Advances in conductive filler-integrated hydrogels and derived aerogels: innovative strategies for electromagnetic interference shielding

Abstract

As wireless communication networks move into higher frequency bands and electronic devices evolve towards higher integration and wearability, the resulting electromagnetic interference (EMI) and pollution have become increasingly severe. Consequently, the development of advanced EMI shielding materials capable of ultra-broadband compatibility, multifunctional and wearability has emerged as an essential and urgent task. Conductive filler-integrated hydrogels (CFHs) demonstrate remarkable application prospects as an innovative multifunctional EMI shielding material, due to their tunable electrical conductivity, excellent flexibility, self-healing ability, and environmental and biological friendliness. This review commences by delving into the EMI shielding mechanisms of CFHs, exploring the crucial factors that affect their shielding performance, including water molecules, porous structures, and the type of conductive fillers. Subsequently, the fabrication methods and comprehensive performances of CFHs incorporating various conductive fillers, as well as different aerogels derived from CFHs, are systematically summarized. Finally, the potential challenges hindering the practical application of CFHs and their derived aerogels are discussed, and perspectives on how to overcome these challenges in future research are provided, thereby offering insightful guidance for the design of novel CFHs and aerogels for efficient and multifunctional EMI shielding.

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Wider impact

The rapid evolution of wireless communication and wearable electronics has triggered an urgent global demand for advanced electromagnetic interference (EMI) shielding materials with ultra-broadband compatibility, multifunctionality, and wearability. Conductive filler-integrated hydrogels (CFHs) have emerged as a promising EMI shielding material due to their exceptional properties, including tunable electrical conductivity, flexibility, self-healing, and environmental sustainability. By exploring the EMI shielding mechanisms of CFHs and their derived aerogels, summarizing their fabrication methods and comprehensive performance with various conductive fillers, and discussing the challenges and perspectives, this review provides a comprehensive understanding of CFH based EMI shielding materials and their potential applications. The insights presented in this review will be valuable for guiding future research toward the development of innovative materials, with broad application prospects for next generation communication devices and wearable electronics, and contributing to the broader goal of reducing EMI and electromagnetic pollution in an increasingly connected world.

1. Introduction

The rapid evolution of wireless communication technologies, especially the transition from 5G to 6G and the exploration of gigahertz (GHz) and terahertz (THz) frequency ranges, has given rise to increasingly severe electromagnetic interference (EMI) and pollution issues.^1,2 6G, in particular, operates at frequencies exceeding 100 GHz, enabling remarkable breakthroughs such as data rates of up to 1 Tbps and ultra-low latencies of less than 0.1 ms.³ While these advancements significantly boost communication efficiency, they also pose unprecedented challenges for EMI shielding materials. At GHz and THz bands, electromagnetic signals are extremely susceptible to interference, which can result in information leakage, transmission failures, and degraded performance, thereby threatening network security, reliability, and privacy.^4–7 To address these issues, EMI shielding materials must possess specific properties. High-frequency compatibility is crucial to effectively attenuate EMWs in these bands. Moreover, with the trend towards miniaturization of electronic devices, the materials need to be ultra-thin yet flexible enough for seamless integration. Additionally, broadband absorption capabilities are required to mitigate multi-frequency interference spanning the entire GHz to THz ranges. Consequently, the urgent development of innovative EMI mitigation strategies has become an imperative in the field. These materials effectively suppress or attenuate EMI through the mechanisms of reflection loss (SE_R), absorption loss (SE_A), and multiple reflection loss (SE_M) of EMWs on conductive/magnetic materials.⁸ They hold broad application prospects in various fields, including wearable electronic devices,⁹ communication technology,¹⁰ and aerospace technology.¹¹ Typically, the mainstream materials for EMI shielding are metallic substances (such as Ag, Cu, and Al) with high electrical conductivity.^3,11–13 Nevertheless, the high density, elevated cost, vulnerability to corrosion, and complex processing intrinsically associated with them hinder their integration into modern electronics.¹⁴ Thus, there is an urgent need to develop EMI shielding materials with ultrathin and lightweight profiles, high-frequency broadband compatibility, harsh-environment adaptability, and flexibility, tailored to the requirements of the 6G era.¹⁵ Recently, a series of novel conductive materials such as carbon nanotubes (CNTs),¹⁶ reduced graphene oxide (RGO),¹⁷ silver nanowires (AgNWs),¹⁸ liquid metals (LMs),^19,20 MXenes,²¹ and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)²² have emerged as promising alternatives to traditional metal materials in the field of EMI shielding. Due to their favorable electrical properties, flexibility, and processability, these materials have attracted significant research attention. They are being investigated as conductive fillers to be incorporated into diverse matrices, including polymers and ceramics.^23–32 Hydrogels consist of crosslinked hydrophilic polymer networks filled with water. Compared to most dry composites, they offer superior application flexibility, adaptability, and customizability, enabling them to meet the stringent demands of EMI shielding in the 6G era and potentially guiding the development of multifunctional integrated miniaturized electronics. But the inherently low shielding efficiency poses an obstacle to their further advancement. To address this issue, incorporating conductive fillers into hydrogel systems has emerged as a highly effective approach for optimizing impedance matching characteristics and significantly improving EMI shielding capabilities while maintaining moderate electrical conductivity.^33–39 Consequently, conductive filler-integrated hydrogels (CFHs) have attracted great attention.

CFHs, an emerging class of conductive polymer composites, are composed of conductive fillers, chemically/physically crosslinked hydrophilic networks, and aqueous cellular architectures.^40,41 Current methods for constructing CFHs mainly involve freeze-casting,⁴² force field-driven assembly,^43–45 self-assembly,⁴⁶ crosslinking,^47,48 in situ reaction,⁴⁹ and three-dimensional (3D) printing.⁵⁰ These methods endow this material with tunable electrical conductivity, fatigue tolerance, mechanical flexibility, and self-healing capabilities—core attributes essential for next-generation flexible EMI shielding systems. Conventional EMI shielding composites depend critically on high filler concentrations for performance and suffer significant shielding degradation under tensile strain due to conductive network failure. In contrast, CFHs offer distinct advantages in biocompatibility, processability, flexibility, and property tunability. The tissue-mimetic mechanical properties derived from their polymer matrices enable conformal engagement with complex surfaces, a critical attribute for adaptive shielding in dynamic environments. Another advance lies in surmounting the fundamental trade-off between filler loading and mechanical performance. By harnessing synergistic interactions within the aqueous environment and polymer chains, CFHs attain high EMI SE at low filler concentrations,^51–55 tackling a long-standing challenge of excessive rigidity in conventional composite materials. In 2018, the introduction of EMI-shielding hydrogel materials marked the emergence of mechanically robust yet flexible materials capable of exhibiting recovery of X-band EMI shielding performance following severe mechanical damage.⁵⁶ Since then, the field has advanced toward multi-band shielding capabilities and multifunctional integration. In 2021, a breakthrough was made with an ultrathin MXene hydrogel that achieved an EMI SE of 45.3 dB across the 0.2–2.0 THz spectrum,⁵⁷ marking the first demonstration of CFHs’ shielding capabilities in the THz regime while showcasing promising health-monitoring functionality as a sensor. This work paves the way for next-generation multifunctional EMI-shielding hydrogels. In 2024, a CFH with ultra-broadband (X-, Ka-, Ku-bands, and THz) EMI shielding and strain-sensing functionalities was synthesized.⁵⁸ Additionally, the biocompatibility of CFHs unlocks new frontiers for integrated EMI shielding in wearable health monitors and implantable devices, where mechanical adaptability and biostability are critical design parameters. Concurrently, the development of a multifunctional conductive hydrogel skin sensor serves as a pioneering example: its biocompatibility was validated through cytotoxicity testing, after which it was engineered to integrate human–machine interaction sensing, photothermal therapy, and multi-band EMI shielding capabilities.⁵⁹ As of 2025, the latest advancements in this field have centered on dynamically tunable EMI shielding and biomimetic-inspired strategies, aiming to enhance material adaptability to complex electromagnetic environments and integrate biological system functionalities. For instance, a temperature-responsive CFH has been developed, featuring reversible on/off switching of EMI shielding performance and rapid self-healing capabilities.⁶⁰ Another pioneering work is the creation of a novel triple-network hydrogel that emulates conductive pathways analogous to neurons and axons, exhibiting exceptional EMI SE across the 8.2–26.5 GHz range.⁶¹ These innovations not only enhance durability under cyclic loading but also meet the growing demand for lightweight, multi-functional, intelligent, adjustable, and sustainable materials in flexible electronics.^62–66 Moreover, the preparation of aerogel systems based on CFHs through post-treatment typically involves pivotal methods such as atmospheric pressure drying,^67–69 supercritical drying,^70–72 and freeze drying.^73,74 This enables the introduction of more interfaces and micropores within the material to amplify the multiple reflections of EMWs, consequently leading to superb EMI shielding performance and ultralight properties. However, the aerogel systems based on CFHs present inferior rheological, viscoelastic, and biocompatible properties compared to CFHs. For example, freeze-drying the sol–gel processed aramid nanofiber (ANF)/CNT hydrogel produced an aerogel with an outstanding EMI SE of 54.4 dB at 568 μm.⁷⁵ Moreover, composites prepared with anisotropic graphene aerogels and epoxy resins exhibited a radial EMI SE of 32 dB and an axial EMI SE of 25 dB at 0.8 wt% graphene content.⁷⁶ Based on these needs, it is essential to consolidate research on CFHs and CFH derived aerogel systems within the context of EMI shielding (Fig. 1). By analyzing their mechanisms and identifying key patterns, we can anticipate their potential applications across various fields, thereby laying the foundation for future advancements.

Fig. 1 An overview of the different classes of CFHs, including their preparation techniques, post-treatment processes, and influence factors of shielding performance.

The developmental trajectory of CFHs has traversed from single-band EMI shielding to next-generation multifunctional and multi-spectrum shielding, with the newest research focalizing on smart-tunable EMI shielding hydrogels. Thus, CFHs exemplify a transformative approach to developing environmentally benign, high-performance, smart-tunable, and multifunctionally integrated EMI shielding solutions that bridge materials science, engineering, and biomedical applications. As the field reaches a critical inflection point, synthesizing these advancements becomes essential for guiding future research and technological translation. In this review, the EMI shielding mechanism of CFHs is thoroughly introduced, and the most recent research findings on the shielding applications of CFHs incorporating various conductive fillers are comprehensively summarized. The EMI shielding performance of novel aerogel systems based on CFHs is analyzed in detail. In the concluding part, the obstacles and future outlooks related to the advancement of CFHs and CFH derived aerogel systems in the context of EMI shielding are deliberated. It is anticipated that these materials will serve as a valuable reference for the development of high-performance EMI shielding materials through the rational design of components and structures, thereby contributing to their efficient utilization.

2. EMI shielding mechanism of conductive filler-integrated hydrogels

EMI shielding materials are commonly utilized to protect electronic devices or biological entities exposed to electromagnetic radiation, primarily aiming to attenuate the penetration of EMWs.⁷⁷ As delineated in the Schelkunoff theoretical framework,^78–80 the quantitative assessment of electromagnetic shielding performance in materials is predominantly determined by the EMI SE (unit: dB), which encompasses three distinct types of losses: SE_R, SE_A, and SE_M. These losses collectively contribute to the dissipation of EMWs and quantify the ratio of the transmitted EMWs to the incoming ones.⁸¹ Consequently, the EMI SE can be calculated by summing up these individual effects (eqn (1)).^79,82–84In eqn (1), P_i (E_i or H_i) and P_t (E_t or H_t) represent the power (electric or magnetic field intensities) of the incident and transmitted EMWs, respectively. With the increase of EMI SE, the shielding efficiency increases.

When the EMI shielding material encounters incident EMWs, impedance matching plays a critical role in the three types of losses: (i) SE_R stems from impedance (Z) mismatch between free space and the shielding medium.⁸⁵ Enhancing the electrical conductivity of the shielding layer amplifies impedance discontinuity at the free space-shielding interface, thereby augmenting EMW reflection and boosting EMI SE. Given the shielding layer's inability to fully reflect EMWs, additional attenuation of the incident radiation within the layer is essential to minimize transmission. This is achieved through the secondary EMI shielding mechanism of absorption. (ii) SE_A occurs via conduction, dielectric loss/polarization loss, and magnetic losses, which are a result of the interaction of EMWs with electrons, electric dipoles, and magnetic dipoles within the material.⁸⁶ Additionally, through impedance matching optimization (Z_S ≈ Z₀), EMWs can penetrate more efficiently into the material, thereby undergoing enhanced absorption and dissipation within its matrix.⁸⁷ (iii) SE_M refers to the multiple reflections of incident EMWs between the front/back surfaces and within the shielding layer, accompanied by energy dissipation at internal interfaces.⁸⁸ Engineered structures (porous architecture, multilayered architecture, multiphase architecture, etc.) incorporate more impedance mismatching interfaces in materials, thereby promoting internal scattering and boosting wave attenuation.⁸⁹ However, the internal multiple reflection effect becomes negligible when the shielding layer's thickness exceeds the skin depth or when its SE_A surpasses 10 dB.⁹⁰ In theory, SE_A, SE_R, and SE_M can be described by eqn (3)–(5),^79,81–83 where f is the frequency of the incident EMWs, μ_r is the permeability, σ_r is the electrical conductivity, δ is the penetration or skin depth, t is the thickness of the shielding material, and Z_S and Z₀ are the shield and free space impedances, respectively.

CFHs are primarily composed of a polymer network, conductive fillers, and water. The internal water content, porous structure, and type of conductive fillers are closely intertwined with SE_R, SE_A, and SE_M, the three core shielding mechanisms, collectively determining the system's EMI SE. Analyzing these synergistic relationships thus offers promising pathways for designing hydrogel materials with superior shielding capabilities.

2.1. The influence of water molecules on the EMI shielding performance of CFHs

The CFHs can be fabricated by incorporating conductive fillers into hydrogels through assembly, blending, and doping techniques. This material can integrate the strong polarization loss of water molecules, reflection/scattering effect, the conductive loss of the fillers, and the interfacial polarization between substances, enabling efficient dissipation of EMWs. Due to its pronounced polarity, water has been extensively proven to effectively attenuate EMWs through multiple mechanisms including high dielectric loss, significant polarization loss, and synergistic absorption-reflection-scattering effects. Conventional conductive polymer composites typically achieve exceptional EMI shielding performance through high conductive filler content, though this approach often compromises strain energy dissipation and increases costs.⁴⁵ In contrast, the CFHs strategically utilize abundant water molecules to reduce reliance on high-concentration conductive fillers, enabling efficient EMI shielding at substantially lower filler loadings. The EMI shielding mechanisms of water in CFHs primarily operate through three coordinated pathways (Fig. 2): (i) synergistic reflection–absorption mechanism: structural mismatch between hydrated polymer networks and free water molecules induces interfacial EMW reflection, reducing wave transmission. Water-mediated dispersion of conductive fillers promotes the formation of percolating conductive networks, optimizing charge migration and enhancing EMW reflection at material surfaces and internal interfaces.^91,92 Water's dipole oscillations significantly enhance high-frequency EMW absorption,⁹³ while dissolved ions and free electrons from conductive fillers generate induced currents via field-responsive movements, converting EMW energy to heat.^94,95 The EMI shielding mechanism of CFHs primarily involves reflection and absorption, characterized by key parameters (reflection coefficient (R), absorption coefficient (A), and transmission coefficient (T)) collectively termed the power factor, with relationships as follows:⁹⁰

R = |S₁₁|² (6)

T = |S₂₁|² (7)

A = 1 − R − T (8)

EMI shielding performance is commonly characterized using a vector network analyzer, which employs S-parameters (S₁₁ and S₂₁) to quantify incident and transmitted waves. When absorption dominates (A > R), incident EMWs are primarily attenuated via in-material energy dissipation. Conversely, a higher R indicates most waves are reflected at the material–air interface. These mechanisms are reflected in CFHs as absorption- or reflection-dominated shielding (Fig. 3). (ii) Polarization loss mechanism: Under an alternating electromagnetic field, water molecules undergo continuous dipole rotation to track field direction changes. This dynamic polarization process dissipates electromagnetic energy via dipole–dipole interactions during dielectric relaxation, converting EMW energy to heat.^96–98 The cumulative polarization losses significantly enhance wave attenuation. When water interacts with conductive fillers, its polarity induces surface charge redistribution, generating eddy currents that enhance conductive loss.^5,99 Polar functionalities (e.g., carbonyl/hydroxyl groups) on conductive fillers form hydrogen bonds with water molecules, amplifying dipole and interfacial polarization within the material and leading to intensified loss of EMWs.¹⁰⁰ Imaginary permittivity quantifies the polarization loss. For instance, Song et al.¹⁰¹ reported that in PVA-based hydrogels, imaginary permittivity approximately decreased from 14 to 2 across the tested frequency range as water content dropped from 100 wt% to 85 wt%, directly reflecting reduced polarization losses. This highlights water's critical role in driving polarization-dependent EMW dissipation in hydrogels. (iii) Scattering effect mechanism: The porous structure of hydrogels is filled with water, which leads to the EMW refraction and scattering at interfaces due to differences in electromagnetic characteristics (e.g., dielectric constant and refractive index) between water and the pore wall or other components.^45,102 This phenomenon alters the propagation direction, increases the propagation distance, and enhances the probability of absorption and dissipation for EMWs. Furthermore, ions and molecules present in water can scatter EMWs, which diminishes wave transmission efficiency.¹⁰³ The occurrence of the scattering promotes the formation of a complex EMW energy distribution within the material,¹⁰⁴ thereby hindering penetration. Furthermore, an increased gradient of electromagnetic parameters within the material also intensifies multiple reflections and scatterings.

Fig. 2 EMI shielding mechanism for water in CFHs.

Fig. 3 Schematic diagram of the absorption/reflection shielding mechanism.

2.2. The influence of porous structure on the EMI shielding performance of CFHs

The majority of CFHs exhibit well-defined porous architectures, typically fabricated through established methods including freeze-casting, self-assembly, and solvent evaporation. In contrast to conventional dense bulk EMI shielding materials, the 3D porous networks in CFHs provide two critical advantages. First, they diversify interfacial heterogeneities that markedly enhance EMW–material coupling effects. Second, they feature optimized impedance matching characteristics. These two aspects work together to enable outstanding absorption and attenuation of incident EMWs. As illustrated in Fig. 4, the EMI attenuation capability exhibited by the hierarchically porous CFH composite can be attributed to three synergistic mechanisms: (i) absorption: the porous structure not only enhances the surface area of the hydrogel but also synergizes with various filler materials, including conductive polymers, carbon-based materials, and metal nanoparticles, to boost the absorption efficiency of EMWs. When the hydrogel interacts with these dielectric materials, the electric field energy induces a polarization effect within the medium. The delayed polarization partially converts the EMW energy into thermal energy, thereby achieving effective absorption. Moreover, the porous framework stretches the duration and range of the electric field, thus enhancing the loss mechanism. Additionally, this structure facilitates better dispersion and increased contact surfaces for conductive components, thereby promoting enhanced conductive loss. (ii) Scattering effect: EMWs propagating through the hydrogel undergo multiple scattering events at the pore walls, as well as at the interfaces between the pores and the substances within them. The dimensions, geometric forms, and spatial dispersion of the pores play a decisive role in dictating the magnitude and directivity of the scattering. This scattering causes the EMW energy to disperse, thereby diminishing the transmission ability of EMWs. As scattering sources for electromagnetic waves, the pore walls influence the propagation paths of these waves. When colliding with the pore walls, the EMWs experience both reflection and scattering, which in turn reduces their intensity. (iii) Multiple reflection: The porous structure provides a multitude of pore walls, offering numerous interior surfaces/interfaces for EMWs.^99,105 Thus, the reflection effect on EMWs is enhanced. Furthermore, by designing holes with a specific shape and orientation (such as honeycomb or hierarchical holes), EMWs can interact more efficiently with the hole walls during propagation. This leads to increased diversity in both reflection times and angles,^106,107 and extends the path-length of the EMWs, which further increases energy attenuation.^21,108 At the multi-layered interface of the porous structure, refraction and reflection occur alternately, producing an interference effect that weakens the propagation intensity of the EMWs. Consequently, this phenomenon ultimately achieves the objective of effective shielding.

Fig. 4 Schematic illustration of the interaction between incident EMWs and CFHs’ porous structure.

2.3. The influence of conductive fillers on the EMI shielding performance of CFHs

Conductive fillers, such as two-dimensional (2D) nanomaterials (e.g., RGO, MXene, MoS₂), one-dimensional (1D) nanomaterials (e.g., CNTs, AgNWs, and CuNWs), zero-dimensional (0D) metal particles (e.g., Al, Ni, Ag, Fe, and Cu) and their hybrids, and conductive polymers (e.g., PEDOT:PSS, polypyrrole (PPy), polyaniline (PANI)), have been incorporated into hydrogel systems to form CFHs. These conductive fillers primarily attenuate the propagation of EMWs through mechanisms of conductive loss and polarization loss.^109–111 Conductive loss is influenced by conductive currents and material resistance, and it involves the gradual attenuation of EMW energy by converting electrical energy into heat energy.¹¹² Moreover, conductive fillers exhibit interfacial polarization and dipole polarization under the electromagnetic field,^113,114 which similarly transform EMW energy through the orientation of polarized filler molecules (Fig. 2 and 3). In the following sections, the differences in achieving EMI shielding with the few classical fillers are detailed.

Although 0D metal particle-based composites can contribute significant magnetic loss to EMW shielding absorption, their inherently deficient dielectric loss limits the achievable absorption bandwidth. 2D nanomaterials manifest distinct anisotropic physicochemical characteristics attributed to their special structure, surface chemical features, quantum size influence, and large specific surface area. Thus, a conductive network is constructed within the hydrogel framework at relatively low concentrations of 2D material fillers while maintaining the mechanical flexibility of the polymer.^115–117 This leads to a substantial augmentation in the conductivity, relative dielectric constant, dielectric loss tangent, and mechanical properties of the CFHs. Moreover, the large-scale surface area of 2D nanosheets, which are abundant in functional groups, offers copious interfaces for multiple internal reflections, further contributing to the improvement of the hydrogel's EMI SE.^113,118

1D nanomaterials exhibit the characteristics of small dimensions, high aspect ratios, and linear arrangement, enabling efficient charge carrier transport along a controllable direction.^119,120 Consequently, they can effectively mitigate the high resistance associated with insulating substrates, making them highly suitable for enhancing electrical conductivity in composite systems. Moreover, since the EMI SE of materials has a positive correlation with their electrical conductivity, 1D nanomaterials are indispensable for developing composites that possess significant electrical conductivity and superior EMI shielding properties.^121,122

Conductive polymers present tunable mechanical properties, ionic and electronic conductivity, and superior processability. Nowadays, processing strategies that utilize conductive polymers as substrates for EMI shielding materials are predominant.^123,124 However, conductive polymers are also employed as conductive fillers to construct a 3D conductive network structure,¹²⁵ imparting EMI shielding characteristics to materials, particularly in the case of PEDOT:PSS. Consequently, by incorporating conductive polymers as fillers into hydrogel matrices (such as PVA, PAA, PAM, etc.), a uniformly distributed 3D interpenetrating conductive network can be constructed.¹²⁶ Additionally, due to low interfacial impedance, high charge injection capability and electrochemical activity, conductive polymers exhibit outstanding ion-to-electron transduction capabilities.^127,128 This enables rapid charge/ion transfer at high frequency, thereby allowing these materials to dissipate absorbed EMWs via enhanced conductive loss.

It can be summarized as follows: (i) 0D nanomaterials exhibit high dispersibility and straightforward synthesis, thereby suiting large-scale production. Their nanoscale dimensions induce multiple interfacial polarizations, synergistically enhancing EMW absorption. But shielding dominated by high loadings often compromises hydrogel flexibility, while their single-loss mechanism and limited high-frequency attenuation restrict EMI shielding applications. These materials are thus ideal for flexible sensor substrates and low-frequency EMI protective coatings. (ii) 2D nanomaterials, characterized by exceptional specific surface area and electrical conductivity, enable synergistic shielding through reflection loss, conduction loss, and polarization loss. However, stacking-induced aggregation and structural defects during large-scale synthesis remain challenges. These materials are therefore ideal for high-frequency/ultrahigh-frequency shielding applications, such as flexible coatings for 5G device enclosures and satellite communication components. Their appearance addresses the problem of poor dielectric loss of single 0D fillers. In contrast, 1D nanomaterials suffer from restricted conductive pathways, limiting their efficacy. To date, 2D nanomaterials dominate as fillers in EMI shielding hydrogel composites due to their superior performance. (iii) 1D nanomaterials, characterized by their high aspect ratio, form cross-scale conductive pathways at low loadings (<10 wt%), enabling simultaneous enhancement of tensile strength and shielding effectiveness in hydrogels through superior axial load transfer. These materials are well-suited for applications requiring high flexibility and mechanical reliability, such as wearable electronics (smart bracelets, e-skin) and stretchable EMI shielding layers. However, their limited dispersibility due to van der Waals interactions complicates processing. Although they exhibit pronounced anisotropy, with excellent axial conductivity but reduced shielding efficiency perpendicular to the filler axis. (iv) Conductive polymers, with their conductivity arising from π–π conjugation and electron delocalization, exhibit the ability to form conductive networks that block and attenuate EMWs. Although nanofillers exhibit higher EMI shielding efficiency, their dispersion and processing in composites remain challenging. However, conductive polymers are easily processable via chemical treatments or structural modifications, circumventing agglomeration and allowing tunable properties. These attributes render them promising for flexible shielding in extreme deformation scenarios, such as foldable displays and soft robotics, as well as medical applications like shielding for implantable sensors. In addition, in 3D conductive polymer networks, 0D, 1D, and 2D fillers often co-exist and work in combination.^129,130 Specifically, 0D nanoparticles can provide numerous conductive sites, 1D nanowires or nanotubes can form conductive pathways, and 2D nanosheets can contribute to large-area conductive networks. This multidimensional integration creates enhanced effectiveness through multiscale electron transport.

Based on their EMI shielding mechanisms, a summary of some typical CFHs regarding their EMI shielding performance, internal structure, and conductive filler is presented in Table 1. Notably, the electrical conductivities of different CFHs differ, primarily attributed to filler content, filler dispersion uniformity, and interfacial interactions between components. While electrical conductivity directly influences EMI SE, material thickness and internal structure are equally critical for determining shielding performance. The data demonstrate that the above-mentioned elements profoundly influence diverse performance indices of CFHs.

Table 1 Topical EMI shielding performances of CFHs with various fillers and structures

Filler	Material	Internal structure	EMI SE (dB)	Thickness (mm)	Conductivity (S m^−1)	Ref.
Note: the frequency range is 8.2–12.4 GHz.
MXene	MXene/PE-CS	Porous	38.4	0.5–2	1.27	58
MXene	HA/MXene@ PPy	Lamellar	59.6	0.0309	20914	131
MXene	TOCNF/CS/MXene	Porous	71	0.6678	—	132
MXene	PVA/MXene	Porous	91	7.5	1.6	133
MXene	TOCNF/CS/MXene	Porous	40.3	0.1894	—	134
MXene	TOCNF/MXene-CGG	Porous	49.37	0.2	3843	135
MXene	PAA/PVA/MXene	Porous	33.6	2	156.24	136
MXene/Ni	MXene/Ni/PVA/PAA	Porous	27.2	—	2.73 × 10-2	137
MXene/Fe^3+	PAAm-PHEMAA/CMC-Fe^3+-MXene	Porous	41	1.5	0.5	138
RGO	RGO-PAA/CS/ACC	Porous	90.63	9.71	22.79	112
RGO	PVA/IL-PPy-RGO	Porous	32.9	1.0	8.3 × 10-3	139
RGO	PAM/RGO	Porous	45.14	4	1.12	140
MWCNT	PAM/CNF/MWCNT	Porous	28.5	2	0.85	56
MWCNT	MWCNT/PAA	Porous	32.8	2.5	238	141
CNT	CNT@ LM/PAM/gelatin	Porous	41.79	4	0.998	142
AgNW	ANF-PVA/AgNW	Lamellar	52	0.3	1.66 × 104	122
PEDOT: PSS/MXene	PAM/SSD/MXene/PEDOT: PSS	Porous	38.7	3	0.91	143
PEDOT: PSS	PVA/PAA-PEDOT: PSS-TA	Porous	22.92	1	—	144
PEDOT: PSS	PEDOT: PSS/PET	Porous	105	0.228	2050	145

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3. Hydrogels with diverse conductive fillers for EMI shielding

Typically, the incorporation of conductive fillers (such as 0D/1D/2D nanomaterials or conductive polymers) into the hydrogel matrix constructs a conductive network. This network not only provides a pathway for low-resistance electron transport but also forms hydrogen bonds with the polymer network simultaneously.^146–150 A series of studies have recently employed the aforesaid approach to fabricate diverse CFHs based on various fillers (e.g., MXene, RGO, CNTs, AgNWs, and PEDOT:PSS). These resultant materials exhibit moderate electrical characteristics. Concurrently, they retain mechanical flexibility, self-healing functionality, processing amenability, and an internally aqueous milieu, rendering them highly suitable for a wide range of EMI shielding applications. Due to their high aspect ratio, robust electrical conductivity, substantial polarization losses, and outstanding mechanical stability, in the majority of work related to the preparation and application of CFHs, MXenes are chosen to enhance the mechanical, electrical, and EMI shielding characteristics of hydrogels. MXene-based hydrogels can efficiently augment the internal multiple scattering, reflection, and absorption of EMWs. This is achieved by leveraging the layered structure of the MXene and integrating it with the potent polarization loss capacity of water molecules, thereby attaining remarkable EMI shielding performance. Moreover, to enhance the applicability of CFHs in wearable electronic devices, implantable medical devices, and the aerospace sector, the EMI shielding capability of the hydrogel can be tailored during its formulation process. This is achieved by judiciously selecting different types of conductive fillers, optimizing their loading capacity, and designing suitable hydrogel network architectures.

3.1. MXene-based hydrogel materials for EMI shielding

As multifunctional materials, 2D MXenes, which encompass transition metal carbides, nitrides, or carbonitrides, are distinguished by outstanding electrical conductivities, extensive specific surface areas, and notable processabilities. The pivotal research conducted by Yury Gogotsi et al.¹⁵¹ first demonstrated that an MXene film, with an extremely thin thickness of merely 45 μm, presented an EMI SE of 92 dB, indicating its substantial potential for attenuating EMWs. Additionally, MXenes are employed to form conductive networks within composite materials. In recent years, an increasing number of researchers have delved into the utilization of hydrogel materials incorporating MXenes within the realm of EMI shielding.

3.1.1. MXene-based cross-linked network composite hydrogels. Composite hydrogels with cross-linked networks exhibit superior electromagnetic properties and mechanical properties compared to conventional hydrogels. The introduction of adhesives or the formation of reversible dynamic coordination in the hydrogel system is conducive to increasing the crosslinking density, boosting the elongation capacity and extending the service lifespan of the hydrogel material. Xia et al.¹⁵² developed an L-citrulline-functionalized Ti₃C₂T_x-MXene/polyacrylamide (PAM)/sodium alginate (SA) hydrogel, using FeCl₂ as a binder to increase crosslinking density and successfully formulating a novel EMI shielding material. This composite hydrogel demonstrated an exceptional combination of mechanical properties akin to human skin and superlative EMI shielding capabilities. After freeze-drying, it revealed a porous structure with marrow-like interconnected pores (Fig. 5a). Given the relatively low electrical and magnetic conductivities of PAM and SA, the PAM/SA hydrogel demonstrated restricted EMI shielding performance, with a value of approximately 1.2 dB (Fig. 5b). Consequently, the integration of MXenes within the PAM/SA substrate substantially boosted the EMI shielding efficacy. With the MXene content increasing from 1.5 to 6.5 wt%, the maximum EMI SE reached 26.8 dB. Fan et al.¹³⁸ employed MXenes in conjunction with a dual-network structure of polyacrylamide–poly-N-hydroxyethyl acrylamide/carboxymethyl cellulose-Fe³⁺, integrating a multitude of hydrogen bonds and complexation interactions to engineer multifunctional hydrogels. Characterized by its porous architecture (Fig. 5c), moderate conductivity, and synergistic effects in an ionic solution environment, this hydrogel showcased remarkable EMI and UV shielding capabilities. The EMI SE improved from 29.4 to 48.8 dB as the MXene loading increased from 0 to 0.52 wt%, attributed to the increasing MXene content that progressively enhanced surface reflection (Fig. 5d). The hydrogel's electromagnetic shielding mechanism was explained, where its porous framework intercepted penetrating EMWs into confined small-pore cell compartments, promoting multifarious scattering and reflection. This reduces secondary pollution threats and substantially augments the total EMI SE. Moreover, MXene nanosheets and the polymer frame jointly amplified polarization and dielectric loss, subsequently dissipating electromagnetic energy more efficiently. The UV transmissivity of the hydrogel was 0% at 365 nm and 86.6% at 550 nm, underscoring its prime UV shielding aptitude and comparable translucency. Additionally, the obtained hydrogel showed commendable self-regenerative properties. EMI values post-cutting of the hydrogel across varying healing durations were measured (Fig. 5e). Post-self-healing spanning 24 hours, the electromagnetic shielding efficiency increased to 32 dB, maintaining 99.9% of the original EMI SE, predominantly ascribed to the rearrangement of the abundant hydrogen bonding and the secondary physical network presented within the framework of the resultant hydrogel. The UV-Vis transmissivity profiles of the pristine hydrogel versus the self-healed variant were depicted (Fig. 5f), substantiating that after 12 and 24 hours of healing, the specimens retained robust shielding effects against UV light within the 320–400 nm spectrum.

Fig. 5 (a) SEM image of the freeze-dried L-citrulline-functionalized Ti₃C₂T_x-MXene/PAM/SA hydrogel. (b) EMI shielding efficiency of the L-citrulline-functionalized Ti₃C₂T_x-MXene/PAM/SA hydrogel at varying MXene contents. (a) and (b) Reproduced with permission.¹⁵² Copyright 2022, American Chemical Society. (c) SEM photographs of MXene-based composite hydrogels at varying scales. (d) EMI SE of the hydrogels at various MXene contents. (e) EMI SE values and (f) UV protection properties and transparency of MXene-based composite hydrogels following self-healing at various times. (c)–(f) Reproduced with permission.¹³⁸ Copyright 2023, Elsevier B.V.

Double-network hydrogels comprise two interpenetrating polymer networks with excellent mechanical properties, self-healing properties and good environmental adaptability.¹⁵³ Zhang et al.¹³⁶ successfully created MXene/PVA/polyacrylic acid (PAA) hydrogels with dense pores by incorporating MXenes into a 3D hydrogel structure formed through the synergistic interaction of PVA and PAA (Fig. 6a). The combined effects of the MXenes and PVA/PAA led to the formation of micron-scale, well-ordered pores, enhancing interfacial interactions. Increasing the MXene loading from 2 to 10 wt% enhanced the hydrogel's EMI SE from 26 to 32 dB, with absorption loss dominating throughout the total SE range and indicating a robust capability to absorb incident EMWs (Fig. 6b and c). Wan et al.⁵⁹ developed HA-PBA/PVA/MXene hydrogels, featuring a polymer network consisting of phenylboronic acid-grafted hyaluronic acid (HA-PBA) and PVA (Fig. 6d). The strong interactions between HA-PBA, PVA, and MXenes, combined with dynamic crosslinking between PVA's hydroxyl groups and HA-PBA's phenylboronic acid groups, endowed the hydrogel with rapid self-healing capabilities. The electrical self-healing performance was evaluated using a circuit with an LED bulb and the hydrogel (Fig. 6e). Due to its robust self-healing potential, the hydrogel showed a minimal loss in SE_T, SE_R, and SE_A values before and after healing, confirming its reliable EMI shielding performance post-damage (Fig. 6f). Liu et al.¹⁰⁰ engineered a double-network hydrogel composed of PEDOT:PSS and PVA using sulfuric acid/titanium carbide MXene treatment (Fig. 6g). At ultralow filler loadings (0.6 wt% MXene and PEDOT: PSS), the hydrogel simultaneously exhibited self-healing capabilities, remarkable extensibility, and exceptional EMI shielding performance, achieving an average X-band EMI SE of 41 dB. The tensile stress–strain curves of the obtained hydrogels after a one-minute self-healing cycle were displayed (Fig. 6h). After healing, their ultimate tensile strength and elongation recovered to 72% and 78%, respectively. Meanwhile, the EMI SE and shielding efficiency of the M₂₀S₁₂PPH (20 mg mL⁻¹ MXene and 12 mg mL⁻¹ H₂SO₄) remained at 94.5% and 98% even after stretching and releasing at 200% strain (Fig. 6i and j). Moreover, MSPPH preserved superior EMI SE even after self-healing. The electromagnetic parameters of the M₂₀S₁₂PPH hydrogel were assessed across the X-band, Ku-band, and K-band to further investigate its EMI shielding mechanism. The SE_T increased linearly with frequency, averaging around 40, 57, and 84 dB in the X, Ku, and K bands, respectively. Meanwhile, because more EMWs propagated through MSPPH and were attenuated, the reflection coefficient dropped and the absorption coefficient ascended (Fig. 6k).

Fig. 6 (a) SEM photograph of the MXene/PVA/PAA hydrogel. (b) Transmission, reflection, and absorption coefficient of MXene/PVA/PAA hydrogels. (c) EMI SE_T, SE_A, and SE_R values of MXene/PVA/PAA hydrogels. (a)–(c) Reproduced with permission.¹³⁶ Copyright 2024, Elsevier B.V. (d) Synthesis of the HA-PBA/PVA/MXene hydrogel. (e) Circuit with the HA-PBA/PVA/MXene hydrogel and a blue LED bulb indicator. (f) EMI SE of the HA-PBA/PVA/MXene hydrogel pre- and post-self-healing. (d)–(f) Reproduced with permission.⁵⁹ Copyright 2024, John Wiley & Sons. (g) Schematic of MPPH. (h) Tensile stress–strain curves of M₁₅S₁₈PPH, M₂₀S₁₂PPH, and M₂₀S₆PPH after 1 min self-healing. (i) Stretching and releasing at various strains and (j) pre-fracture and post-self-healing conditions. (k) EMI SE and transmission, reflection, and absorption coefficient of M₂₀S₁₂PPH. (g)–(k) Reproduced with permission.¹⁰⁰ Copyright 2024, American Chemical Society.

3.1.2. MXene-based hydrogel membranes. Through the preparation of hydrogels in the forms of sheets or films, a large surface area and enhanced strength can be achieved, enabling their application in a wide range of fields. MXenes, two-dimensional conductive materials, enhance the conductivity, EMI shielding performance, mechanical strength, thermal stability, and adsorption/desorption capabilities of hydrogel films when incorporated into them. Lei et al.¹³⁴ designed three-layered composite hydrogels through the intricate integration of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCN), cationic starch (CS), and MXenes. This composite (TC@MT) utilized a self-assembled TOCN/CS hydrogel to encapsulate MXenes onto filter paper, employing electrostatic interactions to facilitate the rapid establishment of a porous framework (Fig. 7a and b). The synthesized composite paper exhibited a nacreous structure (Fig. 7c), with the TOCN/CS hydrogel membrane layer being dense. This design significantly enhanced EMI shielding efficacy by enabling the creation of a nanostructured conductive layer through the robust compatibility and synergistic interactions between TOCN and MXenes. The resulting filter paper displayed a negligible EMI shielding effect initially. However, empowered by this conductive layer, the EMI SE of the hydrogel film notably improved with the addition of MXenes, reaching approximately 31 dB at 50 mg of MXenes (Fig. 7d and e). Further refinement by adjusting the MXene/TOCN ratio led to an increased EMI SE peak of 40.3 dB for TC@M10T variants, with a reflection coefficient of around 0.91 (Fig. 7f), indicating a predominantly reflection-based shielding mechanism. Xu et al.¹⁵⁴ utilized supergravity to fabricate MXene-based nanocomposite hydrogels with highly oriented, heterogeneous structures. This technique enhanced the dynamic control over the movement, pre-orientation, and stacking of MXene nanosheets, producing composite hydrogels characterized by MXene-rich and polymer-rich layers (Fig. 8a and b). Remarkably, these hydrogels, containing only 0.3 wt% MXenes, exhibited extraordinary EMI attenuation capabilities, attaining 55.2 dB at 12.4 GHz (Fig. 8c). This is because supergravity-aligned MXenes form highly oriented structures within hydrogels, enabling continuous conductive pathways at low filler loadings to efficiently dissipate EMWs. When EMWs impinged from the direction of the MXene-rich and polymer-rich layers, the SE_T of the resulting hydrogel was maintained at a similar level, but with different SE_R and SE_A (Fig. 8d and e).

Fig. 7 (a) Schematic for preparing TC@MT hydrogels. (b) Photographs of TC@MT hydrogels. (c) SEM cross-section of TC@MT hydrogels. (d) EMI SE, (e) SE_R, SE_A, SE_T, and (f) absorption and reflection coefficients of TC@MT hydrogels. (a)–(f) Reproduced with permission.¹³⁴ Copyright 2024, Elsevier B.V.

Fig. 8 (a) Schematic of supergravity fabrication of nanocomposite hydrogels with oriented nanosheets. (b) SEM cross-sections of supergravity-steered MXene-based nanocomposite hydrogels at various positions, showing significant MXene orientation. (c) EMI SE of supergravity-steered MXene-based nanocomposite hydrogel (contains 0.3 wt% MXene and 7.8 wt% gelatin) with various structures, and pure hydrogel in the X-band. (d) EMI SE of supergravity-steered MXene-based nanocomposite hydrogels with varying MXene nanosheet orientations. (e) SE_R, SE_A, and SE_T values at 12.4 GHz for the supergravity-steered MXene-based nanocomposite hydrogels under different EMW incidences. (a)–(e) Reproduced with permission.¹⁵⁴ Copyright 2024, John Wiley & Sons.

3.1.3. MXene-based biomimetic porous hydrogels. The development of high-performance hydrogels that mimic natural biological materials is of substantial interest. These bioinspired hydrogels can closely replicate the structures of natural biological materials, offering biocompatibility ideal for biomedical and wearable electronic applications. Their porous architecture provides significant surface area and open channels, facilitating water transport and enhancing electromagnetic wave interactions, making them suitable for electromagnetic material systems. Liu et al.¹³³ created a robust, conductive bioinspired hydrogel with unidirectional micron-sized honeycomb pores using a straightforward, scalable unidirectional freeze-casting and salt leaching technique (Fig. 9a). The process involved using MXene sediment (MS) as the matrix and PVA for crosslinking. The joint influence of PVA and MS on the resilient cell walls not only maintained the stability of the regular pore architecture but also boosted the mechanical pliability and extensibility of the hydrogel. The collaborative interaction of pores, extensive interfaces between MS and PVA, and the conductive network contributed to its high EMI shielding efficiency. The integration of a small amount of silver nanowires further augmented microwave loss through multiple reflections, significantly boosting EMI shielding performance. The study indicated the impact of PVA content on EMI shielding capability. Increased PVA content resulted in lower EMI SE. For a 2 mm thick hydrogel with 20% PVA, the average SE in the X-band was 44.7 dB (Fig. 9b). Rising PVA levels increased the SE_A proportion in the shielding mechanism due to reduced mobile charge carriers, thus decreasing SE_R (Fig. 9c). Reflection, absorption, and transmission power coefficients (Fig. 9d) revealed that higher PVA content improved impedance matching with air, decreasing the reflection power coefficient. Since reflection preceded absorption, the absorption power coefficient trended oppositely. At the hydrogel/air interface, most incident EMWs were initially reflected, with a minor fraction penetrating the hydrogel, dissipating by cell walls and water. Micron-sized pores induced multiple EMW reflections, enhancing interactions with cell walls, boosting SE and reducing transmission. By incorporating ordered porous architectures resembling honeycombs, water, and conductive MS, the hydrogel achieved an SE above 40 dB across an ultra-wideband gigahertz range (8.2–40 GHz) (Fig. 9e). Overall, the synergy of pore structures, MS, water, and PVA accounted for the high EMI shielding performance. PVA and chitosan, enhanced by MXenes, were employed by Liu et al.¹⁵⁵ to form a double-crosslinked network via freeze-thaw cycles and salt precipitation, yielding anisotropic porous wall structures. Chitosan formed the initial rigid network through intermolecular crosslinking, establishing the polymer skeleton. Chitosan, serving as the first network, established a rigid framework through its reversible sacrificial bonds. This framework was capable of dissipating energy through fracture. Meanwhile, PVA, serving as another network, gave rise to a supple structure, maintaining hydrogel integrity even when the rigid network fractured. The combined influences of MXenes, Fe₃O₄, water, micron-scale porous architectures, and numerous interfaces (Fig. 9f) heightened the EMI shielding performance. Given the variation in EMI SE due to porous wall frameworks exhibiting anisotropic behavior (Fig. 9g), a unidirectional freeze sample with a thickness of 4.5 mm displayed an extraordinary EMI SE of 63 dB vertically and 58 dB parallelly. In the vertical direction, more pore walls and interfaces attenuated incident EMWs, compared to random freeze samples with 55 dB EMI SE. Random micro-porous structures had fewer effective interfaces, implying pore morphology influences EMI SE. With increasing thickness, the absorption shielding efficiency rose from 62.1% to 91.0% (Fig. 9h), as more interfaces attenuated EMWs, effectively enhancing SE_A. In summary, bioinspired hydrogels typically boast excellent biocompatibility, stability, sensitivity, and reversibility, positioning them as promising intelligent wearable materials. Their development also offered valuable insights into creating future multifunctional novel electromagnetic shielding materials.

Fig. 9 (a) SEM images of MS-based hydrogels. (b) EMI SE, (c) SE_R, SE_A, and SE_T values, and (d) power coefficients for varying PVA contents of the MS-based hydrogels. (e) EMI SE across X, Ku, K, and Ka bands of the MS-based hydrogels. (a)–(e) Reproduced with permission.¹³³ Copyright 2022, American Chemical Society. (f) The SEM photograph of MXene-PVA/chitosan hydrogels. (g) EMI SE of hydrogels possessing 1.5 wt% MXene with different orientations. (h) SE_T, SE_A, and SE_R of MXene-PVA/chitosan hydrogels. (f)–(h) Reproduced with permission.¹⁵⁵ Copyright 2024, Elsevier B.V.

Although MXene-based hydrogels exhibit excellent performance, MXenes’ proneness to oxidation and deliquescence within hydrogel matrices significantly compromises system stability and durability. Recent research efforts have targeted solutions to these challenges by leveraging established strategies for enhancing MXene's antioxidative stability in humid environments, such as hybrid design¹⁵⁶ and physical/chemical surface engineering.¹⁵⁷ Integrating these methodologies into hydrogel systems is poised to effectively mitigate this critical issue.

3.2. RGO-based hydrogel materials for EMI shielding

Due to the promotion of electron mobility by conjugated systems, RGO exhibits exceptional electrical properties while demonstrating cost-effectiveness and low density,^158,159 making it one of the most prominent two-dimensional carbon nanomaterials. However, RGO tends to irreversibly agglomerate during the preparation process, negatively impacting its electrical conductivity and EMI shielding capabilities. Recently, combining polymers with RGO to form hydrogels has proven effective in enhancing RGO dispersibility,¹⁶⁰ mitigating agglomeration and bettering the material's EMI attenuation function. Consequently, RGO-based hydrogels have garnered significant attention.

RGO-based hydrogels are substances featuring a well-defined network. This network consists of cross-linked hydrophilic structural units that are encircled by a large amount of water and a conductive network. Due to their exceptional electrical conductivity and sufficient retention of water molecules, we can infer that these hydrogels possess significant potential for achieving outstanding EMI shielding performance through the cooperative interplay between conduction loss and polarization loss. Lai et al.¹¹² demonstrated a simple method for assembling RGO/PAA/amorphous calcium carbonate (ACC)/chitosan hydrogels via a biomineralization-derived strategy (Fig. 10a). The produced hydrogel presented exceptional EMI shielding performance. The reasons lay in its porous architecture, steady conductivity, and an environment filled with a significant amount of water (water content around 50–54 wt%). SEM photographs disclosed the even and porous features of the hydrogel (Fig. 10b). The freeze-dried hydrogel with 4.76 wt% RGO achieved a conductivity value of 22.79 S m⁻¹, which fell into the moderate range (Fig. 10c), due to the even distribution of RGO nanosheets within the network constructed by chitosan and PAA chains. Moreover, the SE_T of the resultant hydrogel with 4.76 wt% RGO reached 90.63 dB in the X-band (Fig. 10d).

Fig. 10 (a) Schematic synthesis of RGO/PAA/ACC/chitosan hydrogels. (b) SEM images of the freeze-dried RGO/PAA/ACC/chitosan hydrogels with an RGO concentration of 4.76 wt%. (c) Electrical conductivity of the RGO/PAA/ACC/chitosan hydrogels at different RGO contents. (d) EMI SE_T of the RGO/PAA/ACC/chitosan hydrogels with various RGO concentrations. (a)–(d) Reproduced with permission.¹¹² Copyright 2022, Elsevier B.V. (e) EMI SE of the PVA/I-P-RGO hydrogel at different RGO contents in the X-band. (f) EMI SE of the PVA/I-P-RGO hydrogel at different RGO contents in the THz band. (e) and (f) Reproduced with permission.¹³⁹ Copyright 2022, Springer Nature.

Furthermore, RGO-based hydrogels exhibit great potential for EMI attenuation across wide frequency spectra that cover the GHz and THz ranges. Xiang et al.¹³⁹ synthesized a modified form of RGO by incorporating polypyrrole and ionic liquids, which was subsequently incorporated into the PVA solution for the fabrication of a PVA/I-P-RGO hydrogel via an in situ synthesis approach, achieving effective EMI shielding in both X-band and THz-band. By further increasing the RGO content, the hydrogel presented an EMI SE of 32.9 dB at 9.0 GHz with 3 wt% PPy, 3 wt% RGO concentration and 3 wt% ionic liquid (Fig. 10e). At 1.0 THz, it was shown that the EMI SE reached 22.4 dB (Fig. 10f). This resulted from the generation of interface polarization between conductive fillers and water in the hydrogels, facilitating THz wave absorption within the matrix. The presence of abundant water in the hydrogels enhanced the diffraction of THz waves.⁵⁷

3.3. CNT-based hydrogel materials for EMI shielding

In 1991, Iijima first demonstrated the structural characteristics of CNTs.¹⁶¹ Since their discovery, these 1D nanomaterials have been recognized as contenders for EMI shielding utilization due to their lightweight nature, extraordinary mechanical strength, greater electrical conductivity than most materials, and environmental stability. The integration of CNTs into polymer matrices to create advanced shielding composites has emerged as a promising strategy. However, pronounced entanglement tendencies arising from strong van der Waals interactions and insufficient surface functional groups, which hinder homogeneous dispersion within polymeric systems, limit their practical application. Currently, these challenges in CNT hydrogels are predominantly tackled via three synergistic strategies. First, physical methods (stirring, ultrasonic treatment, grinding, etc.)¹⁶² are employed to optimize the dispersion homogeneity of CNTs. Second, surface modification techniques such as covalent functionalization and polymer wrapping are applied to enhance interfacial compatibility between CNTs and the matrix.¹⁶³ Third, in situ polymerization is implemented to stabilize CNT architectures and reduce aggregation induced by thermal or mechanical stresses.¹⁶⁴ This multi-pronged methodology allows for the construction of percolative conductive architectures with optimized charge transport pathways. As a result, superior EMI shielding performance is achieved through enhanced electromagnetic energy dissipation mechanisms.

In practical applications, where frequent deformation can severely compromise the stability and lifespan of EMI shielding hydrogels, introducing CNTs into polymer networks offers a promising approach to fabricating robust EMI shielding hydrogels. These hydrogels exhibit outstanding EMI shielding performance even after experiencing severe mechanical damage. Zhu et al.⁵⁶ prepared multiwalled carbon nanotube (MWCNT)/PAM/cellulose nanofiber (CNF) composite hydrogels with homogeneous pore structures using simple and effective in situ polymerization, displaying remarkable electrical performance and EMI SEs (Fig. 11a and b). Increasing MWCNT loadings led to an enhancement in the EMI SE of PAM/CNF/MWCNT composite hydrogels. At a 1 wt% MWCNT loading, the material exhibited a conductivity of 0.85 S m⁻¹ and an EMI SE of 28.5 dB (Fig. 11c-e). Additionally, the hydrogel demonstrated an EMI shielding performance with mechanical self-healing capability. More interestingly, it remained virtually unchanged even after undergoing 1000 cycles of bending (Fig. 11f).^165–167 Guo et al.¹⁴² fabricated the CNT@liquid metal (LM)/PAM/gelatin (LMCPG) dual network hydrogel employing the rapid ultraviolet in situ polymerization technique. The resulting product demonstrated outstanding mechanical characteristics, EMI shielding capabilities, and electrical conductivities (Fig. 11g). The SEM of freeze-dried LMCPG hydrogels showed an interconnected and porous structure (Fig. 11h). First, the interfacial interaction allowed the CNT@LM to attain sufficient stability in an aqueous environment. Simultaneously, CNTs were employed to establish connections among the LM droplets within the hydrogel, facilitating the formation of a more efficient conductive network. Subsequently, gelatin and PAM contributed to the creation of both physical and chemical networks, which led to the formation of a stretchable and robust hydrogel elastomer. Therefore, the properties of the hydrogel could be precisely adjusted, endowing it with regulatable EMI shielding capabilities. When the mass of the LM droplets was increased from 1 g to 8 g, the LMCPG hydrogel exhibited an outstanding EMI shielding performance, reaching an EMI SE of 75.69 dB (Fig. 11i). Even after undergoing 2000 cycles of bending, the hydrogel retained 98% of its EMI SE. This can be ascribed to its remarkable mechanical suppleness and durability (Fig. 11j). Simultaneously, the remarkable deformability enabled a multitude of LM droplets to interconnect and establish conductive pathways. Moreover, the existence of CNTs further facilitated the development of the conductive network.^168–170 However, further stretching significantly affected the EMI SE value, attributed to increased LM droplet spacing reducing electrical conductivity. When stretched from 0% to 500%, the EMI shielding performance first improved then deteriorated (Fig. 11k).

Fig. 11 (a) A photograph of the MWCNT/PAM/CNF composite hydrogels. (b) SEM image of the MWCNT/PAM/CNF composite hydrogels at 1 wt% MWCNT content. (c) Electrical conductivity of the MWCNT/PAM/CNF composite hydrogels at different MWCNT contents. (d) EMI SE of the MWCNT/PAM/CNF composite hydrogels at different MWCNT contents. (e) EMI SE_T, SE_A, and SE_R values of the MWCNT/PAM/CNF composite hydrogels at different MWCNT contents. (f) Contrast of the EMI SE of the MWCNT/PAM/CNF composite hydrogels at 1 wt% MWCNT content in the initial state, following 7 days of self-healing, as well as following 7 days of self-healing and undergoing 1000 bending cycles. (a)–(f) Reproduced with permission.⁵⁶ Copyright 2018, American Chemical Society. (g) Preparation process of the LMCPG hydrogels. (h) SEM image of the LMCPG hydrogels. (i) EMI shielding performance of the LMCPG hydrogels with varying LM contents. (j) EMI shielding performance of the LMCPG hydrogels pre- and post-2000 times of cyclic bending. (k) EMI SE subjected to varying tensile strains ranging from 0% to 500%. (g)–(k) Reproduced with permission.¹⁴² Copyright 2023, Elsevier B.V.

3.4. AgNW-based hydrogel materials for EMI shielding

Excellent flexibility, length-to-diameter ratio, specific surface area, and mechanical properties of AgNWs make them ideal for ensuring good electrical stability in electromagnetic shielding applications.^171–173 Therefore, incorporating an appropriate amount of AgNWs into hydrogels can establish conductive pathways and effectively enhance the EMI shielding properties of the hydrogels.

AgNWs can be utilized as a coating on the substrate layer to fabricate thin film hydrogels, allowing for accurate regulation of the material's thickness and the formation of a homogenous coating on a variety of substrates. Zhou et al.¹²² employed electrospinning and vacuum-assisted filtration techniques to synthesize sandwich-like AgNW composite hydrogels. These hydrogels featured layers of PVA hydrogels that were strengthened by ANFs, and there was an additional layer made from a combination of AgNWs and PVA (Fig. 12a). Due to the robust interfacial interactions existing between the ANF-PVA and AgNW-PVA layers, along with the mesh-shaped conductive network formed by AgNWs, the resulting hydrogel exhibited an exceptional electroconductivity of 1.66 × 10⁴ S m⁻¹ (Fig. 12b). Moreover, as the content of AgNWs rose from 0.02 vol% to 0.23 vol%, the EMI SE incrementally climbed from 32 dB to 52 dB (Fig. 12c). Xu et al.¹⁷⁴ used TEMPO, TOCN and AgNWs to prepare a multilayer composite hydrogel. Firstly, AgNWs were deposited onto the uneven surface of sandpaper via spin-coating. Subsequently, TOCN was introduced as the second layer to obtain a TOCN/AgNW hydrogel. Then, the spin-coating process was utilized once again to apply AgNWs onto the TOCN/AgNW hydrogel as the third layer, which exhibited excellent EMI shielding functionality. Ultimately, a supplementary layer of TOCN was introduced and subjected to cross-linking, aiming to obtain TOCN/AgNW /TOCN/AgNW composite hydrogels. The SEM image of the TOCN/AgNW hydrogel revealed that the AgNWs were embedded in the nanocellulose hydrogel, thereby establishing a complete conductive pathway (Fig. 12d). Therefore, with the increase in the density of AgNWs within the EMI shielding layer, the EMI SE could reach as high as 20 dB when the AgNW density was 78 mg m⁻². Moreover, when the density was beneath 78 mg m⁻², the EMI SE value was relatively low and the EMI shielding capability was subpar (Fig. 12e).

Fig. 12 (a) Fabrication procedures of the sandwich-like AgNW composite hydrogels. (b) Electroconductivity of the sandwich-like AgNW composite hydrogels at varying AgNW concentrations. (c) EMI SE of the sandwich-like AgNW composite hydrogels at varying AgNW concentrations. (a)–(c) Reproduced with permission.¹²² Copyright 2021, John Wiley & Sons. (d) SEM image of the microstructure on the surface of the TOCN/AgNW hydrogels. (e) EMI SE of the TOCN/AgNW/TOCN/AgNW composite hydrogels with various densities of AgNWs. (d) and (e) Reproduced with permission.¹⁷⁴ Copyright 2022, Elsevier B.V. (f) Top-view SEM images of the AgNW-based hydrogel composite. (g) EMI SE_T of 50 mg mL⁻¹ AgNW-based hydrogels before and after self-healing. (f) and (g) Reproduced with permission.⁵⁸ Copyright 2022, Elsevier B.V.

In addition, the porous junction based on AgNWs can impart favorable mechanical and EMI shielding attributes to hydrogels. Huang et al.⁵⁸ prepared a 3D porous hydrogel containing AgNWs via directional freezing. They carried out the in situ polymerization of acrylamide (AM) and N-acryloyl-11-aminoundecanoic acid (A-11) within a pre-formed AgNW aerogel. In the top-view SEM images, the well-preserved cellular structure within the hydrogel composite was observed (Fig. 12f), indicating that the hydrogel was successfully fabricated on the AgNW aerogel. Benefiting from the 3D conductive cellular structure, along with the reversible hydrophobic association and hydrogen-bond forming interactions, the hydrogel exhibited outstanding self-healing abilities. At an AgNW content of 50 mg ml⁻¹ (APAA-50) in the X-band, the hydrogel underwent a self-healing process and maintained excellent electromagnetic shielding properties (Fig. 12g).

3.5. PEDOT:PSS-based hydrogel materials for EMI shielding

PEDOT:PSS has undergone comprehensive research and found applications in polymer-based hydrogels for wearable EMI shielding protection equipment, as well as flexible and self-healing hydrogel-based bioelectric devices due to its high solid phase conductivity, exceptional biocompatibility, as well as remarkable chemical and temperature stability. Previous studies by Agnihotri et al.¹⁷⁵ have demonstrated that PEDOT:PSS-based composites at low density (1.041 g cm⁻³) exhibited remarkable EMI SEs (70 dB), showcasing their immense potential for applications in flexible electromagnetic shielding. Moreover, PEDOT:PSS can be combined with other conductive fillers or non-conductive polymers to prepare hydrogels, thereby effectively attenuating EMWs.

In a study by Hao et al.,¹¹¹ the freeze-casting technique was utilized to synthesize a multifunctional Fe₃O₄/PVA/PEDOT:PSS composite hydrogel. This synthesis involved incorporating Fe₃O₄ clusters into a matrix composed of PVA/PEDOT:PSS hydrogel (Fig. 13a and b). The resulting hydrogel exhibited a high stretchability of 904.5%, as the concentration of Fe₃O₄ increased from 1% to 5%. However, its stretchability decreased from 904.5% to 373.5% when the Fe₃O₄ content reached 20%, which can be attributed to the weakened polymer intermolecular force caused by an excess of Fe₃O₄ nanoparticles doped in the hydrogel (Fig. 13c). Additionally, internal PEDOT:PSS and Fe₃O₄ nanoparticles established a 3D conductive network, resulting in a superb EMI SE exceeding 46 dB (Fig. 13d). This conductive pathway not only enhanced the hydrogel's conductivity but also significantly improved its reflection efficiency of EMWs. Furthermore, the EMI SE of the synthesized hydrogel was maintained at 99% even when subjected to a strain of 800% (Fig. 13e), due to its exceptional mechanical properties. Zhou et al.¹⁴⁴ designed PEDOT:PSS-PVA/PAA-tannic acid (TA) dual-network hydrogels (Fig. 13f). Benefiting from their cell porous structure, polymer framework, and interaction between water molecules and hydrophilic polymer chains, these hydrogels showcased remarkably efficient EMI shielding performances. At 7 vol% PEDOT:PSS content, the EMI SE reached up to 22.92 dB, blocking 99.49% of the EMWs (Fig. 13g). Furthermore, as the amount of PEDOT:PSS augmented, the SE_R increased slightly from 3.2 dB to 4.3 dB, and the SE_A increased from 11.5 dB to 18.4 dB (Fig. 13h). This implied that absorption contributed more significantly to the attenuation of EMWs than reflection did.

Fig. 13 (a) Fabrication of the Fe₃O₄/PVA/PEDOT:PSS composite hydrogels. (b) SEM photographs of the Fe₃O₄/PVA/PEDOT:PSS composite hydrogels. (c) Tensile stress–strain curve for the Fe₃O₄/PVA/PEDOT:PSS composite hydrogels with various Fe₃O₄ concentrations. (d) EMI SE of the Fe₃O₄/PVA/PEDOT:PSS composite hydrogels with varying components. (e) EMI SE of 5% Fe₃O₄/PVA/PEDOT: PSS composite hydrogels under varying tensile strains. (a)–(e) Reproduced with permission.¹¹¹ Copyright 2021, The Royal Society of Chemistry. (f) Schematic preparation of PEDOT:PSS-PVA/PAA-TA composite hydrogels. (g) EMI SE of PEDOT:PSS-PVA/PAA-TA composite hydrogels with various PEDOT:PSS concentrations. (h) Comparison of SE_T, SE_A and SE_R of PEDOT:PSS-PVA/PAA-TA composite hydrogels with various PEDOT:PSS concentrations. (f)–(h) Reproduced with permission.¹⁴⁴ Copyright 2022, The Royal Society of Chemistry.

Incorporating PEDOT:PSS into PAM hydrogels results in exceptional mechanical characteristics and enhanced electromagnetic performances, attributed to the establishment of conductive pathways and hydrogen bonding within 3D polymer networks. Zhu et al.¹⁴³ employed sodium sulfate decahydrate (SSD), MXene/PEDOT:PSS mixed-component fillers, and PAM organohydrogels to fabricate multifunctional organ hydrogels (PMP-SSD PCOHs) through one-step photoinitiation strategies (Fig. 14a), demonstrating excellent compressive-resilience properties and EMI shielding properties. The SEM images of PMP-SSD PCOHs showed a porous network structure, which included the filled SSD crystals (Fig. 14b). Due to the interaction of the polar groups of MXene/PEDOT:PSS with the active functional groups of the PAM chain to enhance mechanical strength, the obtained hydrogel exhibited enhanced corresponding compressive stress with increasing PEDOT:PSS/MXene filler content (Fig. 14c). Additionally, when the filler content was only 1.8 wt%, the outstanding EMI shielding performance reached 38.7 dB (Fig. 14d), due to the conductive network constructed by MXene/PEDOT:PSS filler and the 3D network of the hydrogel. Meanwhile, the reflection power coefficient values of PMP-SSD PCOHs were 0.55, 0.61, and 0.66 respectively, indicating that the EMI shielding capability of PMP-SSD PCOHs encompassed both absorption and reflection (Fig. 14e).

Fig. 14 (a) Synthesis procedure of PMP-SSD PCOHs. (b) SEM images of PMP-SSD PCOHs. (c) Compressive stress–strain curves of PAM hydrogels and PMP-SSD PCOHs at different mass percentages of MXene/PEDOT:PSS and SSD. (d) EMI SE of PMP-SSD PCOHs at different mass percentages of MXene/PEDOT:PSS and SSD. (e) Transmission, absorption, and reflection coefficient of PMP-SSD PCOHs at different mass percentages of MXene/PEDOT:PSS and SSD. (a)–(e) Reproduced with permission.¹⁴³ Copyright 2024, American Chemical Society.

4. CFH derived aerogels for EMI Shielding

In the domain of EMI management, materials that can shield against EMI while minimizing the reflection of EMWs are indispensable. These materials not only prevent the leakage of electromagnetic signals but also safeguard sensitive electronic equipment from external interference. Consequently, they exert a pivotal influence on curbing electromagnetic radiation and alleviating pollution, which is critical for sustaining the normal operation of electronic devices and protecting the electromagnetic environment.

Aerogels, self-supporting 3D porous materials, are characterized by high porosities, large pore sizes, and ultra-low densities. Transforming a shielding material into a porous aerogel and precisely regulating its internal structure through freeze-drying technology can significantly minimize the reflection of EMWs. The porous structure of the aerogel enables it to scatter and absorb electromagnetic waves instead of merely reflecting them into the surroundings. This multi-functional behavior is highly beneficial for effective EMI shielding. A prime example is the research carried out by Kong et al.⁵¹ They integrated PNF with MXenes and synthesized the heterostructure MXene@Ni via an in situ growth method. In this unique structure, the MXene@Ni/PNF component functions as an EMW-absorbing layer. It encompassed materials with high magnetic or dielectric losses, which were capable of converting the energy of incident electromagnetic waves into heat or other forms of energy. Simultaneously, the MXene/PNF part served as an EMW-reflecting layer. Its conductive properties allowed it to bounce back the electromagnetic waves, thus creating a multi-layer shielding mechanism. Subsequently, through a painstaking layer-by-layer freeze-drying process, the (MXene@Ni/PNF)-(MXene/PNF) aerogel was successfully fabricated. This innovative aerogel demonstrated extraordinary capabilities in efficiently absorbing, reflecting, and re-absorbing electromagnetic waves, effectively enhancing the EMI shielding performance.

Inspired by this successful structural design, researchers have delved deeper into the development of aerogels. One common approach for preparing composite aerogels is drying based on CFHs. During this process, the solvent inside the hydrogel is removed while maintaining the 3D network structure. This provides physical support for the conductive filler. As a result, stable conductive channels are established, and the interfacial bonding between the hydrogel substrate and the conductive particles is strengthened. This gives rise to a decrease in the interfacial resistance, concomitantly resulting in an elevation of the conductivity.

Notably, by leveraging the collaborative impact of remarkable electrical conductivity and the pore configuration, the impedance-matching properties can be optimized. This optimization enhances the reflection and multiple scattering of incident EMWs. Moreover, it serves to facilitate the interaction occurring between EMWs and the pore surface, resulting in further attenuation of the waves. Additionally, drying promotes the tight entanglement of polymer chains, which in turn enhances the intermolecular forces. Ultimately, by using post-treatment methods such as annealing,^176,177 dipping & curing,^178,179 blade coating^180,181 and in situ reaction^182,183 on the dried CFHs, it emerges as a material endowed with superb mechanical and electromagnetic characteristics for EMI shielding, aiming to assume a central role in the domains of military electromagnetic stealth and aircraft systems.

4.1. Aerogels prepared by MXene-based hydrogels for EMI shielding

Recently, MXenes have been hybridized with metal ions, magnetic nanoparticles, carbon materials and polymers to form hydrogels, which are subsequently dried into aerogels. These composite aerogels exhibit superior electrical properties and abundant scattering interfaces, rendering them ideal candidates for EMI shielding applications.

Composite hydrogels containing MXene and polymer substrates (such as SA, CS, and PVA) attain lightweight properties through post-treatment processes like drying, and possess characteristics such as chemical stability and workability, making them a significant choice for advanced-performance EMI shielding materials in the coming period. Wu et al.¹⁸⁴ fabricated an SA/MXene aerogel with exceptional conductivity and compressibility. The manufacturing procedure entailed the creation of hydrogels via the directional-freezing of an SA/MXene suspension. Subsequently, the ice crystals within the hydrogels sublimated as a consequence of freeze-drying, thereby leading to the formation of the corresponding aerogels (Fig. 15a). SEM imaging analysis demonstrated that the SA/MXene aerogel presented a macroscopically chaotic structure. Nevertheless, within particular localized areas, it showcased relatively well-ordered polygonal pores, with the aperture sizes spanning from tens of micrometers (Fig. 15b and c). Combined with the well-preformed interconnecting network of MXene nanosheets, conductivity led by the SA with copious and equitably dispersed carboxylic groups, the SA/MXene aerogel displayed the highest EMI SE of more than 72 dB at 95.24 wt% MXene content and an absorption-led shielding mechanism (Fig. 15d and e). Additionally, the obtained aerogels were employed to fabricate MXene foams by applying a thin polydimethylsiloxane (PDMS) coating. Encouragingly, the PDMS coating synergistically improved the aerogel's structural resilience and conductivity while retaining its porous architecture. Thus, PDMS-coated SA/MXene foams exhibited compressible EMI shielding performance with an initial EMI SE of 53.9 dB. Following 3 and 500 cycles at 30% strain compression, the EMI SE decreased to 48.5 dB and 48.2 dB respectively. A synthetic approach for the rational fabrication of MXene/CS composite aerogels was proposed by Wu et al.¹⁸⁵ This strategy involved the assembly of corresponding hydrogels via a freeze-drying process, followed by an appropriate thermal treatment (Fig. 16a). Firstly, the cross-linking of the MXene/CS mixed solution to form hydrogels could be achieved by the addition of glutaraldehyde. Then, the hydrogel underwent directional freeze-drying and an appropriate thermal treatment was carried out in an Ar atmosphere, causing the synthesis of hybrid carbon aerogels. Their interconnected spherical cell structures were attributed to the space occupied by the ice template. Meanwhile, increasing the MXene nanosheet concentration led to coarser pore walls in the hybrid aerogels due to the enwrapping between TiO₂ nanocrystals and MXene nanosheets. At an MXene concentration of 0.1322 vol%, the aerogel calcined at 800 °C displayed an exceptional EMI SE (61.4 dB) (Fig. 16b). A higher content of MXene nanosheets led to a greater number of heterostructures, thereby enhancing the interactions with incident EMWs, and the presence of insulating nanoparticles promoted the interaction with EMWs. Furthermore, the aerogel demonstrated enduring stability in EMI shielding, remaining effective even after exposure to a hygrothermal environment for 30 days. Profiting from ultra-low weight and outstanding water-repellent characteristics, it consistently retained a remarkable EMI SE of 40.5 dB at a thin thickness of 3 mm, which effectively blocked 99.99% EMWs (Fig. 16c). Guo et al.¹⁸⁶ employed PVA as a gelling agent for MXenes to synthesize PVA/MXene hydrogels. Subsequently, the hydrogels underwent directional freezing and were then freeze-dried to produce aerogels with superb EMI shielding capabilities. Meanwhile, they investigated the relationship between the thickness of cell walls, the size of pore channels and the shielding capacity of aerogels. The EMI SE_T of the obtained aerogels at 12.4 GHz dropped from 40.6 dB to 30.4 dB, as the temperature was decreased from −40 °C to −196 °C (Fig. 16d), which was caused by the decrease of average pore diameter and the wall thickness of the resultant aerogels. The presence of sufficient pores in the aerogels induced multiple reflections, which contributed to the saturation of EMI SE. Then, the dominant factor influencing EMI SE shifted towards wall thickness-controlled attenuation of EMWs, with thicker walls proving advantageous for EMWs dissipation. Consequently, the result illustrated that the EMI SE augmented in tandem with the increase in the size of pore channels and the thickness of cell walls (Fig. 16e).

Fig. 15 (a) The preparation of the SA/MXene aerogel and its PDMS-coated foam. (b) and (c) SEM images of the SA/MXene aerogel. (d) EMI SE of the SA/MXene aerogel. (e) SE_T, SE_R, and SE_A of the SA/MXene aerogel with various MXene contents. (a)–(e) Reproduced with permission.¹⁸⁴ Copyright 2020, Elsevier B.V.

Fig. 16 (a) The fabrication of the MXene/CS composite aerogels. (b) EMI SE of the MXene/CS composite aerogels at different MXene concentrations calcined at 800 °C in the X-band. (c) EMI SE of the MXene/CS composite aerogels following storage under conditions of 50 °C and 45–65% relative humidity for various durations of days. (a)–(c) Reproduced with permission.¹⁸⁵ Copyright 2022, Elsevier B.V. (d) EMI SE_T of PVA/MXene aerogels frozen at different temperatures. (e) Correlation between the EMI SE and the size of pore channels as well as the thickness of cell walls. (d) and (e) Reproduced with permission.¹⁸⁶ Copyright 2021, Elsevier B.V.

In addition, for fabricating high-performance MXene-based composite aerogels, carbon-based materials are employed as carriers. These carriers work together with MXene and other components to optimize the electromagnetic parameters of the materials, enabling the resulting composites to exhibit excellent EMI shielding performance. Zhao et al.¹⁸⁷ utilized graphene oxide (GO) sheets as an efficient gelation agent and assembled MXene/RGO hybrid hydrogels through a hydrothermal reaction, where RGO sheets served as the inner framework and MXene sheets were tightly attached to the cell walls of RGO. Based on the resultant hydrogels, MXene/RGO hybrid aerogels (MGA) with uniform cellular microstructure were created through the process of directional freezing, which was then followed by freeze-drying (Fig. 17a and b). MGA presented an impressive conductivity (1085 S m⁻¹) at the bulk density of 44 mg cm⁻³ (Fig. 17c). Upon being combined with epoxy resin, the MGA/epoxy composite aerogels with a low MXene concentration (0.74 vol%) still presented an excellent electroconductibility of 695.9 S m⁻¹, while demonstrating a superior EMI SE of 56.4 dB (Fig. 17d). Moreover, the MGA/epoxy composite aerogels achieved not only satisfactory EMI shielding performances with low filler loading, but also exceptional shielding with small thickness. By increasing the thickness to 1.5 and 2.0 mm, the average values of SE were rapidly enhanced to 39.0 and 52.7 dB (Fig. 17e), offering high flexibility to adjust the EMI attenuation performance.

Fig. 17 (a) The fabrication of the MGA. (b) SEM photographs of the MGA. (c) Contrast of electroconductivities for the MGA and GA at various bulk densities. (d) EMI SE of the MGA/epoxy composite aerogels. (e) Consequence of sample thickness on the EMI SE of the aerogels at 0.74 vol% MXene concentration. (a)–(e) Reproduced with permission.¹⁸⁷ Copyright 2018, American Chemical Society.

4.2. Aerogels prepared from RGO-based hydrogels for EMI shielding

Aerogels prepared from graphene-based hydrogels, ultralight materials with 3D microporous networks, dissipate electromagnetic waves via interfacial scattering and conduction loss within their conductive frameworks. They can be prepared through freeze-drying or capillary-induced assembly for graphene hydrogels. During the preparation process, the integration of polymer, magnetic, and electrical materials not only remarkably bolsters the mechanical robustness but also efficiently elevates the EMI shielding performance.

The inherent fragility of graphene aerogels limits their practical applications. Due to their mechanical reinforcement, thermal/chemical stabilities, biodegradability, and cost-effectiveness, cellulose and its derivatives have been integrated with graphene to fabricate composite hydrogels and derived aerogels to ameliorate the above problems. Cellulose/graphene aerogels thus demonstrate potential for next-generation EMI shielding materials characterized by low weight, pliability, and high-level shielding performance. Chen et al.¹⁸⁸ fabricated cellulose/RGO/Fe₃O₄ aerogels by first producing cellulose/GO hydrogels through the dispersion of GO sheets in the cellulose matrix and subsequently infiltrating vitamin C into the hydrogel to obtain cellulose/RGO hydrogels via in situ reduction, then undergoing freeze drying after immersion in FeCl₃/FeCl₂ solution (Fig. 18a). Their SEM images present open and highly porous structures (Fig. 18b). The EMI SE of the obtained aerogels reached a high value of 40.1 dB at 8 wt% RGO content (Fig. 18c), due to the enhancement of the efficiency of absorption and reflection for EMWs. This enhancement was caused by the presence of a conductive filler network and the improved dispersion of Fe₃O₄ particles by virtue of RGO. Liao et al.¹⁸⁹ proposed a facile approach to prepare GO-CNF/PMMA composite aerogels by mixing cellulose nanofibrils (CNF), GO, as well as polymethylmethacrylate (PMMA), followed by emulsification, gelation, and freeze-drying processes. Subsequently, annealing treatment was employed to obtain the RGO-CNF/PMMA aerogel with a regular cellular 3D skeleton framework (Fig. 18d). The compressive stress–strain curves demonstrated its ability to withstand an intense compressive strain amounting to 99.3%, indicating a high-level compressive strain and outstanding fatigue-resistant property (Fig. 18e). Furthermore, the obtained aerogel presented a satisfactory EMI SE of 36.75 dB due to the conductive skeletal structure constructed by RGO, which caused multiple scattering events of incident EMWs, leading to effective energy attenuation and conversion into heat energy (Fig. 18f). Erfanian et al.¹⁹⁰ successfully demonstrated a robust aerogel composed of electrochemically synthesized GO (EGO) and TEMPO-oxidized cellulose nanofibrils (TOCNF) (EGO/TOCNF aerogel) using high-resolution 3D printing via direct ink writing for precursor hydrogel and subsequent freeze-drying. TOCNF facilitated the establishment of connections between neighboring EGO nanosheets, thereby evidently forming mutually linked 3D porous architectures within the aerogels (Fig. 18g). As shown in compressive stress–strain curves, the aerogels at different EGO/TOCNF ratios showed compression modulus ranging from 250 to 1096 kPa (Fig. 18h). Additionally, the EMI SE increased from 24.9 to 55.5 dB by enhancing the EGO/TOCNF ratio from 25/75 to 75/25, attributed to the augmented electrical conductivity of the shield and increased availability of mobile charge carriers for interaction with EMWs (Fig. 18i).

Fig. 18 (a) The preparation of the cellulose/RGO/Fe₃O₄ aerogels. (b) SEM images of the cellulose/RGO/Fe₃O₄ aerogels. (c) EMI SE of the cellulose/RGO/Fe₃O₄ aerogels at a thickness of 0.5 mm in the X-band. (a)–(c) Reproduced with permission.¹⁸⁸ Copyright 2020, American Chemical Society. (d) SEM micrographs of the RGO-CNF/PMMA aerogel. (e) Stress–strain curves of the RGO-CNF/PMMA aerogels at varying compressive strains. (f) Comparison of EMI SEs for the RGO-CNF/PMMA aerogels. (d)–(f) Reproduced with permission.¹⁸⁹ Copyright 2021, Elsevier B.V. (g) SEM images of the EGO/TOCNF aerogels. (h) Stress–strain curves of the EGO/TOCNF aerogels. (i) The average EMI SE of the EGO/TOCNF aerogels. (g)–(i) Reproduced with permission.¹⁹⁰ Copyright 2023, Elsevier B.V.

The microstructure of aerogels prepared by graphene-based hydrogels can be precisely controlled through manipulation of their constituent hydrogel 3D architectures,^187,191,192 a strategy that enables systematic enhancement of EMI shielding performance. Li et al.¹⁹³ designed a novel approach for the fabrication of thermally annealed graphene aerogels (TGAs) with multilayer structures, achieving high EMI shielding performances. This was accomplished through the application of mechanical compression to the graphene hydrogel. Subsequently, the sample underwent freeze-drying, and was then thermally annealed at 900 °C (Fig. 19a). The uniaxially mechanical compression disrupted the original graphene cell wall, leading to a transformation of the hydrogel's microstructure from a honeycomb structure to a multilayer structure, which was fundamental in building an effective conductive network (Fig. 19b). As shown in its SEM images, it displayed composite microstructures comprising two distinct types of hierarchical layered architectures (Fig. 19c). Consequently, TGA-C5% (compressed to a height equivalent to 5% of the initial one) exhibited remarkable electroconductivity of 181.8 S m⁻¹ and EMI SE of 43.29 dB (Fig. 19d and e). Furthermore, a comparison was made between TGA-12.5 (with a GO density of 12.5 mg mL⁻¹) without undergoing compression and TGA-C40% (Fig. 19f). Interestingly, the latter showed superior performance, suggesting that the influence of mechanical compression on the EMI SE of TGAs primarily stemmed from alterations in density.

Fig. 19 (a) Fabrication procedure of the TGA. (b) Schematic illustrating reconstruction of the aerogels’ microstructure. (c) SEM micrographs and different shape images of TGA. (d) Electrical conductivities of TGAs at varying compression. (e) EMI SE_T, SE_A, and SE_R of TGA-C5%. (f) Contrast of the EMI SEs of TGA-C40% and TGA-12.5. (a)–(f) Reproduced with permission.¹⁹³ Copyright 2020, American Chemical Society.

To achieve significant improvements in both conductivity and EMI SE of aerogels prepared by graphene-based hydrogels, researchers have developed hybrid filler systems combining magnetic materials (e.g., Fe, Cu, Ni, and their oxides) with graphene and other conductive components. This synergistic design enhances EMW attenuation through complementary loss mechanisms. Subsequently, polymer matrices (e.g., epoxy,¹⁹⁴ polyurethane,¹⁹⁵ and PDMS¹⁹⁶) are infiltrated into the architecture of aerogels prepared by graphene-based hydrogels to further optimize EMW reflection capabilities while providing structural reinforcement. Huangfu et al.¹⁹⁷ demonstrated a nanocomposite aerogel with outstanding EMI shielding performance. In the preparation process, the Fe₃O₄/GO hydrogel was initially synthesized by combining functionalized Fe₃O₄ nanoparticles with GO and subsequently adding L-ascorbic acid, followed by freeze-drying and thermal annealing which made it transform into the thermally annealed graphene aerogel. Finally, through the template-casting method, they fabricated the composite aerogel (epoxy/Fe₃O₄/TAGA) (Fig. 20a). The epoxy resins presented smooth and dense morphologies thanks to their uniform dispersion in Fe₃O₄/TAGA's pores, which proved excellent interfacial compatibility between the epoxy resins and the Fe₃O₄/TAGA aerogel (Fig. 20b). Moreover, the EMI SE of the aerogels increased from 24 to 31 dB as the Fe₃O₄/TAGA loading increased from 1.2 to 2.7 wt%. This improvement is attributed to robust conductive networks formed by Fe₃O₄/TAGA within the epoxy matrix, which enhance the EMI SE (Fig. 20c). Similarly, Yang et al.¹⁹⁸ fabricated an epoxy/copper nanowire-thermally annealed graphene composite aerogel (epoxy/CuNWs-TAGA) through a two-step process. First, a freeze-drying procedure was carried out, which was followed by thermal annealing treatment. Subsequently, the epoxy resin was infiltrated into the 3D framework. The SEM images revealed that the pores within the 3D CuNWs-TAGA framework were filled with epoxy resin, and the CuNWs were evenly dispersed in the resultant aerogels (Fig. 20d), ensuring the exceptional comprehensive performance of the epoxy/CuNW-TAGA. When the mass fraction of CuNW-TAGA was 7.2 wt%, the obtained aerogels exhibited the maximum EMI SE (47 dB) and electroconductivity (120.8 S m⁻¹) (Fig. 20e and f), due to repeated reflection and absorption of EMWs in TAGA, which was caused by 3D CuNW-TAGA conductive networks. Song et al.¹⁹⁹ employed preconstructed GO hydrogels to prepare the corresponding aerogels via freeze-drying. After loading the surface functionalized FeNi alloy particles onto the aerogels’ skeleton, epoxy/RGO-FeNi composite aerogels were fabricated by in situ reduction and subsequent impregnation of epoxy resin with vacuum assistance (Fig. 20g). The fracture surfaces of epoxy/RGO-FeNi composite aerogels presented a highly oriented hexagonal honeycomb structure, which was significant for the enhancement of EMI shielding (Fig. 20h). It exhibited a remarkable EMI SE (46 dB) at 2.1 wt% RGO-FeNi content (Fig. 20i), benefiting from the establishment of the honeycomb-like structure and the strong dielectric loss, which optimized the impedance matching of the aerogels and improved the hysteresis loss of EMWs.

Fig. 20 (a) The synthesis of the epoxy/Fe₃O₄/TAGA. (b) SEM images of the epoxy/Fe₃O₄/TAGA. (c) EMI SE of the epoxy/Fe₃O₄/TAGA with different TAGA loading in the X-band. (a)–(c) Reproduced with permission.¹⁹⁷ Copyright 2019, Elsevier B.V. (d) SEM micrographs of epoxy/CuNWs-TAGA. (e) Electroconductivity and (f) EMI SE of the epoxy/CuNWs-TAGA at varying concentrations of CuNW-TAGA. (d)–(f) Reproduced with permission.¹⁹⁸ Copyright 2020, Elsevier B.V. (g) Fabrication of epoxy/RGO-FeNi composite aerogels. (h) SEM images of epoxy/RGO-FeNi composite aerogels. (i) EMI SE of epoxy/RGO-FeNi composite aerogels at various contents of RGO-FeNi. (g)–(i) Reproduced with permission.¹⁹⁹ Copyright 2022, Springer Nature.

The polymer-infiltrated composite aerogels prepared by graphene-based hydrogels construct a complete and uniform 3D conductive network at an ultralow filler loading, thereby leading to the attainment of remarkable EMI SE and conductivity, where absorption serves as the predominant shielding mechanism. Li et al.²⁰⁰ demonstrated a PDMS/RGO/AgNW aerogel with a low-loading filler by constructing 3D conductive skeletons resulting from freeze-drying RGO/AgNW hydrogel, followed by infiltrating PDMS into the above skeletons (Fig. 21a). The intact filler skeleton consisting of a RGO/AgNW bicontinuous network was clearly observable in the resultant composite aerogels (Fig. 21b). Therefore, PGAC with only 0.76 wt% RGO/AgNW loading presented superior conductivity of 10.6 S cm⁻¹ and EMI SE of 34.1 dB (Fig. 21c and d). Additionally, its average SE_T, SE_A, and SE_R in the X-band were 34.1, 31.1, and 3.0 dB, respectively (Fig. 21e). Throughout the entire frequency spectrum, the ability of the SE_A to attenuate EMWs was notably greater compared to that of the SE_R. Gao et al.²⁰¹ prepared a graphene/PDMS aerogel with a remarkably low loading of graphene. Firstly, GO hydrogels formed by a bidirectional freeze casting technique were freeze-dried to obtain GO aerogels, followed by thermal reduction and PDMS infiltration (Fig. 21f). As shown in the SEM images, graphene/PDMS aerogels demonstrated a visible porous structure at a high graphene content (Fig. 21g), and lamellar structure at a low graphene content (Fig. 21h). Under liquid nitrogen (LN) freezing conditions, a superb EMI SE (42 dB) was detected in a 0.49 wt% graphene/PDMS aerogel (Fig. 21i). More importantly, an exceptional EMI SE (65 dB) was shown by the composite aerogels after annealing the GO aerogels at 2500 °C (Fig. 21j), surpassing even that of metallic foils and solid materials containing abundant conductive fillers. In summary, it is commonly recognized that the hydrogel-based EMI shielding composites with low filler loading and comprehensive performances have significant potential for application in civil-military fields that are both economically and functionally promising.

Fig. 21 (a) Preparation process of PDMS/RGO/AgNW aerogels. (b) Morphologies of PDMS/RGO/AgNW aerogels. (c) Electrical conductivity, (d) EMI SE, and (e) the average SE_T, SE_R, and SE_A of PDMS/RGO/AgNW aerogels at various RGO/AgNW concentrations. (a)–(e) Reproduced with permission.²⁰⁰ Copyright 2019, Elsevier B.V. (f) Schematic illustrating fabrication of the graphene/PDMS aerogels. (g) and (h) SEM photographs of the graphene/PDMS aerogels. (i) EMI shielding performance for the graphene/PDMS aerogels at varying conductive filler content under LN freezing. (j) High EMI SE of 0.42 wt% graphene/PDMS aerogels. (f)–(j) Reproduced with permission.²⁰¹ Copyright 2020, Elsevier B.V.

4.3. Aerogels prepared by CNT-based hydrogels for EMI shielding

Benefiting from the low density, high electrical conductivity, remarkable strength, and significant conductive loss of CNTs,^202–205 researchers have found that the incorporation of CNTs into hydrogel systems creates conductive pathways and cross-linked networks. Subsequently, the hydrogels prepared into aerogels in various forms (such as cellular monoliths and thin films) can exhibit outstanding shielding efficiency and increased mechanical performance, albeit with some compromise in flexibility and biocompatibility compared to hydrogels.

Aerogels prepared by CNT-based hydrogels exhibit a densely cellular architecture that induces multiple reflections of incident EMWs, enabling the advancement of low-density and high-efficiency EMI shielding composites. However, the current reliance on energy-intensive vacuum drying to preserve the structural integrity and stability of these porous frameworks poses a significant barrier to their scalable application. Recently, Zeng et al.³⁴ employed a simple, energy-efficient, and freezing-thawing-drying approach to fabricate large-area CNT aerogels with unidirectional microhoneycomb pores via ambient pressure drying of nanofibrillated cellulose (NFC)/CNT hydrogels (Fig. 22a and d), thereby providing innovative solutions for addressing the aforementioned challenges. The SEM images revealed the presence of ordered and aligned pore channels, as well as randomly distributed isotropic pores in both longitudinal and transverse directions (Fig. 22b and c). Thus, CNT aerogels exhibited high transverse and longitudinal compression moduli of 47.5 and 85.9 kPa, respectively, thanks to the substantial aspect ratios of the CNT and NFC which effectively established physical crosslinking within the system (Fig. 22e). Moreover, it showed a high EMI SE of more than 70 dB, while a declining trend in SE was observed as the thickness decreased, attributed to the reduced overall amount of CNTs (Fig. 22f). They also explored the shielding performance disparity induced by orientation at angles of 0° and 90°. The difference was significant at the density of aerogels from 13 to 63 mg cm⁻³ (Fig. 22g), because CNT conductive paths and oriented pores were effectively preserved within the cell walls of the aerogels.

Fig. 22 (a) Synthesis of the CNT aerogels. (b) and (c) SEM micrographs of the longitudinal plane and transverse plane for the CNT aerogels. (d) Optical images of the large-area CNT aerogels. (e) Longitudinal and transverse compressive curves of the CNT aerogels. (f) EMI SE of the CNT aerogels at various thicknesses. (g) EMI SE of the CNT aerogels at various densities. (a)–(g) Reproduced with permission.³⁴ Copyright 2020, The Royal Society of Chemistry.

The aerogel films prepared by CNT-based hydrogels not only retain the unique porous properties of aerogels, but also integrate the advantages of the film material. Therefore, they possess the advantages of high stiffness, high porosity, fatigue resistance and high conductivity. This method has great prospects for developing electromagnetic interference shielding materials suitable for complex environments. Hu et al.⁷⁵ fabricated a hybrid aerogel film with environmental adaptability and superb EMI shielding capability, which involved freeze-drying of CNT/ANF hydrogels synthesized by blade coating and sol–gel technologies. Then, they coated it with fluorocarbon resin to obtain the CNT/ANF aerogel film (Fig. 23a). The adaptable and thin aerogel was capable of undergoing substantial deformation, and its cross-section presented an interconnected 3D porous network architecture (Fig. 23b and c). Benefiting from the fiber reinforcement, the obtained aerogel film demonstrated enhanced mechanical performance compared to pure ANF aerogels and exhibited a maximum strength of 5.2 MPa under a CNT loading of 27 wt%. However, with increasing CNT content, the strength decreased. This resulted from the aggregation of CNTs at high concentrations (Fig. 23d). The resultant aerogel film with 40 wt% CNT content also displayed an exceptional EMI SE (54.4 dB) at 568 μm thickness and outstanding electrical conductivity (230 S m⁻¹) (Fig. 23e–g). The aforementioned electromagnetic performance can be ascribed to the establishment of porous conductive networks dominated by CNT content.

Fig. 23 (a) The preparation of CNT/ANF aerogel films. (b) SEM photograph of CNT/ANF aerogel films. (c) Images of the CNT/ANF aerogel films. (d) Stress–strain curves of CNT/ANF aerogel films under various CNT concentrations. (e) Electrical conductivity, (f) EMI SE, and (g) SE_R, SE_A, and SE_T of the CNT/ANF aerogel films under various CNT concentrations. (a)–(g) Reproduced with permission.⁷⁵ Copyright 2020, American Chemical Society.

Based on recent representative research, we reveal the distinct property profiles between hydrogels and aerogels. Hydrogels, characterized by superior stretchability and self-repairing capabilities, show great promise for wearable electromagnetic shielding applications. Yet, their environmental stability remains a critical challenge. For instance, the above MSPPH hydrogel maintained 94.5% of its EMI SE of 41 dB even after undergoing 200% strain cycling and can self-heal to recover 72% of its ultimate tensile strength post-fracture,¹⁰⁰ demonstrating remarkable flexibility and self-regeneration. However, its water retention and mechanical strength were suboptimal. Even MXene organohydrogels,⁶² which exhibited relatively higher environmental stability, suffered significant shrinkage after 7 days of storage. When subjected to freezing at −25 °C for three hours, their SE_T dropped sharply from 30.8 dB to 14.4 dB. In contrast, aerogels exhibit lightweight and thermal insulation advantages, making them well-suited for EMI shielding in aerospace and high-temperature industrial applications. For example, MXene/CS-based aerogels presented a high EMI SE of 61.4 dB at a low density of 11.9 mg cm⁻³.¹⁸⁵ Additionally, the aerogel also provided significant thermal insulation (62.65 mW m⁻¹ K⁻¹ at 800 °C under Ar₂). The EMI shielding and thermal insulation performance can be retained even after storage in a hygrothermal environment for 30 days. However, aerogels often face drawbacks related to limited deformability and inherent mechanical brittleness, which restrict their practical applications. Therefore, this review highlights that future research should consider the composite modification of the above two materials to develop EMI shielding products that synergize their respective benefits.

5. Conclusions and prospects

In this review, the latest research progress in CFHs and their derived aerogels for EMI shielding is systematically reviewed. The fundamental concepts and EMI shielding mechanisms are first elucidated, followed by a detailed discussion of the key factors influencing the EMI shielding performance of CFHs. Subsequently, the fabrication methods and comprehensive performance of CFHs based on diverse conductive fillers are comprehensively summarized. Benefiting from their tunable electrical properties, CFHs can achieve outstanding EMI shielding performance, with the EMI SE reaching up to 125 dB when filled with RGO after composition and structure optimization,¹¹² while the modification of fillers, the filler concentrations, and the structure design all exert a substantial influence on the performance of CFHs. Moreover, CFHs can conformally adhere to objects featuring arbitrarily complex geometries and rapidly recover from damage. As a result, their potential applications in wearable electronics and artificial skin appear highly promising. Furthermore, CFH derived aerogels are also systematically reviewed, focusing on their preparation strategies and EMI shielding capabilities. Notably, when CNT-based hydrogels are dried, the resultant aerogels can achieve an EMI SE of 54.4 dB at a relatively thin thickness of 568 μm,⁷⁵ exhibiting ultralight properties and outstanding shielding capabilities. In general, the mechanical pliability and biocompatibility of the aerogels are inferior to CFHs. Consequently, they are less suitable for wearable and implantable devices but their ultralight properties and thermal insulation make it more appropriate for applications in the military and aerospace fields. Despite the fact that CFHs possess favorable comprehensive performance such as high biocompatibility, remarkable EMI shielding efficacy, and notable flexibility, rendering them with extensive application prospects in flexible wearable electronics and electronic skin, achieving the practical implementation of CFHs in complex environments for stable EMW shielding still presents numerous challenges that demand resolution. The specifics are outlined as follows.

5.1. Performance optimization of CFHs

Although conductive fillers can enhance the electrical conductivity and EMI shielding capability of hydrogels, there is still a significant performance gap compared to metallic materials such as silver, aluminum, and copper. Consequently, to meet the growing demand for high-performance shielding materials in future communication networks and highly integrated electronic devices, it is crucial to further improve the shielding performance of CFHs. The EMI SE of CFHs is greatly influenced by the composition, dispersion, and loading ratio of conductive fillers, as well as the network structure of hydrogels. Therefore, the shielding performance of hydrogels can be optimized from both the composition and structure aspects. First, optimizing the composition and loading ratio of conductive fillers, while balancing the mechanical properties of the hydrogels, is essential for improving the EMI shielding efficiency of CFHs and CFH derived aerogels. Furthermore, suitable structural designs,^206–208 including the hybridization of different nano-fillers, the chemical modification of fillers, and the employment of material assembly techniques to generate heterogeneous structures, as well as designing polymer network structures with appropriate mechanical properties, can further enhance their comprehensive performance.

5.2. Enhancing stability of CFHs

Due to their high-water content, hydrogels exhibit high sensitivity to environmental factors such as humidity and temperature. Over extended periods of use, hydrogels may suffer from performance degradation due to water evaporation or swelling effects, thereby compromising their stability in EMI shielding applications. Additionally, the interfacial interactions between conductive fillers and the hydrogel matrix may evolve, potentially leading to reduced conductivity and EMI SE. Their mechanical strength and toughness will also be significantly affected. To improve the long-term stability of CFHs and maintain their EMI shielding performance, various strategies can be employed, including applying hydrophilic/superhydrophobic surface modifications, integrating inorganic materials and performing surface modifications, depending on the specific conditions.^209,210 Furthermore, introducing functional materials with anti-oxidation,²¹¹ anti-UV,²¹² and anti-hydrolysis²¹³ can prolong their service life and preserve their EMI SE.

5.3. Scalable fabrication of CFHs

Conductive fillers, particularly high-performance materials such as graphene, CNTs, liquid metal, and MXene, are often expensive and require intricate preparation processes. Consequently, the controlled macroscopic preparation of CFHs and CFH derived aerogel systems involves relatively high costs. This poses economic challenges for large-scale applications, especially in civil and commercial sectors such as medical protection, flexible electronics, and wearable devices. The aforementioned problems could be alleviated through well-established products as starting materials and the simplification of the hydrogel fabrication process through technological advancements. This approach could lead to the development of high-performance conductive fillers that are not only cost-effective but also readily applicable in practical scenarios.

5.4. Intelligent response development of CFHs

Emerging research has increasingly focused on developing intelligent EMI shielding materials with dynamic effectiveness.^60,214 Subsequent efforts could involve integrating phase-change materials or shape-memory polymers with conductive fillers to fabricate composite hydrogels. These materials can undergo reversible phase transitions in response to external stimuli, enabling efficient and tunable shielding. This approach offers novel strategies for advancing adaptive EMI shielding and smart material development.

In summary, to facilitate the practical application of CFHs, the pivotal strategies involve refining the preparation methods, innovating the filler systems, and optimizing the composition and structural design to enhance the performance, stability, and cost-efficiency of the materials. It is of great significance to develop novel CFHs and CFH derived aerogel systems to explore their shielding capacities. It is believed that the challenges can be overcome through subsequent research, leading to significant improvements in their comprehensive performance and expansion into a broader spectrum of applications. This comprehensive review will provide fresh insights and guidance for the design of high-performance CFHs, thereby driving the advancement of next-generation EMI shielding materials.

Author contributions

Z. Z. contributed to writing the original draft, visualization, and conceptualization. Y. Z. was involved in writing the original draft, visualization, and conceptualization. Y. W. provided supervision and carried out writing review and editing. Y. H. was responsible for writing review and funding acquisition.

Data availability

All data discussed in this review are derived from publicly available scientific publications cited throughout the paper. The relevant datasets are accessible through the respective journals, as indicated in the reference list. No primary data were generated or analyzed specifically for the purpose of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from the National Key R&D Program of China (MoST 2020YFA0711500), the National Natural Science Foundation of China (NSFC, 22475111), the 111 project (B18030), and the State Key Laboratory of Polymer Materials Engineering (sklpme2023-3-13).

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Footnote

† These authors contributed equally to this work.

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