Electrospinning Device for Nanofibers| Aramid Nanofiber/MXene-ReinforcedPolyelectrolyte Hydrogels for Absorption-Dominated ElectromagneticInterferenceShielding and Wearable sensing

Northwestern Polytechnical University's Professor Liu Xuqing & University of Manchester's Dr. Chen Liming: Aramid Nanofiber/MXene-Reinforced Polyelectrolyte Hydrogels for Electromagnetic Shielding and Wearable Sensing

With the rapid development of flexible electronic devices and high-density integrated circuits, electromagnetic radiation from electronic equipment has become a new environmental interference source following noise and water pollution, which can easily cause performance degradation or even functional failure of surrounding devices. To address electromagnetic pollution, it is crucial to develop electromagnetic interference (EMI) shielding materials that combine high shielding efficiency with flexibility. However, traditional conductive composite materials cause strong electromagnetic wave reflection due to impedance mismatch, achieving shielding effects at the cost of secondary pollution. Meanwhile, existing materials struggle to optimize impedance matching to enhance electromagnetic wave absorption while maintaining high conductivity. Research shows that polymer hydrogels based on the synergistic effects of porous structures, conductive networks, and polar groups can attenuate electromagnetic waves through multiple reflections, conductive loss, and polarization loss mechanisms, providing new insights for developing absorption-dominated EMI shielding materials.

Recently, Professor Liu Xuqing from Northwestern Polytechnical University and Dr. Chen Liming from the University of Manchester published their latest research titled "Aramid Nanofiber/MXene-Reinforced Polyelectrolyte Hydrogels for Absorption-Dominated Electromagnetic Interference Shielding and Wearable Sensing" in *Nano-Micro Letters*. The researchers utilized the unique properties of the polyelectrolyte 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and chitosan (CS), as well as the conductivity differences between aramid nanofibers (ANFs) and the two-dimensional material MXene, to construct ANF/MXene-reinforced polyelectrolyte hydrogels. These composite hydrogels exhibit outstanding EMI shielding performance, excellent mechanical properties, superior adhesion, and reliable human motion signal monitoring capabilities.This innovative design leverages the hydration of hydrophilic polar groups in polyelectrolyte molecular chains to generate intermediate water (IW) with high migration freedom, enhancing the ionic conductivity of the hydrogel while promoting the polarization relaxation and molecular rearrangement of water molecules under electromagnetic fields, thereby achieving absorption-dominated EMI shielding performance.

Fig. 1: Preparation process and design strategy of ANF/MXene-reinforced polyelectrolyte hydrogels

As shown in Figure 1, polar groups such as -SO3H and -NH2 on polyelectrolyte chains form bound water (BW) through hydration, which reconstructs hydrogen bonds with free water (FW) to produce highly mobile IW. This unique IW structure facilitates electromagnetic field-induced polarization relaxation and molecular rearrangement of water molecules, while the ion conduction pathways provided by polyelectrolytes significantly improve the system's conductivity. Experiments confirm that the hydrogel achieves efficient adhesion to human skin through non-covalent interactions such as electrostatic attraction and hydrogen bonds. The internally formed three-dimensional conductive network and activated water molecules work synergistically, demonstrating the integrated potential of absorption-dominated EMI shielding and wearable sensing functionalities.

Fig. 2: Basic characterization of composite polyelectrolyte hydrogels

As shown in Figure 2, SEM and rheological test results indicate that the polyelectrolyte hydrogels synergistically reinforced by ANF and MXene have more regular pore structures, with the introduced fillers simultaneously improving storage modulus and loss modulus. DSC tests were used to verify the differences between activated water and bulk water in the composite hydrogels. The ice crystals formed by bulk water undergo endothermic melting at approximately 0°C, while the broad absorption peak of the A5M1.5PC hydrogel at -19.81°C is attributed to the merging of IW and FW peaks under high water content. Notably, the strong directional hydrogen bonding interactions between polar groups in the hydrogel chains and water molecules restrict water molecule movement during cooling, preventing the detection of BW in the curve. Additionally, composite hydrogels rich in IW can lower the boiling point.

Further qualitative analysis of IW and FW via Raman spectroscopy shows that the O-H stretching vibration region of the A5M1.5PC hydrogel spectrum is deconvoluted into four sub-peaks based on hydrogen bond strength: peaks at 3226 cm⁻¹ and 3343 cm⁻¹ are attributed to FW molecules with two protons and two electron pairs participating in hydrogen bond formation, while peaks at 3468 cm⁻¹ and 3608 cm⁻¹ correspond to IW molecules weakly or non-hydrogen-bonded to adjacent water molecules. This demonstrates that activating water molecules through polyelectrolytes is an effective strategy to reduce the deformation and displacement resistance of water molecules caused by polarization relaxation in electromagnetic fields.

Fig. 3: Mechanical properties of composite polyelectrolyte hydrogels

Through systematic mechanical tests, the authors revealed the ANF/MXene synergistic reinforcement mechanism (Figure 3). The results show that ANF/MXene-reinforced composite hydrogels exhibit excellent mechanical strength and fracture elongation. Among them, ANF contributes more significantly to mechanical strength enhancement, while an appropriate amount of MXene helps form a uniform composite network structure. Based on hydrophilic/hydrophobic differences, the A5M1.5PC hydrogel forms a water-rich matrix phase and a hydrophobic ANF-enriched phase internally. Hydrophilic MXene forms stable connections with ANF and the hydrogel matrix through hydrogen bonds and electrostatic interactions, respectively, acting as a stress-transfer bridge to fully transfer matrix stress to the ANF reinforcement phase, thereby optimizing overall mechanical performance. Additionally, lap-shear test results prove that the composite hydrogel has strong adhesion, enabling seamless integration with the human body to ensure high-fidelity physiological signal acquisition.

Fig. 4: EMI shielding performance of composite polyelectrolyte hydrogels in the X-band

The authors investigated the effects of thickness, fillers, stretching, and water molecule state on the X-band EMI shielding performance of the composite hydrogels (Figure 4). Both the total shielding effectiveness (EMI SET) and absorption effectiveness (SEA) of the composite hydrogels significantly improve with increasing thickness, while the average reflection effectiveness (SER) shows minor changes. Filler composition has a notable impact on shielding performance: the filler-free A0M0PC exhibits the lowest shielding effectiveness; in single-filler systems, MXene outperforms ANF in shielding improvement due to its dominant conductive loss from forming a continuous conductive network; the dual-filler A5M1.5PC achieves the highest shielding effectiveness due to multiple internal reflections and interfacial polarization loss caused by the significant conductivity difference between MXene and ANF. When stretching is applied, shielding effectiveness decreases with increasing stretch ratio, attributed to thickness reduction and MXene conductive network breakage. Notably, the thickness change in the central region slows as tensile strain increases, resulting in a larger effectiveness drop when the stretch ratio increases from 0% to 50% compared to the 50% to 100% stage. To clarify the specific role of water molecules in EMI shielding, comparative tests were conducted on dried and frozen composite hydrogels. Drying drastically reduces shielding effectiveness and weakens the thickness effect, confirming that water molecules are key to the excellent shielding performance of the composite hydrogels. Ultra-low temperature (-80°C) freezing crystallizes IW and FW, limiting their polarization relaxation and displacement response to electromagnetic fields, leading to a significant drop in shielding effectiveness for the frozen state. These results indicate that activated water with weak intermolecular forces is more conducive to electromagnetic wave attenuation. The core mechanism by which polyelectrolyte chains optimize electromagnetic shielding performance by weakening hydrogen bond strength between water molecules is validated.

Fig. 5: EMI shielding performance of composite polyelectrolyte hydrogels in the THz band

As shown in Figure 5, the composite hydrogels exhibit outstanding shielding performance in the THz band. THz-TDS tests show that within the 0.2-3 THz range, the maximum EMI SET reaches approximately 110 dB, with absorption rates as high as 90%-99%. Unlike the X-band, THz shielding effectiveness is primarily dominated by the hydrogel matrix. THz tests on dried composite hydrogels reveal that they can still significantly consume incident waves but show weaker shielding effectiveness in the low-frequency THz range. The porous structure enables effective dissipation of high-frequency, short-wavelength THz waves even under water-free conditions, fundamentally because permanent dipoles in water molecules cannot respond to high-frequency electromagnetic field changes. Comprehensive analysis shows that water molecules exhibit superior electromagnetic wave attenuation capabilities in the X-band and low-frequency THz regions.

Fig. 6: EMI shielding mechanism of composite hydrogels

The authors systematically summarized and graphically analyzed the EMI shielding mechanism of the composite hydrogels, which involves a multi-level synergistic process (Figure 6).

Fig. 7: Strain sensing performance of composite hydrogels

The composite hydrogels demonstrate exceptional sensing performance and can serve as highly sensitive strain sensors for real-time motion monitoring (Figure 7).

Paper link: https://link.springer.com/article/10.1007/s40820-025-01791-4