Recent Advances in Stretchable and Permeable Electrodes for Epidermal Electronics

Epidermal electronics is an emerging wearable platform that involves attaching deformable forms of devices to the skin. Epidermal electrodes represent a vital component of this technology, as they provide a direct electronic interface with the skin for sensing and stimulation. However, most of the current electrodes are built on non‐permeable elastomer substrates, which can limit their long‐term, continuous operations in a non‐invasive manner. Fortunately, recent advancements in conductive materials and fabrication techniques have enabled high‐performance epidermal electrodes that are comfortable to wear. In order to track the latest progress, this review article first introduces the designs of permeable structures and the preparation of conductive electrodes. The subsequent discussion elaborates on effective strategies to achieve desirable properties, such as high conductivity, stretchability, skin adhesion, and biocompatibility. The emerging applications of permeable epidermal electrodes are also summarized. Finally, this review concludes with the current challenges and future directions of breathable epidermal electrodes.

long-lasting wear.To meet this demand, permeable electrodes have been developed by incorporating conductive materials into permeable substrates.However, there is a trade-off among the electronic performance, mechanical deformability, and permeability of these epidermal electrodes.For instance, while an ultrathin solid membrane can offer improved breathability and softness, it may come at the cost of reduced stretchability.Additionally, the porous structures and rough surfaces of textiles pose significant fabrication challenges in producing permeable and stretchable electrodes with high resolution and conductivity.Consequently, the production of epidermal electrodes that possess all the essential properties remains a daunting task.Fortunately, recent advancements in functionalization techniques and conductive materials have enabled high-performance epidermal electrodes with excellent wearing comfort.These permeable electrodes are designed as either ultrathin solid membranes or porous microstructures.They utilize thin film coating and nanomaterial assembly strategies to construct conductive pathways without obstructing moisture transport.21][22] This review provides an overview of the latest developments in stretchable and permeable electrodes and their applications in epidermal electronics.The article begins by introducing the designs of permeable structures, followed by a detailed discussion on the preparation of conductive electrodes.The subsequent section elaborates on effective strategies to achieve desirable properties such as high conductivity, stretchability, skin adhesion, and biocompatibility.Additionally, we summarize the emerging applications based on permeable epidermal electrodes.The review concludes with current challenges and future directions of breathable epidermal electrodes.

Structural Designs of Permeable Electrodes
Stretchable electronics are typically constructed on compliant substrates made of solid elastomers.[6] Due to reduced sweat and hot air release, the disrupted mass and heat transfer tend to induce unpleasant skin sensations, such as dampness and clamminess.][25] In addition, the buildup of skin byproducts can impair the electrode's conformal contact with the skin, leading to delamination and signal degradation. [13,14]Stretchable and permeable electrodes have been developed to overcome these challenges.Based on the structural features, these electrodes are classified into ultrathin solid membrane and porous scaffold designs.

Ultrathin Solid Membrane-Based Permeable Electrodes
Stretchable electrodes are commonly constructed by assembling conductive materials on solid elastomer substrates. [26]Although elastomers contain rich free volume as the diffusion pathways for gas molecules, thick substrates can still impede gas permeability and stream evaporation from the skin, making it difficult for the body to maintain thermoregulation. [27,28]The commercially available elastomers, such as PDMS and Ecoflex, are often considered impermeable substrates with thicknesses exceeding 100 μm. [11]To address this issue, an ultrathin layout of elastomer membrane has been adopted for stretchable and breathable epidermal electronics. [19,29]This approach involves the deposition of conductive nanomaterials onto permeable membranes to establish percolation networks without affecting permeability.For example, Yang et al. have developed a breathable nanocomposite electrode by transfering patterned AgNWs onto a 150 nm-thick styrene-ethylene-butylene-styrene (SEBS) film. [19]The resulting electrode shows a high permeability of 580 g m −2 d −1 , which is attributed to the ultrathin SEBS film and porous silver nanowires (AgNWs) network.As illustrated in Figure 1a, the ultrathin design also has low mechanical stiffness to achieve conformal lamination on the human skin using van der Waals forces alone without the need for adhesives or tapes.However, the reduced mechanical robustness of the membrane can cause fractures, making it difficult to detach the devices without causing structural damage. [1,30]As a result, these devices are often regarded as disposable electronic tattoos.To address this issue, a recent study proposed a promising strategy by reinforcing the elastomer membrane with nanofibers. [29]Specifically, a ≈100 nm-thick electrospun polyurethane nanofiber mat is dip-coated with a thin PDMS layer, as shown in Figure 1b.The resulting ultrathin composite membrane can be easily handled and support liquids that are 79 000 times its weight.Subsequently, 70 nm-thick Au is deposited on the ultrathin film to create electrocardiogram (ECG) electrodes.In addition to the ultrathin PDMS film, the permeability is primarily attributed to the nanovoids in the Au layer.Alternatively, the ultrathin membranes can be combined with stretchable porous substrates for enhanced durability. [11,31,32]The porous substrate provides a strain-limiting and mechanically reinforced support for these electrodes.For example, Jang et al. spin-cast the fabric with a thin silicone gel and then attach conductive materials to it (Figure 1c). [11]The electrode can withstand repeated attachment and detachment more than 100 times without adverse effects, which can record high-quality EP signals.In contrast, similar manipulations of the silicone membrane alone result in immediate mechanical failure.Currently, it is still challenging to fabricate multilayered permeable electronics based on ultrathin membranes since stacking multiple layers may significantly reduce the permeability.

Porous Scaffold-Based Permeable Electrodes
In addition to solid films, stretchable porous scaffolds have been widely studied as breathable platforms for permeable epidermal electronics.Based on their building components, porous membranes can be classified into fiber-based and sponge-based substrates.Fiber-based membranes are either woven fabrics [33,34] or non-woven fabrics, [35,36] while sponge-based membranes are fabricated by templating, [37][38][39] phase separation, [40][41][42] breath figure method. [43,44]By introducing interconnected micropores, these membranes can easily achieve excellent breathability and low Figure 1.Ultrathin Planar Membrane-Based Permeable Electrodes.a) Optical images of an ultrathin device laminated on human skin and subsequently peeled off from the skin.Reproduced with permission. [19]Copyright 2020, John Wiley & Sons, Ltd. b) Fabrication and characterizations of free-standing polyurethane-PDMS nanofilms.Reproduced with permission. [29]Copyright 2021, Proceedings of the National Academy of Sciences.c) A schematic illustration of a representative system on a fabric.Optical images showing attachment and detachment of the device from the skin.Reproduced with permission. [11]Copyright 2014, Springer Nature.mechanical stiffness. [45]To date, various techniques have been developed to integrate conductive materials into porous substrates, including spray coating, [46] vacuum deposition, [47] and chemical deposition. [48]Porous membranes can not only be employed as support scaffolds but also as the backbone [37,49] or the template [23,50,51] of conductive nanostructured networks.Li et al. developed a physical deposition method to create a liquid metal micromesh over an elastomer sponge, [37] involving sequentially depositing a thin Cu adhesion layer and a gallium layer.As shown in Figure 2a, this conformal deposition harnesses the styreneisoprene-styrene sponge as a porous backbone to generate the mesh-like coating, which gives rise to textile-level steam permeability for conformable perceptions during long-term attachment to the skin.
Textiles possess intrinsic 3D fibrous structures that offer exceptional gas and liquid permeability.In textile-based electrodes, the conductivity and stretchability are largely controlled by the junctions between adjacent fibers.Regular knitted textiles have been frequently employed as substrates for permeable electrodes.Electroless metallization was used for the controlled deposition of conductive materials on the knitted structures. [2,52,53]The conductivity of the electrodes is determined by the surface coverage of the metallic layer on individual fibers and the physical contact between adjacent conductive fibers.During stretching, the contact pressure at the junctions between yarn loops increases, and the contact resistance at these junctions decreases, leading to a decrease in the overall resistance.However, the highly rough surfaces of the fabrics with interloped yarns make it difficult to realize high-resolution device fabrication and integration.An alternative approach to address this trade-off is to develop micro/nanofiber membranes using electrospinning techniques as substrates.Nonetheless, the pristine fiber membranes are limited in stretchability due to the irreversible sliding of physically stacked nanofibers.To enhance the mechanical stretchability and robustness, the reinforcement of fiber junctions has been achieved using different methods, such as thermal treatment, [54,55] chemical cross-linking, [56] physical interlocking, [57,58] and co-solvent welding. [59]s opposed to planar membranes, the thickness of the porous scaffold has a minor impact on its permeability.Accordingly, porous electrodes can be easily implemented without the constraints of ultrathin thickness requirements.][61][62][63][64] For example, Ma et al. fabricated multilayer and permeable electronics by alternatively printing liquid metal layers and depositing electrospun SEBS fiber mats, which allowed for physiological signal monitoring and electrothermal therapy (Figure 2b). [64]Zheng et al. developed a permeable and moisture-wicking electronic skin in the form of a trilayer structure with wettability gradients (Figure 2c).This design allows stable and long-term use even under sweating conditions. [21]However, multilayered devices tend to fail under complex deformations, such as stretching, shearing, and torsion, due to different mechanical properties among these layers. [65]To mitigate this issue, Yang et al. developed a Janus textile electrode based on a medical adhesive (MA)-reinforced all nanofiber network multilayers for EP signal monitoring. [62]The electrode demonstrates robust properties under severe bending and sweat penetration, thanks to the use of MA to improve the interfacial bonding (Figure 2d).Likewise, Gao et al. utilized hot pressing to melt  [37] Copyright 2022, American Chemical Society.b) Schematic illustration and optical images of a trilayer-structured device with liquid metal electrodes in each layer.Reproduced with permission. [64]Copyright 2021, Springer Nature.c) Schematic illustrating the structure of a moisture-wicking, breathable membrane.Reproduced with permission. [21]Copyright 2022, John Wiley & Sons, Ltd. d) Schematic showing the architecture of the Janus textile electrode.The cross-sectional SEM images reveal the medical adhesive for reinforcement.Reproduced with permission. [62]Copyright 2022, John Wiley & Sons, Ltd.
polycaprolactone fibers with low melting pointing to provide a reliable interface for the multilayered electronic. [36]

Conductive Material Selection for Permeable Electrodes
The selection of the conductive material for electrodes directly impacts their performance.[68] Nevertheless, many electrodes are limited in their permeability.Achieving adequate permeability presents a significant challenge in processing these conductive materials.To this end, solid conductive nanomaterials and polymers have been introduced to permeable substrates, forming a percolated conductive network without obstructing the pathways for moisture transport.In contrast, the liquid metal is inherently deformable, but acts as an impenetrable barrier for gases and liquids.Accordingly, numerous efforts have been devoted to developing permeable liquid metal electrodes in porous morphology.This section provides a comprehensive discussion of permeable electrodes based on various conductive materials, including film coating and nanomaterial assembly strategies.

Film Coating Strategy
Metal films are widely used for EP signal acquisition due to their superior conductivity and low skin-electrode impedance.However, a major drawback of these films is their non-permeable and rigid characteristics. [69]To create permeable conductors, ultrathin metal films have been deposited on permeable substrates using techniques such as physical vapor deposition, [23,47,70] sputtering, [56,71] and electroless deposition. [72]For instance, a 100 nm-thick thin Au layer was conformally deposited on an electrospun polyurethane fibers to prepare the ultrasoft electrode. [47]hese fibers were initially covered with a thin parylene layer before depositing the Au layer to enhance the structural continuity of the Au layer at the fiber junctions (Figure 3a).Likewise, Wang et al. fabricated a permeable nanomesh conductor by depositing a Au layer on PU-PDMS core-sheath nanomesh with high durability. [73]The ultrasoft electronics enable reduced mechanical mismatch with the skin for seamless attachment.In addition, mesh-like metallic permeable conductors can be created using soluble nanofiber mats as the template. [23,50,51]he process involves depositing a polyvinyl alcohol nanofiber mat with Au layers, which is then transferred onto the skin and removed through dissolution.A mesh-like and permeable  [47] Copyright 2019, Springer Nature.b) Schematic showing the preparation of nanomesh conductors.c) Optical images of nanomesh conductors attached to a finger under bending and relaxed states.(b,c) Reproduced with permission. [23]Copyright 2017, Springer Nature.
electrode was formed on the skin, allowing for real-time and high-precision electromyogram recording (Figure 3b,c). [23]However, this substrate-free approach has limitations in handling and reusability.
Liquid metals, such as gallium and gallium-based alloys, are gaining popularity as an alternative to rigid metal films due to their competitive conductivity and liquid-like deformability.[76][77][78][79][80] In a typical fabrication process, a liquid metal layer was coated on a soft fiber mat and activated into a porous microstructure under ≈1800% strain (Figure 4a,b). [64]The resulting liquid metal fiber mat exhibits excellent stretchability and steam permeability.However, this approach has demonstrated limited loading capacity of liquid metal and relatively high sheet resistance due to the poor wettability of liquid metal on the textile.To address this limitation, an Ag layer was electrolessly deposited on the textile that enables reactive wetting with liquid metals. [48]A superelastic, permeable, and highly conductive liquid metal electrode was formed after mechanical activation.Nevertheless, the post-treatment procedure still increases the fabrication complexity and is only compatible with ultradeformable substrates.Without any post-treatment, Ma et al. have demonstrated the direct fabrication of a liquid metal micromesh on electrospun microfiber textile as a highly permeable and ultrastretchable conductor through the assistance of reactive wetting. [35]Specifically, a wetting layer of Cr/Cu film was thermally evaporated on the nonwoven textile.The liquid metal was then drop cast on the textile, followed by high-speed rotation to remove the excess liquid metal, as depicted in Figure 4c,d.The resulting liquid metal micromesh has a low sheet resistance of 0.38 Ω sq −1 , ultrahigh stretchability of >1000% strain, and similar steam permeability to standard textiles.Furthermore, the liquid metal micromesh can be easily patterned into arbitrary features for device fabrication using selective laser ablation.However, achieving high-resolution patterning remains a significant challenge for porous liquid metal electrodes.To this end, Zhuang et al. have proposed a transfer printing method to create stretchable liquid metal microelectrodes on fibrous substrates.The microelectrode boasts 2 μm resolution and 75 000 electrodes cm −2 density, as shown in Figure 4e-g. [80]he technique involves creating Ag micropatterns on dextranmodified wafers using photolithography.The Ag micropatterns were transferred on the fiber mat by dissolving the sacrificial dextran layer.Finally, liquid metal is selectively coated onto Ag micropatterns to form the permeable microelectrodes.
Conductive polymers, such as PANI, PPy, and PEDOT:PSS, have shown promise for epidermal electronics due to their biocompatibility, mechanical flexibility, and adjustable chemical/physical properties. [81]In particular, the PEDOT:PSS film has been extensively used for various biological applications. [82]This conductive polymer has a unique nanoscale void structure resulting from the spatial arrangement of its two components. [83,84]onetheless, PEDOT:PSS-based electrodes face practical challenges, such as insufficient skin adhesion, low stretchability, and limited conductivity. [68,85,86]Ultrathin PEDOT:PSS membranes have been developed to improve their adhesion to the skin.For instance, Jeong et al. fabricated an epidermal electrode by spray coating PEDOT:PSS onto an ultrathin nanofiber mat.The subsequent thermal treatment significantly raises the conductivity from 0.5 to 125 S cm −1 (Figure 5a,b). [87]This ultrathin electrode can be conformally attached to a finger without extra adhesive, as illustrated in Figure 5c.In addition, PEDOT:PSS has been combined with other conductive materials to create hybrid electrodes.In Figure 5d,e, Fang et al. constructed a submicron-thick, hybrid electrode composed of AgNWs and PEDOT:PSS in a solutionbased process.This free-standing, sandwich-structured electrode has a low sheet resistance of 16.8 Ω sq −1 and is conformal, water-permeable, and noninvasive to the skin. [88]Interestingly, the nanocomposite is optically transparent and provides aesthetically pleasing epidermal electrodes (Figure 5f).Furthermore, Cao et al. prepared a permeable and self-adhesive electrode by dropcasting a blend of PEDOT:PSS, poly(vinyl alcohol), tannic acid, images of the electrospun fiber mat (left), the liquid metal-coated fiber mat (middle), and the strain-activated permeable electrode (right).(a,b) Reproduced with permission. [64]Copyright 2021, Springer Nature.c) Fabrication process flow for the liquid metal micromesh on the SEBS microfiber textile.d) SEM images of the microfiber textile before (left) and after (right) liquid metal coating.(c,d) Reproduced with permission. [35]Copyright 2022, American Chemical Society.e) Schematic of the fabrication process to create liquid metal microelectrodes.f) Optical images showing the high-density microelectrode at 75 000 electrodes cm −2 .g) Corresponding SEM image displaying the liquid metal pattern on the fibers that form highly porous microstructures.(e-g) Reproduced under the terms of a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). [80]Copyright 2023, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.and ethylene glycol, achieving a conductivity of 122 S cm −1 and a mechanical stretchability of 54%, as shown in Figure 5g-j. [58]TA promotes strong bonding to the skin due to the hydroxyl group and the hydrophobic benzene ring, while PVA improves mechanical softness and energy dissipation.

Nanomaterial Assembly Strategy
Metal and carbon-based nanomaterials have been widely used to create stretchable and permeable conductors.[91][92][93][94][95] Carbon-based materials are popular choices for epidermal electrodes due to their high stability, low toxicity, and low mass density.CNTs are particularly promising with excellent mechanical strength, high surface areas, and large aspect ratios.For example, Zhou et al. created a permeable conductor of MWCNTs on electrospun SEBS fibers using sequential vacuum filtration and ultrasonic treatment (Figure 6a). [96]To establish a robust conductive network, dopamine is employed as the binder for MWCNTs to adhere to SEBS fibers.Nevertheless, there are some drawbacks associated with adding a binder, such as reduced conductivity and blocked porous structure. [97]Taylor et al. created permeable and washable electromyogram (EMG) electrodes by interweaving carbon nanotube threads with textiles, as shown in Figure 6b. [92]Graphene is also an attractive candidate material to create epidermal electronics on various substrates, such as polyimide, [38] PU nanomesh, [98] and textile. [99]For example, Wang et al. used laser writing to create programmable graphenebased electrodes on protective clothing for ECG signal recording (see Figure 6c,d). [99]By combining porous graphene with textiles, the electrode achieved high permeability and low sheet resistance of 10.6 Ω sq −1 .
Metals are known to exhibit higher conductivity than carbonbased conductors.In particular, AgNWs have been widely used for permeable electrodes owing to their high conductivity, stretchability, and large aspect ratios. [46,49,55,100,101]For example, Wang et al. fabricated a permeable electrode of 4 Ω sq −1 .by spray depositing AgNWs onto nanofiber textiles (Figure 7a). [100]owever, the AgNW network is easily fractured by dynamic motions due to its weak adhesion to the nanofibers.To this end, Jiang et al. fabricated a highly conductive (9190 S cm −1 ) and durable electrode by spin-casting AgNWs over electrospun  a-c) Reproduced with permission. [87]Copyright 2021, American Chemical Society.d) Schematic of the fabrication for hybrid electrodes consisting of AgNWs and PEDOT:PSS.e) Cross-sectional SEM image of the hybrid electrode.f) Transmittance versus wavelength for AgNW and hybrid electrodes.(d-f) Reproduced with permission. [88]Copyright 2020, American Chemical Society.g) Conformal attachment of a permeable electrode based on PEDOT:PSS, poly(vinyl alcohol), tannic acid, and ethylene glycol.h) Conductivity as a function of PEDOT:PSS loading.i) Conductivity versus tannic acid loading.j) Normalized resistance and stress of the electrode with respect to strain.(g-j) Reproduced with permission. [58]Copyright 2022, John Wiley & Sons, Ltd.
nanofibers and then applying hot-press treatment. [55]This posttreatment step effectively improves the adhesion between the AgNWs and the nanofibers (Figure 7b).As opposed to surface coating methods, AgNWs have been embedded into the nanofiber scaffolds to form an interlocked network through bottom-up coassembly [46,102] or vacuum filtration. [49]Fan et al. created a hierarchical nano-network electrode by electrospinning polyamide nanofibers and electrospraying multi-level AgNWs simultaneously (Figure 7c). [46]The thick AgNWs are the leading conductors that extend throughout the nanofiber scaffold, while the thin, more flexible AgNWs form branched conductive pathways.The permeable electrode has a stretchability of up to 500% strain, stable resistance during strain fatigue tests, and excellent washability.

Strategies of Electrode Performance Optimization
Epidermal electrodes provide a non-invasive way to collect EP signals from the human body.It is essential to construct a robust electronic interface between the electrodes and the skin for high-quality signal recording.Extensive efforts have been made to develop epidermal electrodes with desirable characteristics, such as high conductivity, large stretchability, mechanical compli-ance, strong tissue adhesion, and biocompatibility.In particular, the addition of permeable features may sometimes affect other properties.For example, the ultrathin planar membrane often achieves improved breathability and softness at the price of reduced stretchability.As a result, fabricating epidermal electrodes with all the essential properties remains a daunting challenge.This section discusses the design strategies to improve electrode performances.

Conductivity
High conductivity is crucial for epidermal electrodes as it helps to reduce signal loss.If the conductive layer thickness is difficult to measure or define, sheet resistance can be used for comparison.It is challenging to create highly conductive epidermal electrodes in permeable designs.Various conductive materials, such as metal films, metallic nanostructures, and conductive polymers, have been introduced to permeable substrates.The commonly used fabrication approach includes spray deposition, vacuum filtration, dip-coating, ultrasonic treatment, laser writing, and electroless deposition. [36,46,96]Generally, metallic materials tend to show the highest conductivity, whereas conductive polymers are less conductive.The electrode performance is also affected Reproduced with permission. [96]Copyright 2021, John Wiley & Sons, Ltd. b) CNT-based electrodes integrated with clothing.Reproduced with permission. [92]opyright 2021, American Chemical Society.c) Schematic illustration of the laser-induced graphene pattern (left) and the optical image of a bird-shaped graphene pattern (right).d) Graphene-based electrodes on textiles for ECG recording.(c,d) Reproduced with permission. [99]Copyright 2020, American Chemical Society.
by the conductive material loading.The sheet resistance is lowered by depositing additional conductive materials onto permeable substrates.Another important factor is the choice of conductive materials.According to the percolation theory, high aspect ratio materials can establish a more interconnected network, which results in better conductivity and stretchability.For instance, carbon black nanoparticles were deposited on polymeric fibers through ultrasonic cavitation, producing a high sheet resistance in thousands of ohms.On the other hand, CNTs achieved a much lower sheet resistance in tens of ohms. [103]urthermore, it is worth noting that the structures of permeable substrates significantly impact the conductivity when compared to ultrathin planar membranes.Permeable electrodes on porous substrates tend to have a higher sheet resistance due to the reduced conductive pathways.As-deposited conductive materials are often subjected to post-processing.Thermal annealing has proven effective in enhancing the conductivity of metal nanostructures. [55,104]

Stretchability
The skin undergoes varying elongations in different body positions on a daily basis.In fact, the maximal tensile strain may sometimes exceed 100% at joints. [105]It is therefore essential for epidermal electrodes to maintain high conductivity during stretching.Generally, stretchable electrodes are often made possible by structural designs or material innovations, [106] and many of these strategies have been adopted to enable or enhance the stretchability of permeable electrodes.
[109][110][111][112][113] As a typical example, a strain-insensitive and permeable conductor with a wrinkled structure was developed by sputtering gold on shrinkable electrospun fluorine rubber fiber mats. [108]This approach used the pre-stretching effect of the fibers during electrospinning that caused their area shrinkage by 35-40% after releasing from the substrate.The resulting conductors demonstrated high stretchability over ≈170% strain and limited resistance change by a factor of ≈0.8 under 60% strain (Figure 8a).Sun et al. reported a porous graphene electrode with a high permeability of up to 180 g cm −1 h −1 based on sugar-templated elastomer sponges (Figure 8b). [38]The electrode adopted a serpentine design to achieve a limited resistance change of ≈80% at 60% strain Figure (8c).When combined with kirigami cuts, the electrode has further improvement in stretchability, exhibiting a limited resistance change of 6% at 1000% strain (Figure 8d).In a wrinkle and serpentine design strategy, Xu et al. prepared stretchable electrodes by spray coating Ag NWs onto a pre-stretched sponge (Figure 8e,f).The electrode showed a slight change in resistance of 17% under 100% strain (Figure 8g). [107]Wu et al. introduced a textile-based design to create epidermal electrodes, [114] involving conformal coating gold films on ultrasheer knitted textiles using solution-based metallization.The resulting electrode has a low sheet resistance of 3.6 ± 0.9 Ω sq −1 and a minor change in resistance of up to 200% Reproduced with permission. [100]Copyright 2019, The Royal Society of Chemistry.b) SEM image of AgNW-based electrode (left).Schematic illustration of the strong interactions between AgNWs and fibers.Reproduced with permission. [55]Copyright 2019, John Wiley & Sons, Ltd. c) Fabrication (left) and characterizations (right) of AgNW-based electrode in the form of a hierarchically interactive nano-network.Simultaneous electrospinning polyamide nanofibers and electrospraying AgNWs generate these electrodes.Reproduced with permission. [46]Copyright 2020, The Royal Society of Chemistry.
strain (R/R 0 < 2), as shown in Figure 9a-d.Wang et al. used selective laser ablation to prepare an open mesh of AgNW/elastomer composite that retains stable conductance during tensile deformations (Figure 9e,f). [112]esides the aforementioned structural optimizations, developing new materials may also enhance the stretchability of the permeable electrodes.Generally, the conductive filler choice may affect the stretchability of the resulting composite electrodes.High aspect ratio fillers, such as AgNWs, CNTs, and graphene, are preferred for achieving excellent stretchability due to their interconnected network with rich conductive pathways. [46,92,96,99]][76][77][78][79] The fabrication of permeable liquid metal electrodes involves stencil printing and subsequent mechanical activation, which shows stable resistance even under extremely large strains. [64]Specifically, the resistance has only 4.1% increase when stretched to 1800% strain (Figure 9g), due to oxidation-induced microstructure changes during mechanical activation.

Skin Adhesion
Strong adhesion is essential for the conformal and robust contact between the electrodes and the skin.The minimized gap at the interface increases contact area and reduces the impedance, which is vital in acquiring high-quality signals. [115]In addition, the skin undergoes frequent deformations during daily life.Therefore, weak adhesion may lead to relative displacement between electrodes and skin, disturbing the recorded signals with motion artifacts. [116]Various methods have been developed to enhance skin adhesion by using bio-adhesives, microstructured "dry adhesives", and pressure-sensitive adhesives. [67,117]The permeability requirement brings challenges and opportunities for preparing epidermal electrodes with strong interface adhesion to the skin.
Ultrathin membranes have found extensive uses in permeable electrodes due to their low stiffness and skin conformability.These membranes are capable of achieving decent adhesion to the skin through van der Waals interactions. [118]Stretchable porous membranes are another critical platform for permeable epidermal electronics.However, it can be challenging for porous electrodes to maintain stable adhesion with the skin due to their reduced contact areas.To ensure high-quality signal collection, the electrodes should be fixed on the skin with external forces. [100,119,120]Recently, a fabric electrode was fabricated by sandwiching an Ag NWs conducting network between a polytetrafluoroethylene (PTFE) non-woven fabric and an electrospun silk fibroin nanofiber (Silk NF) membrane (Figure 10a,b). [121]In this electrode, the hydrophobic PTFE non-woven fabric is used as the outer self-clean layer to prevent external water pollution, the Ag NWs act as the stable conductive layer for biopotential monitoring, and the silk NFs are used as the flexible skin contact layer for absorbing sweat and providing firm skin adhesion.Interestingly, the fabric electrode has enhanced skin adhesion (13 N m −1 ) on wet skin, likely due to the reduced modulus of Silk NFs to increase the contact area (Figure 10c).
Biocompatible adhesives can help in achieving skin adhesion for epidermal electrodes.Various commercially available adhesives, such as water-soluble tapes, silicone adhesives, medical liquid bandages, and acrylic tapes, have been utilized to enhance the interface adhesion of epidermal electrodes. [11,64,122,123]However, there is often a trade-off between the permeability and adhesion of these electrodes.Thick adhesives usually provide strong adhesion but limited permeability.To address this issue, permeable electrodes have been developed based on thorough-hole patterns. [43,124]A porous adhesive of amphiphilic polymer is fabricated with a pore size of <≈10 μm via the breath figure method  [108] Copyright 2021, Springer Nature.b) Optical images showing the as-prepared epidermal device and its lamination on the skin.c) Change in normalized resistance versus tensile strain for the serpentine-shaped graphene conductor.d) Normalized resistance changes of the serpentine-shaped graphene conductor with additional kirigami cuts.(b-d) Reproduced with permission. [38]Copyright 2018, John Wiley & Sons, Ltd. e) Schematic showing the spray deposition of Ag NWs onto pre-stretched porous SEBS substrates.f) Optical image of a serpentine pattern (left) and SEM image showing wavy structured Ag NWs on the porous SEBS.g) Corresponding resistance change as a function of tensile strain.(e-g) Reproduced with permission. [107]Copyright 2020, Proceedings of the National Academy of Sciences.
(Figure 10d), [43] which adheres to the human skin using its reactive succinimide groups (Figure 10e).The resulting electrodes demonstrate high permeability without disrupting transdermal water loss and low contact impedance comparable to standard Ag/AgCl gel electrodes.Due to the impenetrable nature of the adhesive layer, this design may still block a significant fraction of sweat pores on the skin and reduce the wearing comfort.
Apart from bio-adhesives, various microstructures can be generated on epidermal electrodes to improve their adhesion based on physical mechanisms. [125,126]Min et al. created a bioinspired structure of miniaturized octopus suckers in a stretchable patch of conductive polymer composites.This patch features a hexagonal mesh structure that ensures both water and air permeability (Figure 10f).The octopus-like structures on the mesh help the patch maintain strong skin adhesion even underwater by efficiently draining excess water trapped at the interface (Figure 10g). [126]However, the construction of a complex structure will not only complicate the preparation of the device but also make the realization of mass production challenging.
Notably, the adhesion force helps in securing epidermal electrodes on the skin for reliable recording.However, excessive ad-hesion can lead to electrode failure and tissue damage during the detaching process, particularly in the case of wounded skin. [127]herefore, it is crucial to regulate the skin adhesion of epidermal electrodes.

Biocompatibility
Biocompatibility is a crucial consideration of epidermal electrodes during long-term applications.Natural biopolymers like silk and leather are attractive options for creating permeable and biocompatible electrodes. [93,128,129]Biocompatible and conductive polymers, such as PPy and PEDOT:PSS have been introduced for electrical engineering. [129,130]For example, conformal silk-based electrodes were prepared by electrospinning silk and dip-coating biocompatible PEDOT:PSS, showing a high stretchability of >250% and a high water-vapor transmission rate of ≈117 g m −2 h −1 under sweaty conditions. [129]Furthermore, biocompatible carbon-based nanomaterials, such as CNT and graphene, have also been introduced to biodegradable and permeable substrates for epidermal electrode fabrication.For example,  (a-d) Reproduced with permission. [114]Copyright 2020, Cell Press.e) Optical images of solid and hollow mesh conductors.f) Normalized resistance as a function of applied strain.(e,f) Reproduced with permission. [112]Copyright 2020, The Royal Society of Chemistry.g) Normalized resistance of a liquid metal permeable electrode as a function of applied strain.Inset: Optical images of the electrode without stretching and stretched to 1000% strain.Reproduced with permission. [64]Copyright 2021, Springer Nature.a biocompatible and multifunctional patch was prepared by dispersing highly conductive CNTs onto biodegradable silk nanofibers, [131] which allows for EP signal recording and drug delivery.Gold has been extensively used for its high biocompatibility and conductivity. [18,31]Kwon et al. fabricated a larger-area breathable epidermal system consisting of polyimide, gold, silicone elastomeric membrane, and strain-limiting fabric. [31]n these experiments, the absence of skin reactions can be verified through visual examination and infrared thermography.The cytotoxicity tests can further confirm the biocompatibility of the electrodes at the cellular level.Although AgNWs have been commonly used for permeable electrodes, they are toxic to living cells and highly corrosive in biological environment.AgNW-based electrodes are therefore incompatible with open wounds and damaged skin sites.To this end, Jeong et al. have prepared a permeable and biocompatible conductor involving electroplating an Au layer over Ag NWs embedded in nanofiber scaffolds. [132]Likewise, Au nanoparticles have been coated on the liquid metal-based electronic textile that suppresses the release of Ga ions. [133]The resulting textile possesses excellent electrochemical performances and biocompatibility, making it suitable for multimodal biomedical applications.

EP Signal Sensing
EP signals, including ECG, EMG, electroencephalogram (EEG), and electrooculography (EOG), are potential changes associated with ion transfers of cellular activities. [134]Recording and analyzing EP signals are essential for healthcare monitoring and HMI.Generally, electrodes for EP signal recording can be divided into invasive and noninvasive types.The former refers to electrodes implanted into human bodies or attached to the skin, while the latter refers to those used in an noninvasive manner.Invasive electrodes record EP signals with a higher signal-to-noise ratio (SNR).However, the biosafety of electrodes and physical damage should be taken into consideration carefully.Noninvasive electrodes, namely epidermal electrodes, show more convenience and safety than invasive ones. [135]Accordingly, we provide a detailed discussion on EP sensing by permeable epidermal electrodes.
ECG is a diagnostic tool that captures electrical signatures of cardiovascular health.ECG monitoring facilitates the prescreening, identification, and management of heart diseases. [21,75,129]pidermal electrodes featuring high thermal-wet comfortability were created by adding conductive polymers PEDOT:PSS into glycerol-plasticized porous silk fiber mats. [129]These electrodes have low skin contact impedance comparable to commercial gel electrodes.High-quality ECG signals are collected under both resting and sweaty states.Interestingly, the attached electrodes do not induce a noticeable elevation in skin temperatures, significantly reducing the discomfort caused by the poor heat dissipation of traditional electrodes.Ding et al. developed an in situ deposited skin-adhesive liquid metal particle (ALMP) for fabricating comfortable and mechanically stable epidermal electronics (Figure 11a). [75]ALMP electrodes achieve low contact impedance and high SNR through their conformability and selfadhesiveness to the skin(Figure 11b,c).The concurrent measurement of seismo cardiograms (SCG) can provide essential information about heart mechanics, such as myocardial activity and valve motions, which are absent from ECG and useful for detecting cardiac complications.Ling et al. have developed a novel cardiac monitoring system that utilizes multifunctional porous  (a-c) Reproduced with permission. [121]Copyright 2022, Composites Science and Technology.d) Schematics of adhesive and breathable monolayer porous film.e) Photographs of the properties of the prepared film.(d,e) Reproduced with permission. [43]Copyright 2022, American Chemical Society.f) Schematics of hexagonal mesh-patterned conductive polymer composite (CPC) film with the Octopus-inspired Structure (OIS) attached to wet human skin.g) Desirable characteristics of the CPC film with the OIS.(f,g) Reproduced with permission. [126]Copyright 2020, American Chemical Society.
PI meshes interfaced with the skin (Figure 11d). [111]This bimodal system records both ECG and SCG signals simultaneously (Figure 11e,f), providing vital information about the evolving condition of the heart.
Monitoring human muscle activity is crucial for sports health, rehabilitation training, and neuro-medicine. [136,137]Although epidermal EMG is an effective method for monitoring muscle activity, the lack of permeability in most epidermal electrodes presents a significant challenge.The non-breathable electrodes tend to block sweat glands for uncomfortable perceptions.In sports-related application settings, these traditional EMG electrodes easily accumulate sweat in their interface with the skin, resulting in skin irritations, signal degradation, and electrode damage.To this end, highly permeable EMG electrodes have been developed to return the natural perspiration and thereby satisfy the demands for daily uses.In addition, recent studies have introduced a new generation of epidermal electrodes featuring directional water transport that actively pumps sweat away from  (a-c) Reproduced with permission. [75]Copyright 2022, American Chemical Society.d) Images showing the pattern of the permeable electronic.e) Schematic illustration of mobile data acquisition circuit and graphic data display of ECG and SCG on the smartphone.f) ECG (top) and SCG (bottom) measured simultaneously using porous mesh bioelectronics.(d-f) Reproduced with permission. [111]][61][62][63] Xu et al. created a permeable electrode in a trilayer architecture consisting of an active Au layer, a hydrophobic TPU nanofiber mat, and a hydrophilic cellulose nanofiber mat (see Figure 12a,b).Due to its porous microstructure and wettability gradients, the resulting electrode can unidirectionally pump sweat away from the skin. [18]These sweat-wicking electrodes can operate well under heavy perspiration, whereas commercial electrodes essentially fail with accumulated sweat (Figure 12c).
EEG signals generated by brain activities provide vital insights into neurological disorders and human emotions.These signals are typically classified into five frequency bands, namely  (d-f) Reproduced with permission. [139]opyright 2023, Elsevier.g) Images of EOG electrodes worn around human eyes.h) Schematic showing the locations of GET EOG sensors laminated on the face.i) Schematic of representative EOG signals corresponding to different eye movements and blinking.(g-i) Reproduced with permission. [148]opyright 2018, Nature Research.
wave (0-4 Hz),  wave (4-8 Hz),  wave (8-12 Hz),  wave (12-40 Hz), and  wave (≥40 Hz), which correspond to different mental states. [138]However, detecting EEG signals from the human brain is challenging due to their weak strength (measured in μV scales) and the need for long-term and accurate monitoring.][141] A recent study fabricated liquid metal textile electrodes involving coating liquid metal on silver decorated SEBS textile. [139]The prestretching activation is essential to ensure excellent gas permeability and stretchability to 1000% strain of the liquid metal electrodes.These E-textile electrodes are capable of acquiring highfidelity EEG signals (Figure 12d,e).At a relaxed state with the eye closed, the EEG background typically exhibits the posteriorly dominant  rhythm with a prominent oscillation of 8-12 Hz, corresponding to brain activities such as meditation and mindfulness that reduce stress levels.The  rhythm significantly attenuates when the eyes are opened, demonstrating the dynamic activity of the  rhythm during repeated eye-closing and eye-opening (Figure 12f).
EOG is a technique that records EP signals between the cornea and the retina of the human eye, widely used in brain science research, mental disease diagnosis, and sleep studies.EOG is also employed for patients with neural system disorders such as progressive neuro-motor degenerative diseases.In these applications, EOG has two significant advantages over EEG and EMG.First, the EOG signal has stable and easily detectable waveforms.Second, analyzing EOG signals is less complex due to their linear correlation with small-range eye movements. [142]ince EOG electrodes are applied to the delicate skin around the eyes, the EOG electrodes should be soft and permeable enough to avoid skin damage, fatigue, and irritation.[145][146][147] Alternatively, electronic tattoos are also used for EOG recording.Ameri et al. have developed an imperceptible sensor based on ultrathin and transparent graphene electronic tattoos (GETs). [148]The GET can be directly mounted around the eyes without hindering blinking or facial expressions (Figure 12g).The GET can record precise EOG data with an angular resolution of 4°of eye movement, which allows the accurate interpretation of eye movements (Figure 12h,i).

HMI
Body movements activate muscle fibers and generate electrical signals that can be recorded with epidermal electrodes.These EMG voltage signals are unique for different gestures, which makes it possible to use them to control external devices.This technology can be used for gesture recognition and prosthetic control. [20,110,149]A deep understanding of EMG signals is essential to enable HMI that transforms gestures into control commands.Machine learning algorithms are often implemented to classify the recorded EMG data.Collecting abundant EP signals is crucial for reliable prosthetic control.Therefore, stretchable and permeable electrodes are urgently needed for HMI to achieve long-term EP recording.Zhao et al. fabricated a smart arm sleeve as an epidermal HMI to acquire control commands according to hand gestures. [20]A four-channel sensing system is created on elastomeric microtextile and then sewn onto an arm sleeve as an integrated EMG sensing wearable (Figure 13a).Regarding the epidermal sensing capabilities, the recovery force of the arm sleeve allows conformal contact of the textile electrodes with the skin to acquire stable and high-fidelity signals.Complex hand motions are identified as voltage waveforms that reflect contraction levels of corresponding muscles (Figure 13b,c).Individual gestures are correlated with EMG waveforms at multiple channels due to the synergistic contractions of a group of muscles.The gesture recognition system powered by the machine learning algorithm demonstrated here is the enabler of smart HMIs capable of delivering complex commands (Figure 13d).Won et al. introduce the Kirigami approach to pattern a highly conductive and transparent electrode into diverse shapes of stretchable electronics with multivariable reconfigurability for E-skin applications. [110]These kirigami engineered patterns impart tunable elasticity to the electrodes, which can be designed to intentionally limit strain or grant ultrastretchability.Owing to soft and ultrathin but highly stretchable characteristics, the electrode conformally covers the curvilinear and irregular surfaces of the human body, facilitating measurement of EMG on both forearms with dynamic motions in real-time for controlling a drone in an advanced form for the demonstration of HMI.Besides EMG signals, EOG signals have also been used for external mechanical equipment. [150,151]For example, permeable and self-adhesive electrodes based on PEDOT served as HMIs to control virtual aircraft. [150]The processed signals can control the virtual aircraft to fly along the trajectory of "S", according to the ocular motions in real-time (Figure 13e).Remarkably, the augmented hydrogen bonding generation and chain rearrangements by dopants mitigate the mechanical resilience of the electrodes, endowing them with stress relaxation behavior.Although the surrounding skin can be stretched or compressed during ocular movements, electrodes remain stable adherence without affecting biosignal acquisition.

Wound Dressing
Skin wounds are commonly encountered in daily lives and affect millions of people every year. [152]The wound healing process involves four stages: hemostasis, inflammation, proliferation, and dermal remodeling. [153]It is helpful to monitor the heal-ing status using smart dressings that can detect pH, temperature, hydration, glucose, ROS, and uric acid. [154]Antimicrobial dressings are particularly important as bacterial infection often hinders wound healing. [12]However, wound dressings that lack permeability may result in the accumulation of moisture and sweat, hampering accurate sensing over time.In addition, the buildup of skin byproducts may cause delamination of the dressing and increase the risks of infection.][160] Yeon et al. developed a hydration sensor using a sweat pore-inspired perforated method for skin regeneration monitoring. [13]The dumbbell through-hole patterns of the sensor provide sweat-permeable channels and enhance its mechanical reliability and conformability.Long-term health monitoring tests were successfully performed, including tracking skin regeneration with no disturbance and daily activity checkups over one week (see Figure 14a,b).Brown et al. created an adhesive-free, stretchable, and permeable wound bandage based on a soft, thin silicone fiber mat, allowing diffusive wound exudate flow and passive gas transfer.The embedded sensors can continuously measure multiple inflammatory biomarkers such as lactate, glucose, pH, oxygen, and wound temperature (see Figure 14c). [158][163] Tang et al. introduced a bactericidal patch based on a triboelectric nanogenerator featuring excellent flexibility, permeability, and wettability.The patch utilized electrospun polymer tribo-layers and a chemical vapor-deposited polypyrrole electrode. [163]Electrical stimulations harvested mechanical motions and positive charges on the polypyrrole surface to kill over 96% of bacteria and accelerated wound healing due to the synergistic effects of cell membrane disruption and enhanced gene growth factor expressions (see Figure 14d,e).

Conclusion
In this review, we have systematically summarized the advancement of stretchable and permeable electrodes.The permeable electrodes are divided into two types: ultrathin solid membranes and porous scaffold-based designs.Furthermore, this review discusses in detail the choice of conductive materials, including film coating and nanomaterials assembly strategies.Effective strategies for achieving desired properties, including high conductivity, stretchability, skin adhesion, and biocompatibility, are also elaborated upon.Additionally, we provide an overview of the emerging applications of stretchable and permeable electrodes, such as EP signal sensing, HMI, and wound dressing.Despite the remarkable progress in this field, some challenges still need to be addressed for the practical applications of stretchable and permeable electrodes in healthcare monitoring, HMI, and wound dressing.
First, it is rather difficult to create epidermal electrodes that provide high performance and wearing comfort.Achieving optimal functionality requires careful consideration of various factors, including permeability, conductivity, stretchability, skin adhesion, robustness, softness, optical transparency, and  [20] Copyright 2021, American Chemical Society.e) The trajectory of aircraft controlled by eyeball movements.Reproduced with permission. [150]Copyright 2021, American Chemical Society.lightweight.However, there are trade-offs among these properties.For instance, while ultrathin planar membranes can improve permeability and softness, they tend to reduce stretchability and robustness.Currently available epidermal electrodes are still lacking in several aspects and have ample room for improve-ment.Additionally, permeable electrodes have gained popularity in research fields, but commercialization is still in its early stages.Single-channel sensors are not ideal for capturing multiple EMG signals from the human body, as the voltage responses are easily affected by slight variations in electrode placement.Reproduced with permission. [13]Copyright 2021, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.c) Schematic illustration of stretchable and permeable patches with complex sensors for chronic wounds.Reproduced with permission. [158]Copyright 2022, American Chemical Society.d) Wound area statistics at different time points.e) Sketches of wound areas to visualize the healing process in different groups.(d,e) Reproduced with permission. [163]Copyright 2023, American Chemical Society.
Multichannel EMG arrays can help improve the reliability and spatiotemporal resolution of EMG recordings.It is highly desirable to manufacture these electrodes in a scalable, cost-effective, and environmentally friendly manner.Advanced materials, structural designs, and manufacturing methods are also required for breakthroughs in permeable epidermal electrodes.
Stretchable and permeable electrodes play a vital role in standalone epidermal electronic systems that involve sensing, power supply, and data management.These systems combine multiple electrodes and sensors to achieve multifunctional health assessment and disease diagnosis.Soft power sources are necessary for their sustained operation.Therefore, developing highperformance energy conversion/storage devices is crucial for the success of stand-alone systems.In addition, the electrodes can couple with the weak bioelectricity from skin surfaces, which need to be amplified and filtered for further processing.In healthcare applications, a tremendous amount of data is collected during continuous recording, which requires data storage, conditioning, and analysis.Developing new processors and intelligent algorithms can potentially improve data processing efficiency and reduce power consumption. [164]Therefore, multidisciplinary approaches are highly desired for the development of advanced epidermal systems.
In summary, there is still room for improvement in stretchable and permeable epidermal electrodes to make them more practical.Despite the challenges, the potential benefits of developing a top-performing, integrated permeable electrode system are immense, as it could enable real-time healthcare monitoring and reliable HMIs.This technology could be a game-changer for nextgeneration healthcare.Interdisciplinary research may be necessary to overcome existing obstacles and unlock the full potential of epidermal electronics.

Figure 2 .
Figure 2. Porous Scaffold-Based Permeable Electrodes.a) Schematic illustration of the liquid metal micromesh prepared by physically depositing liquid metal on the sponge.Optical and SEM images showing a representative flower-shaped pattern of the liquid metal micromesh.Reproduced with permission.[37]Copyright 2022, American Chemical Society.b) Schematic illustration and optical images of a trilayer-structured device with liquid metal electrodes in each layer.Reproduced with permission.[64]Copyright 2021, Springer Nature.c) Schematic illustrating the structure of a moisture-wicking, breathable membrane.Reproduced with permission.[21]Copyright 2022, John Wiley & Sons, Ltd. d) Schematic showing the architecture of the Janus textile electrode.The cross-sectional SEM images reveal the medical adhesive for reinforcement.Reproduced with permission.[62]Copyright 2022, John Wiley & Sons, Ltd.

Figure 3 .
Figure 3. Solid metallic film-based permeable electrodes.a) Schematic illustrations of the nanomesh device.Reproduced with permission.[47]Copyright 2019, Springer Nature.b) Schematic showing the preparation of nanomesh conductors.c) Optical images of nanomesh conductors attached to a finger under bending and relaxed states.(b,c) Reproduced with permission.[23]Copyright 2017, Springer Nature.

Figure 4 .
Figure 4. Liquid metal film-based permeable electrodes.a) Schematic diagram of a typical liquid metal fiber micromesh fabrication process.b) SEMimages of the electrospun fiber mat (left), the liquid metal-coated fiber mat (middle), and the strain-activated permeable electrode (right).(a,b) Reproduced with permission.[64]Copyright 2021, Springer Nature.c) Fabrication process flow for the liquid metal micromesh on the SEBS microfiber textile.d) SEM images of the microfiber textile before (left) and after (right) liquid metal coating.(c,d) Reproduced with permission.[35]Copyright 2022, American Chemical Society.e) Schematic of the fabrication process to create liquid metal microelectrodes.f) Optical images showing the high-density microelectrode at 75 000 electrodes cm −2 .g) Corresponding SEM image displaying the liquid metal pattern on the fibers that form highly porous microstructures.(e-g) Reproduced under the terms of a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).[80]Copyright 2023, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.

Figure 5 .
Figure 5. Conductive polymers-based permeable electrodes.a) Schematic of hydrothermal treatment for the PEDOT:PSS film coated on a fiber mat.b) Corresponding conductivity before and after the hydrothermal treatment.c) Optical image showing PEDOT:PSS-based permeable electrodes conformally attached to the hand (left) and SEM image displaying the porous microstructure of the electrode.(a-c) Reproduced with permission.[87]Copyright 2021, American Chemical Society.d) Schematic of the fabrication for hybrid electrodes consisting of AgNWs and PEDOT:PSS.e) Cross-sectional SEM image of the hybrid electrode.f) Transmittance versus wavelength for AgNW and hybrid electrodes.(d-f) Reproduced with permission.[88]Copyright 2020, American Chemical Society.g) Conformal attachment of a permeable electrode based on PEDOT:PSS, poly(vinyl alcohol), tannic acid, and ethylene glycol.h) Conductivity as a function of PEDOT:PSS loading.i) Conductivity versus tannic acid loading.j) Normalized resistance and stress of the electrode with respect to strain.(g-j) Reproduced with permission.[58]Copyright 2022, John Wiley & Sons, Ltd.

Figure 6 .
Figure 6.Carbon nanomaterials-based permeable electrodes.a) Schematic of the fabrication process to create MWCNTs/fiber-based electrode.Reproduced with permission.[96]Copyright 2021, John Wiley & Sons, Ltd. b) CNT-based electrodes integrated with clothing.Reproduced with permission.[92]Copyright 2021, American Chemical Society.c) Schematic illustration of the laser-induced graphene pattern (left) and the optical image of a bird-shaped graphene pattern (right).d) Graphene-based electrodes on textiles for ECG recording.(c,d) Reproduced with permission.[99]Copyright 2020, American Chemical Society.

Figure 7 .
Figure 7. Metallic nanomaterials-based permeable electrodes.a) Schematic of the electrode fabrication involving spray depositing AgNWs onto an electrospun fiber mat (left) and SEM image of the resulting electrode (right).Reproduced with permission.[100]Copyright 2019, The Royal Society of Chemistry.b) SEM image of AgNW-based electrode (left).Schematic illustration of the strong interactions between AgNWs and fibers.Reproduced with permission.[55]Copyright 2019, John Wiley & Sons, Ltd. c) Fabrication (left) and characterizations (right) of AgNW-based electrode in the form of a hierarchically interactive nano-network.Simultaneous electrospinning polyamide nanofibers and electrospraying AgNWs generate these electrodes.Reproduced with permission.[46]Copyright 2020, The Royal Society of Chemistry.

Figure 8 .
Figure 8. Strategies to enhance the stretchability of permeable electrodes.a) Normalized resistance changes of gold-coated conductors based on different substrates.Reproduced with permission.[108]Copyright 2021, Springer Nature.b) Optical images showing the as-prepared epidermal device and its lamination on the skin.c) Change in normalized resistance versus tensile strain for the serpentine-shaped graphene conductor.d) Normalized resistance changes of the serpentine-shaped graphene conductor with additional kirigami cuts.(b-d) Reproduced with permission.[38]Copyright 2018, John Wiley & Sons, Ltd. e) Schematic showing the spray deposition of Ag NWs onto pre-stretched porous SEBS substrates.f) Optical image of a serpentine pattern (left) and SEM image showing wavy structured Ag NWs on the porous SEBS.g) Corresponding resistance change as a function of tensile strain.(e-g) Reproduced with permission.[107]Copyright 2020, Proceedings of the National Academy of Sciences.

Figure 9 .
Figure 9. Strategies for enabling or enhancing the stretchability of permeable electrodes.a) Normalized resistance changes of a gold-coated ultrasheer fabric as a function of strain along the wale and course directions.b) SEM image of the gold-coated ultrasheer fabric stretched to 40% strain.c) Normalized resistance changes of gold-coated spandex fibers as a function of stretching strain.d) Sem image of a gold-coated spandex fiber stretched to 40%.(a-d)Reproduced with permission.[114]Copyright 2020, Cell Press.e) Optical images of solid and hollow mesh conductors.f) Normalized resistance as a function of applied strain.(e,f) Reproduced with permission.[112]Copyright 2020, The Royal Society of Chemistry.g) Normalized resistance of a liquid metal permeable electrode as a function of applied strain.Inset: Optical images of the electrode without stretching and stretched to 1000% strain.Reproduced with permission.[64]Copyright 2021, Springer Nature.

Figure 10 .
Figure 10.Strategies to improve skin adhesion of permeable electrodes.a) Microstructure of the fabric electrode.b) Optical and optical microscopy images of the fabric electrode attached to the skin.c) Adhesion force of the fabric electrode on different skin conditions revealed by 90-degree peeling tests.(a-c)Reproduced with permission.[121]Copyright 2022, Composites Science and Technology.d) Schematics of adhesive and breathable monolayer porous film.e) Photographs of the properties of the prepared film.(d,e) Reproduced with permission.[43]Copyright 2022, American Chemical Society.f) Schematics of hexagonal mesh-patterned conductive polymer composite (CPC) film with the Octopus-inspired Structure (OIS) attached to wet human skin.g) Desirable characteristics of the CPC film with the OIS.(f,g) Reproduced with permission.[126]Copyright 2020, American Chemical Society.

Figure 11 .
Figure 11.ECG signal sensing by permeable electrodes.a) SEM images and the corresponding schematic illustration of the ALMP (left), film-based LM electrode (middle), and adhesive-based LM electrode (right) on the skin replica.b) Skin contact impedances of three electrodes.c) Overlay multiple waveforms detected with ALMP (left), film-based LM electrode (middle), and the adhesive-based LM electrode (right) on the skin replica.(a-c)Reproduced with permission.[75]Copyright 2022, American Chemical Society.d) Images showing the pattern of the permeable electronic.e) Schematic illustration of mobile data acquisition circuit and graphic data display of ECG and SCG on the smartphone.f) ECG (top) and SCG (bottom) measured simultaneously using porous mesh bioelectronics.(d-f) Reproduced with permission.[111]Copyright 2023, John Wiley & Sons, Ltd.

Figure 12 .
Figure 12.EMG, EEG, and EOG signal sensing by permeable electrodes.a) Schematic illustration of the design of antigravity directional perspiration electrode.b) Optical image of the antigravity directional perspiration electrode for EMG sensing.c) EMG signals collected by commercial Ag/AgCl-PI array electrodes (up) and antigravity directional perspiration array electrodes (down) under dry and sweating conditions.(a-c) Reproduced with permission. [18]Copyright 2022, John Wiley & Sons, Ltd. d) Schematic diagram of the EEG measurement position.e) Recorded EEG signals from volunteers engaged in eye closed and open states and corresponding the fast Fourier transform -processed frequency distributions.f) Time-frequency spectrograms of the EEG signals recorded during cyclic eye closing/opening, revealing the dynamic activity of the alpha rhythm at ≈10 Hz.(d-f) Reproduced with permission.[139]Copyright 2023, Elsevier.g) Images of EOG electrodes worn around human eyes.h) Schematic showing the locations of GET EOG sensors laminated on the face.i) Schematic of representative EOG signals corresponding to different eye movements and blinking.(g-i) Reproduced with permission.[148]Copyright 2018, Nature Research.

Figure 13 .
Figure 13.Applications of permeable electrodes in HMI.a) Schematic illustration of the smart HMI-powered machine learning algorithm to use hand gestures for wireless control of a four-wheel car.b) Output voltages at individual channels of the epidermal EMG sensing sleeve associated with various hand gestures, including hand close, hand open, wrist extension, wrist flexion, ulnar deviation, and radial deviation (from left to right).c) 2D diagram to visualize the data set of 240 recorded hand gestures that fall into six distinctive color-coded clusters.d) Optical images showing the motion of a four-wheel car under control commands wirelessly set by hand gestures.(a-d) Reproduced with permission.[20]Copyright 2021, American Chemical Society.e) The trajectory of aircraft controlled by eyeball movements.Reproduced with permission.[150]Copyright 2021, American Chemical Society.

Figure 14 .
Figure 14.Applications of permeable electrodes in wound dressing.a) Photographs of the inflamed skin region right after erythema and after two weeks.b) Hydration level of the inflamed skin as a function of the duration of e-skin lamination.(a,b) Reproduced under the terms of a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).Reproduced with permission.[13]Copyright 2021, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.c) Schematic illustration of stretchable and permeable patches with complex sensors for chronic wounds.Reproduced with permission.[158]Copyright 2022, American Chemical Society.d) Wound area statistics at different time points.e) Sketches of wound areas to visualize the healing process in different groups.(d,e) Reproduced with permission.[163]Copyright 2023, American Chemical Society.