Design Intelligent Pressure Sensing Devices and System Integrated with Conductor‐Coated Porous Composites

Porous composite pressure sensors have the advantage of a wide deformation range owing to their structural characteristics. It is one of the building blocks for wearable intelligent devices and systems. Conductor‐coated porous composites (CCPCs), consisting of a conductive coating and an insulating elastomeric skeleton, exhibit low modulus, high compressibility, and a facile fabrication process. Pressure sensors developed based on CCPCs provide specific intelligent wearable sensor applications, including high‐resolution pressure detection and multimodal sensing under extremely compressing environments. Here, representative works on the fabrication, performance, and applications of CCPC devices are reviewed. The “state‐of‐the‐art” compilation of the most sophisticated design strategy of CCPCs with durable, low hysteresis, high sensitivity, and wide detection range is summarized, which also ensures instructive significance for the design of other pressure devices based on soft matter and rigid conductors. Finally, the intelligent device and system scenarios of CCPC devices are introduced.


Introduction
The skin provides humans with the ability to effortlessly sense the breeze and strong impact. Under pressure, the skin precisely senses electrical impulses converted from pressure stimuli through mechanoreceptors, while maintaining flexibility and DOI: 10.1002/adsr.202200091 stretchability. In order to simulate the pressure-sensing performance of soft matter, which is obviously different from silicon-based pressure devices and strained metals, research on flexible pressure sensors has been continuously carried out. Flexible pressure sensing technology [1][2][3] has developed rapidly in recent years. It is widely used in focus areas such as wearable medical devices, [4,5] robotic haptics, [6,7] and artificial intelligence. [8,9] Flexible pressure sensors are classified into capacitive, [10][11][12] resistive, [13][14][15] piezoelectric, [16][17][18] and triboelectric. [19][20][21] Among them, resistive devices have been widely used because of the advantages of simple preparation and convenient signal acquisition ). Flexible pressure sensors usually consist of electrodes and sensitive layers. The sensitive layer supports the main pressure-sensing performance of the device. The sensitive layers of flexible resistive devices are generally composed of soft matter and advanced conductor. Although filler-elastomer composites based on electropercolation [22] and tunneling [23] exhibit attractive flexibility, severe thermal noise and limited sensitivity limit their generalization. Bao et al. first proposed the device sensitization strategy of micropatterns construction in capacitive pressure sensor design, [24] which was extended to thin-film piezoresistive devices fabrication. Micropattern designs such as micropyramids, [25][26][27] microdomes, [28][29][30] micropillars, [31][32] and microcones [33] have been reported. Incomplete contact between the sensitive layer and the electrodes increases the initial resistance of the piezoresistive device. The contact area between the micropatterns array and the electrode increases during the deformation process, and the electrical properties of the device change abruptly, thus obtaining high sensitivity. However, the narrow detection range limits their application due to the compression set saturation of the microstructure. In addition, the need for patterning and fabrication processes of micro/nanostructures adds additional device fabrication costs. Different from the change of the contact relationship between the sensitive layer and the electrode, the porosity design controls the electrical path by changing the looseness and density of the piezoresistive composite. Porous piezoresistive composite materials with a sponge-like structure [34] are divided into pure conductive sponges, [35][36][37][38] composite conductive sponges, [39][40] Figure 1. Summarization of fabrication, performances, and applications of conductor-coated porous composites (CCPC). Image for "Hysteresis": Reproduced with permission. [79] Copyright 2019, Wiley-VCH. Image for "Sensitivity": Reproduced with permission. [97] Copyright 2021, Springer Nature. Image for "Working rang": Reproduced with permission. [58] Copyright 2022, Wiley-VCH. Image for "Pressure detection": Reproduced with permission. [48,68,95] Copyright 2017, American Chemical Society. Copyright 2022, Wiley-VCH. Copyright 2021, Royal Society of Chemistry. Image for "Multimode": Reproduced with permission. [87] Copyright 2022, American Chemical Society.
conductive sponges impregnated with elastomers, [45][46][47][48] and conductive material-coated sponges.  Different from other aerogels, conductor-coated porous composites (CCPCs) consist of a conductive coating and an insulating elastomer backbone. CCPC shows attractive large-scale production potential because of readily available raw materials and facile preparation process. The low production cost is also predictable because of the small amount of expensive advanced nanomaterials, which are only used in thin coatings on the surface of the elastomeric backbone.
Here, this review aims to provide a comprehensive survey of CCPC devices (Figure 1). The details of the preparation process, the latest reports on performance optimization, and interpretations for advanced applications are summarized, which also ensures instructional significance for the design of other pressure devices based on soft matter and rigid conductors.
Carbon materials are considered to be materials with a higher degree of commercialization. Therefore, these materials generally exhibit low cost and easy availability. In terms of electrical properties, carbon-based materials have high intrinsic conductivity. However, these materials are difficult to disperse uniformly due to -forces. Agglomerates often exhibit limited electrical properties due to high contact resistance with the metal electrodes. Furthermore, obtaining uniform carbon-based conductive coatings is challenging due to lack of substrate adhesion. Graphene exhibits outstanding brittleness, with a fracture energy of only 16 J m −3 . [99] There is a risk of coating cracking [48,51,62,67,68,69,75,81,89,94] and peeling off under high pressure. Carbon nanotubes own high modulus and hollow structure, which remain stable and provide a piezoresistive mechanism by deforming even under large pressure loads. However, the deformation relaxation of nanotubes may cause CNT-based CCPC sensing to exhibit high hysteresis. Nanometallic materials exhibit excellent electrical conductivity and hydrophilicity. Metallic materials are easier to disperse than carbon materials while having low interfacial contact resistance. However, silver-based nanomaterials and gallium-based liquid metals are chemically unstable and easy to be oxidized. Liquid metals are difficult to coat on the skeleton surface due to the high surface energy. It is necessary to modify the substrate and the liquid metal for better compatibility. Last but not least, metal-based materials are still expensive to mass-produce, which means that CCPC devices with nanometal coatings are difficult to apply in practice. Conductive polymers are usually attached to the surface of elastomeric backbones by in situ polymerization. Conductive polymers exhibit good substrate attachment by covalent bonding. Conductive polymer coated CCPC devices usually exhibit good coating uniformity. The consistency of sensing performance of the device can be guaranteed. However, the stability of the sensor is limited by the degradability of the polymer. PEDOT:PSS exhibits high electrical conductivity, but is heavily affected by humidity. MXenes are obtained by acid etching of transition metal carbides and nitrides (TMCs and TMNs). MXene-based materials have a multilayer structure and exhibit higher piezoresistive properties than graphene during compression. [100] However, MXene-based materials could be easily affected in humid and chemical environment leading to poor stability. [101] Moreover, the prohibitive cost limits their application in CCPC devices.
Some unconventional materials were also reported. Polyvinyl alcohol (PVA) -based electrolytes have potential as conductors coated with elastomeric backbones to fabricate flexible piezoresistive sensors. [94][95][96] However, the electrolyte coating is moisture sensitive. As the ambient humidity changes, the electrolyte-based CCPC is prone to change in resistance. Therefore, packaging of the device is necessary. Besides conductive coating materials, studies on the coating of porous frameworks with semiconducting materials have also been reported. [97,98] Sea urchin-structured semiconductor composites exhibit unprecedented bulk piezoresistive properties. [98] Figure 2. Common materials used in the fabrication of CCPCs. Advanced conductive materials are used as the main components of the coating. Image for "Polyurethane sponge": Reproduced with permission. [93] Copyright 2021, American Chemical Society. Image for "Melamine sponge": Reproduced with permission. [85] Copyright 2018, American Chemical Society. Image for "Porous PDMS": Reproduced with permission. [54] Copyright 2021, Springer Nature.
As one of the most promising coating materials, conducting polymers may receive additional attention in future CCPC device designs. First, the conductive polymer is uniformly immobilized on the surface of the elastomeric backbone by in situ polymerization of the precursor. Coating-skeleton is bound by covalent. During compression, the conductive coating remains stable. Unlike brittle coating materials such as graphene, conductive polymers may not show obvious cracking. In addition, the polymerization is beneficial to large-scale production through the regulation of temperature. During soaking of the precursor solution, no additional squeeze-release procedure is required. Nevertheless, polymer degradation, moisture resistance, and limited electrical conductivity still need to be addressed.
In terms of the availability of base materials, PU and MS sponges have been commercialized and are easy to access. PU tends to have hydrophilic properties, [94] and its lower surface energy is suitable for the coating of conductive materials. Through parameter adjustment in the foaming process, polyurethane sponge can achieve high compressibility, excellent resilience, and adjustable modulus, and is widely used in insoles, seat cushions and sofa fillers. Piezoresistive devices fabricated by covering with conductive substances show outstanding applications in these fields of application. On the other hand, the existence of bonds on the MS surface is suitable for the attachment of low-dimensional carbons and conductive polymers. However, the coating performance of some hydrophilic materials is poor, which is attributed to the low surface energy of MS. Last but not the least, PDMS prepolymer can be purchased commercially. PDMS is added to the platinum catalyst to undergo a crosslinking reaction, which facilitates the researchers to control the pores of the porous base of the elastomer. Porous PDMS is generally prepared by template method. [54,57,58,95] The PDMS prepolymerization solution enters the inside of the sugar cube, or a large amount of salt is mixed into the PDMS pre-polymerization solution. After the PDMS is cross-linked, the sugar cube is dissolved by boiling water to obtain a PDMS porous sponge. Compared to PUS and MS with hydrogen bonds, the covalently bonded PDMS exhibits low mechanical hysteresis. [79,95] Ecoflex has a high degree of deformation ability while it's the hysteresis is large. [102] In addition, natural porous materials have also been applied to the practice of conductive coated sponge devices, such as natural latex, [69] but the low reproducibility and uncontrollable preparation process limit their application.

Fabrication Methods of CCPCs
The reported CCPCs are mostly prepared by dip-coating method. Therefore, in this section, we focus on discussing the morphological characteristics of CCPCs prepared by different dip-coating ways and the issues in the process of dip-coating conductive inks with elastomeric skeletons. According to the dip-coating steps and the kinds of materials used, the preparation methods of CCPCs are classified into one-step dip-coatingand multi-step dip-coating. The one-step dip coating method is simple, which is realized by immersing the porous elastomer directly in the  [98] Copyright 2020, Elsevier. b) CB@PUS. Reproduced with permission. [48] Copyright 2022, Wiley-VCH. c) PVA/H 2 SO 4 @PUS. Reproduced with permission. [94] Copyright 2021, Royal Society of Chemistry. d) PVA/H 3 PO 4 @PDMS. Reproduced with permission. [95] Copyright 2021, Royal Society of Chemistry. Schematic diagrams of CCPCs prepared by multi-step dip-coating method. e) LMNP@PDA-PUS. Reproduced with permission. [70] Copyright 2019, American Chemical Society. f) Au/Cu@VTMS-PDMS. Reproduced with permission. [74] Copyright 2016, Wiley-VCH. g) (MMT/G) n @PEI-MS. Reproduced with permission. [65] Copyright 2021, American Chemical Society. dispersion of the configured conductive material. One-step dip coating method is widely applied due to its simple process and lab-accessible facilities. Rigid material coating and polymer electrolyte coating exhibit different surface morphologies. For the rigid material coating, as shown in Figure 3a, the typical NiO nanofibers were aggregated and fixed on the skeleton of the sponge. [98] As shown in Figure 3b, the conductive carbon black was attached to the PUS surface nonuniformly. [48] Device durability under large deformations may be limited by nonuniform coating and lead to instability. The attachment of conductive polymers to the elastomeric backbone is attributed to the in situ polymerization of monomers. The skeleton is soaked in the precursor solution. Through the regulation of catalyst and temperature, the conductive polymer coating is deposited on the surface of the skeleton through the in-situ polymerization of the precursor. Excellent coating stability and adhesion uniformity are exhibited due to the covalent bonding of the conductive polymer coating to the elastomeric substrate. Here, the dip-coating preparation method involving multiple conductive materials is also attributed to the single-step dip-coating method. Generally, the attachment of multiple conductive materials will bring higher sensitivity to CCPC devices. However, this coating strategy increases the instability of the device coating.
A polymer electrolyte is a soft conductor that relies on the movement of ions in a polymer network to conduct electricity. Hydrogen bonds cross-link the polymer chains. Ions move in the polymer network under the action of an electric field, making the polymer electrolyte exhibit excellent conductivity. Compared with inorganic hydrophobic materials, organic hydrophilic polymer electrolytes exhibit natural compatibility with polymer elastomer backbones. [108] The hydrophilic porous PUS was soaked in the polymer electrolyte solution to prepare the porous sensor. Different types of electrolytes cause the morphology to vary. As shown in Figure 3c, as the concentration of H 2 SO 4 increases, microcrack morphology of PU surface starts to appear on the surface. [94] As shown in Figure 3d, PVA/H 3 PO 4 @PDMS exhibits a structure of filamentous interconnections inside the framework. [95] The prepared CCPC based on PVA-based electrolyte coating exhibited coating uniformity. The ions that provide conductivity will not agglomerate and form a uniform conductive coating www.advancedsciencenews.com www.advsensorres.com layer, which is proved by energy dispersive spectroscopy (EDS) experiments. [94][95][96] Encapsulation of the device is necessary due to the moisture sensitivity of the polymer electrolyte.
Here, multi-step dip coating is defined as a dip coating method involving other nonconductive substances. Due to the involvement of functional insulation materials, extra dipping steps was added. Multi-step dip coating is divided into adhesive-assisted adhesion, wet-chemical deposition and electrostatic self-assembly. During the implementation of adhesive-assisted adhesion or wetchemical deposition, surface modification of the porous framework is required. Typically, advanced conductive materials exhibit limited binding forces on the surface of porous elastomeric backbones.
Typically, advanced conductive materials exhibit limited binding forces on the surface of porous elastomeric backbones. Adding viscous substances on the surface of the substrate to increase the coatability of conductive materials is one of the alternatives. Weng et al. modified polydopamine (PDA) on the surface of PUS and prepared CCPC with thick AgNW coating by dip coating. [70] Huang et al. tuned liquid metal with 3-mercaptopropionic acid and ultrasonic treatment, [74] thereby, LMNPs with good compatibility were obtained. As shown in Figure 3e, LMNPs were coated on PUS and PDA-modified PUS surfaces, respectively. The modified PUS surface obtained a denser conductive coating.
Wet-chemical deposition of metals on the surface of modified elastomeric skeleton has also been reported. Wet-chemical deposition avoids the involvement of viscous substances. The surface of the elastomeric substrate is modified through the participation of additional groups. During the reduction of the metal precursor solution, micron-sized metal particles are deposited on the surface of the porous substrate. Yin et al. reported a CCPC prepared by a two-step dip coating. A simple solution dip-coating method completes the self-assembly of Au nanoribbon thin films onto an amine-functionalized polyurethane (PU) framework. [73,74,75,77,81,82,83] As shown in Figure 3f, Liang et al prepared a dense metal coating by electroless plating. [74] Porous PDMS was prepared using the sugar template method. The PDMS was modified by salinization of vinyltrimethoxysilane (VTMS), followed by a typical in situ free radical polymerization step with methacryloxyethyltrimethyl ammonium chloride (METAC). The modified porous PDMS was used as a framework for metal deposition, and a dense Au/Cu layer was immobilized on the PDMS surface.
Electrostatic self-assembly is a layer-by-layer deposition thin film fabrication technique, which is also used in the preparation of CCPCs. [50,65] A dense conductive coating is formed by electrostatic adsorption by alternately dipping the sponge substrate in a dispersion of oppositely charged materials. Liu et al. dip-coated MS sponges in positively charged polyetherimide (PEI) solutions and negatively charged montmorillonite (MMT) aqueous dispersions to form a MMT layer. [65] MMT has better charge-material binding ability as a soft phyllosilicate mineral than MS sponge substrate. Afterward, another layer of GO was prepared by electrostatic self-assembly based on MMT@MS. As shown in Figure 3g, the surface of the MMT/GO@MS sponge skeleton exhibited a dense and uniformly distributed conductive coating.
Compared with the single-step dip-coating method, the multistep dip-coating method shows higher sample performance su-periority. The CCPCs prepared by the multi-step dip coating method showed better coating compactness, uniformity and stability. Nevertheless, the involvement of a large number of functional materials brings additional costs. In addition, the cumbersome step process may affect the promotion of the process.
Nondip coating methods for CCPCs are mainly focused on physical vapor deposition (PVD). PVD is a common coating technology. Under vacuum conditions, the material source is vaporized into atoms or molecules, or partially ionized into ions. Through low-pressure gas, a film with a Certain special function is deposited on the surface of the substrate. However, PVD is not a routine method in the preparation of conductive coating sponge sensitive layer. Typically, the deposited films exhibit limited binding forces on sponge substrates. Wu et al. pre-sputtered a layer of titanium on the surface of PU sponge as a base layer, in order to solve the insufficient bonding force between gold and the substrate. Au/Ti@PUS sensitive layer with dense conductive coating was prepared. However, there is a mechanical mismatch between the rigid conductive coating and the flexible substrate. During the compression-releasing process, the stress in the coating is difficult to release, leading to the generation of cracks. Besides, the Au/Ti@PUS devices exhibit large mechanical hysteresis, which may be related to the dense metal deposition. Compared with the dip coating method, the stability of the CCPCs sensitive layer prepared by the nondip coating method is worth testing. High cost and limited large-scale production potential also limit the promotion of the process.
In this section, typical coating materials, as well as substrate materials used to prepare CCPC are summarized. In addition, the preparation method of CCPCs with coating stability and uniformity is reviewed, which provides a strategy for fabricating durable devices.

Performance Optimization of CCPC Pressure Sensor
Mechanical and electrical hysteresis during the pressure loading process affects the sensing performance of the device. Therefore, reviewing the causes of hysteresis, and the related improvement strategies is essential. In Figure 4a, a typical structure of CCPC devices are shown. A porous pressure-sensitive layer is fixed between the electrodes. During compression, the conductive skeletons contact with each other and create additional conductive paths, which manifest as a monotonous decrease in resistance with increasing pressure. The following two formulas represent the response sensitivity of the pressure device There, ΔR is the difference between the pressure-loaded and initial resistance, and R 0 is the initial resistance There, ΔI is the difference between the pressure-loaded and initial current, and I 0 is the initial resistance. The circuit that detects the change in resistance is simple, and most of the work  [94] Copyright 2021, Royal Society of Chemistry. d) SEM images of the skeleton changes of PEDOT:PSS@MS under compression-release and the corresponding stress-strain curves. Reproduced with permission. [85] Copyright 2018, American Chemical Society. e) Schematic diagram of pressure response hysteresis. Reproduced with permission. [79] Copyright 2019, Wiley-VCH. f) Preparation process and hysteresis performance of PPy@PDMS with uniform pores. Reproduced with permission. [79] Copyright 2019, Wiley-VCH.
is defined in terms of resistance change. However, the electrical changes under high pressure are difficult to observe and thus cannot accurately determine the working range. Considering the requirements for defining parameters such as the detection range and linearity of the device, ΔI/I 0 is more suitable to measure CCPC devices.
Three-dimensional porous CCPCs have a wide strain range. During the straining process, the CCPCs sensitive material exhibits obvious changes in electrical properties. Therefore, CCPCs have potential as sensitive layers of strain sensors. The sensitivity parameter gauge factor (GF) of the strain sensor is defined There, ΔR is the difference between the pressure-loaded and initial resistance, and R 0 is the initial resistance. L 0 is the initial height of the device, and L is the height of the device after deformation. The factor is defined as (L − L 0 )/L 0 .
Based on the brief introduction the working mechanism of the CCPC device, this chapter will specifically discuss how to optimize the device's hysteresis, monotonicity, sensitivity, detection range. Table 1 summarizes the performance parameters of CCPC fabricated from different compositions and preparation methods.

Hysteresis Performance Optimization Strategies
As shown in Figure 4b, the stress-strain curve of the elastomeric sponge is illustrated. As the porous structure is compressed, the stress response region is divided into elastic region, plateau region (plastic region), and dense region. In the elastic region, stress and strain follow Hooke's law. In the plateau region, the skeletons of the elastomers come into contact, plastic deformation occurs, and the stress rises slowly. When the porous structure is densified, the skeleton is almost entirely in contact, and the stress rises rapidly with the increase of strain. Sponge substrates usually exhibit hysteretic linearity, which originates from the viscoelastic properties of the polymer. When the elastomer deforms, its polymer chains rub against each other, causing the sponge substrate to dissipate energy during compression-release cycles. The stress generated during the release of the elastomeric sponge is less than that under compression loading. The mechanical hysteresis coefficient is defined as There, the A loading and A unloading are the area of loading and unloading under the curve of stress/strain. The stress generated during the compression of the elastomeric sponge substrate is related to the pore size, porosity, and type of material. Polyurethane and melamine sponges with hydrogen bonds participating in the cross-linking rely on the breaking and recombination of sacrificial bonds during the deformation process, which has excellent toughness but exhibits high hysteresis characteristics in compression and release. In contrast, PDMS sponges cross-linked by silicon-oxygen covalent bonds exhibit relatively low hysteresis.
As shown in Figure 4c, PVA/H 2 SO 4 electrolyte was coated on the surface of PU sponge. [94] Both PU and PVA/H 2 SO 4 @PUS exhibited standard stress-strain curves for elastomeric sponges. At 20%, 40%, 60%, and 80% deformation, the stresses of pure PU sponge are 2.53, 4.25, 6.07, and 14.72 kPa. After electrolyte coating, the sponge exhibited more significant stress under compression due to the decrease in pore size. At 20%, 40%, 60%, and 80% deformation, the stresses of PVA/H 2 SO 4 @PUS are 2.74, 4.93, 7.51, and 16.20 kPa. In addition, the hysteresis of the electrolyte elastomer composite increases due to the intervention of additional viscoelastic soft matter. As shown in Figure 4d, PEDOT:PSS@PUS was prepared and exhibited a similar increase in compressive stress and hysteresis. Yao et al. designed a PVA/H 3 PO 4 @PDMS device with low mechanical hysteresis and a filamentous interconnection structure. [85] At strain values of 20%, 40%, 60%, and 80%, the mechanical hysteresis coefficient of the compressive strain of the PVA/H 3 PO 4 @PDMS are 7.78%, 9.98%, 12.64%, and 16.36%.
Mechanical hysteresis prevents the compressed porous composite from returning to the same height, thereby changing the initial value of resistance and causing electrical hysteresis. In addition, the instability of the coating is also one of the reasons for the electrical hysteresis. The materials bonded by intermolecular forces on the skeleton's surface cracked, slipped, and fell off during compression, causing the rearrangement of the conductive paths inside the composite. The electrical hysteresis of pressure sensing is demonstrated in Figure 4e. The electrical hysteresis coefficient is defined as There, the A loading and A unloading are the area of loading and unloading under the curve of ΔR/R 0 -P. Due to the unstable signal response, the device is difficult to be applied in engineering applications. Oh et al. prepared PPy-attached PDMS via a liquid template method. [79] PDMS has relatively little hysteresis. The droplets are dispersed in the PDMS pre-polymerization solution, and uniform pores are distributed inside the prepared elastomer. Friction between coatings is improved during compression. In addition, covalent bonding occurs between PPy and PDMS, which avoids slippage and detachment of the coating material. Therefore, the PPy@PDMS pressure sensor exhibits only 2% hysteresis under a loading of 100 kPa.
Low hysteresis is one of the critical requirements for a sensor to be used in practical applications. Regarding material choice, PDMS is the least hysteresis available elastomeric framework materials. The hysteresis of a material is often related to modulus and toughness. Low hysteresis materials often relate to high modulus and low toughness. In actual use, performance parameters are often to be compromised. The electrical hysteresis of CCPC devices is caused by the slipping and detachment of coating materials. During the compression of the 3D structure, the slippage of the coating material can lead to the rearrangement of the electrical paths. During the release of the backbone, the conductive coating cannot remain stable, causing the initial resistance of the device to change. For electrical hysteresis, enhancing the bonding of conductive coatings to substrates is a general approach. A covalent bond between the coating-substrate  NiO@MS Dip-coating -0.375 kPa −1 (10−35 kPa) [98] is necessary. Coatings bonded by intermolecular forces are prone to irreversible position changes under pressure overload. Therefore, in situ polymerization-attached conductive polymers may be more potential coating materials in the design of low electrical hysteresis CCPC devices in the future. The conductive polymer is evenly bonded to the substrate surface through covalent bonds, which avoids the irreversible slippage of the coating material. In addition, adjusting the pore size, porosity, and pore uniformity of the substrate also reduces the friction between the coatings, thereby obtaining CCPC devices with low hysteresis.

Nonmonotonic Response of CCPCs
Porous compression-resistant devices often exhibit monotonic pressure response curves. Under pressure loading, the porous structure is extruded and densified gradually. During the compression, the conductive path of the device increases continuously, manifested by a monotonic decrease in resistance. CCPC is prepared by attaching a conductive material to an elastomeric backbone. During the compression of CCPC devices, the positive resistance effect under minor strains appears, which is dif-  [50] Copyright 2016, Wiley-VCH. d) Gold/Ti@PUS. Reproduced with permission. [68] Copyright 2017, American Chemical Society. e) MXene/CS@PUS. Reproduced with permission. [89] Copyright 2019, Elsevier. f) MCNT/rGO@PUS. Reproduced with permission. [62] Copyright 2018, American Chemical Society. g) AgNW/CNF@PUS. Reproduced with permission. [67] Copyright 2019, American Chemical Society. h) Ag@Natural rubber. Reproduced with permission. [69] Copyright 2020, American Chemical Society.
In the elastic region, the skeletons of the sponges are not yet in contact. The nonuniform deformation of the backbone provides the stress generated by compressing the sponge. As shown in Figure 5a, the modulus matching of the dense, rigid coating to the elastomeric skeleton caused the coating to crack. When the skeleton is bent, the coating on the front side cracks, reducing the number of conductive paths. The conductive path of the coating on the backside of the skeleton is unchanged. Therefore, in the elastic region, the CCPC with the nonuniform deformation of the skeleton as the primary sensing mechanism exhibits an increase in resistance during compression. As the CCPC com-presses further, the skeletons come into contact. In the plateau region, the stress of the sponge rises or slows down. Skeleton contact replaces nonuniform deformation as the main factor of conduction path change. As shown in Figure 5b, the response nonmonotonicity of CCPCs tends to occur at the junction of the elastic and plateau regions. Under deformations, the devices have different sensing mechanisms, showing an opposite resistance response trend. As shown in Figure 5c,e,g, with the participation of natural polysaccharides such as chitosan (CS) and cellulose nanocrystals (CNC), CCPCs are more likely to exhibit coating cracking. [50,89,67] The charged polysaccharides can adsorb conductive materials. In addition, these hydroxyl-rich materials adhere easily to hydrophilic polyurethane substrates. Therefore, a dense conductive coating is formed on the surface of the porous elastomer skeleton. However, these polysaccharides exhibit high modulus and high brittleness due to the nonrotatable glycosidic bonds. In the elastic region of sponge strain, the coating cracks as the backbone flexes, which is attributed to the mismatch of mechanical properties between the coating and the substrate. As shown in Figure 5d, Wu et al. sputtered a Ti layer on the PU's surface to improve the performance. [68] Ti is used as a base layer and has good adhesion on the surface of PU. Subsequently, dense Au was successfully attached to the Ti surface by sputtering, resulting in a highly conductive Au/Ti@PUS composite. The Au/Ti coating cracked due to the high modulus of Ti. The Au/Ti@PUS sensor exhibits a positive resistance phenomenon under small pressure. As shown in Figure 5f, Tewari et al. repeatedly dip-coated PU sponges in multi-walled carbon nanotubes (MCNTs) and hydrophilic reduced graphene oxide (rGO) sheet inks to prepare CCPCs with dense coatings. [62] The hydrophilic rGO sheet exhibited good adhesion on the PU surface. However, the high brittleness of rGO induced cracks in the layer. MCNT/rGO@PUS exhibited nonmonotonic response within a narrow pressure range. As shown in Figure 5h, Wang et al. used tetrahydrofuran to swell a natural latex sponge to prepare an elastomeric backbone with a rough surface. [69] Through the deposition of AgNPs, the porous composite exhibits excellent electrical conductivity and positive resistance effect similar to crack morphology.
The mechanism is discussed in the following. Positive resistance response occurs in CCPC devices with cracked coatings. In engineering applications, devices that are not monotonic are generally considered are unsatisfactory. Coating cracks arise due to the mechanical mismatch between the brittle coating and the ductile elastomer backbone. Under pressure loading, brittle coating materials may slip and fall off, resulting in coating cracks. As the pressure increases, the number of irreversible cracks gradually increases. In order to avoid the positive resistance phenomenon, coating materials with certain flexibility and toughness should be used. Natural polysaccharides such as chitosan and cellulose nanocrystals have certain viscosity and can be used as adhesion aids for coating materials. However, the rigidity and brittleness of polysaccharides easily induce cracks in the coating. Therefore, polysaccharide binders should be avoided. In addition, excessive conductive material adhesion is also one of the reasons for coating cracks. Due to the large modulus gap with the substrate, the subsequent conductive coating will be subjected to more stress during deformation, resulting in cracking. In future CCPC device design, it is necessary to find a modulus matching coating-skeleton. It is worth noting that previous attempts have focused on heterogeneous coating-skeleton bonding strategies. The use of homogeneous materials may be the key to avoiding crack morphology. For example, PDMS is used as a binder to immobilize the conductive material and fix it on the porous PDMS backbone. A homogeneous design of the coatingbackbone may avoid the mechanical mismatch between the materials.

Sensitivity Enhancement Strategies
The formula for calculating the resistance of a conductor is R = L/S, where is the resistivity, L is the length, and S is the cross-sectional area. CCPC devices are essentially porous conductors sandwiched by two electrodes. Under pressure loading, the relative conductivity changes with the deformation of the shape. The reported sensitivity of CCPC is usually within 1 kPa −1 . [34] In this subsection, we mainly discuss strategies to improve the sensitivity of CCPCs. Bao et al. constructed microstructures on the surface of the sensitive layer for the first time to prepare ultra-highsensitivity thin-film sensors. [24] A similar concept is introduced in the performance optimization of CCPCs. Through contact isolation, a contact resistance is preset between the electrode and the sensitive layer, which can effectively increase the initial resistance of the device (as shown in Figure 6a). In initial conditions, the contact area between the sensitive layer and the electrodes is limited. During compression, there is entire contact between the sensitive layer and the electrodes, thus the sensitivity increases with contact resistance decreases. Wang et al. increased the sensitivity of PEDOT:PSS@MS by 10 −6 times by presetting the copper wire array between the sensitive layer and the electrodes. [84] As shown in Figure 6b, Yue et al. prepared a MXene coated sponge sensor with 442 kPa −1 sensitivity by presetting PVA nanowires between the sensitive layer and the electrode. [92] It is worth noting that the sensitivity of devices fabricated by this strategy is often lower under small-pressure loading than under large-pressure loading. This may be attributed to the insignificant point contact between the sensitive layer and the electrode under small pressure.
Metamaterials have exotic properties that are valuable for designing devices with superior properties. Initially coined in the field of electromagnetics, the concept of metamaterials has recently been attributed to a class of artificially engineered materials that exhibits extraordinary properties not available or not easily obtainable in nature. Metamaterials are already used in the fields of electromagnetism, [104] optics, [105] and acoustics. [106] As the latest branch of metamaterials, mechanical metamaterials [107] exhibit special functions in compression deformation and pressure response. The value of Poisson's ratio is the negative of the ratio of transverse strain to axial strain. Porous polymeric foams usually exhibit a positive Poisson's ratio close to 0, since the cells tend to collapse in compression. Through special structural design, elastomeric frameworks with negative Poisson's ratio effects are realized. The application of mechanical metamaterials with negative Poisson's ratio in the design of CCPC devices has been reported, which has been proven to effectively improve the sensitivity of the device. As shown in Figure 6c, Huang et al. reported a porous metamaterial substrate obtained by pre-straining PU sponge under heating conditions. [48] Compared with ordinary PU sponges, the PU with negative Poisson's ratio exhibited earlier skeleton densification and more obvious stress concentration. Under the same strain loading, the metamaterial PU-based CCPC showed higher sensitivity. The design of hierarchical contact and structure could also be used to improve sensitivity. As shown in Figure 6d, Guo et al. assembled and reduced graphene oxide (GO) on the surface of PU, and in situ polymerized PANI nanohair on this basis. [81] CCPCs usually exhibit low sensitivity under small forces (< 1 Pa), which is attributed to the asynchronous deformation of the backbone and coating. Under the loading of a small force, the nanohairs on the coating surface contact each other. Such PANI/C-RGO@PUS exhibits an obvious electrical response under the loading of 25 mg. Figure 6. Sensitivity enhancement strategy for CCPC pressure sensors. These strategies are summarized as: Contact isolation between electrodes and sensitive layers: a) Copper wire isolation. Reproduced with permission. [84] Copyright 2020, Wiley-VCH. b) PVANWs isolation. Reproduced with permission. [92] Copyright 2018, Elsevier. Negative Poisson's ratio structure. c) CB@PUS with Negative Poisson's ratio. Reproduced with permission. [48] Copyright 2022, Wiley-VCH. Hierarchical contact during compression: d) Microstructure and working mechanism of PANIH/C-RGO@PUS. Reproduced with permission. [81] Copyright 2018, Wiley-VCH. Synergistic effect of hierarchical structured materials: e) Microstructure and pressure response curves of Fe 2 O 3 /C/SnO 2 @PUS. Reproduced with permission. [97] Copyright 2021, Springer Nature.
Relative to electrode contact and skeleton deformation strategies, efforts to improve the piezoresistive properties of conductive coating materials have also been reported. As shown in Figure 6e, Wang et al. prepared Fe 2 O 3 /SnO 2 /C composites with sea urchin-type structure by hydrothermal method. [97] In compression, the synergistic effect of Fe 2 O 3 /C, Fe 2 O 3 /SnO 2 , and SnO 2 /C enables the composite to exhibit high bulk piezoresistive properties. CCPC based on Fe 2 O 3 /SnO 2 /C composite exhibits a sensitivity of 680 kPa −1 .
After reviewing the works on sensitivity improvement of CCPC devices, sensitivity improvement strategy is discussed. The sensitivity of CCPCs is defined as (ΔR/R 0 )/P, where ΔR is the resistance change under compression, R 0 is the initial resistance, and P is the applied pressure. Discussions about electrical design and mechanical design are carried out by the sensitivity formula to increase the sensitivity of CCPC effectively. First, the initial resistance of the device is varied. Contacts are reduced by adding isolation between the sensitive layer and the electrodes. As the deformation increases, there is complete contact between the sensitive layer and the electrodes. Second, materials with higher piezoresistive properties are used as the coating of CCPCs, thereby increasing the ΔR under compression. Finally, the morphology of the substrate or coating is changed. More conductive path increases are obtained under the same deformation through stress concentration.

Working Range Extension Strategies
For flexible pressure devices, sensitivity and working range are two crucial parameters. However, it is a dilemma to obtain both high sensitivity and wide working range simultaneously. In this section, the working range extension strategies for the CCPCs will be reviewed. Figure 7a illustrates the general dilemma between sensitivity and working range. When the sponge is compressed to the dense region, the contact area of the skeleton is gradually saturated. The conductive coating is confined in the skeleton under compression. At this stage, the device often shows a decrease in sensitivity or even no response, which is attributed to the failure of the conduction mechanism under high pressure. To solve the above issue, Gao et al. demonstrated that the high-pressure sensitivity of MXene is maintained in a narrow strain range (Figure 7b). A pressure sensor based on MXene film was fabricated. When the MXene film is compressed to a certain range, the sensitivity will drop sharply. [101] However, recalled in Figure 4b, the stress of the sponge-based pressure device rises rapidly after densification. Therefore, the sensitivity of CCPCs saturates at high pressure.
Notably, CNT-coated CCPCs often exhibit a wide pressure detection range. As shown in Figure 7c-e, Kim et al. treated porous PDMS substrates with oxygen plasma and built dense CNT coatings on them. [52] During compression, CNT@PDMS exhibits 10 Pa to 1.2 MPa. Generally, the elastomeric sponges reach the dense region within 100 kPa. It is worth noting that CNT@PDMS exhibits a sensitivity drop to 0.01 kPa −1 after 100 kPa, but still has a pressure response, which is attributed to the pressure resistance and continuous bulk piezoresistivity of CNTs. Wang et al. modified PANI/CNT on the surface of PDMS. [82] PANI/CNT@PDMS has improved sensitivity com-pared to CNT@PDMS and exhibits a linear sensitivity of 0.918 kPa −1 from 0 to 130 kPa. Bae et al. addressed the lack of conduction mechanism of sponge-based devices under high pressure. The sugar surface was attached with CNTs and used as a template. The mixed CNT and PDMS prepolymers solution infiltrate the sugar template and self-crosslinks. By washing away excess sugar with boiling water, a PDMS sponge with CNT coating was constructed. CNTs were also embedded inside the sponge, While the sponge has not yet shown sufficient conductivity. As shown in Figure 7e, as the compression continues, the CNTs on the surface of the skeleton come into contact, resulting in a decrease in the contact resistance inside the device. When the sponge is compressed to a dense degree, the CNTs inside the framework and the CNTs on the surface are connected to each other, and the bulk resistance of the framework decreases. Under different stages, the sponge device has a dual sensing mechanism and exhibits a pressure detection upper limit of 3.84 MPa. [58] After reviewing the works on response range extension, the design of CCPC devices with high operating range is discussed. The sensitivity saturation of CCPCs is attributed to the failure of the conduction mechanism under large pressure loading. Due to the intensified densification of the porous structure under compression, the coating is confined in the middle of the skeleton. However, the excellent piezoresistive properties of advanced coating materials such as MXene are only maintained in a narrow strain range. As the contact saturation of the conductive framework becomes saturated with deformation of the coating, additional conductive paths cannot be formed. Relying on its own unique structural characteristics, CNT could provide a continuous resistance change under pressure, thereby delaying the sensitivity saturation of the device. To meet the demand of wider pressure detection range, an additional large pressure transmission mechanism is designed. The method of coating the conductive coating with the conductive elastomer combines the two sensing mechanisms of the skeleton contact and the piezoresistive change of the skeleton body, so an ultra-wide pressure detection range can be realized.

High-Resolution Pressure Detection
The advantage of CCPCs is the low detection limit, high sensitivity, and wide working range. CCPCs unique porous structure could enhance their capability to detect tiny pressure change even under large pressure with high resolution. These highperformance sensors meet emerging requirements and have been used in exciting demonstrations including wearable electronics in the fields of intelligent health monitoring, motion detection, and human-machine interaction.

Physiological Signal Detection
Porous structure provides high-resolution pressure detection in the physiological signal detection. Du et al. designed an in situ polymerized polypyrrole/halloysite Nanotube/silver nanoflower modified porous sponge device. A standard pulse waveform is detected while the device is secured to the subject's radial artery. [75] [101] Copyright 2017, Springer Nature. c) Wide working mechanism of CNT@PDMS. Reproduced with permission. [52] Copyright 2019, American Chemical Society. d) High linearity and wide working range of PANI/CNT@PDMS. Reproduced with permission. [82] Copyright 2020, Royal Society of Chemistry. e) The CNT@CNT embedded PDMS based on the additional high pressure response mechanism has high sensitivity and ultra-wide pressure detection range. Reproduced with permission. [58] Copyright 2022, Wiley-VCH. Figure 8a, percussion wave (P-wave), tidal wave (T-wave) and dicrotic wave (D-wave) can be clearly observed in the pulse response curve, which are closely related to diseases such as hypertension and arteriosclerosis. Ballistocardiogram (BCG) is the change of external pressure or surface displacement of the human body caused by the beating of the heart and the flow of blood in the aorta. It reflects the mechanical properties of the heart and is a noncontact, nonsense heart monitoring method.

As shown in
However, it is difficult to detect BCG signals using CCPCs because CCPC devices exhibit low pressure resolution under large pressure loading. Luo et al. fabricated the detection of BCG signals by designing a self-adapted CCPC device with a gradient structure. [103] In Figure 8b, the device can still clearly distinguish the BCG signal of the human body under the load of 100 kPa, which is due to the adaptability brought about by the modulus difference of the gradient structure.  [75] Copyright 2021, Royal Society of Chemistry. b) Ballistocardiogram signal. Reproduced with permission. [103] Copyright 2022, Wiley-VCH. CCPCs for human activity signal detection: c) Voice signal Reproduced with permission. [68] Copyright 2017, American Chemical Society. d) Arm flexion signal. Reproduced with permission. [92] Copyright 2018, Elsevier. e) Plantar pressure signal. Reproduced with permission. [52] Copyright 2019, American Chemical Society. CCPCs for human-computer interaction: f) Robot Control. Reproduced with permission. [95] Copyright 2021, Royal Society of Chemistry. g) Slip detection for robotic grasping. Reproduced with permission. [48] Copyright 2022, Wiley-VCH.

Human Motion Signal Detection
The process of human movement covers the stretching of muscles and the activities of joints, accompanied by changes in pressure. Porous structure provides more compressible range for the wearable and flexible stress detection devices monitoring human activities in real time. Due to the high sensitivity, wide detection range and modulus close to that of human skin, CCPC devices are suitable for the development of human motion monitoring. Bao et al. divided the range of human activities into 1 Pa to 1 kPa (e.g., human skin sensing), (2) low-pressure regime from 1 to 10 kPa (e.g., gentle manipulation of items), (3) medium-pressure regime from 10 to 100 kPa (e.g., human weight and joint movements), and (4) high-pressure regime > 100 kPa (e.g., plantar pressure). [24] As shown in Figure 8c, Wu et al. fabricated Gold@PUS with a cracked coating and exhibited a pressure detection limit of 0.568 Pa. [68] During compression, the coating on the surface of the Gold@PUS skeleton cracks and closes, bringing about an additional small pressure transmission mechanism. Gold@PUS was fixed on the subject's throat. As the vocal cords vibrated, the sensor exhibited distinct and repeatable speech signatures, demonstrating the potential of the device in the field of voice recognition. As shown in Figure 8d, Yue et al. designed a spongebased piezoresistive sensor with MXene coating. [92] MXenesponge exhibited a high sensitivity of 442 kPa −1 , which could clearly distinguish the active states of the subjects. The MXenesponge sensor is connected in series with a circuit built into the Bluetooth system, which transmits the electrical current signal to the phone. Although the CCPC has been widely used in the Figure 9. Application of multi-mode CCPCs. Dual-modality detection of pressure and temperature: IR patterns and response curves for a) cold and b) hot surfaces. c) Response curves for self-powered devices. Reproduced with permission. [40] Copyright 2022, American Chemical Society. Triple-modality detection of pressure, strain, and temperature: d) Response curves in different modes. e) Working mechanism. f) Demonstration in pressure detection mode and corresponding response curves. g) Demonstration in temperature detection mode and corresponding response curves. Reproduced with permission. [41] Copyright 2022, American Chemical Society. assessment of human joint activity pressure, the accurate evaluation of the pressure value of the sole is still difficult. Limited by the limited piezoresistive properties of coating materials, the narrow pressure detection range limits the evaluation of large pressures such as plantar pressure.
Kim et al. designed CNT@PDMS composites with thick CNT coatings. [52] The composite exhibits a wide pressure response range from 10 Pa to 1.2 MPa, which is suitable for human plantar pressure detection. As shown in Figure 8e, sensors were assembled in the smart insole to evaluate the pressure distribution in the forefoot, midfoot, and heel. Pressure sensors allow accurate localized pressure detection independent of surrounding disturbances. The generated voltage output is transmitted by the pressure sensor to the data acquisition board and collected by a personal computer for real-time evaluation of human foot pressure.

Human-Machine Interaction
Human-machine interface refers to the contact or interaction between human and machine in information exchange and function. Among them, robot control and tactile endowment of robot skin are represented. CCPCs have low detection limit, high sensitivity, and fast response time, which is suitable as a carrier for interaction between human and machine. As shown in Figure 8f, Yao et al. fabricated a PVA/H 3 PO 4 @PDMS pressure sensor with a filamentous interconnection structure by a simple dip-coating method. [95] In compression, the device exhibited a high GF of 5.51 and proved suitable for human-machine control. PVA/H 3 PO 4 @PDMS was fixed at the joints of fingers. As the joint flexes, the conductive compound is squeezed and exhibits a drop in electrical resistance. A microprocessor-based smart car equipped with a robotic arm is used as a demonstration object for human-computer interaction. A custom circuit board detects the resistance of the sensor in real time. The bending motion of the four fingers results in a change in resistance. Control circuit board distinction Variation of resistance in different channels and controlling the grasping motion of the robotic arm and the motion of the vehicle.
As shown in Figure 8g, Huang et al. designed a PU-based metamaterial with a negative Poisson's ratio. [48] A sponge-based sensor with high sensitivity is realized by CB attachment. CB@PUS with negative Poisson's ratio exhibits a high strain sensitivity of 4.62. CB@PUS is installed on the fingers of the bionic hand to detect the contact relationship between the manipulator and the water cup. When the bionic hand is not in contact with the cup, all sensors exhibit a constant resistance. At 4.0 s, the bionic hand held the cup, and the sensor was compressed, resulting in a decrease in resistance. At 10.0 s, the bionic hand holds and lifts the cup. As the cup fills with water, slippage occurs, which causes an increase in the compressive strain on the sensor and a decrease in the resistance of the response. Simple signal processing is used to distinguish between stationary and slipping grasp states.
This chapter reviews the representative work of highresolution CCPCs in the fields of human physiological signal detection, human motion detection, and human-computer interaction, and clarifies the huge and extensive application potential of flexible, soft, and porous devices. Notably, CCPC is prepared by a simple dip-coating method, which has low fabrication cost and extended potential for large-scale production.

Multimodal Sensing
Sensors based on the collapse of porous structures upon compression are generally considered suitable only for single-mode sensing. Changes in the electrical path during compressionrelease affect the resistance of the device. Strain and temperature are also factors that affect device resistance. However, the resistance changes caused by multiple variables are difficult to decouple, and the multimodal sensing of the device is difficult to achieve. Gao et al. realized a dual-mode CCPC with thermoelectric and piezoresistive mechanisms by coating PEDOT:PSS and CNTs on the surface of MS sponges. [40] Sensors with low thermal conductivity (0.077 W m −1 K −1 ) and high electrical conductivity (8.3 S m −1 ) can transduce temperature and Pressure stimuli to independent resistance and voltage signals without the requirement for complex decoupling analysis. As shown in Figure 9a,b, cold and hot water are used to create a temperature difference. In the process of the sensor being compressed, the voltage signal change caused by the thermoelectric effect and the fluctuating resistance signal change are displayed. By decoupling the temperature signal and the pressure signal, the dual-mode CCPC can be realized. As shown in Figure 9c, the surface and bottom of the porous composite generate a temperature gradient under the irradiation of light. Therefore, the CCPC with a temperature difference mechanism has the function of generating electricity by itself under the temperature difference caused by light.
Lo et al. designed a PEDOT:PSS-coated PDMS-based elastomer device and decoupled compression, strain, and temperature signals by measuring the magnitude and trend of changes in resistance and capacitance. [41] As shown in Figure 9d,e, under compression, the resistance of the device decreases monotonically, while the capacitance increases monotonically. Under strain, the resistance of the device increases monotonically, while the capacitance decreases monotonically. Whereas when strain is not considered, temperature affects the resistance of the device but not the capacitance. As shown in Figure 9f, the device is fixed on the bionic hand. The device exhibits a trend of decreasing resistance and increasing capacitance during compression. As shown in Figure 9g, when the device is placed on a hot surface, the resistance of the device drops rapidly while the capacitance hardly changes.
In this section, reports on CCPCs with multimodal sensing are reviewed. The resistance of CCPC devices is affected by compression, strain, and temperature. However, the resistance signal mixed with multiple factors is difficult to decouple, making multimodal devices difficult to design. In this paper, the work of standardizing strain and temperature by different electrical parameters is introduced, which provides ideas for the design of multimodal CCPC devices.

Conclusions and Outlook
In the past few years, great progress has been made in CCPC pressure sensors composed of conductive coatings and porous elastomer skeletons prepared by dip-coating. In this review, we summarize the existing problems in the preparation of CCPCs and the corresponding process optimization methods. The optimization strategy mainly focuses on the selection of conductive material and skeleton, as well as the process selection during dip coating. Representative work on realizing high performance CCPC pressure sensors is discussed. Design schemes for devices with low hysteresis, response monotonicity, high sensitivity, and wide operating range are analyzed to guide other pressure devices composed of soft matter and rigid conductive materials. In addition, advanced applications are review in detail to demonstrate the versatility of CCPC devices. Representative work on multimodal CCPCs is introduced, and signal decoupling strategies for multiple physical variables are summarized and generalized to guide the design of piezoresistive devices for complex variable detection.
It is worth noting that resistive flexible pressure sensors based on blend films of advanced conductors and polymers have been initially commercialized. Real time body pressure mapping, tactile sensing array can be realized by these devices and has commercial applications.
The CCPC device is a special resistive pressure sensor with a 3D porous structure sensitive layer. Compared with other 3D devices, CCPC devices exhibit a small amount of advanced conductors which is beneficial to reduce production costs. Compared with rigid silicon-based and strained metal pressure sensors, the sensitivity of current CCPC devices is sufficient for applications. For a wide device operating range, the substrates with high modulus and high toughness can meet its needs. Therefore, conventional device specifications are no longer the reason for limiting the application of CCPC devices. The key optimization direction of CCPC devices in the future may be in hysteresis and durability. Stable, repeatable, and long-lasting pressure feedback is necessary during the operation of flexible devices. For electrical hysteresis: 1) It is necessary to find advanced materials with stable performance, easy coating, and strong bonding with the substrate. Coating-substrate bonding strategies based on weak intermolecular forces cannot meet the hysteresis and durability requirements for practical applications. Under pressure loading, weak coating cohesion can lead to irreversible slippage of the coating material, resulting in permanent changes in the electrical properties of the device. Conductive polymers may be one of the most promising alternative materials at present. 2) For the porous elastomer framework, parameters such as pore size, porosity, modulus, and toughness need to be determined. The porous elastomer needs to be subject to additional mechanical modeling/simulation studies, which can help determine the acceptance criteria for the hysteresis performance of CCPC devices. For durability: 1) It is worth noting that the heterogeneous coating-skeleton structure can hardly avoid mechanical mismatch. Under pressure overload, the coating inevitably slips and falls off. It is difficult for CCPC devices to remain stable under repeated loading of high pressure. In future CCPC device designs, the homogeneous combination of coating-skeleton may be the key to mechanical matching. The elastomer was modified into a conductive elastomer and then fixed on a homogeneous porous framework, resulting in an electrically graded coatingsubstrate structure with similar mechanical properties The design of this homogeneous porous piezoresistive sensitive layer is worth trying. 2) In the existing reports, the research focuses on the design of the pressure sensitive layer. However, in actual use, the contact relationship between porous CCPCs and electrodes is one of the key issues for the practical application of the device. Sensitive layer materials usually show weak bonding force with electrodes. The contact resistance between the interfaces changes simultaneously during the deformation of the device. It is necessary to fix the sensitive layer and electrodes by conductive adhesive. To avoid changes in device mechanical properties, homogeneous binders are a competitive choice.