A Novel Polyacrylamide/Sodium Alginate/Polypyrrole Composite Hydrogel for Fabricating Flexible Sensors for Wearable Health Monitoring

Abstract

Conductive hydrogels that simultaneously exhibit high mechanical robustness, reliable electrical conductivity, and interfacial adhesion are highly desirable for flexible sensing applications; however, achieving these properties in a single system remains challenging due to intrinsic structure–property trade-offs. Herein, a multifunctional conductive hydrogel (ASP hydrogel) is developed based on a polyacrylamide (PAM)/sodium alginate (SA) double-network architecture using a gallic acid (GA)–Fe³⁺–pyrrole (Py) coupling strategy. In this design, GA provides metal-coordination sites for Fe³⁺, while Fe³⁺ simultaneously serves as an oxidant to trigger the in situ polymerization of pyrrole, enabling the homogeneous integration of polypyrrole (PPy) conductive networks within the hydrogel matrix. The resulting ASP hydrogel exhibits a markedly enhanced fracture strength of 2.95 MPa compared with PAM (0.26 MPa) and PAM–SA (0.22 MPa) hydrogels, together with stable electrical conductivity and reproducible strain-dependent electrical responses. Moreover, the introduction of dynamic metal–phenolic coordination and hydrogen-bonding interactions endows the hydrogel with intrinsic self-healing capability and strong adhesion to diverse substrates. Rather than relying on simple filler incorporation, this work demonstrates an integrated network design that balances mechanical strength, conductivity, and adhesion, providing a versatile material platform for flexible strain sensors and wearable electronics.

1. Introduction

Wearable health monitoring technology enables continuous, real-time, and personalized tracking of physiological conditions in daily life, facilitating early disease detection, improved management of chronic illnesses, and reduced healthcare costs [1]. Flexible sensing materials with biocompatibility [2], long-term stability, and high sensitivity in complex environments are key components of next-generation sensing systems for wearable devices and soft robot [3]. Hydrogel electrolytes with excellent ionic transport capability and stable mechanical performance have been rapidly developed for use in energy storage devices such as supercapacitors [4].

Hydrogels are polymer materials that form three-dimensional network structures through chemical crosslinking (e.g., covalent bonds) [5] or physical crosslinking (e.g., physical crystallization, coordination bonds, or hydrogen bonds) [6] of single or multiple homopolymers or copolymers. Due to their high water content, excellent softness, and tunable structure, hydrogel materials exhibit unique advantages in the field of flexible sensors.

Conductive hydrogels (P-CHs) are prepared by incorporating various conductive materials into polymer network hydrogels [7]. By combining the biomimetic properties of hydrogels with the physiological and electrochemical characteristics of conductive materials, these hydrogels exhibit high conductivity and redox activity, enabling the detection of bioelectrical signals and the application of electrical stimulation to regulate cellular activities, including migration, proliferation, and differentiation [8]. Constructing conductive networks within the hydrogel matrix allows the material to retain the intrinsic flexibility [9] and skin compatibility of hydrogels [10] while achieving efficient signal transmission through the conductive network. Their excellent signal conductivity, flexibility, multifunctionality, self-adhesion, strain sensitivity, and biocompatibility make conductive hydrogels promising materials for motion monitoring, health management, and human–machine interaction. Consequently, they are attracting increasing attention in the field of smart wearable devices.

However, hydrogels face a trade-off between mechanical and electrochemical properties, limiting their widespread application in flexible wearable sensors. Increasing the crosslinking density can enhance toughness and stability but often compromises the uniformity and continuity of conductive components [11]. Similarly, excessive incorporation of conductive fillers can lead to phase separation, causing interfacial delamination under tensile, compressive, or cyclic loading and reducing fatigue resistance. Achieving simultaneous improvement of mechanical strength and electrical conductivity within a single hydrogel system remains a critical challenge in current research on flexible sensing materials.

Biomass materials are abundant, widely available, biodegradable, highly water-retentive, and renewable, making them ideal for hydrogel fabrication [12]. Sodium alginate (SA) is a linear, anionic, unbranched natural polysaccharide extracted from brown algae, composed of β-D-mannuronate (M) and α-L-guluronate (G) residues [13]. It is widely available, non-toxic, and degradable, making it a preferred material for hydrogel preparation [4]. The numerous free hydroxyl and carboxyl groups in SA make it highly water-soluble; however, its strong hydrophilicity causes rapid swelling and poor mechanical performance in aqueous environments, limiting its direct application in flexible electronic devices [14]. Double-network (DN) hydrogels, a unique class of interpenetrating polymer networks, consist of two distinct and asymmetric networks and have been shown to effectively enhance mechanical strength and fracture toughness. The two networks differ in crosslinking density, stiffness, and molecular weight. Typically, the first network is highly crosslinked and brittle; after its formation, it swells in a solution of the second network monomers, which then polymerize to form a less crosslinked, more ductile network [15]. Previous studies have employed sodium polyacrylate, sodium alginate, and graphene oxide as raw materials to fabricate composite hydrogel fibers with excellent electrical conductivity and sensing stability via wet spinning [16]. In this study, a composite network is constructed by combining sodium alginate with synthetic monomers of controllable crosslinking to improve the overall mechanical performance of the hydrogel.

Polyacrylamide (PAM) hydrogels can be prepared by polymerizing high concentrations of acrylamide monomers [17]. The abundant amine groups in PAM can chemically react with SA to form amide bonds, resulting in a semi-interpenetrating network hydrogel. Additionally, the numerous hydrogen bonds formed by the amide groups contribute to excellent mechanical properties [18].

To impart conductivity to hydrogels, conductive polymers are incorporated into the system [19]. Polypyrrole (PPy) is a widely used heterocyclic compound with a conjugated π-electron system, high conductivity, and excellent stability, making it a common conductive filler for hydrogel fabrication. As a semiconductor polymer, PPy is easy to synthesize and exhibits both electrical and thermal stability, leading to its broad applications in biomedicine, including drug delivery, gene delivery, neural interfaces, biosensors, artificial muscles, and tissue engineering [20]. While doping hydrogels with PPy can provide good conductivity, exogenous doping or filler incorporation often results in uneven dispersion within the hydrogel matrix and may even cause macroscopic phase separation [21]. Uniform dispersion can also be achieved through certain physical methods. For example, ultrasonic agitation may be employed to promote the homogeneous distribution of PPy within the hydrogel matrix, thereby facilitating the formation of conductive pathways. However, such treatments may simultaneously lead to damage to the hydrogel’s underlying network structure [22]. Therefore, it is necessary to construct in situ-formed PPy conductive pathways that are uniformly distributed and strongly integrated with the hydrogel network.

Metal coordination bonds play a key role in enhancing hydrogel mechanical performance and establishing energy-dissipating mechanisms due to their dynamic reversibility [23], rapid responsiveness, and relatively high bond strength. Introducing Fe³⁺ into a hydrogel system allows it to form dynamic coordination bonds with carboxyl groups in SA while acting as a strong oxidizing agent to catalyze the in situ polymerization of pyrrole monomers. To provide additional active sites for metal ion adsorption and complexation, gallic acid (GA) is incorporated into the hydrogel [24]. GA, a hydroxybenzoic acid extracted from plants, consists of a benzene ring substituted with three hydroxyl groups and one carboxyl group [25]. It exhibits strong antioxidant properties and can coordinate with metal ions. Its combination of carboxylic acid and phenolic functionalities allows it to participate in acid-base neutralization reactions or undergo alkylation to form ethers. As a naturally amphiphilic molecule containing π-conjugated groups and carboxylic acid moieties, GA theoretically has the potential to form self-assembled hydrogels [26].

Conventional conductive hydrogels based on PAM–SA or similar polymer matrices often achieve electronic conductivity by physically embedding pre-synthesized polypyrrole particles or by simple in situ polymerization of pyrrole monomers within a preformed network. However, these approaches can lead to challenges such as inhomogeneous PPy distribution and aggregation due to its poor solubility and lack of specific molecular guidance during polymerization, which in turn impedes the formation of continuous conductive pathways [27]. Moreover, embedding conductive polymers like PPy as fillers without additional coordination motifs frequently results in a trade-off between conductivity and mechanical strength due to weak interfacial bonding, as highlighted in comprehensive reviews of conductive hydrogel composites [28]. Previous PAM–SA–PPy conductive hydrogels typically achieve conductivity either by physical incorporation of pre-synthesized PPy particles or by simple in situ polymerization without additional coordination motifs. In such systems, PPy aggregation or weak interfacial bonding with the polymer matrix often limits mechanical integrity and sensing stability. In comparison, the GA–Fe³⁺–Py coupling strategy employed here enables PPy to grow in situ under the guidance of metal–phenolic coordination sites, leading to a more continuous conductive pathway and improved network integration. Moreover, unlike conventional PPy-filled hydrogels that mainly focus on conductivity enhancement, the present system simultaneously introduces reversible coordination and hydrogen-bonding interactions, which contribute to energy dissipation, autonomous self-healing, and universal adhesion.

Although PAM/SA-based double-network conductive hydrogels and PPy-incorporated systems have been reported, most existing approaches rely on either physical doping of conductive fillers or sequential fabrication strategies, which often suffer from poor dispersion, phase separation, or an inherent trade-off between mechanical robustness and electrical conductivity. In contrast, the present work proposes a GA–Fe³⁺–Py coupling strategy, in which gallic acid serves as a multifunctional molecular linker to anchor Fe³⁺ ions through metal–phenolic coordination, while Fe³⁺ simultaneously acts as an oxidant to trigger the in situ polymerization of pyrrole. This integrated strategy enables the homogeneous formation of a PPy conductive network within a PAM/SA double-network hydrogel, while introducing dynamic coordination bonds that contribute to mechanical reinforcement, intrinsic self-healing, and strong interfacial adhesion. Rather than focusing on a single performance metric, this work emphasizes the synergistic balance among mechanical strength, electrical conductivity, and adhesion/self-healing properties, providing a unified design concept for multifunctional conductive hydrogels in wearable sensing applications.

2. Results and Discussion

2.1. Gelation Mechanism

Recent studies have demonstrated that advanced hydrogel architectures and properties can be achieved through either mechanically programmed or chemistry-driven strategies. For instance, anisotropic dual-network hydrogels with muscle-like mechanical properties have been constructed via mechanically induced training, where the asymmetric responses of rigid and flexible polymer chains to external stimuli give rise to self-arranged structures, resulting in significantly enhanced storage modulus and crack resistance, as well as potential in wearable sensing and directional cell growth applications [29]. In parallel, polyphenol–metal chemistry has emerged as an effective approach for regulating polymerization kinetics and functional integration in hydrogel-based sensors. Representative examples include tannic acid (TA) and Fe₃O₄ nanoparticle-assisted catalytic systems, in which dynamic Fe²⁺/Fe³⁺ redox cycling accelerates free-radical generation, enabling rapid formation of super-stretchable and highly sensitive hydrogels [30].

Distinct from both mechanically trained and redox-catalyzed systems, the present work adopts a chemistry-driven network integration strategy based on GA–Fe³⁺–Py coupling within a PAM/SA matrix. Here, the metal–phenolic coordination not only guides the in situ polymerization of PPy but also reinforces interfacial interactions through synergistic coordination and hydrogen bonding. This integrated network design allows simultaneous optimization of mechanical robustness, electrical conductivity, self-healing capability, and adhesion without reliance on external mechanical training, catalytic nanoparticles, or surface microstructuring, offering an alternative and scalable pathway toward multifunctional conductive soft materials.

After polymerization, the precursor solution transformed into a self-supporting hydrogel. Based on the reported gelation code classification, the prepared hydrogel corresponded to Code F, indicating successful gel formation. As shown in Figure 1a, the hydrogel is reinforced and toughened by designing a double-network structure, while the conductive filler pyrrole is introduced. Dynamic interactions within the network enable uniform dispersion of polypyrrole, enhancing the hydrogel’s conductivity. According to the gelation results at different ratios (Figure 1b), increasing the SA content helps form a more homogeneous gel. Physical doping of PPy often leads to uneven dispersion or aggregation of the conductive material, whereas chemical dispersion effectively resolves these issues. By carefully selecting the monomers, crosslinkers, and metal ions, a stretchable conductive hydrogel is synthesized. As illustrated in Figure 1c, the AM–SA hydrogel precursor exhibits good transparency and fluidity, broadening its potential applications as a flexible sensor across various fields.

Figure 1d illustrates the design mechanism of the ASP composite hydrogel. Upon heating, the initiator APS decomposes to generate free radicals, initiating the chain-growth polymerization of AM monomers. The bifunctional nature of MBA serves as a covalent crosslinking point, linking the growing PAM chains to form a covalently crosslinked network. The carboxyl groups (-COOH) in SA chemically react with the amine groups (-NH₂) in PAM to form amide bonds. However, synthesized polypyrrole (PPy) is difficult to disperse uniformly within the hydrogel matrix. To address this, gallic acid is added to introduce carboxyl groups, which adsorb and complex Fe³⁺. The Fe³⁺ then oxidizes pyrrole in situ to form PPy, achieving uniform incorporation throughout the hydrogel.

2.2. Structural Characterization and Analysis

2.2.1. Fourier Transform Infrared Spectra

Fourier-transform infrared (FTIR) spectroscopy was used to characterize the chemical evolution of SA, AM, Py, and the synthesized ASP hydrogel (Figure 2a). Characteristic absorption bands of polypyrrole were observed at ~1547 and ~1456 cm⁻¹, corresponding to C=C and C–C stretching vibrations in the pyrrole ring, and at ~1235–1359 cm⁻¹, corresponding to C–N stretching vibrations, consistent with literature reports for PPy structures [31]. Additionally, in composite hydrogels with PPy incorporation, the intensity of the peak near 1650 cm⁻¹—attributed to the pyrrole C=C vibration—became more pronounced, further indicating the presence of pyrrole-derived structures within the hydrogel matrix [32]. These FTIR features, together with the retention of characteristic SA/PAM peaks, support successful PPy formation and its homogeneous integration without disrupting the base hydrogel network.

After forming the composite, new intermolecular hydrogen bonds appear, causing a shift in this peak toward lower frequencies. The ASP hydrogel shows both new absorption peaks and retention of original peaks, confirming the successful synthesis of the ASP hydrogel.

2.2.2. SEM

After confirming the successful synthesis of the ASP hydrogel and the incorporation of pyrrole, the microstructure of each hydrogel was systematically characterized by SEM. As shown in Figure 2b–d, the surface of pure PAM hydrogel exhibits numerous irregular pores with relatively thick walls. SEM analysis shows that with increasing SA content, the pore size of the hydrogel gradually increases and the pore distribution shifts toward a more dispersed morphology. For example, previous studies on PAM/SA hydrogels demonstrate that pore diameters significantly increase upon the addition of alginate compared with pure PAM hydrogels [33]. These structural changes are often ascribed to enhanced interactions between the polysaccharide and acrylamide networks, such as hydrogen bonding and electrostatic interactions, which promote the formation of a more homogeneous three-dimensional network with modified pore architecture. Similar compositional effects on pore morphology have been observed in related alginate-containing composite hydrogel systems where component ratios significantly influence pore size distribution [34]. With increasing SA content, the pore size of the hydrogel gradually enlarges, and the pore distribution shifts from a relatively ordered morphology to a more dispersed and homogeneous network, resulting in an overall smoother gel surface. Similar trends have been observed in PAM/SA composite systems: for example, SEM analysis of PAM/SA hydrogels shows that the average pore size of pure PAM hydrogels (~19.9 μm) increases significantly when SA is incorporated, indicating clear changes in network structure associated with the SA component [33].

As shown in Figure 2e–g, the uniform dispersion of pyrrole in the ASP hydrogels forms continuous conductive pathways. With increasing pyrrole content, the internal conductive network of the hydrogel becomes progressively denser and more ordered. Among them, ASP₂ exhibits the most regular arrangement of its conductive network, consistent with its highest observed electrical conductivity.

2.3. Mechanical Properties

The mechanical properties of conductive hydrogels are critical for their practical applications in flexible electronics and wearable devices. As shown in Figure 3a, compared with pure AM hydrogel, the elongation at break of the hydrogel significantly increases with the addition of SA. This outstanding mechanical performance arises from the presence of hydrogen bonds and ionic interactions within the polymer network, combined with the stabilizing effect of PPy as a conductive filler. The synergistic effect of these factors imparts excellent mechanical strength to the hydrogel. During stretching, the entangled polymer chains first disentangle and then elongate under increasing stress. Subsequently, the hydrogen bonds and ionic interactions within the polymer network break, dissipating energy. Meanwhile, the dispersed PPy within the hydrogel matrix helps release stress. If PPy is poorly dispersed, it tends to aggregate randomly, creating stress concentration sites that make the hydrogel prone to fracture under load and damage the network structure, resulting in a significant decrease in hydrogel strength.

As shown in Figure 3b, ASP hydrogels prepared with different mass ratios all exhibit excellent compressive mechanical performance. Under applied force, the hydrogels undergo significant compression, and upon removal of the load, the samples rapidly recover to their original shape within a short time, showing almost no residual deformation. These results indicate that the internal network structure of the ASP hydrogels possesses high elasticity and stability, endowing the material with superior compressive resilience.

2.4. Sensor Performance

In flexible sensor applications, the hydrogel matrix requires not only excellent mechanical properties but also high electrical conductivity. Electrochemical impedance spectroscopy (EIS) is used to evaluate the conductivity of hydrogels with different PPy contents. As shown in Figure 4a, the hydrogel’s conductivity increases with higher PPy content, and the impedance decreases from 120 Ω for AM–SA–PPy 0.5 g to 63 Ω for AM–SA–PPy 3 g. These results confirm that PPy forms a uniform conductive network within the hydrogel matrix, facilitating efficient electron transport and significantly enhancing the hydrogel’s electrical conductivity.

Figure 4b shows a photograph of the hydrogel used as a wire to connect a power supply and light a small bulb, confirming the hydrogel’s good electrical conductivity. The results demonstrate that the ASP hydrogel can stably transmit current and exhibits excellent conductive performance. Figure 4c–e illustrate the effect of stretching the hydrogel samples on the bulb’s brightness. As shown, when the composite hydrogel is stretched, the bulb gradually dims; when the hydrogel returns to its original state, the bulb brightness is restored. The hydrogel’s electrical response closely follows the mechanical deformation, with almost no delay. During stretching, the electron transport paths lengthen and the cross-sectional area decreases, causing the hydrogel’s resistance to increase and the bulb brightness to diminish.

2.5. Swelling Ratio

Figure 5a shows the effect of different PPy contents on the swelling behavior of the composite hydrogels. As illustrated, AM–SA hydrogels exhibit a rapid swelling rate, a high equilibrium swelling ratio, and a short time to reach swelling equilibrium. In contrast, hydrogels containing GA–Fe³⁺–PPy generally require more than 30 h to reach swelling equilibrium.

The AM–SA–PPy double-network hydrogel exhibits slower swelling behavior compared with the AM–SA interpenetrating network hydrogel. This is because the double-network structure effectively restricts the diffusion of water molecules, enabling the hydrogel to absorb water more gradually. As the PPy content increases, it occupies the pores within the hydrogel, leading to a reduced swelling ratio and lower water uptake.

2.6. Self-Healing Performance

Figure 5b–g illustrate the structural and performance behavior of the AM–SA–PPy composite hydrogel before and after self-healing. When the intact hydrogel is cut and the pieces are brought into contact, the fracture surfaces gradually rejoin under natural conditions, restoring the hydrogel into a single, continuous piece without visible interface traces at the macroscopic level.

To further evaluate self-healing, the repaired hydrogel was subjected to bending, twisting, and stretching tests. The results show that under external force, no re-cracking or structural instability occurs at the fracture site, and the overall shape remains well-maintained, demonstrating high structural integrity and mechanical stability. This indicates that the healed hydrogel can withstand a certain degree of mechanical deformation and stretching without breaking.

This self-healing behavior is primarily attributed to the abundant reversible hydrogen bonds within the hydrogel network. After fracture, polymer chain segments at the interface recontact and dynamically reorganize, allowing the network to spontaneously repair through hydrogen bond breaking and reformation. Therefore, the AM–SA–PPy composite hydrogel exhibits excellent intrinsic self-healing capability, providing reliable structural stability for applications in flexible devices and wearable sensors.

2.7. Adhesion Performance

The adhesion of flexible sensors enables them to form effective contact surfaces with the tested object without external force, thereby achieving excellent sensing performance. As shown in Figure 6, the hydrogel was prepared as a cylinder with a diameter of 1.7 cm and a height of 0.5 cm. By bringing the AM–SA–PPy hydrogel into contact with different materials, it demonstrated strong adhesion to both hydrophilic and hydrophobic surfaces, including leaves, plastic, cardboard, glass, and stainless steel, as well as adhesion to mouse organ tissues.

The excellent adhesion of the AM–SA–PPy hydrogel is primarily attributed to physical interactions arising from its composition and chemical structure, including hydrogen bonding, metal coordination, hydrophobic interactions, and the synergistic effects of multiple interactions between the hydrogel and the substrate surface. Overall, this strong adhesion endows the AM–SA–PPy hydrogel with greater potential for applications in biosensors, wearable devices, and other related fields.

3. Conclusions

In this study, an ASP composite hydrogel was successfully fabricated by integrating sodium alginate (SA), polyacrylamide (PAM), and polypyrrole (PPy) through a synergistic network design. The PAM network formed via radical polymerization provides structural integrity, while the incorporation of SA introduces additional physical interactions that enhance toughness and stability. The in situ formation of PPy endows the hydrogel with reliable electrical conductivity, enabling stable and distinguishable electrical responses under mechanical deformation. Importantly, the combination of dynamic coordination interactions and hydrogen bonding within the hydrogel network contributes to effective energy dissipation, intrinsic self-healing behavior, and strong adhesion to diverse substrates. These integrated properties allow the hydrogel to maintain mechanical robustness and sensing stability under repeated deformation, highlighting its suitability for flexible strain-sensing and wearable electronics applications. Although no biological evaluations are included in the present work, the soft mechanical characteristics, adhesion capability, and electrical responsiveness of the ASP hydrogel suggest potential relevance for future biointerface-related applications, such as soft tissue–interfacing sensors. Further studies incorporating biocompatibility assessments and in vivo-relevant evaluations will be necessary to fully explore its applicability in biomedical contexts.

4. Materials and Methods

4.1. Materials

N,N′-methylenebisacrylamide was supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Solid ferric chloride was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Sodium alginate (SA), acrylamide (AM), pyrrole (Py), ammonium persulfate (APS), gallic acid (GA), anhydrous ethanol, and potassium bromide were all obtained from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). Male mice with a body weight of 30.0 ± 0.4 g were purchased from Hunan SJA Laboratory Animal Co., Ltd. (Changsha, China). All materials were used as received without further purification.

4.2. Preparation of Hydrogel

4.2.1. Preparation of PAM/SA Semi-Interpenetrating Network Hydrogels

N,N′-methylenebisacrylamide is accurately weighed and dissolved in 30 mL of ethanol, followed by heating in a 70 °C water bath until completely dissolved. A specified amount of acrylamide (AM), 0.5 g of sodium alginate (SA), and 0.3 g of the N,N′-methylenebisacrylamide crosslinker solution are then added to 20 mL of deionized water and stirred at 60 °C until fully dissolved. Subsequently, 0.1 g of ammonium persulfate is added as the initiator, and the mixture is magnetically stirred at 500 rpm at room temperature until homogeneous, yielding a hydrogel precursor solution. Finally, the solution is transferred to an oven (Changzhou Jintan Liangyou Instrument Co., Ltd., Changzhou, China) and maintained at 70 °C for 1 h to form the PAM/SA hydrogel.

4.2.2. Preparation of PAM/SA/PPy Conductive Hydrogels

An ASP conductive hydrogel is prepared using a one-pot method. First, 1.5 g of sodium alginate (SA) is dissolved in 20 mL of deionized water and stirred at 60 °C. Then, 3 g of acrylamide (AM) and a predetermined ratio of crosslinker and initiator are slowly added to the SA solution to obtain a transparent AM–SA precursor. Next, 0.1 g of gallic acid (GA), 0.3 g of FeCl₃, and varying amounts of pyrrole monomer are added to the uniformly dispersed AM–SA solution and stirred at room temperature until all components are fully dissolved. The resulting solution is slowly poured into a mold consisting of a 1 mm thick silicone spacer sandwiched between two glass plates. Finally, the mold is placed in an oven at 70 °C for 1 h to form the AM–SA–GA–Fe³⁺–PPy conductive hydrogel, referred to as the ASP conductive hydrogel.

4.3. Characterization of Hydrogel

Fourier-transform infrared (FT-IR) spectra are measured using an Avatar 370 infrared spectrometer (Thermo-Nicolet, Waltham, MA, USA) over a wavenumber range of 400–4000 cm⁻¹ with a resolution of 8 cm⁻¹. Prior to testing, the hydrogels are fully freeze-dried and kept clean. SA, AM, and the freeze-dried ASP hydrogel are ground into powders and mixed with potassium bromide (KBr) at a mass ratio of 1:50 to form pellets for measurement. The surface morphology is observed using a Sigma 300 scanning electron microscope (SEM, Zeiss, Oberkochen, Germany). The hydrogel samples are freeze-dried, fractured in liquid nitrogen, and then coated with gold. The gold-coated hydrogels are mounted on the SEM sample stage, and their microstructure is examined at a scanning voltage of 10 kV.

4.4. Mechanical Property

Tensile tests of the hydrogels are conducted at room temperature and 60% relative humidity using an RGM-6005 electronic universal testing machine (Shenzhen Ruigeer Co., Ltd., Shenzhen, China). Dumbbell-shaped hydrogel samples are prepared with dimensions of 50 ± 2 mm in length, 8 ± 0.2 mm in width, and 2 ± 0.05 mm in thickness. The ends of each dumbbell sample are secured in the machine’s grips, and the initial distance between the grips is recorded as the sample’s initial length. Tensile testing is performed at a stretching rate of 100 mm/min, and the stress–strain curves of the hydrogel samples are recorded. Additionally, compression tests are performed on ASP hydrogels with varying gallic acid to Fe³⁺ mass ratios using a 25 kg load. The height of each hydrogel sample is measured with a ruler before and after applying and removing the load to evaluate its compressive mechanical performance.

4.5. Sensing Performance Testing of the Hydrogel

The sensor performance is evaluated by assembling a closed circuit consisting of a power supply, a resistance box, a small light bulb, and the composite hydrogel. By stretching the hydrogel to different extents, the brightness of the light bulb is observed to visually assess its sensing capability. Electrochemical impedance spectroscopy (EIS) measurements are carried out using a CHI660C electrochemical workstation (Shanghai Chenhua Co., Ltd., Shanghai, China) over a frequency range of 0.01 Hz to 100 kHz. Hydrogel samples are cut into circular shapes (thickness: 1.0 mm, diameter: 10.0 mm), and copper plates are used as electrodes in contact with both ends of the hydrogel. The electrodes are connected to the electrochemical workstation, and Nyquist plots are recorded to determine the hydrogel’s electrical resistance.

4.6. Hydrogel Swelling Performance Testing

The prepared hydrogel is placed at room temperature for 12 h and weighed to obtain its initial weight (W0). Using the room-temperature water immersion method, the hydrogel samples are soaked in water to allow swelling. At 6 h intervals, the hydrogel is removed, surface water is gently blotted off, and the sample is weighed to obtain the swollen weight (Ws). This procedure is repeated until the hydrogel weight no longer changes, indicating that swelling equilibrium has been reached, and the swelling curve is recorded. The swelling ratio (S%) of the hydrogel samples is calculated using the following formula:

$$S\%={\displaystyle \frac{\mathrm{W}\mathrm{s}-\mathrm{W}0}{\mathrm{W}0}}\times 100\%$$

where (Ws) (g) is the weight of the hydrogel after swelling in water, and (W0) (g) is the weight of the dry hydrogel, respectively.

4.7. Hydrogel Self-Healing Behavior Testing

The prepared composite hydrogel samples are cut in half at the center, and the cut surfaces are carefully aligned and brought into contact. The samples are left at room temperature for 24 h to allow the self-healing process to proceed fully. After healing, the hydrogel samples are subjected to simple tensile, bending, and twisting tests to visually assess the overall structural integrity and the retention of mechanical properties after self-repair.

4.8. Hydrogel Adhesion Performance Testing

The prepared hydrogel is brought into contact with a variety of substrates to systematically evaluate its adhesion performance on both hydrophilic and hydrophobic surfaces. Selected substrates include tissue paper, stainless steel, plastic, metal, glass, wood, and stone. The hydrogel’s adhesion is assessed by observing its attachment, stability, and any detachment on each surface, providing a comprehensive evaluation of its interfacial adhesion capabilities.