Highly Sensitive Conductive Hydrogel Enabled by Nanoscale Synergy of Carboxylated MXene and Polypyrrole for Multifunctional Applications

ABSTRACT

Conductive hydrogels are pivotal for flexible electronics. However, their sensitivity is often limited by the agglomeration and inefficient conductive network formation induced by the single‐component nanofillers. To address these issues, we developed a nanoscale synergy strategy employing a hybrid filler system composed of two‐dimensional carboxylated MXene (C‐MXene) nanosheets and one‐dimensional polypyrrole (PPy) chains within a polyacrylamide hydrogel. The C‐MXene nanosheets formed a robust conductive scaffold that mitigates agglomeration, while the interwoven PPy chains bridge adjacent nanosheets, promoting efficient electron transport under strain and enhancing mechanical integrity. This synergistic interaction results in a hydrogel with an exceptional electrical conductivity of 5.5 mS/cm and a high strain sensitivity with a gauge factor of 7.52, enabling accurate monitoring of physiological movements. Leveraging the abundant binding sites provided by the carboxylated MXene and its inherent gas adsorption capability, we further extended this multifunctional platform to ammonia detection. These findings underscore that our nanoscale synergy strategy offers a powerful route to integrating multiple functionalities into a single, easily processable hydrogel platform.

The nanoscale synergy between carboxylated MXene nanosheets and in‑situ polymerized polypyrrole chains creates a highly conductive hydrogel with exceptional strain sensitivity (gauge factor of 7.52) and strong adhesion. This multifunctional platform enables accurate monitoring of human motions and efficient room‑temperature ammonia detection, offering a versatile strategy for flexible electronics.

1. Introduction

Conductive hydrogels have emerged as pivotal materials for next‐generation flexible electronics, with significant applications in wearable sensors [1], human‐machine interfaces [2], and environmental monitoring [3]. Introducing conductive fillers, such as inorganic ions [4], carbon‐based materials [5], conductive polymers [6], and metals [7], into hydrogels represents the most common approach for imparting conductivity. However, electrolyte‐based conductive hydrogels often exhibit low sensitivity under stretching, as their electrical response primarily arises from geometric changes under deformation. In contrast, nanocomposite conductive hydrogels demonstrate superior strain sensitivity due to resistive and tunneling effects [8], alongside enhanced mechanical properties through nano‐reinforcement.

Among various conductive nanofillers, including nanodots [9], nanoparticles [10], nanowires [11], nanotubes [12], and nanosheets [13], two‐dimensional (2D) nanosheets are particularly promising due to their high aspect ratios and large specific surface areas, which enable exceptional sensing sensitivity under mechanical deformation [14]. MXene, an emerging class of 2D transition metal carbides/carbonitrides [15], has attracted extensive interest due to its high electrical conductivity, excellent mechanical properties, and exceptional hydrophilicity. The abundant hydrophilic functional groups (e.g., ‐OH, ‐F, ‐O) on MXene nanosheets facilitate strong physical interactions with hydrogel networks, while their superior aqueous dispersibility promotes the formation of continuous conductive pathways, imparting both conductivity and strain sensitivity [16]. Furthermore, the rich surface chemistry of MXenes enables outstanding gas sensing capabilities [17].

Despite these advantages, MXene‐based hydrogel sensors [18, 19, 20] often face challenges such as nanosheet aggregation and oxidation, which compromise their mechanical and sensing stability. This aggregation primarily results from Van der Waals forces and hydrogen bonding that promote MXene restacking, which not only disrupts charge‐transfer pathways and degrades electrical conductivity but also induces stress concentration, ultimately leading to mechanical embrittlement. In addition to restacking, MXene faces degradation via oxidation [21]. Dissolved oxygen can oxidize MXene nanosheets into metal oxides, degrading their lamellar structure and undermining sensing stability [22]. To mitigate these issues, several strategies have been explored including hydrogen bonding aqueous molecules such as sulfonated polyurethane nanospheres [23], grafting catechol groups on MXene nanosheets via coordination bonds through hydrophobic interactions to suppress oxidation [24, 25].

Another promising candidate is polypyrrole (PPy), one of the most widely studied conductive polymers, renowned for its biocompatibility, high conductivity, ease of processing, and low cost [26]. However, PPy‐based hydrogels often exhibit limited mechanical strength [27], which restricts their durability in flexible devices. To address this, researchers have developed PPy‐based composites with other nanomaterials. For instance, a reduced graphene oxide/PPy composite hydrogel was prepared through room‐temperature self‐assembly and high‐temperature oxidative polymerization, which achieved a fracture compression stress of 0.35 MPa [28].

Conventional approaches relying on single‐type nanofillers (0D nanoparticles, 1D nanowires/tubes, or 2D nanosheets) [29] frequently encounter common bottlenecks, such as filler agglomeration, inefficient conductive network formation under strain, and limited mechanical reinforcement. Consequently, developing a multifunctional hydrogel that integrates high conductivity, excellent sensitivity, and robust mechanical properties remains a significant challenge [30, 31].

Herein, we propose a nanoscale synergy strategy to overcome these limitations by constructing a hybrid‐dimensional filler system within a polyacrylamide (PAM) hydrogel. This system integrates two‐dimensional carboxylated MXene (C‐MXene) nanosheets with in situ polymerized one‐dimensional polypyrrole (PPy) chains. In this design, the 2D C‐MXene nanosheets serve as a robust, anti‐aggregation conductive scaffold, while the interwoven 1D PPy chains effectively bridge adjacent nanosheets. The carboxylation of MXene enhances its colloidal stability and provides active sites for strong interfacial interactions. Crucially, the in situ polymerization of PPy fosters a uniform, interpenetrating three‐dimensional conductive network, leading to superior electrical conductivity (especially under strain) and enhanced mechanical properties compared to single‐filler systems.

2. Experimental Section

2.1. Materials

All chemical reagents and materials were used as received without further purification. Specifically, we employed the following chemical agents including lithium fluoride (LiF, >99.9%), concentrated hydrochloric acid (HCl, 36%–38%, Zhuzhou Xingkong Chemical Glass Co., Ltd., Zhuzhou, China), titanium aluminum carbide MAX phase powder (Ti₃AlC₂, 400 mesh, Foshan Xinkesi Technology Co., Ltd., Foshan, China), ethanol (C₂H₅OH, analytical reagent grade, 99%), sodium hydroxide (NaOH, 98%), 4‐aminobenzoic acid (C₇H₇NO₂, analytical reagent grade, 99%), sodium nitrite (NaNO₂, ≥97%, Sigma‐Aldrich Trading Co., Ltd., Shanghai, China), acetone (C₃H₆O, analytical reagent grade), acrylamide (AM, 99%), pyrrole (Py, 98%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China), ferric chloride (FeCl₃, 98%), N,N'‐methylenebisacrylamide (MBAA, 99%), 2‐hydroxy‐4'‐(2‐hydroxyethoxy)‐2‐methylpropiophenone (photoinitiator 2959, >98%), and Carboxylated MXene (denoted as C‐MXene, prepared following our previous study [32]). The deionized water used in this work was prepared using the Millipore Ultrapure System. Gloves and other consumables utilized in this study were procured from Shanghai Yansu Technology Co., Ltd. (Shanghai, China; mall.shiyanjia.com).

2.2. Preparation of Hydrogels

The C‐MXene/PPy composite hydrogels were synthesized via a two‐step process involving in situ polymerization followed by UV‐initiated crosslinking. First, the C‐MXene powders were dispersed into 10 mL of deionized water to obtain the C‐MXene solution at a concentration of 1 mg/mL. Then, 5 µL of pyrrole (Py) was added into the above C‐MXene solution under magnetic stirring for 1 h at room temperature. Subsequently, 30 mg of FeCl₃ dissolved in 5 mL of water was added dropwise under ice‐bath conditions. After stirring for 8 h, the color of the solution changed from black to dark green, indicating the in situ polymerization of Py on the MXene nanosheets to obtain C‐MXene/PPy₅. This procedure was repeated with 10, and 15 µL of Py to obtain the C‐MXene/PPy₁₀, and C‐MXene/PPy₁₅ hybrid composites, respectively. Each mixture was then blended sequentially with AM (3 g), MBAA (5 mg), and photoinitiator 2959 (30 mg) under magnetic stirring for 40, 20, and 20 min after each addition to form a homogeneous precursor solution. The solution was then transferred into a petri dish, degassed, and exposed to Ultra‐violet (UV) light (365 nm) for 1.5 h to crosslink the acrylamide, yielding the final hydrogels designated as cMP₅H, cMP₁₀H, and cMP₁₅H. The “cM” stands for C‐MXene, “P” for PPy, “H” for hydrogel, and the subscript number indicates the added volume of Py. For example, the cMP₁₀H means that the C‐MXene/PPy₁₀ hydrogel. In addition, a PPy‐free C‐MXene hydrogel (cMP₀H) was also prepared for comparison following the same protocol without the loading of PPy.

In addition, to investigate the effect of C‑MXene on the gas sensing capability of the conductive hydrogels, the cMPH hydrogels with different C‑MXene loadings were prepared for the ammonia adsorption tests. The preparation procedure followed the same protocol as described previously. The content of pyrrole (Py) was kept constant by adding 10 µL in each case, while only the concentration of the C‑MXene solution was varied (0.5mg/mL, 1.0 mg/mL, and 1.5 mg/mL). The resulting hydrogels were designated as cM_0.5P₁₀H, cM₁P₁₀H, and cM_1.5P₁₀H, respectively. For comparison, we also prepared three control hydrogels: the one with non‑carboxylated MXene (MH), the one with C‑MXene but without PPy (cM₁P₀H), and the one with PPy but without C‑MXene (cM₀P₁₀H).

2.3. Characterization

2.3.1. Fourier Transform Infrared Spectrometer (FTIR) Characterization

The FTIR spectra of the C‐MXene powder, lyophilized C‐MXene/PPy mixture and cMPH hydrogel films were obtained using an FTIR spectrometer (Bruker Tensor 20) over a wavenumber range of 500–4000 cm⁻¹ at a resolution of 4 cm⁻¹ to identify the chemical structures and interfacial interactions.

2.3.2. Scanning Electron Microscope (SEM) Characterization

The microstructural morphology of the cMPH was examined using a digital scanning electron microscopy (SEM, Tescan Mira3). Before imaging, the hydrogels were frozen in liquid nitrogen, fractured, lyophilized, and then sputter‐coated with a thin layer of gold for conductivity. After that, the prepared samples were imaged under a voltage of 3 kV.

2.3.3. Mechanical Property Testing

The tensile properties of the prepared hydrogels were evaluated using a universal testing machine (SAAS, Sansi, Shenzhen, China) with a load of 100 N at room temperature. The hydrogels were cut into rectangular strips (30 mm × 10 mm) with a thickness of 1 mm. The tensile speed was set as 20 mm/min. Five samples of each group were evaluated.

2.3.4. Adhesion Testing

The adhesion performance of the cMPH films was evaluated following the previous studies [33]. Briefly, the prepared hydrogels were sized into a square shape (10 mm in width and 2 mm in thickness) and sandwiched between two substrates, which was then stretched using the above‐mentioned tensile machine at a rate of 20 mm/min until complete detachment to quantify the adhesive strength. Five samples of each group were evaluated.

2.3.5. Strain Sensing Test

The strain sensing behavior of the cMPH hydrogels was characterized via a dual display LCR meter (879B, B&K Precision) being connected with the tensile machine to record the in‐real time resistance of the hydrogels under stretching under different tensile deformations. The strain sensing capability was evaluated by analyzing the relative resistance of the hydrogels based on the following Equation (1) and (2) [21]:

$$\frac{\mathrm{\Delta }R}{R_0}=(R-R_0)/R_0$$ (1)

$$\frac{\mathrm{\Delta }I}{I_0}=(I-I_0)/I_0$$ (2)

Where, R ₀ and R represents the initial and real‐time resistance, respectively. The gauge factor (GF), a critical parameter to indicate the sensitivity of the hydrogels, was determined using the Equation (3) [34]:

$$GF=\left(\mathrm{\Delta }R/R_0\right)/\epsilon$$ (3)

Where, ε represents the applied strain. The sample size was 30 mm in length, 10 mm in width and 1 mm in thickness.

2.3.6. Electrical Conductivity Measurement

Hydrogel square specimens (10 mm in width) were sandwiched between platinum electrodes. The thickness (L) was measured using vernier calipers. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CHI 760E, Chenhua) over a frequency range of 10 Hz to 10⁶ Hz. The resistivity (ρ) was calculated as:

$$\rho =\frac{L}{AR}$$ (4)

Where, R was the impedance magnitude derived from the Nyquist plots, and A was the electrode contact area. Triplicate measurements were performed with averaged results.

2.3.7. Motion sensing Performance Evaluation

The motion sensing performance was characterized using the above electrochemical workstation. The relative current changes in hydrogel strips (3 cm in length,1 cm in width, 1 mm in thickness) with embedded nickel‐chromium wires were monitored. The sensors were encapsulated with 3M VHB tape and attached to human joints (fingers, wrists), eyebrows, and cheeks to monitor biomechanical motions. Informed written consent was obtained from the volunteer for the experiments.

2.3.8. Gas Sensing Performance Evaluation

The ammonia sensing performance of the hydrogel specimens was evaluated by monitoring their real‐time resistance response in a sealed chamber. First, the hydrogel samples (2 cm in length, 1 cm in width and 1mm in thickness) with embedded nickel‐chromium wire electrodes were placed in a closed chamber. Then, controlled concentrations of NH₃ were introduced via a syringe injection system. The relative resistance change (ΔR/R₀) was continuously recorded using the above‐mentioned electrochemical workstation to assess the effects of both C‐MXene content and NH₃ concentration on the sensing behavior.

3. Results and Discussion

3.1. Design strategy

As illustrated in Figure 1, the multifunctional hydrogel was constructed based on a designed nanoscale synergy between 1D PPy and 2D C‐MXene, which was tailored to foster strong interfacial coupling rather than a simple physical mixture. Specifically, the acidic surface of C‐MXene facilitated the protonation and ordered polymerization of pyrrole, while hydrogen bonding between the carboxyl groups of C‐MXene and the amino groups of the pyrrole rings promoted the aligned growth of PPy chains on the MXene nanosheets, resulting in a tightly integrated MXene‐PPy heterostructure. The subsequently introduced polyacrylamide network further immobilizes the conductive architecture. This tight coupling, achieved through strong interfacial interactions such as hydrogen bonding, enables the formation of a coherent and long‐range three‐dimensional conductive network, which is fundamental to the enhanced mechanical robustness and multifunctional sensing capabilities of the hydrogel simultaneously.

FIGURE 1.

Schematic illustration of the synthesis of the cMPH multifunctional sensor.

Furthermore, the intrinsic physicochemical properties of PPy and C‐MXene directly enable its pronounced gas sensing function. Both components contribute high specific surface areas for exceptional adsorption. Specifically, upon NH₃ exposure, the functional groups on MXene serve as Lewis acid sites for chemisorption, while the nitrogen atoms in PPy engage in proton transfer with NH₃, causing PPy dedoping and a substantial resistance change. The conjunction of the high conductivity of MXene and the hole‐transport properties of PPy would create efficient charge‐transfer pathways that amplify the signal from NH₃ adsorption, resulting in high sensitivity and stability of the resultant hydrogels.

3.2. Morphological and Structural Characterization

SEM was used to characterize the microstructural evolution of MXene (Figure 2a–c). It can be found that the pristine MXene and carboxylated MXene (C‐MXene) exhibited similar, smooth nanosheet morphologies (Figure 2a,b). In contrast, the C‐MXene/PPy composite showed abundant nanowire clusters on the nanosheet surfaces (Figure 2c), confirming the successful in situ polymerization of Py on the surface of C‐MXene. EDS elemental mapping (Figure 2d) further verified the in situ growth, showing a uniform nitrogen signal (from PPy) overlaid on the C‐MXene substrate (rich in C, F, Ti, O).

FIGURE 2.

Morphological and structural characterization: SEM images of (a) MXene, (b) C‐MXene, and (c) C‐MXene/PPy powders, (d) EDS elemental mapping of the C‐MXene/PPy powder, (e) FTIR spectra of MXene, C‐MXene, and C‐MXene/PPy powders, (f) SEM image of the cMP₀H, (g) FTIR spectra of cMP₀H and cMP₁₀H, (h) SEM image of the cMP₁₀H.

FTIR spectroscopy was employed to analyze the chemical structures and interactions (Figure 2e,g). Compared to original MXene, the C‐MXene spectrum exhibited an intense peak at 1685 cm⁻¹ (C = O stretching), confirming the successful carboxylation [25]. The broad O–H band around 3200 cm⁻¹ also intensified and broadened in C‐MXene, attributable to the increased surface hydroxyl population from –COOH groups and their enhanced hydrogen‐bonding capability [35]. In the cMP₁₀H hydrogel, additional peaks appeared at 1316 cm⁻¹ (N–H bending), 1036 cm⁻¹ (C = N stretching), and 633 cm⁻¹ (C–H/N–H wagging) [36]. Compared to the cMP₀H, the observed peak shifts arise from PPy incorporation and hydrogen bonding between the N–H groups of PPy and the O/F groups of MXene [37].

XPS analysis (Figure S4) was performed to compare the chemical states of cMP₀H (without PPy) and cMP₁₀H (with PPy). In the C1s spectrum, the peak corresponding to C = O groups was notably enhanced in cMP₁₀H relative to cMP₀H, indicating the presence of additional carbonyl species from the in situ polymerized PPy or hydrogen‐bonding interactions with the carboxyl groups of C‐MXene. The N1s spectrum of cMP₁₀H revealed a distinct new peak that is absent in cMP₀H, which is attributed to the pyrrolic nitrogen (–NH–) of PPy, confirming successful polymerization. Furthermore, the O1s spectrum showed an increased intensity of the C–O component in cMP₁₀H compared to cMP₀H, suggesting the formation of hydrogen bonds between the –COOH groups of C‐MXene and the –NH– groups of PPy. These spectral changes provide direct evidence for the strong interfacial coupling between C‐MXene and PPy in the cMPH hydrogel.

SEM was further utilized to identify the loading affect of PPy on the network architecture and the results are illustrated in Figure 2f and h. It can be found that the hydrogel cMP₀H exhibited a porous structure with partially occluded pores and a smooth pore surface. In contrast, the cMP₁₀H exhibited more open pores with distinct, fimbriated edges, which might be attributed to the hydrophobic PPy chains preferentially aggregating at the pore boundaries (hydrophobic interfaces) within the hydrophilic PAM/C‐MXene network. The confinement within PAM pores (50–200 nm) restricted the diffusion of PPy chains (≈ 0.5 nm in diameter), effectively localizing the growing polymer to the pore peripheries. The volumetric shrinkage of PPy during helical chain formation generated localized stresses, deforming the pore edges into fringed structures. The formation of this distinctive, interconnected porous structure with PPy localized at the pore boundaries would facilitate the establishment of continuous conductive pathways throughout the hydrogel matrix.

3.3. Hydrogel Performance

3.3.1. Electrical and Tensile Properties

The electrical conductivity of the composite hydrogels was investigated and the results are shown in Figure 3a. Compared to pristine MXene, the conductivity of the C‐MXene exhibited a slight decrease due to the introduction of insulating –COOH groups. When being loaded into the hydrogels, the cMP₀H hydrogel exhibits very low conductivity, which might be attributed to the isolation of the C‐MXene nanosheets via the insulating PAM matrix and absorbed water that prevent the formation of the continuous conductive network. In contrast, incorporating PPy dramatically enhanced the conductivity of the cMPH series, which demonstrated a clear positive correlation to the PPy content. This enhancement can be ascribed to the formation of a more interconnected conductive architecture, as directly evidenced by the distinct fimbriated pore edges and the localized PPy aggregation at pore boundaries observed via SEM (Figure 2f, and h). These observations suggest that the one‐dimensional PPy chains, preferentially assembled at these pore interfaces, effectively bridge the isolated two‐dimensional C‐MXene nanosheets within the PAM network, which effectively mitigates the random stacking of nanosheets and connects conductive pathways, leading to the establishment of a robust 3D conductive network, which accounts for the orders‐of‐magnitude increase in conductivity.

FIGURE 3.

Comparison of the properties of cMPH composites with different PPy loadings: (a) Electrical conductivity, including pristine MXene and C‐MXene powders as references, (b) Mechanical performance shown by stress‐strain curves, Quantitative analysis of (c) tensile modulus and (d) toughness, (e) Tensile cycling and (f) corresponding energy dissipation and hysteresis of cMPH at 400% strain, (g) the mechanical stability of cMPH after subjecting for 300 cycles of loading‐unloading process.

As shown in Figure 3b–d, the PPy content also significantly affects the mechanical properties of the hydrogels. The tensile strength, fracture elongation, toughness, and tensile modulus all show a declining trend as the PPy content increases from 5 to 15 µL (cMP₅H to cMP₁₅H), which might be attributed to the poor compatibility and weak interfacial adhesion between the rigid PPy particles and the flexible PAM matrix, leading to stress concentration around the rigid fillers under tension.

The gradual decline in mechanical performance with excessive PPy underscores the importance of optimizing the filler ratio for effective synergy. For subsequent multifunctional motion sensing applications, the cMP₁₀H was selected as the optimal formulation, offering a balanced combination of retained mechanical integrity and significantly enhanced conductivity.

In addition to the static mechanical properties, the cyclic loading–unloading behavior of the cMPH hydrogel was evaluated to assess its resilience and energy dissipation capacity. As shown in Figure 3e, the cMPH hydrogel was subjected to consecutive tensile loading–unloading cycles at a fixed strain of 400% without resting between cycles. The first cycle exhibits a pronounced hysteresis loop, indicating efficient energy dissipation through the reversible breaking and reformation of hydrogen bonds between the C‐MXene nanosheets and the PPy chains. The stress–strain curve of the second cycle shows a reduced hysteresis area compared to the first cycle, which is typical for soft materials due to the Mullins effect, yet the overall shape remains stable in subsequent cycles.

The corresponding energy dissipation and hysteresis ratio are quantified in Figure 3f. The energy dissipation per unit volume decreases from 2.95 kJ·m⁻³ in the first cycle to 1.2 kJ·m⁻³ in the second cycle, and then gradually stabilizes at 1.3 kJ·m⁻³. The hysteresis ratio follows a similar trend, decreasing from 17.7% to 7.9% after the first cycle and remaining nearly constant thereafter. This behavior suggests that the hydrogel quickly reaches a steady state after an initial conditioning period, which is favorable for practical sensing applications requiring stable and reproducible mechanical response.

The long‐term mechanical stability of the cMPH hydrogel was further investigated by subjecting it to 300 consecutive loading–unloading cycles at a strain of 50% (Figure 3g). The excellent fatigue resistance is attributed to the robust interfacial interactions between the C‐MXene and PPy, as well as the resilience of the polyacrylamide network. This high cyclic stability is crucial for the reliable long‐term operation of wearable strain sensors.

To further validate the proposed nanoscale synergy mechanism, we prepared two physically mixed control hydrogels: cMPH‐mix (C‐MXene + PPy physically blended in PAM) and MPH‐mix (pristine MXene + PPy physically blended in PAM). As shown in Figure S2, the in situ polymerized cMPH hydrogel exhibits markedly superior mechanical properties, electrical conductivity, and strain sensitivity compared to both physically mixed counterparts. The stress–strain curves (Figure S2a) and the corresponding tensile modulus and toughness (Figure S2b) demonstrate that physical mixing fails to achieve effective load transfer and energy dissipation, whereas the synergistic cMPH displays significantly enhanced stiffness and toughness. Likewise, the electrical conductivity of cMPH (Figure S2c) is substantially higher than that of the mixed samples, indicating that the in situ formed PPy chains effectively bridge adjacent C‐MXene nanosheets to create a continuous conductive network. The gauge factor curves (Figure S2d–f) further confirm that the physically mixed hydrogels exhibit lower strain sensitivity, while the synergistic cMPH delivers a high and well‐defined gauge factor across a broad strain range. Collectively, these results provide unambiguous evidence that the nanoscale synergy enabled by in situ polymerization of PPy on C‐MXene is fundamentally distinct from and far superior to simple physical blending.

3.3.2. Adhesion Evaluation

Adhesion is a critical attribute for flexible electronic sensors, as robust self‐adhesion would ensure accurate and stable signal acquisition. As shown in Figure 4a, the cMP₁₀H demonstrated excellent adhesive capability, conformably adhering to diverse substrates including stone, plastic, ceramic, glass, and steel, demonstrating its exceptional adhesive performance. This strong and substrate‐dependent adhesion originates from the interfacial interactions such as hydrogen bonding, coordination bonds, and hydrophobic interactions between the functional groups (e.g. –NH₂, –COOH) in the hydrogel and the substrate surfaces (Figure 4b). Quantitative lap‐shear tensile tests (Figure 4c) revealed its adhesion strength of 152.3 kPa (glass), 129.6 kPa (wood), and 77.8 kPa (steel) (Figure 4d). Compared to metals, the higher adhesion to glass and wood, can be attributed to the hydroxyl‐rich surface of glass and the micro‐roughness of wood, which enhance contact area and interaction sites. In contrast, the smoother and chemically inert surfaces of metals offer fewer anchoring sites, resulting in relatively lower adhesion strength.

FIGURE 4.

Adhesion performance of the cMP₁₀H. (a) Adhesion to various substrates, (b) schematic illustration of the adhesion mechanism, (c) schematic diagram of lap shear adhesion test, (d) adhesion strength curve of hydrogel to different substrates.

3.3.3. Motion Sensing Performance

The cMP₁₀H combines excellent mechanical flexibility, strong self‐adhesion, and good conductivity, making it a promising candidate for strain sensors to detect human motion through electrical signal changes. Figure 5a,b demonstrate the sensor's stable and repeatable signal output under various deformation conditions. At a low strain of 15%, the output signal remained steady, and the ΔR/R₀ increased progressively and consistently with increasing strain. Similar stability was observed under different strain rates, confirming its reliable signal repeatability (Figure 5c).

FIGURE 5.

Performance characterization of the cMPH sensor: Response signals of the cMPH at (a) 15%–60% and (b) 100%–300% strain, (c) Resistance changes upon different stretching rates. (d) ΔR/R₀ versus strain with its fitting curve, (e) Comparison of the gauge factor (GF) in this work with those reported in previous studies [1, 38, 39, 40, 41, 42, 43], (f) Resistance variation over 500 loading‐unloading cycles.

The sensitivity of the sensor was quantified by the gauge factor (GF) as demonstrated in Figure 5d. It was found that the curve can be fitted by two linear regions including a GF of 2.98 (y = 2.98x‐94.32, R² = 0.95) in the 0–300% strain range, and a GF of 7.52 (y = 7.52x‐1533.21, R² = 0.99) in the 300–800% strain range, which might be attributed to the synergistic effect of the 1D PPy and 2D MXene within the elastic matrix. Initially, the reversible deformation of the porous network causes a moderate, linear resistance change. At higher strains, the gradual disconnection of the PPy‐facilitated MXene bridging pathways, which are critical for long‐range conductivity, triggers a more pronounced and linear increase in resistance, yielding the higher GF. These results indicate that the sensor possesses a wide sensing range, high sensitivity, and good linearity, which was further highlighted compared to previous studies across different strain intervals (Figure 5e). In addition to the gauge factor comparison shown in Figure 5e, a more comprehensive comparison of key performance metrics (including conductivity, elongation at break, response time, and adhesion strength) between our cMPH hydrogel and recently reported MXene/conductive polymer‐based hydrogels is summarized in Table S1. The cMPH hydrogel exhibits a balanced combination of high sensitivity (GF = 7.52), good conductivity (0.55 S·m⁻¹), ultrahigh stretchability (1063%), and strong adhesion (152.3 kPa to glass), outperforming most of the listed counterparts in terms of overall multifunctionality. Furthermore, the sensor exhibits outstanding cyclic stability over 500 loading‐unloading cycles at 50% strain (Figure 5f), with negligible variation in ΔR/R₀. This durability directly reflects the structural integrity of the designed 3D conductive hydrogel, where the intertwined polymer network and the strong interfacial interactions prevent irreversible damage to the conductive elements during dynamic deformation, which provides high reliability for the hydrogel in the advanced wearable sensing applications. In addition, the water retention behavior of the cMPH hydrogel under ambient conditions (25 °C, 70 % RH) is shown in Figure S1, where a gradual weight loss due to water evaporation was observed over 3 days.

To preliminarily assess the biocompatibility of the cMPH hydrogel for on‑skin wearable applications, a skin patch test was conducted on a healthy human volunteer with informed consent. The hydrogel was attached to the inner forearm skin for 1 hour. As shown in Figure S3, no erythema, edema, or any visible signs of irritation were observed on the skin after removal of the hydrogel, indicating that the cMPH hydrogel is non‑irritating and safe for direct contact with human skin. This preliminary result supports the potential of the cMPH hydrogel for wearable health monitoring devices.

To evaluate its practical performance in human motion monitoring, the cMPH strain sensor was attached to different parts of the human body for real‐time detection of biomechanical signals. As shown in Figure 6a,b, when being attached to the volunteer's wrist, the sensor exhibited a rapid response time of approximately 700 ms during fast bending. Furthermore, when mounted on the index finger joint (Figure 6c), it produced distinct and repeatable ΔR/R₀ responses corresponding to different bending angles, while maintaining a stable baseline at rest. The sensor was also capable of detecting larger‐scale limb movements such as wrist flexion (Figure 6d), as well as subtle facial motions including frowning and smiling (Figure 6e,f). These results collectively demonstrate its high fidelity in monitoring a broad spectrum of activities, from macroscopic joint motions to subtle physiological expressions, confirming its strong potential for wearable health and motion‐sensing applications.

FIGURE 6.

Application of cMPH in human motion monitoring: (a) Response behavior of the cMPH, (b) Magnified view of the response time in (a), Real‐time resistance changes of the cMPH sensor during (c) finger bending at different angles, (d) wrist flexion, (e) frowning, and (f) smiling.

3.3.4. Gas Sensing Performance

While the previously demonstrated mechanical, adhesive, and strain‐sensing properties of the cMPH series are primarily governed by the robust PAM network and the synergistic conductive pathway established by the C‐MXene/PPy combination, the ammonia sensing performance is critically dependent on the specific surface chemistry and the density of accessible binding sites. Therefore, to specifically maximize the gas‐sensing efficacy, we further fine‐tuned the content of C‐MXene—the primary provider of carboxyl (–COOH) adsorption sites—while maintaining a constant optimal PPy loading (10 µL, as determined from earlier electrical conductivity studies).

First, the fundamental advantage of this design was confirmed by comparing the ammonia adsorption capacities of key samples (Figure 7a). The carboxylated MXene hydrogel (cM₁P₀H) exhibited significantly enhanced adsorption over unmodified MXene (MH), validating the role of –COOH groups as binding sites. More importantly, the composite cM₁P₁₀H demonstrated far superior performance than its single‐component counterparts (cM₁P₀H and cM₀P₁₀H), directly evidencing the synergistic effect between C‐MXene and PPy in creating a high‐surface‐area, conductive architecture optimal for gas sensing. In detail, the in situ deposited PPy nanowires act as effective “nanospacers”, hindering the restacking of MXene sheets and promoting further delamination. Consequently, the accessible surface area and the exposure of active sites are substantially increased, offering more interfaces for ammonia adsorption. Additionally, the conductive network co‑constructed by C‑MXene and PPy efficiently transduces the chemical adsorption events into pronounced electrical resistance changes, thereby greatly enhancing the sensor sensitivity.

FIGURE 7.

(a) Adsorption of 1% vol ammonia by hydrogels with different components, (b) resistance change of hydrogels with different C‐MXene contents under 4% vol ammonia, (c) relative resistance changes of cM₁P₁₀H at different ammonia concentrations, (d) linear fitting of the resistance response versus ammonia concentration.

Subsequently, to identify the optimal composition for ammonia adsorption, the responses of hydrogel sensors with different C‑MXene loadings were evaluated under 4 vol% NH₃ (Figure 7b). The cM_1.5P₁₀H delivered the highest response (ΔR/R₀ = 76.2%), indicating an optimal balance between the density of –COOH groups (for NH₃ capture) and the porosity/conductivity of the composite (for mass transport and signal transduction). This specific optimization underscores that gas‐sensing represents a distinct, surface‐sensitive application scenario for our material platform.

Subsequently, the responses of hydrogel sensors with different C‑MXene loadings were measured under 4 vol% ammonia (Figure 7b). Among them, the cM_1.5P₁₀H sensor delivered the highest response, with a relative resistance change of 76.2%. To investigate the concentration‐dependent behavior, the optimal sensor was tested across a range of 1–4 vol% NH₃ (Figure 7c). The response showed a good linear relationship with ammonia concentration (R² = 0.95, Figure 7d), confirming its reliable quantitative detection capability within this range. These results underscore the promising potential of the cMPH composite for practical gas‑sensing applications.

4. Conclusions

In summary, we have successfully demonstrated a nanoscale synergy strategy to fabricate high‐performance cMPH sensor. This strategy leverages the complementary roles of 2D C‐MXene as a supportive skeleton and 1D PPy as conductive bridges, forming a robust 3D network that yields exceptional overall properties: high conductivity (5.5 mS/cm), superior strain sensitivity (GF of 7.52), strong adhesion (152.3 kPa to glass), remarkable toughness (120 kJ·m⁻³), and extreme stretchability (1063%). The carboxyl groups on C‐MXene further endowed the hydrogel with excellent room‐temperature ammonia detection capability. This work validates the power of the nanoscale synergy strategy in integrating multifunctionality into a single, easily processable hydrogel platform for advanced flexible electronics.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73884‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.