et al. introduced highly disordered graphene nanosheets onto the surface of CNT fibers.The Young's modulus of the hybrid fiber is increased to 154 GPa, and various stable complex structures can be formed, such as coil springs and curved snakes. [105]he 2D carbon nanomaterial graphene was first proposed in 2004, and its ideal resistivity ≈10 −8 Ω m −1 , which is lower than most metals, so it can be used as an electrode.However, the high tensile strength of graphene leads to fracture at low strain, which makes it challenging to exploit its extraordinary electronic properties in flexible electronic devices.To enable graphene conductors with excellent strain-related properties, Bao et al. created graphene nanoscrolls between stacked graphene layers. [106]nder mechanical strain, some scrolls bridge the fragmented regions of graphene to maintain the percolation network, resulting in excellent conductivity under high strain.The use of MGG electrodes maintained 60% of their original current output at 120% strain.In addition, graphene with a wrinkled structure possesses many attractive properties, including enhanced stretchability, steerable chemical reactivity, and engineered modifiable energy bandgap.Hong et al. fabricated multilayer graphene on thermoplastic polystyrene (PS) substrates coated with a sacrificial layer of water-soluble poly(4-styrenesulfonic acid) (PSS) via a rolling-based transfer process.The highly compliant PSS layer provides a conformal contact between the PS substrate and graphene during wrinkle formation, enabling easy transfer of the graphene wrinkled structure to the flexible substrate. [107]Therefore, its low sheet resistance makes it a promising candidate for transparent electrodes.

Metal-Based Materials
Although flexible electrodes based on carbon nanomaterials have been developed, most materials are still less conductive than metals.Compared with carbon nanomaterial electrodes, metal-based nanomaterial electrodes often have higher conductivity and even redox properties, but are undeniably weaker than carbon nanomaterials in flexibility.In view of these special properties, metalbased nanomaterials can be applied to some specific IFMS devices.Khodagholy developed a flexible electrode grid made of gold-coated titania nanowires embedded in a silicon matrix. [108]xperiments prove that it can be used in neural interfaces, exhibiting long-term stability and high electrode density, which provide a feasible electrode technology path for IFMS-based humancomputer interaction technology.Intrinsically stretchable rubbery semiconductors with high mobility are crucial for realizing high-performance stretchable electronics and integrated devices for many applications involving large mechanical deformation or stretching.Yu et al. enhanced the effective carrier mobility of poly(3-hexylthiophene-2,5-diyl) incorporated into a silica matrix by metallic carbon nanotubes (m-CNTs).The AuNPs-AgNWs/PDMS elastomeric conductors were fabricated by embedding silver nanowires (AgNWs) into PDMS, followed by conformal coating of gold nanoparticles (AuNPs) on the surface by a galvanic replacement process. [109]This surface decoration lowers the energy barrier between the electrode and the semiconductor, thus ensuring ohmic contact.Transistors and their arrays based entirely on intrinsically stretchable electronic materials were thus developed, maintaining electrical properties without substantial loss even when subjected to 50% stretching.In addition, the redox reactions of metals are also crucial in IFMS devices.Liu et al. designed a copper and zinc primary battery as a pressure sensor, which requires metal electrodes to trigger redox reactions to achieve self-powered continuous detection of human physiological signals, human joint motion, and information transmission. [67]Indium tin oxide (ITO) is a kind of transparent conductive oxides (TCDs).Due to its best combination of conductivity and transparency, it has become the most important transparent conductive material.Pan et al. coated ITO on PET substrates as electrodes and ionogel matrix as thin-film capacitive sensing material (called ionoelectronic thin film).Utilizing the capacitive interface at the ionic-electronic contact, iontronic thinfilm sensors offer large capacitance per unit area (5.4 μF cm −2 ) and ultrahigh sensitivity (3.1 nF kPa −1 ), more than a thousand times higher than conventional solid-state sensors. [110]n addition, since Gogotsi reported a 2D material in 2011 and named it "MXene," it has been rapidly developed and widely used. [111]Thanks to the transition metals and abundant tunable functional groups of MXene, it has high electronic conductivity and plasticity.Therefore, it is an excellent flexible electrode material.For example, Fu et al. achieved good pressuresensing performance and stable electrodes against mechanical bending through uniform Ti 3 C 2 T x electrodes. [112]He et al. studied Ti 3 C 2 T x -based flexible electrodes obtained by uniformly compounding Ag NWs and nanosheets. [113]

Conductive Polymers
Conductive polymer (CP)-based materials have shown great potential as electrode materials for IFMS devices due to their unique advantages, including good electrical conductivity, flexibility, relative cheapness, and ease of synthesis. [114]Polypyrrole (PPy), polyaniline (PANI), polythiophene, and poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are common conductive polymers. [115]The flexibility of CP gives it the potential to form curved geometries or nanofibers with high aspect ratios.Park et al. formed a stretchable electrode from PEDOT:PSS mixed with an aqueous polyurethane dispersion (PUD) and achieved a sensitivity of 10.3 kPa −1 when the pressure-induced geometric change experienced by the electrode reached a maximum at an elongation of 40%. [116]In addition, the mixture of PEDOT:PSS and metal-based nanomaterials can also effectively improve the conductivity and stretchability of electrodes.Compared with pristine NW electrodes, the surface roughness of hybrid electrodes is significantly reduced and the contact performance between NWs is improved.Kim et al. fabricated large-area, smooth, and flexible AgNWs/PEDOT:PSS composite electrodes. [117]The electrode exhibited a light transmittance of 84.3% and a sheet resistance of 10.76 Ω sq −1 .After 200 stretching and folding cycles, the sheet resistance of the composite electrode decreased only by 5.3%.
In general, the selection of flexible or stretchable electrodes in IFMS devices remains a challenge in the fabrication of flexible sensors.Except for flake and filament metals (such as zinc, copper, silver, gold, and aluminum) that can be directly used in electrodes, the preparation of most functionalized electrodes needs to be realized by different electrode engineering.These processes include: casting, coating, evaporation, sputtering, screen printing, inkjet printing, and laser-assisted patterning.In short, in order to flexibly respond to the challenges of diverse application scenarios of future IFMS equipment, it is necessary to select electrode materials and preparation processes according to actual conditions.

Ion Transport Layer
As shown in Table 2, in a typical IFMS device structure, the ion transport layer can be called a sensing layer, a pressure sensitive layer, an ion conductor, or an ion electrolyte layer.Compared with traditional electronic systems, the ion transport layer is inspired by the receptors of human skin.The receptors of the human sensory system are based on ionic dynamics. [21]A large number of mechanoreceptors are distributed in the dermis, so the spatial distribution of strain on the skin can be clearly sensed.By simulating the biosensing mechanism of ion migration, the ions in the ion transport layer will migrate directionally when receiving external stimuli, thereby providing detectable electrical signals through different mechanical conduction mechanisms. [118]The ion transport layer is the most important part of the IFMS device and has the following characteristics: i) the ion transport layer generates ion migration under external mechanical or electrical stimulation and has good ion mobility; ii) the ion transport layer is ductile and can deform with the external substrate; iii) the ion migration in the ion transport layer can be fed back by electrical signals through the different action characteristics of the electrodes (polarization, induction, adsorption, redox). [119]

Liquid/Liquid-Solid Integration
Ionic liquids or electrolyte solution-nanofluidic membranes are mostly used in the ion transport layer of IFMS devices based on liquid environment.From the perspective of sensing mechanism, the use of ionic liquid is achieved by establishing EDL to obtain significant interfacial capacitance.The use of electrolyte solution-nanofluid membranes is based on the EKEC sensing mechanism.
Ionic Liquids: Ionic liquids (ILs) are also called room temperature ionic liquids, organic ionic liquids, etc.Compared with the simple ions cof classical inorganic molten salts (such as NaCl), the presence of bulky and asymmetric ions in ionic liquids prevents them from crystallizing easily, resulting in a liquid state at room temperature or low temperature (<100 °C), so also known as low temperature molten salt. [120]ILs are usually a salt composed of organic cations and inorganic anions.As ionic compounds, they possess many unique physical and chemical properties, such as thermal stability, negligible volatility, nonflammability, electrochemical stability, wide electrochemical window, and high ionic conductivity. [121]As shown   Reproduced with permission. [122]Copyright 2017, American Chemical Society.b) Microfluid system sensing device.Reproduced with permission. [24]Copyright 2015, American Chemical Society.c) Novel multichannel pulse monitoring platform.Reproduced with permission. [25]Copyright 2022, Springer.d) Ionoelectronic graphene tactile sensing device.Reproduced with permission. [26]opyright 2020, Wiley-VCH.
in Figure 3a, typical ionic liquids are composed of bulky and asymmetric cations (imidazolium, pyrrolidinium, pyridinium, ammonium, and phosphonium) and inorganic anions (such as halides, triflate, tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate). [122]ue to the liquid nature of ILs, elastomeric encapsulation is often required to exert their physical and chemical properties.It is an effective way to fabricate highly stretchable transparent strain sensors by combining the advantages of microfluidic systems and ionic liquids.A microfluidic channel network embedded in a thin elastomeric PDMS layer was filled with a binary mixture of ionic liquids (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [BMIM][Ntf 2 ] and 1 -butyl-3methylimidazolium acetate [BMIM][Ac]) to produce transparent strain sensors (Figure 3b). [24]When the transparent microfluidic strain sensor is deformed by an external force, it produces distinct electrical signal changes due to different motions such as stretch-ing, bending, pressing, and twisting.Another advantage of ionic liquids as active filling material in elastic substrates is their extremely low Young's modulus, which is 22 orders of magnitude lower than that of typical elastomers.Therefore, the durability and long-term stability of IFMS can be greatly enhanced. [123]ncapsulating ionic liquids with elastomers can often lead to ion transport layers with high Young's modulus and optical transparency.Meanwhile, the nature of ionic liquids determines that the mechanotransduction mechanisms of IFMS devices are mostly piezoresistive or interfacial iontronic sensing.As a proof-of-concept, Wang et al. proposed a multi-channel pulse monitoring platform based on TCM pulse theory and wearable electronics. [25]The pulse sensing platform simultaneously detects the pulse status of three pulse positions (Chi, Cun, and Guan) and provides 3D pulse mapping, which vividly reveals the shape of pulse length and width, making up for the shortcomings of traditional single-point pulse sensors (Figure 3c).However, for  [128] Copyright 2021, Macmillan Publishing Ltd.b) Serosa-Mimetic structure sensing device.Reproduced with permission. [28]Copyright 2022, Wiley-VCH.c) Bionic ion channel sensing device.Reproduced with permission. [29]Copyright 2016, American Chemical Society.
tactile sensors, all previous studies using ILs in tactile sensors have used liquid droplets confined in closed cells, where the liquid is in contact with top and bottom electrodes.These methods are ineffective for fabricating sensitive sensors with high signalto-noise ratio due to the high initial capacitance caused by the formation of EDL at the interface between the ionic liquid and the electrode. [110,124]Li et al. reported a flexible ionoelectronic graphene tactile sensor with superior sensitivity, consisting of a top floating graphene electrode and an ionic liquid droplet immobilized on the bottom graphene grid electrode (Figure 3d). [26]The unique cell design in this work can induce dynamic properties in the ionic liquid droplet when the top floating graphene electrode touches it, and generate spreading behavior under gentle touch.Tactile sensors achieve superior sensitivity during this transition step.
Electrolyte Solution-Nanofluidic Membrane: The IFMS device based on electrolyte solution-nanofluid membrane is an application of nanofluidics.MacKinnon, who won the Nobel Prize in Chemistry in 2003, reported the structure of potassium ion channels and the molecular basis of potassium ion conduction and selectivity in 1998, laying the foundation for the study and understanding of nanofluids from the perspective of bionics, known as bioinspired nanofluidics. [125,126]Nanofluidics is the study and application of fluid and mass transport in nanometer-sized confined regions, where intermolecular forces, namely short-range steric interactions and hydration, and longrange van der Waals forces and electrostatic interactions begin to dominate. [127]The pore size of nanofluidic channels is usually in the nanometer or subnanometer range, which not only facilitates the transmembrane ion flux but also can effectively limit the charge polarization.The nanofluidic channel can be simplified as two adjacent solid surfaces, and in the electrolyte solution, the fixed charges on the solid surfaces are neutralized by the counterions through the Coulomb force.In this way, an EDL consisting of a stern layer (inner) and a diffusion layer (outer) is formed.The EDL is related to the Debye length by which the spatial extent of the electrostatic effects of unshielded charges can be estimated (Figure 4a). [128]Nanofluidic channels with tailored ion transport kinetics enable harvesting of renewable osmotic energy between seawater and river water.[131][132] However, there are few reports on the integrated system of electrolyte solution-nanofluid membrane in the field of IFMS devices.Its conduction mechanism mainly depends on the EKEC mechanism. [69]Specifically, the stimulation of external pressure can cause the transport of counterions in the electrolyte solution in the nanoflow channel, thereby generating directional net charges and promoting the generation of flowing voltage and current.In 2016, Han et al. developed a bionic ion channel pressure sensor. [29]The biological ion channel system that senses external stimuli is basically composed of receptors and nanopores.This assembled ion channel has a sensitivity of up to 5.6 kPa −1 with a response time of 12 ms and a frequency of 1 Hz, and the sensor has more than 10 000 loading/unloading cycle stability (Figure 4c).Lei et al. reported a 2D kinetic energy conversion device based on MXene membranes. [133]The generation of flowing current was observed when the electrolyte solution passed through the MXene film under external pressure.At a pressure of 5 kPa, the flowing current density is close to 1.3 mA m −2 .However, membranes made of typical 2D materials have inherent drawbacks for practical applications, including long ion diffusion distances, low ion selectivity, and poor persistence in water.Liu et al. proposed an EKEC-based pressure sensor inspired by the serous membrane structure to address these inherent drawbacks (Figure 4b).The specific description has been mentioned in Section 2.2.4. [28]he design of the ion transport layer is mainly inspired by bioinspired nanofluidic iontronics. [134]In biological systems, the transmission and storage of ions in nerve signals is mainly regulated by ion channels.Artificially developed controllable ion channels usually simulate the function of biological ion channels in the nano-confined space and construct nanofluidic devices with similar functions.Currently, IFMS devices designed and constructed based on nanofluidic membranes and ionic solutions aim to mimic the pressure-regulating function of ion channel.However, unlike the signal carriers (electrons and holes) of current electronic devices, ion-based IFMS devices operate more stably during dynamic life processes. [135]Also, the diverse ion classes provide more optional functionality.This provides the future field of human-computer interaction with characteristics closer to the real state of living organisms.Therefore, IFMS devices based on the EKEC mechanism have great development prospects.

Solid/Quasi-Solid
Solid/quasi-solid ion transport layers have natural advantages in the assembly of IFMS devices, such as higher ion conductivity of one or even several orders of magnitude higher than that of liquid electrolytes and less liquid leakage.A similar trend has been observed in the field of battery and supercapacitor research, which are often referred to as solid/quasi-solid electrolytes. [40]According to the current application categories in IFMS, such ion transport layers can be divided into ion gels, ion polymers, and hydrogels.
Ionic Gels: Ionic gels, also known as ionic liquid gels, are composed of polymers or inorganic network matrices, in which encapsulated ILs serve as dispersion media.Since the first report of ionic gels by Watanabe et al. in 2005, the literature has reported numerous studies on this class of materials. [136]139][140][141] In theory, ILs endow ionic gels with unlimited possibilities by combining different organic cations and anions.ILs assist in the formation of ionic gels in multiple ways and regulate their struc-ture and properties by providing hydrogen bonding, electrostatics, and host-guest interactions (Figure 5a).
Ionic gels with high transparency, excellent mechanical properties, and remarkable recoverability are emerging materials for the fabrication of IFMS devices.However, phase separation between polymer networks and ionic liquids is a major obstacle in the preparation of ionic gels due to the incompatibility of normal polymer structures with ionic liquid structures.A one-pot copolymerization method for the preparation of tough ionic gel materials was proposed by Lu et al. [142] The synthesized ionic gels showed good mechanical properties (tensile stress ≈4.5 MPa, tensile modulus ≈3.2 MPa, excellent transparency (91%), and high thermal stability (>300 °C) due to the synergistic effects of hydrophobic ionic liquid microregions, ionic bonding interactions, and molecular entanglement, and resistive sensor and capacitive sensor based on ionic gel have been further developed (Figure 5b).In addition, in order to make better use of the high ionic conductivity of ionic gels to overcome the trade-off between sensitivity and detectable range when ionic skin senses mechanical strain, Hong et al. developed a porous ionic gel (Figure 5c). [143]his gel can be effectively deformed by closing the pores even under small pressure and induces a large change in the gelelectrode contact area, resulting in a significant difference in the electric double layer capacitance.After optimizing the mechanical properties by tuning the gel parameters, a high sensitivity of ≈152.8 kPa −1 , a broad pressure sensing range (up to 400 kPa), and excellent durability (>6000 cycles) are achieved.IFMS devices based on ionic gels tend to exhibit higher sensitivity than those based on ionic liquids.
In recent years, as ionic gel has attracted more and more attention as a flexible conductive material, it is still a great challenge to integrate multiple functions into one gel, which can be widely applied in various complex scenarios.Yue et al. prepared multifunctional ionic gel by one-step photoinitiated polymerization of 2,2,2-trifluoroethyl acrylate and acrylamide in hydrophobic ionic liquids (Figure 5d). [144]Among them, the rich non-covalent interactions (including hydrogen bonds and ion-dipole interactions) make ionic gel have excellent mechanical strength, elasticity, and rapid self-healing ability at room temperature, while the fluorinerich polymer matrix has high resistance to water and various organic solvents, showing tough underwater adhesion on different substrates.Wearable strain sensors based on ion gels can sensitively detect and distinguish large body movements, such as limb bending, walking, and jumping, as well as subtle muscle movements, such as pronunciation and pulse.
[147][148] Unlike ILs, which are liquid at room temperature, most of the PILs reported so far are solid.According to the polarity of their ionizable functional groups, PILs can be divided into: i) polycationic (such as chitosan, polyquaternium, polyacrylamide); ii) polyanionic (such as sodium alginate, polystyrene sulfonic acid); iii) amphoteric polyelectrolytes, that is, both cations and anions are linked to the polymer backbone through covalent bonds.Compared with general nonionic polymers, PIL-based polymers have higher ionic conductivity (lower than ILs), wider electrochemical window (up to 5 V), high thermal stability (350 °C), non-flammability, and overcome the  [136] Copyright 2005, American Chemical Society.b) Tough ionic gel sensing device.Reproduced with permission. [142]Copyright 2022, Elsevier.c) Porous ionic gel sensing device.Reproduced with permission. [143]Copyright 2021, American Chemical Society.d) Multi-functional ionic gel sensing device.Reproduced with permission. [144]Copyright 2021, Wiley-VCH.mobility of ILs.The synergistic effect makes PIL a versatile and unique polyelectrolyte suitable for a variety of applications.
The unique ionic structure and excellent ionic conductivity of PILs make them an ideal sensing material.Sun

et al. prepared a new ion-sensing fabric with interfacial iontronic sensing by electrostatic spinning of poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) ([PBVIm][TFSI]
) and polyacrylonitrile (PAN) with hydrophobic properties (Figure 6b). [34]he sensitivity was 0.18 kPa −1 in the pressure range of 0.2 kPa, and the signal did not decay after ten repeated washes.Recently, Zu's group synthesized a transparent, stretchable, self-healing PIL gel with a modulus/conductivity dual gradient using a layerby-layer gel method (Figure 6c). [149]By changing the concentration of ionic liquid monomers, the modulus, compressive strength, and conductivity of each layer of gel can be effectively tuned.Thus, the response of the low modulus layer to slight pressure and the high modulus layer to high pressure is ensured.The resulting gradient PIL pressure sensor has an extremely wide detection range from 10 Pa to 1 MPa.In addition, PILs have lower ionic conductivity than ILs, mainly because only the counteri-ons of polyanionic and polycationic PILs can move freely on the backbone.However, this brings with it a different zeta potential signature. [150,151]Suo et al. demonstrated a stretchable ionic device using ionic elastomers, in which polycations or polyanions are immobilized on the elastic network, but their counterions are mobile, thus constituting ionic diodes.The polyanion/polycation heterojunction leads to the formation of an ionic double layer (IDL), acting like a depletion layer at the p-n semiconductor junction, thereby exhibiting complete non-Faraday rectification.The ionic elastomer-based device was able to convert mechanical motion into an electrical signal, yielding a peak open-circuit voltage of 46 ± 2 mV and a short-circuit current of 0.18 ± 0.01 μA cm -2 under cyclic uniaxial stretching from  u = 1.2 to 1.5. [152][155][156] Hydrogels are quasi-solid-state materials with a 3D cross-linked hydrophilic polymer network, and their special  [147] Copyright 2013, Elsevier.b) Electrospinning.Reproduced with permission. [34]Copyright 2019, American Chemical Society.c) Polyionic liquids gradient structure sensing device.Reproduced with permission. [149]Copyright 2022, American Chemical Society.network structure, biocompatibility, and high ionic conductivity make them good candidates for IFMS devices.In view of the wide range of application fields of hydrogels, this paper mainly focuses on their characteristics and applications as stretchable ion conductors in IFMS devices.Hydrogel ionic electronics mainly rely on non-Faradaic processes. [48]Generally, hydrogels that can be applied to IFMS devices contain cross-linked polymer networks serving as scaffolds, water molecules, and freely movable ions.This makes it an elastic ionic conductor.Typically, the pore size of the polymer network is about 10 nm, which is much larger than the size of water molecules or hydrated ions. [157]In iontronicsbased hydrogel devices, mobile ions and electrons form an EDL at the interface between the hydrogel and the metal, coupling the ionic current in the hydrogel and the electronic current in the metal.In addition, under normal circumstances, the resistance of the hydrogel changes with stretching following the square law relationship R/R 0 =  2 , where R 0 is the resistance in the normal state, and R is the resistance of the hydrogel stretched  times, which is much lower than most electronic conductors (Figure 7a).
In 2014, Suo et al. developed a new sensory sheet based on ionic conductors, called "ionic skin" (Figure 7b). [22]Highly stretchable strain sensors are demonstrated by using NaClcontaining polyacrylamide hydrogels as ionic conductors and acrylic elastomers as dielectrics.This is a typical mechanoconduction mechanism based on interfacial ion sensing.It easily measures strains from 1% to 500% and pressures down to 1 kPa.In addition, existing electronic skins differ from natural skin in charge carriers as well as water content, often resulting in limited biocompatibility.Hydrogel-based ionic skin can bridge the gap between artificial skin and human skin to some extent.Liu et al. added polycationic electrolyte (poly(4-styrene sodium sulfonate)) and polyanionic electrolyte (poly(diallyldimethylammonium chloride) to the interpenetrating agarose-polyacrylamide network); two separate hydrogels were assembled to form an overall "diode-like" device, which made directional ion transport possible (Figure 7c). [158]This artificial ionic skin (Alskin) can convert mechanical stimuli into four types of electrical signals (resistance, capacitance, open-circuit voltage, and short-circuit current).Proper introduction of conductive materials and ions in the hydrogel matrix can control the conductivity of the hydrogel, making the hydrogel suitable for direct use as the ion transport layer of IFMS devices.Hu et al. developed a  [48] Copyright 2018, Macmillan Publishing Ltd.b) Ionic skin.Reproduced with permission. [22]Copyright 2014, Wiley-VCH.c) "Diode-like" sensing device.Reproduced with permission. [158]opyright 2020, The Royal Society of Chemistry.d) MXene-hydrogel sensing device.Reproduced with permission. [159]Copyright 2022, Wiley-VCH.
multifunctional MXene-hydrogel material containing Ti 3 C 2 T x nanosheets, in which polyacrylamide and PVA were crosslinked as polymer matrix to construct a flexible and stretchable TENG (Figure 7d). [159]According to Wang's hybrid EDL model, the mechanism of the output enhancement is due to the dynamic balance of the EDL between the MXene nanosheets and the water inside the 3D MPP hydrogel.As a result, MPP-TENG has a wide strain range from 50% to 400%, and can recognize different word pronunciations through the unique vibration signal peak of the laryngeal process.

Structural Design
In order to further improve the sensing performance or application mode of IFMS devices, researchers have proposed a variety of internal or external design methods to optimize the performance in many aspects including sensitivity, sensing range, response time, and multi-modal response. [160]According to the dif-ferent mechanical transduction mechanisms in the second chapter of this paper, different device overall structure designs can often respond to different mechanical stimuli.In this subsection, we mainly focus on the microstructure of the sensor rather than the macroscopic design.This is because human skin is the main source of inspiration for IFMS devices, and its complex tactile sensory system contains a variety of unique and well-established biological structures, which are important factors in helping skin mechanoreceptors recognize objects. [161]In view of the vigorous development of IFMS device structure design, this section mainly focuses on bionic structure design, self-healing design, and environmental adaptability design.

Bionic Structure Design
The ability of human skin to directly acquire contact/non-contact information mainly depends on the biological microstructure of the epidermis, dermis, and subcutaneous tissue in the skin, which is composed of rows of "interlocking" micro-ridges.Such a structure helps to concentrate and amplify local pressure, so that Meissner corpuscles can quickly obtain information about external stimuli. [162]Most of the IFMS devices designed based on the bionic structure are inspired by the above functions to obtain the tactile perception function.
Normally, the resting membrane potential in the intracellular equilibrium state is in the range of −40 to −80 mV due to the closure of ion channels.When an outside stimulus causes mechanoreceptors to change shape, ion channels open.This lets Na + move across the cell membrane, creating action potentials.Inspired by the versatility and characteristics of biological cell structures, Kim et al. demonstrated a synthetic multicellular hybrid ion pump (SMHIP). [20]The rationally designed SMHIP consists of ILs ([EMIM][TFSI]) embedded on silica microstructures (dispersed phase) in a TPU elastomeric matrix (continuous phase) (Figure 8a).External stimulation leads to induced ion pumping in the skin of SMHIP ion mechanoreceptors, creating an EDL at the IL-SiO 2 -TPU/electrode interface.However, most of the current research efforts focus on the construction of eskins for tactile perception of a single stimulus.Therefore, Shen et al. combined ionic electronic skin with artificial intelligence (AI) technology to build an intelligent cognitive system, realize human-specific tactile cognition, and achieve a superhuman ability (Figure 8b). [163]The electronic skin based on full-skin bionic is a stacked component of TENG electronic skin and piezoelectric ionic skin, showing a three-layer structure configuration: a double-sided heterogeneous top layer is used to imitate human vellus hair and epidermis; the middle layer of the micro-cone structure (LMS) ion gel is used to simulate the dermis of the skin; the bottom layer uses a single-sided LMS to simulate the subcutaneous tissue of the skin, thereby achieving an ultra-high sensitivity of 8053.1 kPa −1 below 1 kPa, a linear response in the pressure range of 1 to 34 kPa and a sensitivity of 3103.5 kPa −1 .By combining ionic electronic skin with six-layer multi-layer perceptron neural network technology and supporting units including signal acquisition, transmission, processing and display functions, an advanced intelligent material cognition system is constructed, which can realize accurate recognition of different materials.
In addition, learning from nature is an eternal theme for human beings.Flexible electronic skins with high sensitivity and wide-range pressure sensing are highly desired in artificial intelligence and human-computer interaction.Although the introduction of surface microstructures tends to improve sensitivity somewhat, it usually requires expensive microfabrication methods.Guo et al. reported a low-cost ion transport layer using Calathea zebrine as a template leaf (Figure 8c). [164]Calathea zebrine leaves have a highly uniform surface microstructure, forming surface microcones with an average height of approximately 25 μm and an intercone distance of approximately 34 μm.The fabricated sensor exhibits a fast response time within 29 ms, a detection limit as low as 0.1 Pa, and a high sensitivity of 54.3 kPa −1 at ultra-low pressure (<0.5 kPa).On the other hand, internal structures from natural animal skins and fabric leaves are also worthy of reference.They are composed of more than 10% water and rich moisturizing factors such as amino acids, betaine, and bio-mineral ions.These humectants build supramolecular hydrogen bond networks to store water and resist drought.Inspired by nature, Wu et al. developed a biomimetic hydrogel dynami-cally cross-linked by three moisturizing factors, including silk fibroin, betaine analogs, and calcium ions (Figure 8d). [165]In this hydrogel, abundant amide, ester carbonyl, and ionic groups provide abundant hydrogen bonding, ionic associations, and even cation- interactions between polymer chains, internal water, and cooperative biological tissues.Therefore, dynamic crosslinking and stretchable network formation are favored.In addition, Yang et al. developed a multimodal sensing ionic skin capable of tactile sensing, complex wound monitoring, and healing management repair (Figure 8e). [166]Inspired by the phytoplankton dinoflagellates, Kang proposed a sensing layer made of transition metal complex luminophores and ionic liquids capable of producing electrochemiluminescence (ECL). [167]Based on the piezoelectric ionic effect, changes in mechanical stress induce changes in the distribution of ionic luminophores in the film (Figure 8f).The facile fabrication and unique operation of the demonstrated that ECL skin is expected to provide new insights into the material design of electronic skins for human-computer interaction.

Self-Healing Design
The ability of IFMS devices to recover their functionality after accidental damage is critical for long-term use.Restoration of structure and function is automatically triggered upon external damage, increasing durability and longevity. [168]IFMS devices with self-healing ability have gradually become a research hotspot.[171][172] For biomimicry and medical applications, flexibility/stretchability alone is not enough.Wu et al. developed an IFMS device with high sensitivity, mechanical compliance, durability, and self-healable degree, inspired by the shrimp shell structure (Figure 9a). [173]The hydrogel-based IFMS exhibited pressure sensitivity up to 1 kPa and could detect gentle finger touches, human motion, and even small water droplets (mimicking rain).In practical applications, IFMS devices are often required to "sense" wounds and repair themselves.However, most self-healing IFMS devices are generated by incorporating dynamic covalent or physical crosslinks in ion-conducting networks that are reconfigured through chain rearrangements, and such interchain dynamic designs will basically not improve fatigue fracture resistance.Further, Wu et al. designed a fatiguefree yet fully repairable hybrid ionic skin toughened by a highenergy, self-healing elastic nanomesh, similar to the repairable nanofiber interwoven structure of human skin (Figure 9b). [174]his design provides an ultrahigh fatigue threshold of 2950 J m −2 while maintaining skin-like conformability, stretchability, and strain adaptability.Aromatic disulfides with efficient disulfide bond metathesis are embedded into the hard segment to provide room temperature self-healing capability, which is critical for the self-healing of PU nanofibers embedded in the mixed-ionic skin.In addition, Sun et al. developed a PU-based ionic gel structure with self-healing and ultradurability, maintaining reliability over 10 000 uninterrupted strain cycles, and maintaining its  [20] Copyright 2019, Macmillan Publishing Ltd.b) A full-skin bionic electronic skin combined with AI technology.Reproduced with permission. [163]Copyright 2022, Wiley-VCH.c) Ion transport layer using Calathea zebrine as a template leaf.Reproduced with permission. [164]Copyright 2018, Wiley-VCH.d) Bio-inspired ionic skin for theranostics.Reproduced with permission. [165]Copyright 2021, Wiley-VCH.e) Multi-functional ionic skin.Reproduced with permission. [166]opyright 2021, Wiley-VCH.f) Visco-poroelastic electrochemiluminescence skin with piezo-ionic effect.Reproduced with permission. [167]Copyright 2021, Wiley-VCH.
original performance even after 200 days of open-air storage (Figure 9c). [175]Under certain circumstances, IFMS devices require plasticity to cover irregular and dynamic surfaces.The catechol groups in chitosan can form dynamic ester bonds with poly(3acrylamidophenyl)boronic acid.With the assistance of hydrogen bonds, IFMS devices can have strong plasticity and self-healing properties (Figure 9d). [176]The biomanufacturing process per-formed by natural organisms produces many organisms with an elaborate hierarchical structure and specific functions adapted to their native living environment.Drawing on the formation process of oyster reefs, Fu et al. proposed a hydrogel with tunable mineral content and a wide range of properties, including plasticity, 3D printability, self-healing ability (85% recovery in 1 min), stretch, and high ionic conductivity (Figure 9e). [177]igure 9. Self-healing design.a) Ionic skin inspired by shrimp shell structure.Reproduced with permission. [173]Copyright 2017, Wiley-VCH.b) Fatiguefree artificial ionic skin.Reproduced with permission. [174]Copyright 2022, Macmillan Publishing Ltd.c) Mechanically robust, elastic, and healable ionic gels.Reproduced with permission. [175]Copyright 2020, Wiley-VCH.d) Plastic IFMS device.Reproduced with permission. [176]Copyright 2020, Elsevier.e) Wearable ionic skin inspired by oyster reef.Reproduced with permission. [177]Copyright 2021, Elsevier.

Environmental Adaptability
In practical applications, IFMS devices often require users to wear them for a long time to achieve long-term sensing or monitoring functions.However, during this long-term monitoring process, the device is faced with a variety of constantly changing environmental conditions (such as mechanical force, temperature, or humidity), which may cause damage to the structure and function of the flexible IFMS device, seriously affecting its working stability and data reliability.Therefore, developing environment-adaptive IFMS devices is of great significance for practical applications.
Several innovations in structural design have been initiated for stability against failure in mechanically overloaded environments.Structures including fibers, [178] twists, [179] networks, [180] fractals, [181] cells, [182] microdomes, [183] micropillars, [184] micropyramids, [116] porous sponges, [185] origami, [186] and kirigami [187] are commonly incorporated into substrate materials.In each case, the basic design principle is to improve the compression or extension limit of the device by incorporating it into a specific structure, thereby greatly improving the structural integrity and functional integrity of the equipment in the mechanical overload environment.Someya et al. reported an IFMS device that can only measure normal pressure even under extreme bending conditions (Figure 10a), [188] and further simu-lations showed that the fibers changed their relative alignment to accommodate the bending deformation, reducing the strain on individual fibers.Pressure sensitivity is maintained down to a bend radius of 80 μm.This sensor exhibits strong adaptability to mechanical deformation, thus eliminating the influence of abnormal deformation on normal pressure detection.
Under different thermal environmental conditions, the biggest challenge faced by flexible IFMS devices is the unreliability of collected data caused by the decrease of mechanical strength.For example, hydrogel-based devices undergo structural failure at temperatures below 0 °C.Therefore, the environmental adaptability to different temperatures limits the wide application of IFMS devices.For extreme cold environments, the addition of cryoprotectants, such as glycerol, [189] ethylene glycol, [190] sorbitol, [191] ILs, [192] or salts, [193] is a proven strategy.However, the ionic conductivity of hydrogels at low temperature is usually not ideal.How to balance the strength, toughness, ionic conductivity, and antifreeze performance of conductive gels is still an urgent problem to be solved.Jiang's team prepared TEMPO oxidized nanocellulose (CNF) solution and PVA organic hydrogel at low temperature (−20 °C) by sol-gel method (Figure 10b). [194]The introduction of sodium chloride solution into the organic hydrogel system endows the gel with good ion conductivity.This novel gel exhibits superior mechanical properties, high transparency, high ionic conductivity, and frost  [188] Copyright 2017, Macmillan Publishing Ltd.b) Cellulose nanofibrils enhanced, strong, stretchable, freezing-tolerant ionic conductive organohydrogel.Reproduced with permission. [194]Copyright 2020, Wiley-VCH.c) Self-healing electronic skins for aquatic environments.Reproduced with permission. [196]Copyright 2019, Macmillan Publishing Ltd.
resistance.Adding cryoprotectants is a common way to make flexible sensors work well in low-temperature environments.However, in the current research, most of the designs for making IFMS equipment work in extreme temperature environments are concentrated in ultra-low temperature environments, and there are few reports on high-temperature working environments.The operating states of ions in high-temperature and low-temperature environments are quite different.Therefore, it is necessary to study the design strategy of IFMS devices operating in high-temperature environments.
In the long-term operation of IFMS equipment, it is necessary to be able to demonstrate reliability in changing humidity environments under specific application requirements.Ambient humidity can cause flexible devices to lose flexibility, ad-hesion, and breathability.Creating IFMS devices that also function underwater could extend such applications to a variety of aquatic and marine environments. [195]Inspired by transparent jellyfish, Tee et al. reported a diving self-healing skin-like material (Figure 10c). [196]Composed of a fluorocarbon elastomer and a fluorine-rich ionic liquid, the material has tunable ionic conductivity up to 10 −3 S cm −1 and can withstand strains up to 2000%.The material self-heals through highly reversible iondipole interactions, resulting in intrinsic conductivity, transparency, stretchability, and self-healing capabilities under aquatic conditions.This satisfactory humidity environment reliability broadens the way for swimmers, divers, and other underwater workers to provide related biological detection IFMS equipment applications.Reproduced with permission. [197]Copyright 2021, Wiley-VCH.b) The characteristic active EEG signals were extracted in real time by MXene crosslinking based ionic conducting hydrogel.Reproduced with permission. [198]Copyright 2022, American Chemical Society.c) Highly stretchable, transparent ionic touch panel.Reproduced with permission. [6]Copyright 2019, AAAS.d) Cutaneous ionogel mechanoreceptors.Reproduced with permission. [7]Copyright 2021, Wiley-VCH.e) Skin type acoustic sensor detects noise.Reproduced with permission. [199]Copyright 2022, Springer.

Application
Regulation of IFMS devices by ions is the basis of their sensing.Its central property is ion migration under mechanical strain.Different from traditional electronic devices, ion-based IFMS devices usually have higher biocompatibility or features similar to biological functions, thus opening up more application scenarios for flexible mechanical devices, such as human-computer interaction interface and life health application.

Human-Machine Interaction
With the continuous advancement of social digitalization and industrialization, the application of flexible devices in humancomputer interaction interfaces has attracted more and more attention.The human-computer interaction interface based on IFMS equipment can provide interactive responses for users.Among them, the focus of attention is on high stretchability, high toughness, good long-term environmental stability, excellent frost resistance, and tough surface adhesion.Figure 11a shows a hydrogel-based ionic skin.Express the message "I love you" with three consecutive gestures by building a sensory glove. [197]As the hub of information processing and command transmission in the body, the brain produces characteristic and recognizable electroencephalogram (EEG) signals.Figure 11b shows the real-time extraction of characteristic active EEG signals as commands via MXene-based cross-linked ion-conducting hydrogels, enabling intention, motion, and visual interactions. [198]igure 11c shows a polyacrylamide (PAAm) hydrogel ion touch panel based on lithium chloride (LiCl) salt.In a surface capacitive touch system, the same voltage is applied to all corners of the panel, creating a uniform electrostatic field across the panel. [6]igure 12.Life and health application.a) A highly sensitive artificial bionic nerve based on flexible organic electronic devices.Reproduced with permission. [8]Copyright 2018, AAAS.b) Measurement of plantar pressure distribution.Reproduced with permission. [201]Copyright 2022, Elsevier.c) Track the patient's facial pressure distribution.Reproduced with permission. [202]Copyright 2017, Wiley-VCH.d) Imperceptible epidermal-iontronic interface for wearable sensing.Reproduced with permission. [203]Copyright 2018, Wiley-VCH.
When a conductor such as a human finger touches the panel, the touch point is grounded, and a potential difference is generated between the electrode and the touch point.Connected to a monitor via an ion touch panel, it can even recognize written text and drawings.In addition, ultra-sensitive response has great potential for industrial robots.Figure 11d shows a bionic hand attached to an industrial robot capable of detecting the touch of an ultralight feather. [7]Speech is the most intuitive biological signal for daily communication and information dissemination.It is crucial to develop intelligent speech recognition that mimics the human auditory system.Figure 11e shows an all-fiber ionotropic triboelectric mechanoreceptor.It was made into a skin-type acoustic sensor, and an interactive interface that could detect noise levels was built. [199]he sensing essence of IFMS devices is the controlled movement of ions under external stimuli.Compared with the "electronic" language, the "ion" language has more diverse types and interaction forms.At the same time, the hysteresis effect of ion movement brings potential memory effects to ionic circuits, which brings more opportunities for the development and application of human-machine interaction.The development of IFMS in the future will greatly promote the application of artificial intelligence in the field of bionic interaction.

Life and Health Application
The IFMS device is not only satisfied with the design of humancomputer interaction interface, but also aims to reconstruct human body functions and provide users with health-related data under various conditions.In the neural network of the human body, the mechanoreceptors that sense pressure generate different receptor potentials according to changes in pressure, and these signals are collected to generate action potentials on nerve fibers.Nerve fibers form synapses with interneurons in the spinal cord, and action potentials from multiple nerve fibers are integrated through the synapses and transmitted to the brain for subsequent information processing. [200]Bao et al. reported a highly sensitive artificial bionic nerve based on flexible organic electronic devices and successfully simulated the sense of touch (Figure 12a). [8]As a proof of function, the researchers used the artificial neural to distinguish Braille characters, successfully producing a different output for each Braille character.Yang et al. developed an application of ionogel to measure plantar pressure distribution (Figure 12b). [201]When the signal alternates between the minimum and maximum values, it is defined as a walking state, and when the movement fails suddenly and does not respond for a long time, it is judged as a fall.Pan et al. demonstrated an array of nanofabric sensors integrated into a fabric face mask for facial skin pressure mapping.This enables continuous tracking of the patient's facial pressure distribution in real time and with rapid response to prevent intraoperative pressure ulcers (Figure 12c). [202]However, IFMS devices applied to life and health monitoring are susceptible to human parasitic noise and environmental sources (possibly up to hundreds of times the signal).Therefore, novel sensing modalities with high device sensitivity and high noise immunity as well as conformal packaging and body-fitting are still highly sought after.The ionoelectronic pressure-sensing architecture presented by Pan et al. enables characteristic monitoring of various vital signals, such as blood pressure waveforms, respiration waveforms, muscle activity, and artificial touch, indicating its broad applicability in wearable applications (Figure 12d). [203]FMS devices based on ion conduction bring great potential to the development of life and health detection devices.This is because it uses the same "ionic" language as biological systems in the process of information transmission, which is an important means to break the barriers erected between life health detection and artificial systems.

Conclusions and Outlooks
This review covers the evolution of flexible mechanical sensors from "electronic" to "ionic" languages, providing key reference information for the realization of the next generation of biointelligent sensing devices.Different from traditional electronic devices, IFMS devices derive their mechanical strain information from ion migration or ion polarization, which shows their conceptual similarity to biomechanical strain systems.Here, we summarize the basic design principles of ion transport-regulated flexible mechanical sensors, aiming to provide researchers with comprehensive information to develop devices with excellent performance.As the dominant factor, the mode of action of ions determines the mechanotransduction mechanism of the sensing device.Here, active conduction (non-self-powered) and passive conduction (self-powered) are mainly covered.According to the operation mode and properties of these devices, the relevant structural engineering is further highlighted, and the design and manufacture of the ion transport layer are mainly focused.According to different structural forms (ionic liquids, electrolyte solution-nanofluidic membranes, ionic gels, polyionic liquids, and hydrogels), the structural effects of IFMS devices and design strategies for improving performance (bionic structures, self-healing, and environmental adaptability) are summarized.In addition, important applications of IFMS devices are discussed: human-machine interface; life and health applications.Although IFMS devices have made significant progress in recent years, many challenges still remain.Below, we briefly highlight current and future challenges and possible solutions for IFMS devices.i) Integration technology: With the development of the IoT and smart wearable technology, a single IFMS device is required to have the characteristics of miniaturization, flexibility, low power consumption, and integration.However, most devices require a lot of wiring and external connections, thus resulting in their bulky bodies.Additionally, the increased density of interconnect lines leads to parasitic capacitance and resistance, which increases overall impedance and noise levels.The reliability of even large-area sensing arrays is further degraded.Therefore, how to realize the development of IFMS device integration technology is a problem worth considering.A possible solution strategy is to integrate energy storage devices, functional devices, and energy conversion devices into an all-in-one designed IFMS.IFMS devices based on ion transport bring hope to this design; for example, potentiometer-type self-powered devices can abandon external power sources, reduce integrated units and complex circuits, and thus avoid the disadvantages of multi-device integration.ii) Multimodal mechanical sensing: IFMS-based artificial skin and smart machines need to possess multimodal tactile sensing capabilities, such as normal pressure, shear force, and torsional force.However, most IFMS devices currently developed focus on the ability to sense a single type of mechanical stimulus.A promising solution is to automatically learn from a large amount of complex sensor data through machine learning (ML) for intelligent decision-making, thereby endowing IFMS devices with multimodal autonomous perception capabilities.iii) Structural design: Although a large number of design strategies have been developed for the macro-and micro-structure of IFMS devices, there is still a lack of analysis of the relationship between the performance of IFMS devices and the macrostructure (distribution and integration of electrodes and ion transport layers, ductility) and microstructure (such as size, microstructure, ion interaction mode), and the lack of standardized strategies to guide fabrication of IFMS devices with specific perception capabilities.To this end, it is necessary to further establish material systems, devices, modeling, simulation, and data-driven methods to guide the structural design of IFMS devices.iv) Multifunctional recognition: With the development of flexible sensing technology, IFMS devices need to realize multifunctional recognition.In addition to basic mechanical strain recognition, the ability to recognize temperature, humidity, and gases may also be required.However, it is difficult for existing sensing systems to ensure that there is no crosstalk between these multiple stimuli.A typical solution is to decouple multiple stimulus signals, that is, to transduce each stimulus into a separate signal so that their signals do not interfere with each other.v) Service life: Considering the practical application of IFMS devices in daily life and industrial environment, service life is one of the key factors affecting its performance.IFMS devices deform under various mechanical stresses during use, such as normal pressure, tension force, torsion force, and shear force.On the one hand, physical factors that affect the service life of IFMS devices include material fatigue, leakage, cracking, and wear.On the other hand, chemical factors that affect the service life of IFMS devices include irreversible redox reactions, ion concentration polarization, and ion pathway changes.Therefore, in the design of future IFMS devices, these influencing factors should be considered to improve the stability and cycle life of the device.

Table 1 .
Comparison of different mechanotransduction mechanisms of IFMS.

Table 2 .
Summary and comparison of IFMS with different ion transport layers.