3D Sponge Electrodes for Soft Wearable Bioelectronics

By definition, a sponge refers to a soft and porous material, which is typically made from cellulose or synthetic polymers. Structurally, it is 3D offering a high surface‐to‐volume ratio; mechanically, it is elastic and durable; economically, it is inexpensive and can be produced at industrial scales. These attributes promote a strong research interest in exploring 3D sponges for applications in absorption, separation, catalysis, and electronics. This work is dedicated to the discussion of the recent progress in design of mechanically deformable sponge electrodes for their application in soft electronics. First, the characteristics and advantages of sponge electrodes are described, which is followed by the discussion of various active materials for fabricating 3D sponge electrodes, including carbon, metal, conductive polymers, MXenes, and their hybrids. Then, the viable fabrication methodologies are reviewed by comparing their advantages and disadvantages. Furthermore, the applications of 3D conductive sponges in stretchable conductors, sensors, energy storage devices, and integrated systems are discussed. Finally, the challenges and opportunities in future sponge‐based soft electronics are covered.


Introduction
Although optoelectronic devices are becoming more and more miniaturized, they remain rigid and planar in design, incompatible with soft and 3D biological systems.It is expected electronics will evolve from rigid to flexible and eventually to soft bioelectronics. [1]Mechanically soft electronic devices can be conformally integrated with the human body, which can significantly improve the quality of biometrical signals.The emergence of soft electronics creates new opportunities for robotics, artificial intelligence (AI), and health monitoring [2][3][4][5] Various materials and structures have been explored to design soft electrodes for diverse applications.Nanomaterials, ranging from carbon-based materials, [6] metal-based materials [7] to conductive polymers, [8] have been used as conductive fillers for soft electrodes to achieve conductivity under mechanically deformed DOI: 10.1002/aelm.202300334states.Structural designs represent additional strategies to improve stretchability for intrinsically soft materials or to bestow stretchability for intrinsically rigid materials.Examples of viable structural designs include wrinkle structure, [9] serpentine structure, [10] origami/kirigami design, [11] etc. Sponge is a 3D porous material that can serve as a scaffold for designing soft bioelectrodes.Porous electrodes range from microscopic porous silicon [12] and porous gold, [13] mesoporous carbon [14,15] to macroscopic porous graphene, [16] carbon materials, [17] and nickel foam. [18]he previously published reviews have been focusing on the discussions of their applications in batteries [19] or other energy devices. [20]This review article aims to cover the state-of-the-art development of soft sponge electrodes with a focal point on applications in soft bioelectronics.It begins with attributes of 3D sponges in soft electronics, followed by description of various active materials-based sponge electrodes.Then, the representative fabrication methodologies are summarized.Afterward, state-of-the-art literature on utilization of soft sponge electrodes in stretchable conductors, wearable sensors, and soft energy devices are covered.Finally, we discuss the sponge-based integrated soft system, as well as challenges and opportunities.

Why 3D Sponge Electrodes for Soft Bioelectronics?
Soft electronics may include sensors, energy storage devices, interconnection components, and integrated devices, for which to function, active materials need to perform under various mechanically deformed states including bending, twisting, and stretching.However, active materials are typically made of rigid metal and semiconductor materials, which tend to crack and delaminate from elastomeric substrates.A multi-scale soft-hard materials interface design is crucial for achieving outstanding soft bioelectronic devices. [21]ne viable approach to designing soft bioelectronic system is to deposit active optoelectronic materials onto elastomeric surfaces, which may include carbon nanotubes, [22] silver nanowires, [23] gold nanowires, [24] 2D materials, [25] conductive polymers, [8b,26] ionic liquids [27] or liquid metals. [28]These active materials can also be incorporated into elastomers to form percolating networks to form composites.Copyright 2016, American Chemical Society.b) SEM of laser-induced graphene (LIG) on PDMS sponge.Reproduced with permission. [42]Copyright 2018, John Wiley and Sons.c) SEM of SWCNT PU sponge.47d] Copyright 2015, John Wiley and Sons.d,e) SEM of carbon black on PU sponge under different magnifications.Reproduced with permission. [50]Copyright 2016, John Wiley and Sons.
The past several years have witnessed the interest in utilizing 3D conductive sponge electrodes for soft bioelectronics, which can offer several attributes.Typically, commercially available, inexpensive sponges can be used as an elastomeric matrix for depositing active materials to construct bioelectronic electrodes.
The sponge structure, with its interconnected porous and flexible nature, can absorb and redistribute applied mechanical load, hence, sponge skeletons can reduce the likelihood of conductive materials damage or failure under mechanically deformed states. [29]Therefore, sponge electrodes are typically durable and can bear complex deformation such as bending, twisting, and stretching.
The unique 3D porous structure is advantageous for fabricating flexible pressure sensors.When external pressure is applied to the conductive sponge electrodes, the skeleton structure is compressed and the trapped air is squeezed of the pores.This typically promotes skeleton contacts with each other, resulting in decrease in electrical resistance. [30]This characteristic enables the sponge electrode to effectively sense and respond to pressures, offering reliable measurement of pressure over a large range.
13a] In essence, the sponge electrode is 3D conductive network promoting efficient electron and ion transport, leading to low overall resistance.The high surface areas of sponge electrodes can offer high ion adsorption capacity and high loading of active materials, and promote charge transfer between the electrode and electrolyte interface and reduce path lengths for ion diffusion.These properties benefit sponge electrodes in the electrochemical energy storage and conversion systems. [31]n the other hand, sponge-based materials are also advantageous for wearable applications.the porous structure can allow air circulation and moisture evaporation, reducing the buildup of heat and sweat, which is important for long-term usage during physical activities.
In short, 3D sponge electrodes can integrate low electrical resistance, mechanical flexibility, outstanding electrochemical performance, and gas-permeable properties into one single material system, which has implications in a plethora of applications in bioelectronics, including soft biosensors and flexible energy devices.

Carbon Materials-Based Sponge Electrodes
Carbon-based materials, including CNTs, carbon black, graphene, and their composites, are intensively explored in soft electronics. [32]raphene sponges offer several advantages for soft electrodes, including high porosity (up to 98%), large specific areas (≈2620 m 2 g −1 ), and lightweight (0.1-0.6 mg cm −2 ), [16,33] and they can be produced by either template synthesis or templateless synthesis.Template synthesis usually contains chemical vapor deposition (CVD), dip-coating with thermal treatment, electrodeposition, and 3D printing.Figure 1a shows a graphene sponge electrode fabricated by dip-coating commercial polyurethane (PU) sponge into graphene oxide suspension, followed by further hydrothermal treatment to obtain reduced graphene oxide (rGO) sheets.After rGO coating, the overall morphology did not change but the skeleton surface became rougher, which suggested the success of graphene sheets deposition.Such produced sponge electrodes could achieve a conductivity of ≈0.25 S m −1 and a low density of ≈0.027−0.030g cm −3 , which could serve as low-density electromagnetic interface shielding devices. [34]Other templatebased 3D porous graphene, such as CVD-graphene foam, [35] 3D printed graphene, [36] have demonstrated outstanding electrochemical performance in comparison to their bulk counterparts.
Graphene sponge electrodes can also be produced via the template-less synthesis method.Typically, GO or rGO is first dispersed in solvents for a homogenous suspension, which can self-assemble to form 3D free-standing porous structures under controlled conditions.In particular, self-supported graphene film could be obtained via vacuum filtration (VF) method.But this approach typically led to a relatively thick film, where graphene aggregated and restacked. [37]In order to avoid the aggregation and restacking of graphene sheets, a leavening strategy involved in the use of hydrazine vapor was developed, which led to porous freestanding rGO layered paper.Such rGO foams could serve as electrode materials for flexible supercapacitors. [38]ore recently, the CO 2 laser transfer polyimide (PI) into porous graphene with continuous structures are widely used in fabrication of flexible supercapacitors, [39] wearable strain sensors, [40] and biosensors. [41]Figure 1b is a porous laser-induced graphene (LIG), in which the authors first fabricated LIG by CO 2 laser heating on the commercial PI, then transferred LIG onto a porous substrate. [42]They demonstrated breathable skin sensor application.One attribute of this technology is the facile patternable capability of any desired shapes for bioelectronic applications.
Carbon nanotubes (CNTs) are one of the popular materials used in soft stretchable electronics due to their high-aspectratio, [43] good conductivity, [44] superior mechanical performance, and chemical stability. [45]It was reported that CVD could be used to produce CNT sponge electrodes. [46]46a] CNT sponge could also be fabricated by solution-based approach simply by dip-coating porous sponge into CNTs ink. [47]47d] The Van der Waals interaction between CNTs and the sponge surfaces was found to be sufficiently strong to prevent their desorption.The solutionbased coating is uniform and conformal, enabling the formation of a continuous "skeleton/skin" structure -important for facile electron transports.The CNTs-sponge formed by three "dipping and drying" processes could achieve a sheet resistance of 33 Ω sq −1 .This CNT-sponge electrode could be further modified with other active materials for specific applications.CNT can also be deposited into PDMS foams through physical interactions simply by drop casting. [48]The porosity of CNT-decorated foams is similar to that of as-prepared foams, which means CNTs are mainly adsorbed on the surface of the pores and do not change the overall pore volume of the PDMS.
Another method was reported by virtue of the use of sacrificial materials.CNTs could be mixed with polydimethylsiloxane (PDMS) precursor and curing agent, and then the mixture was poured into a mold pre-filled with particulates.Once PDMS was fully cured, the particulates could be washed away with water to leave a porous CNT/ PDMS sponge. [49]In this technology, the resulting materials properties, such as elasticity, conductivity, stiffness, and porosity could be controlled by adjusting the mass ration of conducive materials, water-soluble particulates, and the polymer curing ratio.
Low-cost carbon black (CB) was also used in fabricating sponge electrodes.A CB-based PU sponge was synthesized by a natural polymer-assisted water-based layer-by-layer (LBL) assembly method.The backbones of bare PU sponge are smooth, and after the electrostatic deposition of CB, the surface of sponge skeleton becomes rough and the CB particles were clearly observed (as shown in Figure 1d,e). [50]By increasing the LBL deposition times, CB loading into the sponge increased, leading to an increase in the electrical conductivity and elastic modulus.
Hybrid carbon materials could be achieved in sponge electrode fabrication, as demonstrated by 1D CNTs and 2D graphene. [51]imply by dip-coating PU sponge into MWCNTs and graphene solutions alternatively, a hybrid carbon-based sponge was successfully obtained.It was claimed that plenty of nanogaps between MWCNTs existed, which would help the materials detect slight deformations and offer high sensing capability.The contents of CNTs and graphene in the sponge could be changed by using different concentrations and dip-coating LBL assembly cycles.It was also claimed that the conductivity of CNT/graphene@PU was much higher than CNT@PU or graphene@PU sponges.The authors attributed this to the synergistic effect on electrical conductivity of 1D CNTs and 2D graphene due to the formation of 3D conducting network structure on the sponge skeletons.

Metal Materials-Based Sponge Electrodes
Metallic materials possess intrinsic high electrical conductivity and have been explored to design and fabricate 3D sponge electrodes.29b] A copper sponge monolith with a dimension of 9 cm × 10 cm × 4 cm was fabricated, which could give a low resistivity of 10 ± 5 mΩ cm (Figure 2a).The approach was general, other metals including Au, Ag, and even binary metals (Figure 2b), could be conformally and uniformly deposited onto the PU sponge skeleton.Moreover, this solution-based metal coating method could be further extended to PDMS sponges, to achieve stretchable, compossible, and bendable performance while maintaining good conductivity. [52]n alternative chemical deposition approach was reported to fabricate a nickel-cobalt alloy sponge (Figure 2c). [53]In brief, the elastic porous melamine sponges were chosen as the substrate, which was first modified with a polymeric brush via a dopamineinduced free radical polymerization reaction.This was followed by catalytic metallic ion immobilization via electrostatic attraction.Afterward, electroless deposition of Ni and Co metals could be achieved throughout the sponge skeleton surfaces.The resulting bi-metallic sponge was electrically conductive, magnetically active, and mechanically compressible.It indicated applications in sensing and electromagnetic interference shielding.
Nanomaterials may be used to fabricate conductive sponge electrodes.One of the prime candidates is metallic nanowires which can be suspended in solvents so that simple dip-coating technology can be used.In one example, silver nanowires (Ag-NWs) were used to fabricate sponge electrodes which displayed a slight increase of conductivity under 50% strain and could maintain conductivity of 20 S cm −1 under 100% strain. [54]Hydrophobic gold nanowires (AuNWs) could also be used to produce porous sponges using polymerized high internal phase emulsions (polyHIPEs) scaffolds [55] as shown in Figure 2d.The resulting composite can be patterned and transferred onto skin as "tattoos" in Figure 2e.
In addition, freeze-drying can be used to produce conductive sponge electrodes directly from nanowire solutions.In 2014, our research group successfully fabricated copper nanowires  [29b] Copyright 2013, John Wiley and Sons.c) SEM and elemental mapping of nickel and cobalt of Ni@Co alloy-coated melamine sponge.Reproduced with permission. [53]Copyright 2019, John Wiley and Sons.d) SEM image of AuNWs@polyHIPEs composite.e) The photograph of this composite-based wearable device on a human forearm.Reproduced with permission. [55]opyright 2020, John Wiley and Sons.SEM of microstructure of macropores f) micropores g) in bimodal-porous AgNW nanostructure.Reproduced with permission. [57]Copyright 2018, American Chemical Society.h) Optical images of soft v-AuNWs sponge.i, j) SEM of v-AuNWs sponge under different magnifications.Reproduced with permission. [58]Copyright 2021, Royal Society of Chemistry.
(CuNWs) aerogels using freeze-drying technology.With the use of poly(vinyl alcohol) (PVA), the mechanical property of the sponge electrode could be enhanced while maintaining a conductivity of 0.83 S cm −1 and a low density of 10 mg cm −3 . [56]imilar technology was used to produce bimodal-porous Ag-NWs nanostructure.As shown in Figure 2f,g, the freeze-drying of microsized ice crystals resulted in micro-and macropores of 3D AgNWs nanostructures.This composite could achieve a conductivity of 42 S cm −1 . [57]ecently, our group reported another solution-based electroless gold nanowires (AuNWs) growth technology, which enabled conformal growth of vertically aligned AuNWs throughout the sponge skeletons. [58]This led to highly conductive gold sponges at the macroscopic scale (Figure 2h), which were soft and mechanically deformable.Different from the previous nanowire systems, the nanowires were vertically aligned on the sponge skeleton (Figure 2i,j).The conductivity of the AuNW sponge could be controlled by adjusting the nanowire length by adjusting the growth time, and we could obtain a high conductivity of 5500 S m −1 , [59] in which nanowire length was ≈2 μm.

Other Materials-Based Sponge Electrodes
Other materials such as liquid metal, [60] conductive polymer, [61] and MXene [62] have also been exploited for fabricating sponge electrodes.28a] Figure 3a shows a liquid metal sponge (LMS) fabricated by mixing Galinstan and sugar with uncured Ecoflex, followed by curing Ecoflex and removing sugar.The corresponding optical microscope image in Figure 3b shows the LM droplets were well distributed in Ecoflex.60b] Another LM sponge with excellent electrical conductivity of 10 4 S cm −1 was fabricated by utilizing a vacuum to absorb LM into sponge pores.60c] Conductive polymers such as poly(3,4ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) (Figure 3d,e) [61a] and polyaniline (PANI) nanowire (Figure 3f) [61b] have been used for sponge electrodes by dip-coating or chemical polymerization methods.In addition, MXene was also .73a] Copyright 2021, Elsevier.c) SEM image of PDMS/PUS/LM composite.60c] Copyright 2019, Elsevier.d) Scheme of the structure of PEDOT:PSS sponge electrode.e) Optical micrograph of the PEDOT:PSS PDMS sponge.61a] Copyright 2022, The Authors, American Chemical Society.f) SEM image of PANI nanowires modified conductive elastic sponge.Reproduced with permission. [109]Copyright 2020, Elsevier.g) SEM image of MXene-sponge and corresponding elemental mapping images of MXene-Sponge for C, N, and Ti.62a] Copyright 2018, Elsevier. 62a]

Multi-Materials-Based Sponge Electrodes
Multiple materials can be integrated into sponges to offer multifunctionalities, which can address the limitations of individual materials, while simultaneously retaining their respective advantageous characteristics.For example, carbon-based materials combine with pseudocapacitive materials, such as MnO 2 , PANI, are usually used to improve capacitance of the sponge supercapacitors [27a,64] (Figure 4a,b).
Graphene typically has limited conductivity and mechanical stretchability due to its large interlayer resistance and fragile lamellar structures. [65]To overcome this, researchers have introduced CNTs [51] and metal nanowires [20,66] into the graphene sponge.As shown in Figure 4c, AgNWs could be uniformly distributed between graphene layers without destroying the overall structural integrity. [67]A rGO/tellurium nanowire composite was demonstrated by the freeze-drying approach (Figure 4d).Electron microscopic characterization revealed wrinkled and interconnected walls (Figure 4e).66b] AgNWs/PEDOT:PSS sponge was successfully fabricated through alternatively dip-coating, in which PEDOT:PSS can provide good conductivity and protect AgNWs from corrosion.The hierarchical sea-urchin-like Cobalt Hydroxide (Co(OH)F) arrays were further formed on this hybrid sponge electrode by a hydrothermal method to achieve high-performance energy-related applications. [68]

Fabrication Methods
The conductive sponge electrodes can be fabricated by utilizing conductive ink.It can be achieved via dip-coating, in situ templating, or freeze-drying.In addition, electroless deposition and sputter coating can also be used without the need for conductive ink.

Dip-Coating
Dip-coating method usually requires 3D porous sponges, such as commercial PU sponge, elastomer sponge (PDMS [69] or Ecoflex [70] ), and solution-based conductive fillers, where conductive fillers could be deposited on the backbones of sponge and ultimately formed conductive sponge electrodes.This approach is characterized by its simplicity and straightforwardness.  [93] Copyright 2011, American Chemical Society.c) SEM of graphene/AgNWs sponge composite.Reproduced with permission. [67]Copyright 2014, American Chemical Society.d) Photograph of 3D rGO/tellurium nanowire.e) Low-and f) high-magnification SEM images of 3D rGO/tellurium nanowire.66b] Copyright 2016, American Chemical Society.
29a] The PU sponge was impregnated with a solution of AgNWs in ethanol.Upon evaporation of the solvent, the sponge underwent a visible color change from white to gray, indicating the successful deposition of the AgNWs onto the sponge skeleton.The conductivity of this AuNWs/sponge could be tuned by controlling the density of AgNWs on the surface of sponge skeletons by changing the concentration of AgNWs or the dip-coating times.This method is general and has been applied to other materials, such as CNTs, [27a,47a-c,64a] carbon black, [50] GO, [51,64c] MXene, [62] metal nanowires, [55] conductive polymer, [61a,71] and hybrid materials. [54,68,72]n a dip-coating process, there are typically two steps.The first one involves immersing the sponge into a mixture of solutions, [72a] and the second requires alternatively dip-coating and drying in conductive ink solutions. [54]The advantages of dip coating technology include the generality suitable most soluble materials, large-scale production, and low-cost fabrication.Nevertheless, dip-coated sponges suffer many limitations.The adhesion between conductive materials and sponge skeletons was poor, which often lead to delamination and low performance in bioelectronics.In addition, the uniformity of conductive film on the 3D porous sponge is hard to control.

In Situ Templating
An alternative method to fabricate porous sponge electrodes is in situ templating. [49,73]This method involves several steps.First, conductive materials are mixed with both elastomeric matrix and sacrificial materials (such as sugar and salt).This is followed by molding and curing.Afterwords, the sacrificial mate-rials can be removed to obtain a conductive sponge.As illustrated in Figure 5b, the eutectic gallium indium (EGaIn) liquid metal was uniformly blended with water-soluble sugar particles and Ecoflex precursors.Subsequently, the mixture was cast into a mold.Following the process of vulcanization and dissolution, a porous sponge electrode was produced.73c] By adjusting the relative amount of conductive materials and templating sugar particles, the properties of sponge electrodes could be tuned.Unlike the sponge electrodes produced by dip-coating, the in situ template strategy typically resulted in the sponge electrode with high structural stability in that the conductive fillers were virtually tightly bonded with elastomeric matrix.

Freeze-Drying
Freeze-drying is another viable technology for the fabrication of sponge electrodes.In a typical process, the conductive components, such as CNTs, [74] graphene oxide, [75] metal nanowires, [56][57]76] are dissolved in a solvent to form a suspension. In sme cases, the additives were added to enable the formation of a hydrogel.The resulting suspension or hydrogel was then undergone a freeze-drying process, leading to the formation of a 3D sponge-like conductive composite.Figure 5c depicts a 3D bimodal porous AgNWs composite that is fabricated by introducing macro-sized ice spheres into the AgNWs solution.Subsequent freeze-drying of the mixture led to the removal of the ice spheres, resulting in the formation of additional macropores.This strategy led to the 3D porous AgNWs composite with high electrical conductivities.[57] The spongy aerogel electrodes by the freeze-drying of conductive filler solution generally have poor mechanical stability.Reproduced with permission.[29a] Copyright 2013, John Wiley and Sons.b) Scheme of in situ templating method to fabricate liquid metal sponge.Reproduced with permission.[73c] Copyright 2020, John Wiley and Sons.c) Scheme of 3D bimodal-porous AgNW/PDMS nanocomposites based on freeze-drying.Reproduced with permission.[57] Copyright 2018, American Chemical Society.d) Schematic illustration of freeze-drying-based synthesis process of CuNW/PVA composite aerogel.Reproduced with permission.[56] Copyright 2014, American Chemical Society.
To address this limitation, our group reported a CuNW aerogel with enhanced mechanical toughness and elasticity while maintaining conductivity by introducing a trace amount of PVA (Figure 5d).In the presence of PVA, the composite could return to its original height after 60% compressive strain.Without PVA, the CuNW aerogel demonstrated little shape recovery. [56]Moreover, this PVA/CuNWs aerogel showed high durability at >10 000 loading-unloading cycles.In another report, CNT/graphene aerogels with high elasticity wad demonstrated. [77]It claimed that CNTs could prevent sliding between graphene sheets, leading to enhanced stiffness.Thereby, the graphene/CNT aerogels could fully recover without fracture even after 90% compression.
There are a number of attributes for the freeze-drying technology.In principle, the attainment of a desired pore structure and size could be facilitated by modifying several freeze-drying process variables, such as solvent type, freezing rate and direction, and drying conditions. [78]However, the freeze-drying technique requires specialized equipment and specific conditions, rendering it comparatively complex and expensive compared to other methods.

Electroless Deposition
In the absence of conductive inks, the sponge electrodes may be fabricated via a number of approaches.Electroless deposition is one of them.Figure 6a depicts a typical polymer-assisted electroless metal deposition process, including a radical polymerization process and an electroless deposition process. [52,79]riefly, vinyltrimethoxysilane (VTMS) was modified on the air plasma-treated PDMS sponge through a silanization process.Followed by an in situ radical polymerization process, the VTMS-PDMS sponge was subsequently covalently linked to poly(2-(methacryloyloxy)-ethyl-trimethylammonium chloride).Finally, through the ion exchange and electroless metal deposition processes, a layer of metal was coated on the sponge skeleton. [52]Various metal sponges can be fabricated by this method, including Cu-, Ag-, Ag/Cu-PDMS sponges.
As shown in Figure 6b, the polymeric sponge was firstly modified with amine-group by salinization reactions.Then gold seeds were attached to the sponge skeletons via electrostatic attraction forces.Finally, the nanoseed-modified sponges were submerged in a growth solution containing gold precursors (gold(III) chloride trihydrate), surfactants (4-mercaptobenzoic acid), and a reducing agent (l-ascorbic acid).This led to spontaneous growth of enokitake-like vertically aligned gold nanowires (v-AuNWs) perpendicular to the sponge skeleton surfaces. [58]These v-AuNWs were conformally and uniformly coated throughout sponges with good adhesion.
The electroless plating was also used to successfully produce the Cu sponge electrode. [81]In this work, the PU sponge was first activated by SnCl 2 and PdCl 2 , then immersed into CuSO 4 solution for Cu coating.The Cu coating uniformly covered the skeleton of the PU sponge.The as-prepared Cu sponge could further work as electrode to electroplate another layer of Zn for battery applications.
Recently, the fluoride-assisted in situ synthesis of metal nanoparticles was successfully applied to 3D porous PDMS Reproduced with permission. [52]Copyright 2016, John Wiley and Sons.b) Scheme of solution-based electroless vertical gold nanowires coating technology.Reproduced with permission. [58]Copyright 2021, Royal Society of Chemistry.c) Schematic illustration of the manufacture Au@PU sponge by gold ion sputtering.Reproduced with permission. [84]Copyright 2017, American Chemical Society.
surfaces. [82]By emerging PDMS foam into the ethanolic AgF solution for various incubation times, the amount of AgNPs formed throughout the foam could be controlled.This one-step room temperature deposition of AgNPs on the foam had tunable surface coverage from 0 to 75%, exhibiting tunable piezo-resistive properties.This method can be further extended to 4D printing of the PDMS lenses. [83]

Sputter Coating
Sputter coating is a process that involves the use of a sputtering source and a target substrate in a vacuum chamber.This process allows for precise control of thickness and composition of desired coating materials, making it a versatile technique widely used in optoelectronics.It has been found that the sputtering technique can be utilized to deposit metallic materials onto/into the sponge.Figure 6c shows a gold sponge produced through ion sputtering methodology. [84]In a deposition chamber, the PU sponge was positioned on a positive electrode substrate.The negative gold slice electrode could generate Au atoms through positive ion (Ar+, from argon gas ionization) bombardment.Ultimately, these Au atoms coalesced to form a thin layer of gold film that completely covered the sponge skeletons.Although this gold sponge demonstrated good conductivity, the gold film had relatively poor adhesion with PU sponges leading to crack formation/delamination upon mechanical deformation.
We summarize the advantages and disadvantages of abovementioned fabrication strategies in Table 1, considering the factors including as cost, scalability, adhesion, and performance.While there isn't a one-for-all strategy for fabricating sponge electrodes, it may aid in the selection of the most suitable strategy for specific applications.(Scheme 1)

Soft Bioelectronic Devices Based on 3D Sponge Electrodes
As shown in Scheme 2, 3D soft sponge electrodes have demonstrated versatile applications in the emerging field of soft bioelectronics, including stretchable conductors, sensors, energy devices, and integrated soft devices.Scheme 1. Development of 3D sponge electrodes in soft electronics from 2010 to 2022.From top left to bottom right in sequence is a graphene foam, Reproduced with permission. [114]Copyright 2011, Springer Nature.29b] Copyright 2013, John Wiley and Sons.a gold sponge, Reproduced with permission. [84]Copyright 2017, American Chemical Society.73c] Copyright 2020, John Wiley and Sons.and a vertical gold nanowires sponge, Reproduced with permission. [58]Copyright 2021, Royal Society of Chemistry.A CNT/MnO 2 sponge-based supercapacitor, Reproduced with permission., [93] Copyright 2011, American Chemical Society.29a] Copyright 2013, John Wiley and Sons.a CNTs sponge-based stretchable battery, Reproduced with permission. [100]Copyright 2016, John Wiley and Sons.a LEG on-skin multiplex sensory system, Reproduced with permission. [42]Copyright 2018, John Wiley and Sons.a gold foam-based real-time ECG monitoring system, Reproduced with permission. [80]Copyright 2022, Elsevier.

Stretchable Conductors
7a,85] Stretchable conductors can serve as interconnects among active components such as sensors and energy devices.The challenge is that conventional metallic conductors are rigid and non-stretchable in spite of their high intrinsic conductivity.1b] 3D sponge is advantageous for constructing stretchable conductors in that it has a built-in junction-free, interconnected porosity structure, [29a,b,52,57,60c] which can serve as a skeleton to load conductive materials and to distribute stain, resulting in comparatively stable electrical performance under deformations.
As illustrated in Figure 7a, when a certain tensile strain (ɛ) is applied to the 3D AgNWs sponge/PDMS, the shape deformation of sponge micro-network could make the strain on Ag-NWs network on the sponge skeleton is <ɛ.29a] In contrast, 2D AgNWs film suffered higher stretching at the same level of extension in comparison to 3D AgNWs sponge/PDMS composite.Figure 7b is the corresponding normalized resistance increase (∆R/R 0 ) of these stretchable electrodes under strain.Specifically, the ∆R/R 0 is 160% for 3D AgNWs sponge/PDMS at 100% strain; the corresponding value for the 2D AgNWs-PDMS film is 1190% at the same strain level.It was reported that the AgNW/PDMS composite exhibited a conductivity of 42.36 S cm −1 and a stretchability of 138% (Figure 7c).And the relative resistance change (R/R 0 ) was 1.08 at 40% strain and 1.64 at 120% strain. [57]The sponge-based stretchable conductors could serve as interconnects to integrate LEDs into circuits as shown in Figure 7d.60a] The sponge has a porous structure that allowed the liquid metal to move and distorted freely under different mechanical deformations while maintaining electrical interconnectivity.As a result, the sponge exhibits excellent and consistent mechatronic performance.The researchers embedded an array of LED lamps into the sponge, which enabled the preparation of stretchable LED circuits.The circuits demonstrated stable performance under various strains (as depicted in Figure 7e).
In another study, an Au-Ni graphene sponge composite was utilized as stretchable interconnects to charge a smartphone under stretching, as shown in Figure 7f.The smartphone charged normally when the flexible conductor was at its original length.Then, as the strain increased to 30%, the charging state remained steady with no variations, indicating the stable performance of the flexible conductor as a flexible and stretchable charging interconnect for smart devices. [86]

Wearable Sensors
3D conductive sponges can also be designed to fabricate wearable sensors including physical sensors (pressure, strain, and temperature) and electrophysiological sensors (ECG and EMG).

Pressure Sensors
A wearable pressure sensor is a type of sensor that can convert mechanical pressure associated with human activities into an electrical signal that can be measured and analyzed.Soft wearable pressure sensors can offer the benefits such as comfortability to wear, non-invasive and continuous real-time capturing of biometric data.To date, soft wearable pressure sensors typically rely on one of three sensing mechanisms: resistance, capacitance, and piezoelectricity. [87]onductive porous sponges are excellent candidates for piezoresistive pressure sensors. [51,71]Figure 8a illustrates a wearable foot insole sensor with CNT/PDMS sponge-based flexible pressure sensors integrated.These sensors could function in a wide pressure range of 10 Pa to 1.2 MPa, with a sensitivity of 0.01 -0.02 kPa -1 .Figure 8b presents the dynamic pressure fluctuations recorded at each location over five consecutive walking steps.47a] By controlling the structural properties of AgNPs deposited foam, a pressure sensor with a large dynamic range (up to 120 KPa) was demonstrated to achieve a sensitivity of 0.4 kPa -1 .They could operate for a long duration (41500 cycles) and successfully detected the radial artery pulse wave, allowing for the extraction of clinically relevant parameters. [82]Additional examples of sponge-based pressure sensors include a 3D CNTs/AgNPs sponge, [72a] MXene sponge, [62b] and Fe 2 O 3 /C@SnO 2 sponge [88] with a high sensitivity of several kPa −1 in a broad sensing range.
Microstructural designs could be applied to conductive sponges in order to achieve high sensitivity, and detect both small and large deformations. [50]By applying a compression force to a conductive sponge, dense and fractured microstructures were generated (Figure 8c).When an external pressure was applied, the sponge backbones came into contact, leading to an expansion of the ohmic contact area.Upon the release of pressure, the compressed sponge skeleton recovered, resulting in a decrease in conductivity.Compared to the original sponge, the pressure sensitivity of the sponge with a fractured microstructure was two orders of magnitude higher in the 0 -2 kPa pressure range and one order of magnitude higher in the 2-10 kPa pressure range.30b] In a similar report, a channel crack-based gold sponge was fabricated. [84]This gold sponge sensor achieved a sensitivity of 59 -122 Pa −1 , a detection limit of 0.568 Pa, and detection of both tiny and large human motions.
Sponge-based capacitive sensors can offer a larger capacitance change range than their non-porous dielectric counterparts, hence, giving rise to better sensitivity to mechanical deformation. [89]89b] A CaCu 3 Ti 4 O 12 (CCTO) nanocrystals-PDMS sponge with a huge dielectric constant () was developed to serve as the dielectric layer for a capacitance sensor.This sensor had a 1.66 kPa −1 sensitivity in the range of 0-640 Pa, which was a much higher PDMS sponge-based capacitance sensor. [90]In another report, CNT/PDMS sponge-based capacitive sensor had a , and other similar composites reported in the literature.29a] Copyright 2013, John Wiley and Sons.c) Electrical conductivity of AgNW/PDMS composites under strain.Reproduced with permission. [57]Copyright 2018, American Chemical Society.d) Schematic diagram of the circuits with four LEDs based on CuAg/PU sponge-PDMS ribbons (top), optical images of the as-made LED arrays in twisting, folding, rolling, and stretching states (bottom).29b] Copyright 2013, John Wiley and Sons.e) I-V curves of the liquid metal sponge-based LED circuit at different tensile strains.60a] Copyright 2017, Royal Society of Chemistry.f) Optical images of the Au-Ni@GPUS flexible conductor as a charging interconnect under 0% and 30% strain.Reproduced with permission. [86]Copyright 2018, Royal Society of Chemistry.73b] Figure 8e illustrates a PEDOT:PSS/PDMS sponge-based capacitive sensor.Without pressure, the fringing capacitance was governed by the capacitance of the PDMS sponge and the air inside the pores.When the sponge sensor was under pressing, the air was released, resulting in a larger portion of the volume being made up of PDMS, which has a much higher permittivity than air, leading to an increase in capacitance of the device.The sponge-based fringe sensor could be attached to the fingers to sense the pressure when fingertips grabbed an object (Figure 8f).61c]

Wearable Electrophysiological Sensors
Electrophysiological sensors typically include cardiac electrocardiograph (ECG)), brain electroencephalography (EEG)), and muscle Electromyography (EMG).Unlike the corresponding planar electrodes, 3D sponge electrodes possess porous structures, which enables a more effective contact area with skin, thereby offering low contact impedance and a high signal-to-noise ratio.Figure 9a illustrates the use of PEDOT:PSS sponge electrodes to record ECG signal and EMG signal from both skeletal muscle cells (biceps contractions) and smooth muscle cells (uterine contractions).In addition, this porous sponge electrode could work stably under human motion (Figure 9b).Under motion stage, data from both planar electrodes and commercial Dynamic changes of the pressures measured at each position during five steps of walking.47a] Copyright 2019, American Chemical Society.c) Contact area variation of RGO-PU-HT-P sponge under compressing and releasing.d) The photograph of a tripod on the surface of as-prepared artificial skin to test the pressure-sensing capability.The mapping profile of the pixel signals generated by the tripod.30b] Copyright 2013, John Wiley and Sons.e) Schematic illustration of the porous PEDOT:PSS/PDMS sponge sensor.f) Porous PEDOT:PSS/PDMS sensors were attached to the finger joints of an artificial hand for gesture detection application.61c] Copyright 2022, American Chemical Society.
electrodes showed strong voltage spikes.61a] In another demonstration, we demonstrated gold nanowire sponges could serve as excellent wearable bioelectrodes for collecting ECG waveforms in various dynamic human motion conditions. [80]Compared with traditional wet bioelectrodes, this gold nanowire sponge electrode was dry and deformationinsensitive, rendering it highly suitable for long-term wearable biomonitoring purposes.Notably, this electrode consistently provided stable ECG data across a range of activities, in contrast to gel electrodes that exhibited significant signal distortion during walking (refer to Figure 9c).Furthermore, our electrode is seamlessly integrated with a flexible printed circuit board in a user-friendly bandage-like configuration, enabling convenient on-body monitoring.This wearable system facilitated continuous and non-invasive recording of pertinent signals, which were subsequently utilized for comprehensive analysis of key cardiovascular information.61a] Copyright 2022, The Authors, American Chemical Society.c) The comparison of the sponge electrode and Gel electrode as ECG electrodes under different activities.d) Schematic of the nanowire-based wearable system.Reproduced with permission. [80]Copyright 2022, Elsevier.

Other Wearable Sensors
The sponge electrodes can also be used to fabricate wearable sensors for detecting tensile strain, temperature, and hydration levels.There is an example of the MWCNT/PDMS sponge-based tension strain sensor. [49]The ΔR/R increased along with the increasing tension strain from 0 to 50%.61c] The thermal energy can overcome the potential barrier and facilitate electron hopping between the adjacent grains.This resulted in a decrease in resistance of PEDOT:PSS/PDMS sponge.

Energy Devices
3D conductive sponge electrodes can also be utilized for fabricating energy devices including supercapacitors, batteries, and fuel cells.They offer several attributes. [91]First, the presence of pores in the sponge structure enables facile access of the electrolyte to the electrode surface.Secondly, the continuous conductive network and interconnected hierarchical porosity of the sponge structure reduce path lengths, facilitating efficient charge transfer.Thirdly, the large surface area of the conductive sponge allows for high loading of active materials.

Supercapacitors
Supercapacitors are important energy storage devices, which feature rapid charge-discharge rates, long cycling lives, and ease of construction and integration. [92]A dip-coating CNT sponge was modified with electrodeposited MnO 2 nanoparticles to fabricate a supercapacitor with a specific capacitance of 1230 F g −1 , and a specific power and energy density of 63 kW/kg and 31 Wh/kg, respectively. [93]The capacitances of a PANI/graphene sponge electrode and that of a PANI/graphene film electrode were compared. [94]The interconnected macropores of 3D sponge architecture provided efficient ion and electron transfer in 3D space; whereas, ion migration was confined in a 2D plane for the planer PANI/graphene electrode.64c,95] The utilization of polymeric sponges as elastomeric substrates renders it possible to fabricate flexible, compressible, and even stretchable supercapacitors.47d] Copyright 2015, John Wiley and Sons.c) CV curves of the flexible solid-state supercapacitor device at different bending angles.The background image shows a digital photograph of the flexible Co(OH)F/PEDOT:PSS/AgNW/PUS supercapacitor device.Reproduced with permission. [68]Copyright 2017, John Wiley and Sons.d) CV curves of fully soft v-AuNWs sponge-based supercapacitors under stretching, the insert optical image is v-AuNWs sponge-based supercapacitor under stretching.Reproduced with permission. [58]Copyright 2021, Royal Society of Chemistry.
sponges were assembled into a supercapacitor unit and connected in series.47d] In another report, an all-solid-state Co(OH)F/PEDOT:PSS/AgNW sponge-based supercapacitor exhibited stable performance under an even 120°bending angle with a specific capacitance of 103.7 F g −1 (Figure 10c). [97]Our group has recently reported a gold nanowires sponge, which could be fabricated as fully deformable supercapacitors, such as 50% compression strain, 180°b ending and twisting, and even 66% tensile strain as shown in Figure 10d. [58]Note that the capacitance increased by ≈47.8% when a 66% strain was applied.This could be attributed to the unzipping of gold nanowire arrays, offering higher electroactive surface area exposed to electrolytes under stretched states.

Batteries
Batteries are mainstream energy devices that can store electrical power through redox reactions, offering high energy density, high output voltage, and long-term stability. [98]Current collectors in traditional batteries are made of rigid metals, which are not ideal for on-body wearable applications. [99]With the develop-ment of soft and stretchable electrodes, there are soft and flexible batteries that could sustain deformations such as bending, twisting, and stretching.3D conductive sponge electrodes are favorable candidates for designing flexible batteries.
Figure 11a illustrates the comparison of planar metal foilbased conventional electrodes and the 3D sponge-based stretchable electrode.The traditional plane electrode is unsuitable for stretching due to significant cracks and material delamination during stretching.In contrast, the 3D sponge electrode could bear strain without breaking due to its unique 3D interconnected porous structures.Figure 11b demonstrates 91% and 82% of the capacitance after 500 stretch cycles for the sponge electrodes based Li 4 Ti 5 O 12 (LTO) anode and the LiFePO 4 (LFP) cathode, respectively. [100] sodium-ion full battery has been reported, which was composed of a rGO/PDMS sponge/VOPO 4 cathode, and a rGO/PDMS sponge/hard carbon anode.It was stretchable with a high reversible capacity of 103 mA h g −1 without any strain, and it could be maintained at 96 mA h g −1 and 92 mA h g −1 under 20% and 50% strain, respectively.In addition, it retained 89% of specific capacity after 100 cycles at the 50% strain. [101]A stretchable Zn-MnO 2 battery based on 3D AgNWs sponges demonstrated good conductivity and stretchability.The fabricated device had a 3.6 mA h cm −2 area capacity and could maintain 89% of its initial Reproduced with permission. [100]Copyright 2016, John Wiley and Sons.Photographs of c) the fence-shaped and d) the serpentine-shaped flexible and stretchable ZABs based on Zn sponge before and after stretching.e) Discharge and charge polarization curves of the fence-shaped ZAB under different strains.Reproduced with permission. [81]Copyright 2020, American Chemical Society.capacity even under 100% tensile strain.Under simultaneous stretching and twisting, the batteries could power a red LED light when they were connected in series. [54]Figure 11c,d shows a Kirigami-based design to fabricate fully deformable PU sponge film-based zinc−air batteries (ZABs). [81]A 100% stretchable fence-shaped ZAB and a 160% stretchable serpentine-shaped ZAB were demonstrated.It demonstrated a relatively stable battery performance under 100% strain (Figure 11e).

Fuel Cells
Fuel cell is another potential energy supply for soft electronics. [102]It has been reported a skin-like stretchable fuel cell based on strain-insensitive ultrathin gold nanowires polymer sponge electrode (AuNWs@polyHIPEs).Specifically, AuNWs@polyHIPEs worked as anode, the platinum (Pt)modified AuNWs@poly-HIPEs was the cathode, and ethanol was the fuel.As shown in Figure 12a, with the strain increased to 40%, the performance of this fuel cell only slightly declined, including power density, current density, and open-circuit voltage.The device exhibited stable performance under twisting, stretching, and compression (Figure 12b).The 3D porous electrode could function simultaneously as a fuel host, an electrocatalyst, and a current collector in the fuel cell system. [55]here are also porous sponge scaffolds-based biofuel cells reported with high power density [103] or high energy return on investment. [104]In this work, the CNT buckypaper electrodes were utilized to immobilize lactate oxidase and bilirubin oxidase for a flexible lactate/O 2 biofuel cell, achieving a power density of 520 μW cm −2 . [105]This may be attributed to the large specific surface areas, high electrical conductivity, and flexibility of buckypaper-based bioelectrodes. [106]

Triboelectric Nanogenerators
The triboelectric nanogenerators (TENG) are potential selfpowered energy devices for wearable electronics since they can convert human-associated mechanical energy into electrical energy via electrostatic induction. [107]It has been reported that the porous elastic sponge could work as a triboelectric layer for TENG. [108]It has been demonstrated that the sponge electrodes enhanced the electrical output performance due to an increased effective contact area.
61b,73a,109] Figure 12c shows the schematic of PANI nanowires sponge-based TENG.This system was able to efficiently transform irregular and random mechanical energy into electricity. [109]The conductive sponge could sustain 60% elastic deformation owing to its 3D porous structures.When the compressive deformation increased from 2% to 60%, the corresponding output voltage rose from 142 V to 540 V (Figure 12d), and the current rose from 1.4 A to 6.3 A. By increasing the motion frequency, it was possible to produce an even larger output voltage and current due to the faster contact and separation speeds between the PTFE and polyaniline triboelectric layers.This conductive sponge based-TENG could be  [55] Copyright 2020, John Wiley and Sons.(c) Scheme of PANI sponge-based TENG.d) The output voltage of PANI sponge based-TENG in rotation motion with compression deformation from 2% to 60%.e) Schematic illustration of PANI sponge based-TENG for harvesting irregular and random energy on various flexible object surfaces.Reproduced with permission. [109]Copyright 2020, Elsevier.
utilized on various flexible object surfaces and captured inconsistent and random mechanical energy from daily life to power the LEDs (Figure 12e).

Soft Integrated Devices
An ideal individual soft device should combine energy devices, sensors, signal processing units, and wireless communication units together to form a wearable system that is comfortable and able to offer real-time closed-loop responses. [110]Researchers have attempted to achieve integrated soft devices based on sponge electrodes.An ideal wearable system may be self-powered without the need for battery and recharging, [111] for which soft sponge electrodes are promising components.47c] In order to power the pressure sensor, several supercapacitors were interconnected in serial as illustrated in Figure 13a.The wearable system could distinguish distinct resistance responses to muscular motions.The words such as "Hi," "Hello," and "Bye," led to specific resistive responses with characteristic line profiles (Figure 13b).Additionally, this device could keep track of human motions including jogging, running, and walking.
79c] The hybrid device consisted of a TENG and a supercapacitor.The TENG converted mechanical energy into electricity, which could charge the supercapacitor as the energy storage device.As shown in Figure 13d, the charging rate of the SCs increased with the increase in frequency.The charged SCs could light a LED.
A laser-induced porous graphene on an elastomer sponge substrate-based gas-permeable, multifunctional on-skin electronics was reported (Figure 13e). [42]The patterned laser-induced porous graphene-based electrodes were transferred onto a porous elastomer substrate for a gas-permeable bioelectronic system.The system included four typical bioelectronic sensors, electrophysiological sensors, hydration sensors, temperature sensors, and joule-heating elements.In particular, the prototype devices demonstrated excellent performance in measuring the pertinent physiological information from the human body (Figure 13f).

Conclusion and Future Perspectives
This review summarizes recent advancements in 3D sponge electrode-based soft bioelectronics.It covers the discussions related to the advantages of 3D sponge electrodes, their design and fabrication, and various applications.In the context of big data, patient-centered healthcare, and soft robotics, the interest in soft sponge-based wearable bioelectronics may continue, given their material attributes of high surface area, softness, robustness, and durability.
47c,79c] To date, there are only a few reports on fully Copyright 2019, Elsevier.e) Optical images of the LIG-based on-skin bioelectronic sensing systems.f) Example EMG record from this LEG sponge sensor.Reproduced with permission., [42] Copyright 2018, John Wiley and Sons.integrated sponge-based wearable systems. [80]Rigorous evaluation and validation of such wearable systems is a must prior to their real-world for real-time health monitoring anytime and anywhere. [112]n spite of encouraging progress made to date, there are a number of challenges yet to be overcome for wearable bioelectronic devices, [112] which is also the case for sponge-based systems.First, the precise and controllable patterning and encapsulation of sponge electrodes constitute the intrinsic limitation in the fabrication of soft devices.The soft yet porous nature of the sponges renders it challenging to be compatible with conventional top-down or bottom-up fabrication technologies.The capillary phenomena of porous sponge [113] often leads to poor control over the pattern sizes.Techniques like sputtering offer precise control over shape and dimensions but the resulting structures can be vulnerable to mechanical deformation, which may compromise the desired electrode morphology. [84]The encapsulation is usually required to protect the device from degradation, however, often difficult to achieve for sponge electrodes due to the sacrifice of their functionalities and/or properties.In addition, it is also challenging to establish reliable electrical connections between soft sponge electrodes and other components (soft or rigid).The 3D porous nature and irregular surface of sponge electrode can hinder the formation of consistent and low-resistance electrical connections with other components or conductive traces.Soft and 3D electrical connections are yet to be developed.The scalability of sponge electrode fabrication is also challenging.While the chemical process to produce sponge electrodes may be scalable in principle, it is not straightforward to realize consistent quality and performance at a large scale.To overcome these challenges, multidisciplinary collaboration across disciplinary boundaries is imperative.

Figure 2 .
Figure 2. Metal-based sponge electrodes.a) Optical image of large-scale CuAg/PU sponge/PDMS, b) SEM image of CuAg/PU sponge, the inset shows the cross-section of CuAg film on PU sponge.Reproduced with permission.[29b]Copyright 2013, John Wiley and Sons.c) SEM and elemental mapping of nickel and cobalt of Ni@Co alloy-coated melamine sponge.Reproduced with permission.[53]Copyright 2019, John Wiley and Sons.d) SEM image of AuNWs@polyHIPEs composite.e) The photograph of this composite-based wearable device on a human forearm.Reproduced with permission.[55]Copyright 2020, John Wiley and Sons.SEM of microstructure of macropores f) micropores g) in bimodal-porous AgNW nanostructure.Reproduced with permission.[57]Copyright 2018, American Chemical Society.h) Optical images of soft v-AuNWs sponge.i, j) SEM of v-AuNWs sponge under different magnifications.Reproduced with permission.[58]Copyright 2021, Royal Society of Chemistry.

Figure 5 .
Figure 5. Solution conductive filler-based fabrication of conductive sponge electrodes.a) Scheme of dip-coating strategy for AgNWs sponge fabrication.Reproduced with permission.[29a]Copyright 2013, John Wiley and Sons.b) Scheme of in situ templating method to fabricate liquid metal sponge.Reproduced with permission.[73c]Copyright 2020, John Wiley and Sons.c) Scheme of 3D bimodal-porous AgNW/PDMS nanocomposites based on freeze-drying.Reproduced with permission.[57]Copyright 2018, American Chemical Society.d) Schematic illustration of freeze-drying-based synthesis process of CuNW/PVA composite aerogel.Reproduced with permission.[56]Copyright 2014, American Chemical Society.

Figure 6 .
Figure 6.No prepared-conductive filler-based fabrication of conductive sponge electrodes.a) Scheme of polymer-assisted electroless metal deposition.Reproduced with permission.[52]Copyright 2016, John Wiley and Sons.b) Scheme of solution-based electroless vertical gold nanowires coating technology.Reproduced with permission.[58]Copyright 2021, Royal Society of Chemistry.c) Schematic illustration of the manufacture Au@PU sponge by gold ion sputtering.Reproduced with permission.[84]Copyright 2017, American Chemical Society.

Figure 7 .
Figure 7. Sponge electrode-based stretchable conductors.a) Illustration of stretching effect on the AgNW-sponge/PDMS micro-network and the AgNW nano-network (expansions) of the composite under tensile strain.b)Variation of normalized resistance (∆R/R 0 ) as a function of tensile strain on PUS-RGO-PDMS (black line), AgNW-PDMS film (red line), PUS-AgNW-PDMS (pink line), and other similar composites reported in the literature.Reproduced with permission.[29a]Copyright 2013, John Wiley and Sons.c) Electrical conductivity of AgNW/PDMS composites under strain.Reproduced with permission.[57]Copyright 2018, American Chemical Society.d) Schematic diagram of the circuits with four LEDs based on CuAg/PU sponge-PDMS ribbons (top), optical images of the as-made LED arrays in twisting, folding, rolling, and stretching states (bottom).Reproduced with permission.[29b]Copyright 2013, John Wiley and Sons.e) I-V curves of the liquid metal sponge-based LED circuit at different tensile strains.Reproduced with permission.[60a]Copyright 2017, Royal Society of Chemistry.f) Optical images of the Au-Ni@GPUS flexible conductor as a charging interconnect under 0% and 30% strain.Reproduced with permission.[86]Copyright 2018, Royal Society of Chemistry.

Figure 8 .
Figure 8. Sponge electrode-based pressure sensors.a) Schematic illustration of CNT/PDMS sponge-based flexible pressure sensors for foot insole.b)Dynamic changes of the pressures measured at each position during five steps of walking.Reproduced with permission.[47a]Copyright 2019, American Chemical Society.c) Contact area variation of RGO-PU-HT-P sponge under compressing and releasing.d) The photograph of a tripod on the surface of as-prepared artificial skin to test the pressure-sensing capability.The mapping profile of the pixel signals generated by the tripod.Reproduced with permission.[30b]Copyright 2013, John Wiley and Sons.e) Schematic illustration of the porous PEDOT:PSS/PDMS sponge sensor.f) Porous PEDOT:PSS/PDMS sensors were attached to the finger joints of an artificial hand for gesture detection application.Reproduced with permission.[61c]Copyright 2022, American Chemical Society.

Figure 9 .
Figure 9. Sponge electrode-based electrophysiological sensors.a) Schematics illustrating the use of the PEDOT:PSS/PDMS sponge electrode for ECG and EMG recording applications.b) ECG signals measured under the presence of motion artifacts caused by periodic body movement.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[61a]Copyright 2022, The Authors, American Chemical Society.c) The comparison of the sponge electrode and Gel electrode as ECG electrodes under different activities.d) Schematic of the nanowire-based wearable system.Reproduced with permission.[80]Copyright 2022, Elsevier.

Figure 10 .
Figure 10.Sponge electrode-based supercapacitors.a) Real-time optical images of the resultant four supercapacitor groups showing the compressing and recovering process.b) The galvanostatic charge-discharge curves of the four-supercapacitor group and a single supercapacitor at 0.4 A g −1 .Reproduced with permission.[47d]Copyright 2015, John Wiley and Sons.c) CV curves of the flexible solid-state supercapacitor device at different bending angles.The background image shows a digital photograph of the flexible Co(OH)F/PEDOT:PSS/AgNW/PUS supercapacitor device.Reproduced with permission.[68]Copyright 2017, John Wiley and Sons.d) CV curves of fully soft v-AuNWs sponge-based supercapacitors under stretching, the insert optical image is v-AuNWs sponge-based supercapacitor under stretching.Reproduced with permission.[58]Copyright 2021, Royal Society of Chemistry.

Figure 11 .
Figure 11.Sponge electrode-based batteries.a) Schematic illustration for the comparison of the conventional electrode using metal foil and the stretchable electrode based on the PDMS sponge.b) Discharge-charge voltage profiles of the stretchable LFP cathode with various stretch-release cycles.Reproduced with permission.[100]Copyright 2016, John Wiley and Sons.Photographs of c) the fence-shaped and d) the serpentine-shaped flexible and stretchable ZABs based on Zn sponge before and after stretching.e) Discharge and charge polarization curves of the fence-shaped ZAB under different strains.Reproduced with permission.[81]Copyright 2020, American Chemical Society.

Figure 12 .
Figure 12.Sponge electrode-based other energy devices.a) The power curves of the stretchable AuNWs-based fuel cell at different strain states from 0% to 40%.b) Performance of the skin-like fuel cell attached to skin under various mechanical deformations (inset: corresponding photographic images under mechanical deformation.Reproduced with permission.[55]Copyright 2020, John Wiley and Sons.(c) Scheme of PANI sponge-based TENG.d) The output voltage of PANI sponge based-TENG in rotation motion with compression deformation from 2% to 60%.e) Schematic illustration of PANI sponge based-TENG for harvesting irregular and random energy on various flexible object surfaces.Reproduced with permission.[109]Copyright 2020, Elsevier.

Figure 13 .
Figure 13.Sponge electrode-based integrated soft devices.a) Photograph of the compressible system with a CNT sponge-based pressure sensor attached to the neck and CNT sponge-based SCs placed onto the coat pocket to monitor human motions.Inset photograph shows the circuit diagram of the resistance response recording.b) Recorded resistance response versus time during the pronunciation of polite expressions.Reproduced with permission. [47c] Copyright 2017, John Wiley and Sons.c) The diagram of the compressible hybrid energy collection and storage device.d) Charging curve of the supercapacitor charged by TENGs at various frequencies.The inset is the diagram of electrical circuit.Reproduced with permission. [79c]Copyright 2019, Elsevier.e) Optical images of the LIG-based on-skin bioelectronic sensing systems.f) Example EMG record from this LEG sponge sensor.Reproduced with permission.,[42]Copyright 2018, John Wiley and Sons.

Table 1 .
Summary of pros and cons of the synthesis strategies for sponge electrodes.
• Non-conformal coating • High equipment costs Scheme 2. Application of 3D sponge electrodes in soft electronic areas.