Conventional electromagnetic interference (EMI) shielding materials primarily emphasize either electromagnetic wave (EMW) absorption or reflection, yet fail to address escalating demands for lightweight design, flexibility, and multifunctional synergy required in emerging applications such as Internet-of-Things devices, wearables, and health monitoring systems [1,2]. With the increasing miniaturization and high-density integration of electronic devices, coupled with the complexity of application scenarios, research on EMI shielding materials is rapidly evolving from single-functionality towards multifunctional integration. Conductive hydrogels offer a promising solution, enabling tunable electrical properties through the design of polymer networks and conductive fillers while inherently retaining desirable characteristics like flexibility, adhesion, and biocompatibility [3,4]. Incorporation of diverse conductive fillers, such as silver nanowires [5], graphene [6], carbon nanotubes (CNTs) [7], and liquid metals [8] enables precise customization of electromagnetic characteristics. This unique combination has garnered significant interest not only in biomedical applications, flexible electronics, energy storage, and sensors [9] but also provides a novel framework for developing multifunctional, integrated EMI shielding materials. Nevertheless, a critical challenge remains: the intrinsic hydrophobicity of most conductive fillers compromises their compatibility with hydrophilic hydrogel matrices, potentially degrading mechanical integrity and electrical stability.
F, depending on the etching method) [12]. However, the unsaturated coverage of surface terminal groups often leaves M atoms at the edges or defects incompletely bonded [13]. Consequently, MXene is highly susceptible to oxidation under ambient conditions (dissolved oxygen, light, and heat) and degradation into metal oxides and amorphous carbon [[14], [15], [16]], which significantly compromises its stability during storage and practical applications. Current oxidation-mitigation strategies of MXene primarily focuses on three strategies: (1) Controlling external factors, such as low-temperature storage, inert gas environments, or substituting water with polar organic solvents, can effectively delay oxidation. (2) Surface modification techniques, including silanization [17] and heterocyclic aromatic amine functionalization [18], improve stability by forming covalent bonds between the MXene surface and organic molecules. Although these methods can alleviate oxidation, strict application conditions limit their practical application. (3) Another promising strategy is to passivate reactive M atoms by additives to reduce the oxidative tendency of MXene and provide greater economic efficiency. Commonly employed additives include polyanions (e.g., polyphosphates [19] and polysilicates [20]) and reducing agents (e.g., sodium L-ascorbate [21], β-mercaptoethanol [22], and natural phenolic compounds [23]). Among these, gallic acid (GA), a major natural phenolic constituent abundant in various plants and foods, is widely utilized in the food industry and biomedicine due to its antioxidant and antimutagenic properties [24]. The catechol structure of GA feature adjacent phenolic hydroxyl groups which are particularly susceptible to oxidation, forming a quinone structure. This reaction effectively depletes ambient oxygen, thus conferring antioxidant activity. In addition, the abundance of phenolic groups in GA provides numerous active sites. Combined with the excellent adhesion properties and biocompatibility, GA represents an ideal choice for enhancing the oxidation resistance of MXene. Yan et al. prepared a multifunctional antioxidant fabric based on Fe3+-assisted adhesion of MXene@GA dispersion on cotton fabric surface, achieving excellent EMI shielding (∼35 dB), dual-drive (Joule and solar) heating warmth, and infrared stealth performance [25].Conventional hydrogel fabrication typically requires extended processing times (e.g., freeze-thaw cycling [26]) or external energy input (e.g., UV light [27], heat [28], or ultrasound [29]) to induce polymer network formation. These requirements not only increase energy consumption but also exacerbate MXene oxidation, thereby degrading electrical conductivity. Consequently, developing rapid gelation strategies under mild conditions is crucial for enhancing production efficiency, reducing energy demands and minimizing MXene oxidation. Notably, MXene has been reported to enable rapid gelation with high concentrations of the initiator ammonium persulfate (APS) [30], eliminating the need for external energy input. The underlying mechanism involves the strong reducing capability of MXene, which participates in a redox reaction with APS, accelerating the generation of sulfate radicals. [31] However, this process inevitably intensifies MXene oxidation. To address this limitation, combinations of MXene with various metal derivatives have been proposed such as MXene-Zr4+ [32], MXene-Fe3+ [33], MXene-Cu2+ [34]. These systems leverage the redox characteristics of metal derivatives to further accelerate initiator decomposition, facilitating rapid gelation even at lower APS concentrations. Jiang et al. prepared a fast gelation-forming (52 s) hydrogel sensor by accelerating APS decomposition assisted by Fe3O4 and MXene under room temperature conditions and without external energy input. [35]. This approach effectively circumvents the severe MXene oxidation associated with high APS concentrations and mitigates potential health hazards, offering significant potential for developing conductive hydrogels with high production efficiency and long-term stability.
Herein, we present a multifunctional conductive hydrogel (denoted MMA hydrogel) featuring rapid, energy-efficient gelation synergistically assisted by MXene and CoFe₂O₄. GA was introduced to improve the antioxidant property of MXene in precursor solutions and further corroborated by density functional theory (DFT) calculations. The MMA hydrogels achieves gelation within 30 s at ambient temperature without external energy input, enabling the integration of dual-band EMI shielding, infrared stealth, and flexible strain sensing capabilities. Specifically, the three-dimensional conductive network formed by MXene confers outstanding EMI shielding effectiveness (SE) to MMA hydrogel, reaching 54.21 dB in the X-band and 74.64 dB in the Ku-band, with demonstrated long-term stability. Concurrently, the combination of low-thermal-conductivity glycerol and MXene effectively reduces free water content, retarding heat transfer. Consequently, upon reaching thermal equilibrium on a 90 °C heat source, the hydrogel surface exhibits a radiation temperature of merely 54.4 °C, effectively suppressing IR signatures from objects or human bodies. Furthermore, the MMA hydrogel exhibits excellent flexibility and adhesion, rendering it highly suitable for wearable strain sensors. Sensors fabricated from this material demonstrate high sensitivity (Gauge Factor, GF = 2.06 within 0–75 % strain), rapid response (119 ms), and robust stability. In summary, the MMA hydrogel, characterized by its exceptional oxidation resistance and ultrafast gelation kinetics, establishes a novel paradigm for developing lightweight, flexible, and multifunctional EMI shielding materials.
