PDA@MWCNTs/PEDOT:PSS enhanced polyacrylamide hydrogel for electromagnetic interference shielding, sensing, and infrared stealth applications

Summary

With technological advancement, electromagnetic interference (EMI) waves have begun to compromise equipment performance and human health, prompting significant interest in hydrogel electromagnetic shielding materials. In this study, a multifunctional EMI-shielding hydrogel was prepared via the hydrothermal method by incorporating dopamine-modified multi-walled carbon nanotubes (PDA@MWCNTs) and poly(3,4-ethylenedioxythiophene)-poly (polystyrene sulfonate) (PEDOT:PSS) into a polyacrylamide (PAM) matrix, enabling the formation of a conductive network within the hydrogel. This composite hydrogel exhibits an electromagnetic shielding performance of 39.72 dB in the 8.2–12.4 GHz frequency range, effectively attenuating electromagnetic waves, while maintaining a compressive strength of 3.69 MPa. Additionally, it exhibits infrared invisibility capabilities, maintaining a surface thermal equilibrium temperature of 52.2°C on a heating plate near 80°C. It also demonstrates outstanding motion detection sensing performance, capable of stable and repeatable resistive signal output. This study provides an effective strategy for preparing materials that simultaneously possess EMI shielding, sensing, and infrared invisibility functions.

Graphical abstract

Highlights

• •A multifunctional PAM/PDA@MWCNTs/PEDOT:PSS hydrogel was prepared via a one-pot method
• •PDA@MWCNTs prevent agglomeration and form a conductive network with PEDOT:PSS
• •The total electromagnetic shielding efficiency in the X-band reaches 39.72 dB
• •Possesses excellent infrared stealth capabilities and highly sensitive flexible sensing

Physics; Engineering; Materials science

Introduction

With the rapid advancement of electronic information technology and intelligent electronic devices, the operating frequencies and power levels of electronic equipment are progressively increasing. This trend has given rise to increasingly severe EMI and pollution issues.^1,2 These EMI issues not only compromise the performance of electronic equipment but also pose risks to human health and exert adverse effects on environmental safety.^3,4,5,6 Consequently, the research, development, and application of high-performance EMI shielding materials have attracted widespread attention. Traditional metallic EMI shielding materials comprise highly conductive substances such as iron, silver, and copper,^7,8 which deliver outstanding shielding performance in electromagnetic shielding applications.⁹ However, their inherent characteristics high density, high cost, poor flexibility, and susceptibility to corrosion have hindered their application in the field of flexible electronics.^10,11 Hydrogels readily accommodate modification through the incorporation of other functional fillers. Against this backdrop, conductive hydrogels exhibit promising prospects for application in flexible wearable electronics owing to their outstanding flexibility and favorable electrical conductivity.^12,13 The porous structure of hydrogels enables electromagnetic waves to undergo multiple reflections within them, thereby attenuating and weakening the waves. Furthermore, the substantial number of water molecules within the hydrogel dissipates electromagnetic wave energy through polarization effects, thus enhancing the electromagnetic shielding performance.^14,15 Conductive hydrogels, owing to their electrical conductivity, cause electromagnetic waves to experience impedance mismatch between free space and the surface of shielding materials, thereby reflecting them back. Electromagnetic waves entering the hydrogel, however, can have their energy attenuated through dielectric loss.¹⁶ Therefore, further improving the conductivity of hydrogels can increase the reflection loss of electromagnetic waves. Numerous studies have focused on incorporating substantial amounts of conductive materials into hydrogels, such as carbon nanotubes,¹⁷ MXene,¹⁸ graphene,¹⁹ and silver nanowires,²⁰ to enhance the hydrogel’s conductivity and thereby improve its EMI shielding performance. To balance the flexibility of hydrogels with their EMI shielding performance, optimizing the conductive network within hydrogels must be achieved alongside maintaining excellent strain tolerance. Generally, the continuity and structural integrity of the conductive network can effectively enhance EMI shielding.^21,22,23,24 In addition to increasing the filler content to enhance the continuity of the conductive network, fillers of varying sizes may be selected to create a hybrid conductive network.

Therefore, among numerous conductive fillers, MWCNTs, as one-dimensional conductive materials, possess characteristics such as a high aspect ratio, linear alignment, and excellent conductivity. They enable highly efficient charge transport along controllable directions, making them exceptionally well-suited for enhancing the conductivity of composite hydrogels.²⁵ Owing to the hydrophobic nature of MWCNTs, their poor dispersion in water leads to agglomeration during hydrogel preparation. This prevents the formation of a sufficiently complete and continuous conductive network within the hydrogel, while also adversely affecting the material’s mechanical properties. This can be achieved through various methods to uniformly disperse carbon nanotubes, such as surfactants, acid functionalization, and ultrasonication.²⁶ PEDOT:PSS conductive polymers facilitate the construction of three-dimensional conductive network structures within hydrogels.²⁷ Owing to their high charge injection capacity and electrochemical activity, conductive polymers exhibit outstanding ion-to-electron conversion capabilities. This enables rapid charge/ion transfer within the X-band spectrum, allowing these materials to achieve enhanced conductive losses.^28,29 When incorporated into hydrogels, they hold promise for forming hybrid conductive networks that combine high conductivity with structural stability. PAM, as a hydrophilic three-dimensional networked organic gel, possesses excellent flexibility and a porous structure. It effectively encapsulates water and conductive materials, thereby enhancing the electromagnetic shielding performance of the hydrogel. Yang et al.³⁰ incorporated multi-walled carbon nanotubes into hydrogels to produce PAM hydrogels exhibiting a conductivity of 0.85 s/m and an electromagnetic shielding effectiveness of 28.5 dB.

Herein, this paper designs a hybrid conductive network hydrogel electromagnetic shielding material incorporating MWCNT and PEDOT:PSS conductive substances to establish synergistic interactions. Owing to the poor dispersibility of MWCNTs, PDA@MWCNTs was prepared via the oxidation and self-polymerization of hydrochloride dopamine (DA) under alkaline conditions, thereby imparting additional hydrophilic functional groups. The PAM hydrogel matrix formed by the polymerization of acrylamide (AM) undergoes physical cross-linking with PDA@MWCNTs and PEDOT:PSS through its abundant amide groups, thereby enhancing mechanical properties and the structural integrity and continuity of the conductive network. This paper presents a multifunctional PAM hydrogel composite material exhibiting electromagnetic shielding, sensing capabilities, and moderate thermal insulation properties. It holds significant application potential in the fields of EMI shielding, human motion rehabilitation monitoring, and infrared stealth technology.

Results and discussion

Microstructural characterization of PDA@MWCNTs hydrogel composites

The preparation of fillers and hydrogel composite materials is shown in Figure 1. A series of characterization studies was conducted on the pristine MWCNTs and their composite hydrogels, alongside verification of the successful deposition of self-oxidative polymerization via DA onto the pristine MWCNT surfaces. Figure 2 illustrates the microstructure and chemical composition of the reaction raw materials and their reaction compounds. As shown in Figure 2A, the SEM image of the pristine MWCNTs reveals a relatively dense distribution of carbon nanotubes. Due to the strong intermolecular van der Waals forces, the carbon nanotubes exhibit a pronounced tendency to agglomerate, resulting in a bundled and entangled appearance. To address the agglomeration of MWCNTs, this experiment employed DA compounds that were oxidized and self-polymerized into polydopamine (PDA) within an alkaline environment. This PDA was then loaded onto the surface of the carbon nanotubes, imparting hydrophilic functional groups—specifically hydroxyl and amino groups—to the carbon nanotubes. This modification enabled their uniform dispersion within aqueous solutions. Figure 2B shows the SEM image of PDA@MWCNTs(A@T), revealing that the carbon nanotubes exhibit significant voids and a distinctly fluffy appearance, presenting a marked contrast to the image in a. The EDS elemental mapping of A@T in Figure 2C reveals the presence of carbon, nitrogen, and oxygen uniformly distributed across the surface of the carbon nanotubes. Therefore, the introduction of phenolic hydroxyl and amino hydrophilic groups may reduce the intermolecular van der Waals forces between carbon nanotubes and increase the inter-tube spacing, thereby enhancing their dispersibility in aqueous solutions.

Figure 1.

Schematic illustration of the preparation of dopamine hydrochloride-modified multi-walled carbon nanotubes and M/A@T/S hydrogel composites

Figure 2.

Microstructural characterization of A@T hydrogel composites

(A) SEM image of MWCNTs. (Scale bars, 500 nm).

(B) SEM image of A@T. (Scale bars, 500 nm).

(C) EDS image of A@T. (Scale bars, 200 nm).

(D) SEM image of PAM hydrogel. (Scale bars, 20 μm).

(E) SEM image of M/S. (Scale bars, 20 μm).

(F) SEM image of M/A@T/S. (Scale bars, 20 μm).

(G) XRD pattern of composite hydrogel.

(H) Infrared spectrum of composite hydrogel.

(I) Infrared spectra of AM and A@T.

Figures 2D–2F show SEM images of the PAM, PAM/PEDO:PSS(M/S), and PAM/PDA@MWCNTs/PEDOT:PSS(M/A@T/S) hydrogels, respectively. These images confirm that the PAM hydrogel exhibits a typical porous three-dimensional network structure, with pores appearing either as isolated cavities or interconnected channels. Figure 2D shows thin pore walls, with pores distributed unevenly and exhibiting non-uniform pore sizes. The pore size distribution in Figure 2E is more uniform than that of the pure PAM hydrogel, exhibiting increased pore continuity. The entire cross-section appears smoother, revealing a network-like structure. This conductive polymer contains highly polar sulfonic acid groups that form hydrogen bonds with PAM, thereby restricting the disordered aggregation of PAM chains and rendering the hydrogel structure more uniform. In Figure 2F, the composite hydrogel exhibits a more abundant pore structure, with the majority of pores being interconnected. The phenolic hydroxyl groups in A@T and the sulfonic acid groups in PEDOT:PSS are uniformly distributed throughout the PAM hydrogel, generating hydrogen bonding interactions that restrict excessive cross-linking of additional PAM molecular chains. Consequently, the incorporation of two conductive materials thickens the hydrogel pore walls while simultaneously enriching the pore network, resulting in more uniform pore sizes and forming a more complete and continuous conductive network structure. The XRD pattern of the PAM-based composite hydrogel (Figure 2G) exhibits a broad diffraction peak at 21.5°, which originates from the amorphous PAM matrix, amorphous polydopamine coating, low crystallinity MWCNTs, and amorphous PEDOT:PSS, indicating that the composite hydrogel material possesses an amorphous structure. This suggests that the incorporation of carbon nanotubes and conductive polymers has not significantly altered the crystalline phase structure of the hydrogel. FT-IR testing of hydrogel material samples revealed their chemical composition, as shown in h. All hydrogel materials exhibited four distinct characteristic peaks near 3180 cm⁻¹, 1650 cm⁻¹, 1600 cm⁻¹, and 1450 cm⁻¹, corresponding respectively to the stretching vibrations of N–H and C=O, and the bending vibrations of N–H and C–H. The obvious shift of N–H and O–H peaks confirms the formation of strong hydrogen bonds between PAM (–CONH₂), PDA (–OH/–NH–), and PEDOT:PSS (–SO₃⁻). These peaks correspond to the characteristic peaks of AM shown in Figure 2I. In the PEDOT:PSS hydrogel, the M/S and M/A@T-4/S-2hydrogels exhibit characteristic S=O stretching vibration peaks near 1050 cm⁻¹. The A@T exhibits N–H and C–O stretching vibration peaks at 3130 cm⁻¹ and 1294 cm⁻¹ in the infrared spectrum, respectively, indicating DA self-polymerization loading onto the surface of carbon nanotubes.

XPS characterization of hydrogel samples

To determine the surface chemical composition of the hydrogel, it was further characterized using XPS. The results indicate that both PAM and M/A@T-4/S-2 hydrogels exhibit detectable levels of carbon, nitrogen, and oxygen, with the M/A@T-4/S-2 hydrogel additionally displaying sulfur (Figures 3A and 3E). This indicates that the conductive polymer has successfully combined with the PAM hydrogel. High-resolution C1s XPS spectra (Figures 3B and 3F) reveal that the peak at 287.94 eV corresponds to the amide carbon (N–C=O) in PAM, exhibiting a shift of +0.06 eV in the M/A@T-4/S-2 hydrogel. The peak at 284.8 eV corresponds to C–C/C–H.³¹ Figure 3F exhibits a C–N/C–O bond at 288.6 eV, representing a C–N shift of +0.22 eV relative to Figure 3B; The C–O bond may originate from the phenolic structure of PDA and PEDOT; the peak at 288.92 eV corresponds to the C–S bond in PEDOT:PSS, matching the C–S peak in FT-IR Figure 3H. The positive shift in the amide carbon and C–N bond energies indicates a reduction in electron density within the amide functional group, potentially arising from composite interactions. The fine spectrum of N1s reveals that the peak at 399.6 eV corresponds to the amide nitrogen, whilst the peak at 402.09 eV in Figure 3G corresponds to the R–NH–R chemical bond in PDA.³² The fine spectrum of O1s reveals peaks at 531.45 eV and 531.43 eV corresponding to the carbonyl group within the amide functional group of PAM; The peak at 532.97 eV in Figure 3D corresponds to the O–H bond of bound water in PAM,³¹ while the peak at 533 eV in Figure 3G corresponds to the phenolic hydroxyl group in PDA and the C–O bond in PEDOT. A new S2p peak at 168.04 eV appears in Figure 3E, corresponding to the S2p fine spectrum in Figure 3I. Peaks at 162.89 eV and 164.14 eV correspond to S atoms in PEDOT, while peaks at 167.13 eV and 168.32 eV correspond to S atoms in PSS,³³ thereby further confirming the successful preparation of the composite hydrogel material.

Figure 3.

XPS Characterization of hydrogel samples

(A) Full XPS spectra of PAM hydrogel for C, N, and O. High-resolution XPS spectra: (B) C1s, (C) N1s, (D) O1s. (E) Full XPS spectra of M/A@T-4/S-2 hydrogel for C, N, O, and S. High-resolution XPS spectra: (F) C1s, (G) N1s, (H) O1s, and (I) S2p.

Physical Properties of Hydrogel Materials

Physical property tests were further conducted on the cylindrical composite hydrogel (height 2 cm, diameter 2 cm). Figure 4F illustrates the flexibility test, wherein the hydrogel exhibited no damage after undergoing twisting and curling. Upon removal of external forces, the hydrogel recovers its initial morphology, demonstrating its excellent toughness. Compression testing of the prepared hydrogels, as shown in Figure 4A, revealed that the pure PAM hydrogel could withstand forces up to 526 N, whereas the addition of PEDOT:PSS enabled a maximum force of 604 N. The enhanced performance of this hydrogel may be attributed to hydrogen bonding interactions between the sulfonate groups in the conductive polymer and the amide groups on the PAM hydrogel. This non-covalent interaction effectively disperses stress, thereby improving the hydrogel’s compressive properties. Compared to the compressive strength of 1.71 MPa of the PAM hydrogel, the compressive strength of the M/A@T-4 hydrogel increased to 3.69 MPa (Figure 4B) Owing to the inherently excellent mechanical properties of MWCNTs,³⁴ their incorporation of this material significantly enhances the compressive performance of the hydrogel. Concurrently, the hydrogel containing A@T with polar functional groups can be dispersed relatively uniformly within the PAM matrix, effectively achieving stress dispersion throughout all regions of the hydrogel. Forming hydrogen bonds with the amide groups in PAM, it acts as a “bridge” to firmly link PAM with MWCNTs. This strong interfacial bonding enables stress transfer from the flexible PAM matrix to the relatively rigid filler. Upon incorporation of PEDOT:PSS into the M/A@T-4 hydrogel, its compressive strength decreased to 3.5 MPa. The crystallinity and thermal stability of the hydrogel materials were tested, as shown in Figure 4D and 4E. The peak shape of the pure PAM hydrogel was relatively sharp, indicating the formation of a relatively complete crystalline structure. The crystallization temperatures of the hydrogels after adding fillers were correspondingly elevated, with broadened peak shapes, possibly due to the fillers interfering with the formation of ordered chains within the hydrogels. In the melting curve, due to the predominant water content within the hydrogel material, its melting peak occurs between 0°C and 2°C. By testing the water absorption capacity of the hydrogels, as shown in Figure 4C, after 10 h of water absorption, the PAM hydrogel exhibited a water absorption capacity of 811%, while the M/A@T-4/S-2 hydrogel achieved a water absorption capacity of 1113%. Figure 4F demonstrates its abundant and interconnected porosity. Furthermore, the presence of hydrophilic functional groups in both A@T and PEDOT:PSS enhances the interfacial compatibility between the hydrogel and water, thereby reducing water permeation resistance. Figure 4G illustrates the compression mechanism of hydrogels. Hydrogels based on PAM exhibit a continuous three-dimensional network structure encapsulating numerous water molecules. The incorporation of fillers enhances physical cross-linking interactions, thereby effectively counteracting energy dissipation. This composite hydrogel material exhibits favorable mechanical properties, rendering it well-suited for applications within the field of flexible electronic wearables.

Figure 4.

Physical properties of hydrogel materials

(A) Compression curve of hydrogel.

(B) Compressive strength and compressive modulus of hydrogel.

(C) Water absorption and swelling curve of hydrogel.

(D and E) Crystallization and melting curves of hydrogel.

(F) Flexibility characterization of hydrogel. (Scale bars, 3 cm).

(G) Schematic diagram of hydrogel compression.

Hydrogel electromagnetic interference shielding performance

The electrical conductivity and porous structure of materials play a crucial role in the reflection and attenuation of electromagnetic waves. For conductive materials, incident electromagnetic waves induce the abundant free electrons to oscillate in response to the electromagnetic field, generating surface currents that reflect the incoming waves. This forms an electric current within the conductive material, reflecting the electromagnetic waves. When electromagnetic waves undergo reflection and attenuation within the shielding material, the extensive porous structure increases the number of multiple reflections, thereby enhancing the electromagnetic shielding effectiveness. Figures 5A and 5B display the total EMI shielding effectiveness SE_T and SE_A of the PAM composite hydrogels in the X-band, respectively. The pure PAM hydrogel exhibits an SE_T value of 30 dB and an SE_A value of 25.9 dB. Upon incorporation of PEDOT:PSS and PDA@MWCNT, the M/S-1.15 and M/A@T-4 hydrogels achieve SE_T values of 33.6 dB and 36.7 dB, respectively, accompanied by increases in their SE_A values. When both conductive fillers are introduced simultaneously, the M/A@T-4/S-2 hydrogel attains an SE_T of 39.72 dB and an SE_A of 35.1 dB, indicating that the two conductive fillers can form a synergistic conductive network, thereby enhancing the electromagnetic shielding performance. These results demonstrate that the two conductive fillers increase conduction loss, leading to greater absorption loss in the hydrogels. As both fillers are highly conductive, this indirectly suggests that their addition enables the formation of a more complete conductive network within the hydrogel matrix. Experiments indicate that these hydrogels are predominantly SE_A-based. Considering that frequency variations may cause fluctuations in test data, as shown in Figure 5D, the mean values of SE_T, SE_A, and SE_R for the hydrogels demonstrate that the incorporation of both conductive substances elevated the mean value of PAM from 26.3 dB to 33.6 dB. In the aforementioned electromagnetic shielding efficiency metrics, although SE_A predominates within SE_T, electromagnetic shielding materials exhibiting high absorption efficiency do not necessarily absorb the majority of electromagnetic wave energy. This is because electromagnetic waves must first undergo reflection before penetrating the material: SE_R characterizes the ratio of reflected energy to incident energy, whereas SE_A characterizes the ratio of absorbed energy to energy entering the material. Clearly, the denominators differ in their respective calculations.²⁰ To gain further insight into the mechanism of electromagnetic shielding, the coefficient values for R, A, and T were calculated separately. These coefficients provide a direct indication of the actual energy loss of electromagnetic waves. As shown in Figure 5E, which presents the average values for all three, the R values consistently exceed the A values, while the T values approach zero. This indicates that electromagnetic waves scarcely penetrate the hydrogel material, and that the shielding effect of this material is predominantly achieved through reflection.

Figure 5.

Hydrogel electromagnetic interference shielding performance

(A–C) The electromagnetic interference shielding effectiveness (SET, SEA, SER) of the 3-mm-thick composite hydrogel was measured in the X-band.

(D) Mean values of SE_T, SE_A, and SE_R for hydrogels.

(E) R, T, and A values of hydrogels.

(F) Comparison with previously reported EMI performance in the literature.^{25,30,35,36,37,38,39,40,41,42,43,44}

(G) Electromagnetic interference shielding mechanism of hydrogels.

Figure 5F illustrates the electromagnetic shielding mechanism of the hydrogel material discussed herein. When incident electromagnetic waves reach the surface of the hydrogel material, the impedance mismatch with the surrounding air causes the waves to be reflected, thereby achieving the effect of shielding against electromagnetic interference. When electromagnetic waves enter a porous hydrogel, multiple reflections within it can achieve an attenuation effect. In carbon nanotubes, the delocalized π electron cloud generates directional currents under the influence of electromagnetic fields. Due to the material’s electrical resistance, the high-speed movement of electrons encounters friction, converting electromagnetic energy into thermal energy to achieve conduction losses.⁴⁵ Highly polar water molecules can align themselves along the direction of the electric field within an electromagnetic field. Constrained by the numerous hydrogen bonds within the hydrogel, their rotational speed cannot keep pace with the rapid changes in the electric field, resulting in ‘polarisation relaxation’. This lag phenomenon causes electromagnetic energy to be converted into thermal energy, thereby generating polarisation loss.⁴⁶ In summary, hydrogel materials featuring dual conductive networks exhibit excellent electromagnetic shielding properties. This arises from the mismatch between the spatial output of electromagnetic waves and their surface impedance, coupled with the combined effects of conduction losses from conductive fillers and polariZation losses from water molecules. Ultimately, trace residual electromagnetic waves penetrate the hydrogel material, with multiple electromagnetic shielding mechanisms working synergistically to achieve effective electromagnetic shielding. Consequently, the conductivity and porosity of this hydrogel hold potential applications in sensing and thermal insulation.

Thermal insulation properties of hydrogel samples

Hydrogels must contend with complex environments whilst shielding against electromagnetic interference; consequently, infrared thermal imaging is employed to evaluate the thermal insulation properties of hydrogel materials. Place the hydrogel material on a heating plate maintained at 79°C for 10 min, recording its state at various intervals using a thermal imaging camera. As shown in Figure 6F, with increasing heating time, the surface temperature of the hydrogel material first rises rapidly toward equilibrium, reaching thermal equilibrium at approximately 10 min. As can be observed from a to Figures 6A–6E, the pure PAM hydrogel exhibits the poorest thermal insulation performance, with a temperature difference of 20.5°C relative to the ambient environment. Conversely, the hydrogels incorporating both conductive fillers demonstrate the most effective thermal insulation, achieving a temperature difference of 26.8°C relative to the ambient temperature. Combining Figures 6D–6F, it is concluded that the number and continuity of hydrogel pores may exert a certain influence on the material’s thermal insulation properties. During the heating process, thermal imaging reveals a relatively uniform color distribution across the sample surface, indicating that the highly conductive carbon nanotubes are evenly dispersed. According to the principles of heat transfer, the process is categorized into thermal conduction, thermal convection, and thermal radiation. Given that all hydrogels operate under identical environmental conditions, thermal radiation cannot be the primary factor altering the thermal insulation performance. In hydrogel composites, water serves as the mobile phase, enabling thermal convection during heating. Polyacrylamide polymers and multi-walled carbon nanotubes function as thermally conductive solids, facilitating a “thermal convection-thermal conduction-thermal convection” heat transfer mechanism within the material. Due to differing thermal conductivities, an interfacial thermal resistance arises between the two phases, thereby achieving thermal insulation. Therefore, the dispersion and continuity of pores may play a significant role in the thermal insulation properties of hydrogel materials. In summary, polydopamine-modified MWCNTs and PEDOT:PSS are uniformly distributed within the PAM hydrogel, constructing a continuous porous framework structure. This architecture features abundant solid-liquid interfaces, which increase interfacial thermal resistance and reduce the rate of heat conduction. Figure 6G demonstrates the infrared stealth capability of the hydrogel material under low-temperature isothermal conditions. It can be observed that the thermal imaging surface color is virtually identical to that of the surrounding environment, indicating that this hydrogel material possesses a degree of infrared stealth capability.

Figure 6.

Thermal insulation properties of hydrogel samples

(A–E) Infrared thermal images of hydrogels on a constant-temperature heating plate at 79°C.

(F) Surface temperature versus time plot for the hydrogel.

(G) Infrared thermal imaging of the M/A@T/S hydrogel.

Sensing properties of hydrogel materials

The M/A@T-4/S-2 hydrogel exhibits favourable mechanical properties and electrical conductivity, rendering it suitable for human motion monitoring applications. Figure 7 illustrates the monitoring of changes in skin resistance at different locations on the volunteer. Figure 7A illustrates the changes in electrical resistance signals generated by muscle movements during human frowns. Figure 7B illustrates neck movements during head-down and head-up positions in humans. The monitored resistive signals subsequently change synchronously with neck movements. Additionally, changes in signals at the knee joint can be observed during leg extension (Figure 7G). Figures 7C and 7D display the electrical resistance signals at the elbow and finger joints. This sensor not only detects resistance signals but also determines the degree of joint flexion based on signal intensity, with each amplitude corresponding to relatively stable changes in the resistance signal. Figure 7E depicts rapid and periodic flexion movements at the wrist, demonstrating that the sensor connected to the hydrogel can deliver stable and repeatable resistive signals. Figure 7F shows monitoring of movement at the tarsometatarsal joint during stretching and recovery, with the resistance signal exhibiting no discernible lag phenomenon. This is because the stable and uniform conductive network formed by PDA@MWCNTs and PEDOT:PSS ensures a reliable resistive response under external deformation, hydrogen bonding interactions between PAM, PDA, and PEDOT:PSS endow the hydrogel with excellent mechanical flexibility and structural recoverability, preventing failure of the conductive network during cyclic deformation. The tunable conductive pathways driven by the porous structure enable the hydrogel to convert mechanical strain into sensitive, repeatable, and stable resistance signals, thereby enabling high-performance human motion sensing.

Figure 7.

Sensing properties of hydrogel materials.(ΔR/R₀:relative resistance change rate, ΔR = R−R₀, where R₀ is the initial resistance and R is the real-time resistance) application of M/A@T/S hydrogel strain sensors in monitoring human movement

(A) Glabellar movement.

(B) Neck movement.

(C) Elbow flexion at varying amplitudes.

(D) Finger joint monitoring at different flexion angles.

(E) Wrist flexion monitoring.

(F) Metatarsophalangeal joint flexion/extension monitoring.

(G) Knee flexion monitoring.

Conclusions

This study investigated the synergistic effects of A@T and PEDOT:PSS as conductive fillers and evaluated their performance in electromagnetic interference (EMI) shielding, sensing, and thermal insulation applications. Due to the strong van der Waals forces between carbon nanotubes, agglomeration occurs within the hydrogel, thereby impeding the formation of a continuous conductive network. Employing DA to oxidize and self-polymerize under alkaline conditions endows the MWCNT surface with hydrophilic functional groups. Blending this with PEDOT:PSS enhances the hydrogel’s conductivity, enabling the construction of a continuous, intact conductive network structure. This improves the reflection loss and absorption loss of electromagnetic waves in EMI shielding materials. The M/A@T-4/S-2 hydrogel achieves an EMI shielding effectiveness of up to 39.72 dB in the X-band, and its compressive stress is 3.69 MPa. On a heating plate maintained at nearly 80°C, the surface temperature reached 52.2°C after 10 min of thermal equilibrium. Compared with existing relevant research, this study achieves the multifunctional integration of electromagnetic shielding, motion sensing, and infrared stealth within a single polyacrylamide-based hydrogel, demonstrating an excellent balance between mechanical and functional properties. Future Prospects: We will further optimize the conductive network structure to achieve higher electromagnetic shielding efficiency; introduce self-healing and antibacterial functions to adapt to complex wearable application scenarios. This study provides a feasible approach for the preparation of integrated composite materials that combine electromagnetic shielding, sensing and detection, and infrared stealth capabilities (Table 1).

Table 1.

Comprehensive comparison of EMI shielding composites between this work and 11 literature reports

No	Material system	Conductive filler	SET (dB)	Thickness	Multifunctional integration	Reference
1	PS/PMMA/MWCNT	MWCNT	23.08	2 mm	EMI shielding only	Chen et al.35
2	PLA/PCL/CNT/Ni@CNT	CNT + Ni@CNT	36.7	1 mm	absorption-dominated EMI shielding	Huang et al.37
3	PAI/MWCNT	MWCNT	17.0	1.2 mm	EMI shielding only	Rani et al.25
4	PAM/PVA/GO/LiCl hydrogel	GO + Li+ ions	37.2	3 mm	EMI shielding + anti-dehydration + anti-icing	Zhou et al.39
5	LDPE/MWCNT/GNP/MgFe2O4	MWCNT + GNP + MgFe2O4	34.0	3.5 mm	EMI shielding only	Kumar et al.42
6	MWCNT/polymer composite	MWCNT	28.5	2 mm	EMI shielding only	Bizhani et al.43
7	CNT/polymer composite	CNT	32.4	2 mm	EMI shielding	Wen et al.40
8	Conductive hydrogel	Carbon-based filler	35.8	3 mm	EMI shielding	Li et al.38
9	Multifunctional polymer composite	Hybrid conductive filler	31.6	2.5 mm	EMI shielding	Saeed et al.41
10	Self-healing conductive hydrogel	Carbon filler	33.5	3 mm	EMI shielding + self-healing	Yang et al.30
11	Magnetic chain/polymer composite	Magnetic CNT	35.2	2 mm	absorption-dominated EMI	Feng et al.44
12	PAM/PDA@MWCNTs/PEDOT:PSS hydrogel	PDA@MWCNTs + PEDOT:PSS	39.72	2 mm	EMI shielding + strain sensing + infrared stealth	This work

As shown in Table 1, a comprehensive comparison with 11 typical electromagnetic shielding composites reveals that most existing studies focus solely on a single electromagnetic shielding function, while materials capable of integrating dual functionalities remain scarce. In contrast, the M/A@T-4/S-2 hydrogel prepared in this work achieves the integrated multifunctionality of high-performance electromagnetic shielding, motion sensing, and infrared stealth. At a thickness of 3 mm, the total electromagnetic shielding effectiveness reaches 39.72 dB, outperforming the vast majority of values reported in the literature. The material also exhibits excellent mechanical stability. The synergistic effect of PDA@MWCNTs and PEDOT:PSS constructs a complete conductive network, while hydrogen bonding further enhances the structural stability of the material. The porous structure promotes multiple reflections of electromagnetic waves, thereby endowing the material with infrared stealth characteristics. Collectively, these advantages confer greater innovation and practical value to this study within the field of flexible multifunctional electromagnetic protective materials.

Limitations of the study

In this study, a synergistic conductive network was constructed using PDA@MWCNTs and PEDOT:PSS to investigate the electromagnetic shielding, sensing, and infrared stealth properties of PAM hydrogels. However, due to limitations in current characterization techniques, it is difficult to provide a quantitative analysis of the various loss mechanisms (conductive loss, polarization loss, and multiple reflections). Future research will focus on developing new methods to facilitate a deeper understanding of these mechanisms.

Resource availability

Lead contact

For further information and requests regarding resources and reagents, please contact the primary contact, Changgui Li (3230006390@qq.com), directly. He will be responsible for handling all related matters.

Materials availability

This study did not yield any new unique reagents.

Data and code availability

All data reported in this paper will be shared upon request by the primary contact. The original code is not reported in this paper. Any additional information required to reanalyze the data reported herein may be provided to the primary contact upon request.

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China of China Project (52163001); Guizhou Provincial Science and Technology Plan Project (Qiankehe Platform Talent-CXTD[2021]005; Qiankehe Platform Talent-GCC[2022]010-1); Central Guidance Fund for Local Science and Technology Development (Qiankehe Zhongyindi [2023]035, Qiankehe Fujiqi 2023(001), Qiankehe Zhongyindi [2024]042, Qiankehe Zhongyindi (2025)013, Qiankehe Talent XKBF(2025)005); Guizhou Minzu University Doctoral Start-up Fund Project (GZMUZK[2024]QD77); Guizhou Provincial Scientist Workstation (Qiankehe Platform KXJZ[2024]002), Central Guidance Fund for Local Science and Technology Development (Qiankehe Zhongyindi [2025]013); Guizhou Provincial Department of Education “Hundred Universities, Thousand Enterprises Science and Technology Challenge Project” (Qian Jiao Ji [2025] no. 007); Guizhou Provincial Science and Technology Plan Project (Qiankehe Results [2025]109).

Author contributions

C.L.: material synthesis and validation, data organization, and drafting of the initial manuscript. Z.L.: data organization and manuscript writing. T.Z.: data organization. D.L.: data organization. D.Z.: funding acquisition, supervision and guidance, and manuscript revision and editing. Z.W.: supervision and guidance and manuscript revision and editing. W.G.: funding acquisition and writing-reviewing and editing.

Declaration of interests

No potential conflict of interest was reported by the authors.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Acrylamide	Aladdin	CAS:79–06-1
N,N′-Methylene diacrylamide	Aladdin	CAS:110–26-9
Ammonium persulfate	Aladdin	CAS:7727–54-0
N,N,N′,N′-Tetramethylethylenediamine	Aladdin	CAS:110–18-9
Multi-walled carbon nanotubes	Aladdin	CAS:308068–56-6
Poly(3,4-ethyldioxothiophene)-poly(styrenesulfonate)	Aladdin	CAS:155090–83-8
Software and algorithms
Origin 2024	OriginLab Corporation	https://www.originlab.com/2024
Avantage	ThermoFisher Scientific	https://emportal.swd.thermofisher.com/

Experimental model and study participant details

Experimental models/research subjects (laboratory animals, human subjects, plants, microbial strains, cell lines, primary cell cultures) not adopted in this study.

Method details

Raw materials

Acrylamide (AM, 99%), N,N'-Methylene diacrylamide (MBA, 99%), Ammonium persulfate (APS, 99.99%), N,N,N',N'-Tetramethylethylenediamine (TEMED, 99%), Multi-walled carbon nanotubes (MWCNT, diameter: 20–30 nm, length: 10–30 μm, ≥95%), Tris(hydroxymethyl)aminomethane (Tris, ≥99%), Dopamine hydrochloride (DA, ≥98%), Poly(3,4-ethyldioxothiophene)-poly(styrenesulfonate) (PEDOT:PSS = 1:6, 1.5% in water). All reagents were procured from Shanghai Aladdin Biochemical Technology Co., Ltd. All chemicals were used as received without further purification.

Preparation of PDA@MWCNTs

First, prepare a Tris-HCl solution with a pH of 8.5. Add 0.3 g of MWCNT to 100 mL of this Tris-HCl solution and sonicate to achieve uniform dispersion. Secondly, the MWCNT dispersion was then added to 100 mL of a 3 g/L DA solution and stirred magnetically at 25°C for 24 hours. The reaction product was washed and centrifuged multiple times, then placed in a freeze dryer for drying, yielding PDA@MWCNT.

Preparation of PAM/PDA@MWCNTs/PEDOT:PSS hdrogels

Weigh 80 mg of PDA@MWCNTs into 7 mL of purified water and disperse uniformly by ultrasonication. Weigh 2 g of AM, 0.02 g of MBA, and 0.065 g of APS and dissolve them in 8 mL of purified water by stirring until fully dissolved. Pour this into the ultrasonicated PDA@MWCNTs aqueous solution, and add the conductive polymer PEDOT: PSS. Finally, 20 μL of TEMED was added dropwise to accelerate the reaction. The resulting solution was poured into a polytetrafluoroethylene mold and reacted at 60°C, ultimately yielding the PAM/PDA@MWCNTs/PEDOT:PSS composite hydrogel. As shown in Figure 1: (For clarity, PAM, PDA@MWCNT, and PEDOT:PSS are labeled as M, A@T, and S, respectively, in the figures. The concentration of PDA@MWCNTs is 4 wt%.).

Characterization

The chemical composition and structure of the samples were characterized using Fourier transform infrared spectroscopy (Nicolet IS50). The surface morphology and composition of the samples were examined using a scanning electron microscope (German ZEISS Sigma 300). The crystalline structure of the composite material was determined using an X-ray diffractometer (XRD). Chemical characterization of the surface elements and chemical composition of the sample was performed using X-ray photoelectron spectroscopy (XPS). Prepare the samples into cylindrical specimens and conduct mechanical property testing using a universal testing machine (CMT6104). Thermal stability testing of the samples was conducted using a differential scanning calorimeter (DSC25). The electromagnetic interference shielding performance of the sample was measured in the X-band (8.2–12.4 GHz) using a vector network analyzer (N5225B). The absorption coefficient (A), reflection coefficient (R), and transmission coefficient (T) of the sample were obtained from the following Equations 1, 2, and 3.⁴⁷

$$R={\left|S_{11}\right|}^2$$ (Equation 1)

$$T={\left|S_{12}\right|}^2$$ (Equation 2)

$$A=1-R-T$$ (Equation 3)

The attenuation of electromagnetic waves can be expressed as surface reflection loss (SE_R), absorption loss (SE_A), and multiple internal reflection loss (SE_M), with the calculation formulae as follows⁴⁸:

$$S{E}_R=-10\log \left(1-\mathrm{R}\right)$$ (Equation 4)

$$S{E}_A=-10\log \left(\frac{T}{1-R}\right)$$ (Equation 5)

$$S{E}_T=S{E}_R+S{E}_A+S{E}_M$$ (Equation 6)

When the SE_T value exceeds 10 dB, the SE_M can generally be disregarded.

The water absorption capacity of the hydrogel was tested using the following formula:

$$Q=\frac{m_t-m_0}{m_0}\times 100\%$$ (Equation 7)

Where m₀ denotes the mass of the dry gel, and m_t denotes the mass of the hydrogel after absorbing water for a specified period.

Quantification and statistical analysis

The figures in the main text were generated using Origin 2024 software based on the raw data, and the XPS data analysis was performed using Avantage software.