Semi-Interpenetrating Highly Conductive and Transparent Hydrogels for Wearable Sensors and Gesture-Driven Cryptography

Abstract

Developing conductive hydrogels that balance high conductivity, stretchability, transparency, and sensitivity for next-generation wearable sensors remains challenging due to inherent trade-offs. This study introduces a straightforward approach to fabricate a semi-interpenetrating double-network hydrogel comprising polyvinyl alcohol (PVA), polyacrylamide (PAM), and lithium chloride (LiCl) to overcome these limitations. Leveraging hydrogen bonding for energy dissipation and chemical cross-linking for structural integrity, the design achieves robust mechanical properties. The incorporation of 1 mol/L LiCl significantly enhances ionic conductivity, while also providing plasticizing and moisture-retention benefits. The optimized hydrogel exhibits impressive ionic conductivity (0.47 S/m, 113% enhancement), excellent mechanical performance (e.g., 0.177 MPa tensile strength, 730% elongation, 0.68 MJ m⁻³ toughness), high transparency (>85%), and superior strain sensitivity (gauge factors ~1). It also demonstrates rapid response/recovery and robust fatigue resistance. Functioning as a wearable sensor, it reliably monitors diverse human activities and enables novel, secure data handling applications, such as finger-motion-driven Morse code interfaces and gesture-based password systems. This accessible fabrication method yields versatile hydrogels with promising applications in health tracking, interactive devices, and secure communication technologies.

1. Introduction

Flexible wearable electronics are advancing rapidly, aiming to develop novel devices that can integrate seamlessly with the human body for applications such as human motion and health monitoring, soft robotics, information exchange, and information encryption/decryption [1,2,3,4,5]. A key challenge is developing sensing materials that simultaneously possess excellent electrical performance and tissue-like mechanical properties. Traditional conductors (such as metals or rigid semiconductors) have Young’s modulus orders of magnitude higher than human soft tissues [6]. This mechanical mismatch leads to poor interfacial contact, inaccurate signal acquisition, and significant discomfort during dynamic movements, severely limiting their long-term wearability. In recent years, flexible materials have gained extensive attention for wearable sensors and human–machine interaction [7,8,9,10]. These sensors convert mechanical signals (e.g., subtle stress and strain) into electrical outputs, such as changes in current, resistance, and capacitance [11,12]. To achieve effective human motion monitoring, there is an urgent need to develop flexible sensors with excellent biocompatibility.

As an emerging class of flexible electronic materials, conductive hydrogels stand out due to their unique properties. Unlike conventional polymers (e.g., plastics and rubbers), they consist of a three-dimensional hydrophilic polymer network and mobile ions. They offer low moduli matching human tissues, tunable conductivity, and, through molecular design and component optimization, multifunctionality such as self-healing, adhesiveness, and environmental stability [13,14,15]. These characteristics make them ideal candidates for flexible wearable devices requiring direct body contact. An ideal hydrogel sensor should combine high conductivity to ensure efficient signal transmission and excellent mechanical properties (such as high toughness, stretchability, and fatigue resistance) to withstand repeated deformation, high-strain sensitivity for precise motion capture, and high transparency for visual monitoring. In addition, long-term daily monitoring requires key performance enhancements for hydrogels: ultrathin design to reduce mechanical interference, gas permeability to avoid skin irritation, anti-drying ability for stable signals, and environmental adaptability ensuring reliable operation. Zhang et al. developed a ~10 μm thick polyurethane nanomesh-reinforced hydrogel, which enabled 8 days of high-fidelity electrophysiological monitoring, including ECG, EMG, and EEG. This hydrogel exhibited 2.5 MPa tensile stress, 696% stretchability, 176.8 μJ cm⁻² skin adhesion, and 12-day anti-drying capability [16]. Wang et al. further reported a 2.7 μm thick hydrogel electrode. It tolerates −90 °C, shows 384.1 μJ cm⁻² adhesion, and maintains high gas permeability (air 3.0 × 10⁴ GPU, O₂ 3.2 × 10⁴ GPU) as well as water vapor permeability (1252.3 g m⁻² day⁻¹) for reliable ambulatory monitoring in cold environments [17]. These works highlight that integrating ultrathin structure, multifunctional stability, and long-term wearability is key to advancing hydrogel-based wearable sensors. However, most reported conductive hydrogels have significant limitations. Within the network structure of hydrogels, high water content enables efficient ion migration and conductivity but weakens mechanics, leading to low strength, poor toughness, and brittleness [18,19]. Conversely, enhancing mechanical performance by increasing cross-linking density or incorporating rigid nanomaterials typically comes at the cost of stretchability, flexibility, or ion mobility, which results in reduced conductivity or impaired sensitivity [20]. This inherent trade-off hinders the integration of high conductivity, outstanding mechanical properties, and high-strain sensitivity into a single hydrogel material.

Researchers have explored a variety of strategies to overcome these limitations. To enhance toughness and strength, double network (DN) structures and nanocomposites such as nanocellulose or montmorillonite are employed [21,22]. To boost conductivity, methods include high-concentration electrolytes, conductive polymers, and particles [23,24,25]. While these methods have improved the properties of hydrogels to a certain extent, they often involve complex fabrication processes or are plagued by issues such as uneven dispersion of additives, poor long-term stability, and reduced biocompatibility. Critically, these modification strategies struggle to truly achieve the simultaneous optimization of multiple macroscale performance parameters, including mechanical strength, stretchability, resilience, and electrical conductivity. This limitation hinders their practical applications in complex scenarios.

To address the aforementioned challenges, this work successfully prepared a polyvinyl alcohol/polyacrylamide/lithium chloride (PVA/PAM/LiCl) double-network ionic conductive hydrogel with excellent comprehensive performance. The synergistic effect of physical cross-linking and chemical cross-linking balances toughness and elasticity. Meanwhile, an appropriate lithium chloride (LiCl) concentration not only provides excellent ionic conductivity but also exerts plasticizing and anti-drying effects through interactions with polymer chains, thereby endowing the hydrogel with high conductivity (0.47 S/m), outstanding mechanical properties (including 730% high stretchability, 0.681 MJ m⁻³ high toughness, and rapid resilience), and high-strain sensitivity simultaneously. Leveraging these advantages, we applied this hydrogel to high-precision human motion monitoring, successfully achieving stable capture of various physiological signals, ranging from large-scale joint movements to subtle vocal cord vibrations. Furthermore, we explored its innovative applications in the fields of human–machine interaction and information security. Utilizing its reliable and reproducible electro-mechanical response, we developed a real-time tactile Morse code communication system and realized accurate recognition and differentiation of custom password gestures, which is expected to be applied in information encryption and decryption. This work not only provides a simple and effective strategy for preparing high-performance ionic conductive hydrogels but also demonstrates their great potential in the integration of flexible sensing and secure communication. It opens up a new path for the development of multifunctional flexible electronic devices in the future.

2. Materials and Methods

Materials: Monomer acrylamide (AM), N,N′-methylene dimethyl (acrylamide) (MBAA), Lithium chloride anhydrous (LiCl), Ammonium Persulfate (APS) was purchased from Shanghai Macklin Biochemical Co., Ltd. Shanghai, China. Poly(vinyl alcohol)1788 was purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. All chemicals were used without further purification.

Preparation of the PVA/PAM/LiCl (PPL) hydrogels: PVA (0.4 g) and AM (2.1 g) were dissolved in deionized water (7 mL), followed by stirring for 2 h at 90 °C. Then, LiCl was incorporated into the mixed solution, and the resulting mixture was stirred at room temperature for 30 min. Next, MBA (3 mg) and APS (60 mg) were then added to this aqueous solution. The resulting solution was stirred for 20 min and then injected into a polytetrafluoroethylene mold, degassed under vacuum, and polymerized at 60 °C for 3 h to obtain the corresponding PPL hydrogels annotated as PPL₀, PPL₁, PPL₃, and PPL₅ corresponding to LiCl amounts of 0, 1, 3, and 5 mol/L, respectively. These hydrogels were then stored at 4 °C, and their compositions are listed in

Table 1. The compositions of the PVA/PAM/LiCl (PPL) hydrogels.

Sample	PVA (g)	AM (g)	LiCl (M)	MBA (mg)	APS (mg)	Distilled H2O (mL)
PPL0	0.4	2.1	0	3	60	7
PPL1	0.4	2.1	1	3	60	7
PPL3	0.4	2.1	3	3	60	7
PPL5	0.4	2.1	5	3	60	7

.

Characterization: The chemical functional groups of the samples were analyzed using a Fourier transform infrared (FTIR) spectrometer (iCAN9, TIANJIN ENERGY SPECTRUM TECHNOLOG Co., Ltd., Tianjin, China), with measurements taken over the wavenumber range of 4000−400 cm⁻¹. The microscopic surface morphology of the samples at 15 keV was detected by scanning electron microscopy (SEM, Phenom Nano G2, Eindhoven, The Netherlands). Before use, the sample was freeze-dried and fractured to obtain a cross-section, and then a thin layer of gold was sprayed on it.

Mechanical Property Tests: Mechanical properties were evaluated by a universal testing machine (MARK-10, MARK-10 Corporation, Copiague, NY, USA). Samples were shaped into a long strip with dimensions 60 mm × 10 mm × 2 mm, where the stretched section measures 20 mm × 10 mm × 2 mm. Tensile stress–strain tests were conducted at a rate of 100 mm/min at room temperature. Moreover, strain and stress values were derived via the following formulas [25]:

$$\epsilon ={\displaystyle \raisebox{1ex}{$∆L$}\!\left/ \!\raisebox{-1ex}{$L_0$}\right.}$$

$$\delta ={\displaystyle \raisebox{1ex}{$F$}\!\left/ \!\raisebox{-1ex}{$S_0$}\right.}$$

where ε is the applied strain; ΔL = L−L₀, L and L₀, respectively, represent tensile length and initial length of the samples. δ represents stress; F represents the applied force; and S₀ represents the initial cross-sectional area. The values of tensile elastic modulus (E) correspond to the slope within the 5% to 15% strain range. Meanwhile, toughness (T) represents the area under the curve’s integral [26]. The equation is as follows:

$$\tau ={\int }_{{\epsilon }_0}^{{\epsilon }_f}\delta d \epsilon$$

ε₀ and ε_f, respectively, represent the initial tensile strain and fracture strain and δ represents the stress.

Electrical Measurements: Electrochemical properties were tested with an electrochemical workstation (CHI660E Electrochemical Workstation, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). Conductive hydrogel samples were fabricated into 10 mm × 10 mm × 2 mm sheets and positioned in a mold for electrochemical testing. Conductivity is computed based on the equation below:

$$\sigma ={\displaystyle \raisebox{1ex}{$L$}\!\left/ \!\raisebox{-1ex}{$RS$}\right.}$$

Here, σ denotes conductivity (S/m), R stands for impedance (Ω), and L and S are the distance between the two electrodes (m) and the cross-sectional area of the tested hydrogel (m²), respectively [26].

Sensing Tests: PPL₁ hydrogels prepared as above served as core sensing elements for fabricating a resistive strain sensor. The relative changes in resistance (∆R/R₀) were measured to evaluate PPL’s sensing performance. Samples were trimmed into regular strips of 20 × 10 × 2 mm³ for use in a strain sensor. Conductive copper wires were pasted to both ends of the sensor. The relative resistance change (∆R/R₀) was determined using the volt-ampere method. The measurement setup included a high-precision linear motor (Y400TA100-600, Beijing Aerospace Precision Instrument Technology Co., Ltd., Beijing, China) and an oscilloscope with eight analog channels (MSO44, Tektronix, Inc., Beaverton, USA) [27]. The relative resistance change could be calculated as follows:

$${\displaystyle \frac{∆R}{R_0}}={\displaystyle \frac{R-R_0}{R_0}}={\displaystyle \frac{V-V_0}{V_0}}$$

In the formula, V and V₀ represent the voltage values after strain application and the initial voltage values at both ends of the PPL hydrogel before strain occurs, respectively. The specific testing method is as follows: A long strip of PPL is attached to the linear motor through 3M tape. A voltage divider circuit was constructed by connecting the two ends of a copper wire to the PPL hydrogel and the resistor, respectively. The voltage across the hydrogel was then recorded using an oscilloscope, and a DC power supply served as the power source for the test circuit. Finally, the PPL sensor was assembled and ready for testing. During the test, both ends of the copper wire were fixed with tape to prevent detachment. For motion monitoring and other practical applications, the same components are fixed with medical adhesive tape, and the same method is used for measurement. The gauge factor (GF) of the strain sensor is calculated as follows:

$$GF={\displaystyle \frac{\raisebox{1ex}{$∆R$}\!\left/ \!\raisebox{-1ex}{$R_0$}\right.}{\epsilon }}$$

where ε represents the tensile strain of the sensor.

Statistical Analysis: All quantitative data are presented as mean ± standard deviation (n = 3). Statistical comparisons between different LiCl concentration groups were performed using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test for multiple comparisons. All analyses were conducted using Origin2022. The statistically significant difference between groups was expressed as * p < 0.05, ** p< 0.01, and *** p < 0.001.

3. Results and Discussion

3.1. Formation Mechanism and Characterization of PPL Hydrogels

The fabrication process of PVA/PAM/LiCl hydrogels was schematically illustrated in Figure 1a. PVA/PAM DN hydrogels were synthesized using PVA and acrylamide (AM) via free radical in situ polymerization under an initiator in the presence of LiCl. The hydroxyl group of the PVA chains and the amide group of AM form hydrogen bonds. The addition of LiCl contributed to the high ionic conductivity. In addition, LiCl was attached to PVA molecular chains through ionic interaction with –OH, and its strong hydration strengthened the binding of PVA to surrounding water molecules.

To investigate the microstructure of the as-prepared PVA/PAM/LiCl hydrogel, SEM was used to observe the morphology of freeze-dried hydrogel samples. As shown in Figure 1b, the results revealed that all hydrogels exhibited interconnected 3D porous network structures, and the samples possessed a large number of tiny pores. The aqueous solution can fill the network structure, which serves as a channel for the free movement of conductive ions. The PVA/PAM hydrogel without LiCl addition exhibits smaller pores and a clear framework, where PVA is interspersed and entangled in the PAM network skeleton in a fibrous form. After the introduction of LiCl, the solubility of PVA is enhanced due to the salting-in effect of Li⁺ and Cl⁻ ions. The PVA/PAM/LiCl hydrogel features a distinct porous structure, with its fracture surface presenting an inhomogeneous interconnected network containing both shallow pores and macropores. Moreover, the pore size increases slightly with the increasing LiCl content, and the overall structure is dense and uniform. (Figure 1c).

To confirm intermolecular interactions within the system, FT-IR was utilized to characterize and examine the chemical structure and surface groups of the composite hydrogel. Figure 1d shows the FTIR spectra of the four hydrogels. In the PVA-PAM spectrum, the range of 3200–3600 cm⁻¹ corresponds to the overlapping peak of the stretching vibrations of –OH groups in polyvinyl alcohol (PVA) and -NH₂ groups in polyacrylamide (PAM). The absorption peak at 2940 cm⁻¹ is attributed to methylene groups. The absorption peak of the amide I band is at 1654 cm⁻¹, and that of the amide II band is at 1598 cm⁻¹. The peak of the PAM amide I band (assigned to C=O stretching vibration) around 1654 cm⁻¹ showed significant changes. When the LiCl concentration was 0 M, the peak was located at approximately 1650 cm⁻¹; upon the addition of 5 M LiCl, the peak underwent a blue shift to 1642 cm⁻¹. The magnitude of the blue shift increased with the rising LiCl concentration, indicating that the coordination interaction between Li⁺ ions and C=O groups was enhanced as the Li⁺ concentration increased. No obvious fluctuations in peak position or intensity were observed for the saturated C-H stretching vibration peak in the range of 2800–3000 cm⁻¹ and the PVA C-O-C stretching vibration peak near 1103 cm⁻¹ under different LiCl concentrations. Therefore, the content of LiCl affects the hydrogen bond network structure within the system by regulating the intensity of its coordination interactions with the polar groups of PVA/PAM. A higher LiCl concentration leads to stronger coordination interactions and a more significant weakening of hydrogen bonds. However, the PVA/PAM/LiCl system can form a stable composite network at all tested LiCl concentrations, with only differences in the intensity of intermolecular interactions.

The transparency of hydrogels containing varying LiCl is presented in Figure 1e; the transparency of these hydrogels diminished gradually with increasing LiCl content. The hydrogel without LiCl exhibited a transmittance of over 90% in the visible light region (wavelength range: 400–800 nm), demonstrating high transparency. Although the transmittance values of PPL₁, PPL₃, and PPL₅ decreased to 85%, 76%, and 73%, respectively, after the addition of LiCl, these values still fell within the range required for visual wearable electronic devices. Such high transparency significantly enhanced the practical value and application prospects of the hydrogel in wearable electronic scenarios where visualization is necessary.

Therefore, it can be concluded that the PPL hydrogel holds potential as a strain sensor for monitoring human motion signals. In emergencies, it can be utilized to transmit distress signals via Morse code, thereby enabling effective communication in challenging environments. In addition, it can further realize applications in information encryption scenarios through gesture-driven control (Figure 1f).

3.2. Mechanical Properties

Figure 2a illustrates that the PPL hydrogels exhibit high flexibility and toughness, which can be readily stretched, twisted, and elongated after twisting. Notably, when subjected to stretching, the tip of a pair of scissors barely penetrated the PPL hydrogel, demonstrating its excellent puncture resistance. Additionally, it demonstrated the capacity to support a weight of up to 200 g (Figure 2b). Superior mechanical properties and strong stretchability of PPL hydrogels likely stem from their DN structures. The brittle network of physically cross-linked PVA can impart high toughness via energy dissipation from internal fractures, while the interpenetrating network of chemically cross-linked PAM can deliver strong tensile performance.

A systematic investigation was conducted on how varying PVA and PAM contents influence the mechanical properties of PVA/PAM hydrogels, with a universal testing machine used to run a set of tensile tests. In Figure 2c, the influence of PVA content on the tensile properties of PVA/PAM gel was investigated. When the PVA content is 0.8 g, the PVA/PAM gel exhibits the best strength, which may be due to the formation of appropriate entanglement and cross-linking structures between the polymer chains. In addition, the content of AM has a significant impact on the mechanical properties of PVA/PAM gel. As shown in Figure 2d, a monotonic increase in tensile strength is observed for the PVA/PAM double-network hydrogel when the PAM content is raised from 1.4 g to 2.1 g, with the fracture stress rising from 0.097 MPa to 0.169 MPa and the elongation at break concomitantly decreasing from 1027% to 463%. This behavior is ascribed primarily to the elevated cross-linking density and the synergistic reinforcement–embrittlement effect inherent to the dual-network architecture. Consequently, integrating the fracture stress and elongation data, the formulation comprising 0.4 g PVA and 2.1 g PAM is identified as the optimal composition for the preparation of PVA/PAM DN hydrogels.

To comprehensively elucidate the influence of LiCl on the mechanical characteristics of hydrogels, tensile and compressive performances of PPL hydrogels synthesized with different LiCl concentrations were determined. A range of PVA/PAM/LiCl hydrogels were fabricated with LiCl concentrations of 0, 1, 3, and 5 mol/L, designated as PPL₀, PPL₁, PPL₃, and PPL_5, respectively. To directly observe the mechanical and sensory limits of our hydrogels, we provide the tensile stress–strain curves in Figure 2e, while outcomes of the tensile tests are displayed in Figure 2f,g. With an increase in LiCl dosage, the tensile strength of the hydrogels shows a non-monotonic tendency, first rising and then declining, and reaches a maximum of 0.177 MPa at a LiCl concentration of 1 mol/L (Figure S1b). Concurrently, the elongation at break of the PPL hydrogel decreases slightly. Nonetheless, it remains as high as 730% (Figure S1a). The highly porous 3D structure endows the hydrogels with excellent mechanical properties, as evidenced by the elastic modulus rising from 0.038 to 0.054 MPa at a LiCl content of 1 mol/L (Figure S2a). The fracture energy exhibited only a marginal decline, decreasing from 0.73 to 0.68 MJ m⁻³ (Figure S2b). An appropriate amount of LiCl can strengthen network cross-linking via ionic coordination. The Li⁺ cations dynamically coordinate with the lone electron pairs on oxygen atoms in both the PVA and PAM chains, forming transient but effective ionic crosslinks. This supplementary crosslinking, in addition to the covalent network from MBAA, enhances the overall mechanical strength of the hydrogel. Simultaneously, the presence of a high concentration of dissolved ions increases the polarity of the aqueous phase and strengthens the interchain hydrogen bonding. This effect collectively reduces the mobility and flexibility of the polymer chains, leading to the observed decrease in elongation at break. However, excessive LiCl compromises mechanical performance by disrupting hydrogen bonding, inducing chain collapse, and promoting microphase separation, collectively manifesting as a simultaneous decline in fracture stress and fracture strain [23,27,28].

In addition to mechanical properties, the reliable stability, reusability, and fatigue resistance of hydrogels are equally crucial for wearable electronic sensors to maintain consistent performance during prolonged use. For this purpose, the PPL₁ hydrogel was chosen to examine the durability of the mechanical properties of the hydrogel under prolonged cyclic deformation. Figure 2h presents the stress–strain curves of the PPL₁ hydrogel in cyclic tensile tests under varying strains. The stress–strain profiles for the PPL₁ hydrogel undergoing 300% strain over 10 cycles are shown in Figure 2i. As shown in the figure, minor hysteresis effects were detectable in each loading cycle. The enclosed area of each hysteresis loop quantitatively represented the energy dissipation capacity of the hydrogel under cyclic tensile deformation. The graphical data clearly demonstrated that the hydrogel maintained nearly identical hysteresis loops in the subsequent nine cycles compared to the first stretching cycle, with the dissipated energy remaining essentially constant at approximately 108 kJ m⁻³. This also highlighted the dependable stability and outstanding fatigue resistance of the PPL₁ hydrogel. Figure 2j displays the continuous cyclic loading and unloading curves of the PPL₁ hydrogel subjected to different compressive strain conditions. Similarly, hysteresis loops were observed in compression cycling tests. Additionally, during ten consecutive cycles at 50% strain (Figure 2k), the largest hysteresis occurred in the first cycle, while the loops stabilized and remained nearly identical over the subsequent nine cycles. These findings indicate that the PPL hydrogel possesses a favorable energy dissipation capability and outstanding fatigue resistance during repeated cyclic loading–unloading tests. In conclusion, the PPL hydrogel exhibits excellent mechanical properties, satisfying the requirements for application in flexible wearable devices.

3.3. Conductivity and Sensing Properties of the PPL1 Hydrogel

Good electrical conductivity is also important for the application of hydrogels in flexible sensing. PPL₁ not only enhances the mechanical properties but also improves the ionic conductivity and electrical sensitivity. PPL₁ hydrogel has good electrical conductivity, which is closely related to the content of LiCl. As shown in Figure 3a, the conductivity of the PPL₁ hydrogel first increases and then decreases as the LiCl content rises. When the LiCl content reaches 1 mol/L, the conductivity reaches its maximum value of 0.47 S/m, representing an approximately 113% improvement compared to the conductivity of 0.22 S/m without LiCl addition. The conductive mechanism of hydrogels finds explanation in ion channel theory. As LiCl concentration rises steadily, ion channels open up, allowing more conductive ions to move freely and thereby boosting conductivity. Once LiCl concentration hits a specific threshold, the restricted cross-sectional size of ion channels blocks all ions from passing smoothly. At the same time, collisions among conductive ions lower conduction efficiency. These two factors together cause conductivity to drop. Additionally, a large quantity of Li⁺ is fixed via ionic interactions, leading to a reduced cross-sectional area of ion channels and lower conductivity [26]. PVA/PAM/1M LiCl hydrogel with superior overall performance was chosen to carry out the GF value and the following sensor performance testing. As shown in Figure 3b, the PPL₁ hydrogel was linked to a circuit via wires and a 3V power supply, showing that the LED lamp lit up. Even when stretched, the LED remained on but with a distinct change in brightness. This is because stretching the hydrogel elongates its conductive pathways, increasing the distance between ions and hindering electron transmission within the hydrogel’s pore structure. Such changes raise the hydrogel’s resistance and reduce its conductivity. Copper wires drawn from both ends of the PPL1 hydrogel were used to assemble its strain sensors. The PPL₁ hydrogel sensor exhibits remarkable response performance to diverse strains, precisely capturing electrical signals from subtle strains below 10% (Figure S3) to small strains of 10–50% and further up to large strains of 100–250%, as illustrated in Figure 3c,d. Moreover, it shows superior sensitivity to relative resistance variations under stretching rates from 5 to 35 mm/s (Figure 3e). In addition, the response time of PPL1 hydrogel was also tested. As shown in Figure S4, the response time and recovery time of the PPL1 hydrogel are 70ms and 80ms, respectively, demonstrating excellent electromechanical properties sufficient to respond in real time to human motion monitoring.

The strain GF served as a key indicator for assessing sensor performance. Sensitivity of resistive strain sensors may be represented via GF values, which represent the slope of the relative resistance change versus strain curve. Based on formula GF, it may be interpreted that the material resistance changes under a specific strain. The ΔR/R₀ value rose when the PPL₁ hydrogel was stretched; the strain detection range of PPL₁ strain sensors was able to reach 600%. (Figure 3f). As can be seen from the graph, the relative resistance of the PPL1 hydrogel increases with the strain, indicating that this hydrogel strain sensor exhibits a positive strain effect. That depicts the functional relationship between the relative resistance change (ΔR/R₀) of the sensor and the applied tensile strain. The sensor exhibits a highly linear electromechanical response over the entire tested strain range of 0% to 600%. The linear fitting curve (blue line) yields a fitting equation of y = 0.97 x − 8.01, with a coefficient of determination (R²) as high as 0.99, indicating an extremely strong correlation between strain and resistance change. Such excellent linearity ensures measurement accuracy and simplifies the signal calibration process in practical applications.

The stability of the PPL₁ hydrogel strain sensor was assessed via a continuous load–unload tensile cycling test, carried out under 30% strain for around 600 cycles. Figure 3g reveals that the PPL₁ hydrogel strain sensor showed steady and consistent electrical signals during the continuous tensile cycling test, indicating reliable stability. These findings indicate that the PPL₁ hydrogel, with its excellent strain sensitivity, is well-suited for use in strain sensors designed to monitor movements. This opens up broad prospects for applications in areas like human–machine interfaces, electronic skin, and health monitoring.

3.4. Application of the PPL₁ Hydrogel Sensor

Due to the excellent mechanical properties, good sensitivity, and stable sensing performance of PPL₁ hydrogel, it can be used as a wearable sensor. When in use, it can be conveniently fixed on the skin or joints with adhesive tape to effectively detect various physiological activities of the human body. When mounted on the back of a volunteer’s neck (Figure 4a), the sensor accurately captures subtle head motions, including nodding and tilting. Additionally, the sensor was attached to the index finger to detect various bending motions at different angles (30°, 60°, and 90°) (Figure 4b). Stable, consistent, and repeatable stepwise signals were obtained, demonstrating its ability to accurately distinguish the direction of finger movement. Moreover, when the PPL₁-based strain sensor was mounted on human joints, its electrical output dynamically responded to flexion movements, allowing for active assessment of human motion. Remarkably, the device showed prompt and consistent signal outputs upon large deformations at the wrist (Figure 4c), elbow (Figure 4d), and knee (Figure 4e) joints. Notably, the hydrogel maintained sensitive detection across a wide strain range, attesting to its superior sensing capability and repeatability. When mounted on the laryngeal prominence, the hydrogel-based sensor effectively captured laryngeal vibrations, exhibiting well-defined and repeatable ΔR/R₀ signal patterns corresponding to articulated words including “see you”, “hello”, and “thank you” (Figure 4f,g), thereby highlighting its promising applicability in voice recognition systems. These results demonstrated the excellent sensing properties of the PPL1 hydrogel sensor, thus highlighting its great application prospects in flexible wearable electronic devices.

In addition to detecting electrical signal variations, reliable signal transmission represents a crucial requirement for practical applications. The PPL₁-based sensor exhibits promising potential for information encryption applications due to its excellent sensitivity, outstanding mechanical properties, cost-effectiveness, and scalability for mass production. Morse code, which utilizes sequences of “dots” and “dashes,” is a widely recognized and effective means of communication, encoding alphabetic characters, numerals, and punctuation through distinct patterns formed by these two fundamental signal units. This hydrogel sensor was further utilized for encrypted communication via Morse code. Figure 5a displays the complete Morse code sequences corresponding to all 26 English letters. The coding mechanism was implemented through finger flexion-induced strain signals, where high-amplitude bending (large strain) and low-amplitude bending (small strain) were, respectively, interpreted as dashes (“—”) and dots (“·”). By combining these fundamental signals into predefined waveforms corresponding to specific letters, continuous bending motions could form coherent phrases. Notably, each character exhibited a distinctive signal waveform pattern (Figure 5b,c), enabling information transmission through sequenced phrases, as demonstrated by the successful encoding of words like “BEST” and “GMU”. Therefore, these results highlight how encrypted data transmission could significantly expand the application scope of flexible wearable sensors. In conclusion, the PPL₁ hydrogel developed herein shows considerable potential in information recording and transmission, thus expanding the application scope of hydrogel sensors.

With ongoing technological evolution, expectations for safeguarding personal belongings and households have risen, making intelligent safety systems a growing part of modern living. Capitalizing on the superior information encryption capabilities of the aforementioned PPL₁ hydrogel, we developed a novel intelligent recognition strategy for integration into smart locks to enhance security systems. As a proof of concept, controlled finger-induced stretching of the hydrogel generated distinct relative resistance changes, enabling programmable signal output. The encryption-based identification process operates as follows: the PPL dual-network conductive hydrogel strain sensor serves as the input unit for signal acquisition. When attached to a finger joint, the sensor defines its output as “0” in the initial (unbent) state and as “1” when the finger is bent at 90° and held (Figure 5d). For precise analog-to-digital signal conversion, an STM32F108 microcontroller was employed to process and digitize the acquired analog signals. The decoded results were displayed on an LED screen, demonstrating practical anti-theft password functionality (Figure 5f).

As illustrated in Figure 5e,g, the system successfully decrypted four-digit codes into predefined outputs (e.g., “1010” corresponds to “B.E.S.T”, “0101” corresponds to “G.M.U”), with customizable password mapping. Real-time signal conversion ensured both accuracy and rapid response. To accommodate materials with varying conductive/mechanical properties, the current threshold was adjustable, broadening operational compatibility. A practical demonstration of password theft prevention is provided in Supplementary Videos S1 and S2, confirming effective resistance to information leakage. Furthermore, the PPL1 hydrogel encryption module can be coupled with wireless transmission devices for remote control and human–machine interaction in the future. Beyond smart locks, potential applications include offline banking password entry, taxi fare encryption, and protection of sensitive data (e.g., payment codes, ID/phone numbers). Notably, this system also exhibits promising utility in military-grade encryption scenarios.

4. Conclusions

Multifunctional hydrogels hold significant importance for wearable sensors. In this work, we propose a synthesis protocol for transparent conductive hydrogels with excellent comprehensive properties, using polyvinyl alcohol (PVA), polyacrylamide (PAM), and lithium chloride (LiCl) as raw materials. The hydrogen bonds between PVA and PAM function as the energy dissipation mechanism, endowing the hydrogels with extensibility. The mechanical characteristics of hydrogels were regulated by adjusting the contents of PVA, PAM, and LiCl. Under the effect of LiCl, not only were the tensile strength, compressive strength, and toughness of the hydrogels significantly enhanced, but excellent electrical conductivity was also achieved. The PAM/PVA/1M LiCl hydrogel exhibited outstanding electrical conductivity (σ = 0.47 S/m), favorable linear sensitivity (GF = 0.97), and tensile performance (elongation at break exceeding 700%). As a portable sensor, the hydrogel is capable of monitoring diverse human activities, including joint motions (finger, wrist, and elbow flexion) and slight deformations. Owing to the stable and clear signals displayed by the sensor under different deformations, its application in the field of information encryption and decryption was realized via Morse code. Moreover, we conducted a systematic comparison with recently reported advanced hydrogel-based sensors (Table S1) [27,29,30,31,32]. As summarized, the radar chart clearly demonstrates that our design achieves a superior trade-off between multiple key parameters, including electrical conductivity, mechanical stretchability, and sensing sensitivity (Figure S5). In conclusion, the PAM/PVA/LiCl hydrogel prepared in this work integrates multiple excellent properties, providing insights for the fabrication of flexible sensors and their extended applications in health monitoring, motion recognition, and information encryption transmission.