Strain‐Sensitive Double‐Network Organogel With Strong Adhesion for Movement Monitoring

ABSTRACT

Conductive organogels, as a class of soft materials, have demonstrated great application potential in the field of flexible electronics. However, they often suffer from several drawbacks, including low transparency, poor mechanical properties, low sensitivity, and weak adhesion. In this work, a highly adhesive and stretchable double‐network organogel is constructed by the interpenetration of a nanofiber network self‐assembled from 1,3:2,4‐dibenzylidene sorbitol (DBS) and tough polyacrylamide (PAM) chains in glycerol. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is incorporated to modulate the ionic conductivity of the gel, forming multiple ionic and intermolecular interactions within the gel network. The DBS/PAM‐Li conductive organogel exhibits high and durable adhesion strength to various substrates, as well as good optical transparency (92% transmittance at a thickness of 1.45 mm). Furthermore, when assembled into a strain sensor, the DBS/PAM‐Li organogel exhibits a high gauge factor of 8.72, a fast response time of 133 ms, and the capability to detect both small (with a low detection limit of 0.25% strain) and large deformations. This material can be used to monitor human motion, expanding the applications of flexible gel materials in the field of wearable electronics.

In this work, a one‐step method is designed to fabricate a physicochemical double‐network crosslinked DBS/PAM‐Li conductive organogel. The organogel exhibits high and durable adhesion strength to various substrates. The strain sensor based on this DBS/PAM‐Li organogel can be employed for real‐time detection of various human motions, demonstrating high sensitivity.

1. Introduction

Flexible conductive materials have garnered significant attention due to their irreplaceable roles in cutting‐edge fields such as wearable electronics, soft robotics, and smart healthcare. Research on flexible conductive materials primarily focuses on hydrogels and organogels. Hydrogels have attracted considerable interest due to their excellent flexibility, stretchability, and biocompatibility. However, the practical application of conventional hydrogels in flexible wearable electronics is severely limited by inherent drawbacks, including easy dehydration, poor environmental stability, and weak adhesion [1, 2, 3]. Consequently, organogels have emerged as a promising alternative material. As a class of functional materials featuring three‐dimensional cross‐linked networks and soft and wet characteristics, organogels demonstrate potential application value in multiple fields including adhesives [4, 5, 6], intelligent sensing and human‐machine interaction [7, 8], energy and environment [9], and biomedical engineering [10, 11]. However, organogels generally exhibit low mechanical strength and poor stability [12]. To enhance the mechanical properties of gels, several strategies have been proposed, such as the incorporation of nanomaterials [13], biomimetic multi‐scale regulation, and the construction of double‐network structures [14]. Among these approaches, constructing double networks introduces dissipation mechanisms through reversible sacrificial bonds, such as hydrogen bonds, ionic bonds, and metal‐coordination bonds, which effectively reduces energy dissipation. For instance, Wang [15] fabricated a high‐performance CAF/PAM double‐network gel by utilizing swollen calcium alginate fibers (CAF) as a reversible physical network, which interpenetrates with the chemical network of PAM. This structure significantly enhanced the mechanical properties and imparted good self‐healing capability to the gel. However, its adhesive performance remained weak. Without sufficient adhesion, conventional gel‐based strain sensors require external assistance to achieve intimate contact with substrates during operation, which significantly compromises both signal transmission accuracy and long‐term durability [16].

Despite these advances, most existing double‐network organogels still suffer from several critical limitations: complicated and time‐consuming preparation processes; poor optical transparency, which limits visualization in wearable devices; insufficient sensitivity for detecting subtle human motions; and a difficult trade‐off among mechanical strength, adhesion, and ionic conductivity. For example, Gao et al. recently reported an ultra‐stretchable organogel with rapid self‐healing and self‐adhesion based on microphase separation, but its gauge factor was below 1 and its ionic conductivity was as low as 0.004 mS·cm⁻¹, which is inadequate for high‐precision strain sensing [17]. Sun et al. developed a conductive nanocomposite organohydrogel by incorporating carboxyl‐functionalized carbon nanotubes (C‐CNT) and phytic acid (PA) into a carboxymethyl cellulose nanofiber/polyacrylamide network via radical polymerization combined with a glycerol solvent exchange strategy, aiming to achieve multimodal sensing over a wide temperature range (−30°C–60°C) [18]. In that system, the coupling of C‐CNT and the ionization of PA provided good electrical conductivity, while the coexistence of glycerol and PA promoted the formation of abundant hydrogen bonds, endowing the organohydrogel with excellent stretchability, self‐healing capability, and favorable adhesion. However, its fabrication process involves multi‐step solvent exchange and is rather complicated. Moreover, it suffers from poor sensing sensitivity and low optical transparency due to the introduction of CNTs, which limits its application in visual wearable devices. Therefore, developing a simple and efficient strategy to simultaneously achieve strong adhesion, high transparency, high sensitivity, and excellent conductivity in organogels remains an urgent challenge.

1,3:2,4‐dibenzylidene sorbitol (DBS), a low molecular weight gelator containing multiple hydroxyl groups and benzene rings, can leverage a series of non‐covalent interactions including hydrogen bonding, π–π stacking, and van der Waals forces in organic solvents to induce the self‐assembly of the gelator into a nanofiber network. The resulting gel exhibits stability and thermal reversibility [19, 20, 21]. Glycerol stands out as the solvent of choice for organogels in fields such as pharmaceuticals and flexible electronics, owing to its high stability, versatility, safety, and excellent compatibility [22]. For example, Ge et al. reported that the addition of glycerol increases the adhesion to wood and other surfaces. The adhesion effect of glycerol is attributed to the formation of multiple hydrogen bonds with the chemical groups on the surface, as well as the penetration of the solvent into the surface microstructure, resulting in an interlocking bonding effect [23]. However, most organogels are not intrinsically conductive and require the incorporation of conductive materials to achieve high electrical conductivity.

In this work, we prepared a DBS/PAM‐Li organogel with a double‐network structure using a simple one‐step method, employing glycerol as the solvent and rapid UV‐initiated polymerization. The gel is formed by the interpenetration of a self‐assembled (1,3:2,4‐dibenzylidene sorbitol) DBS nanofiber network in glycerol and tough polyacrylamide (PAM) chains. This physicochemical dual cross‐linking endows the gel with high adhesiveness, environmental stability, suitable mechanical properties, and high transparency. The incorporation of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) modulates the ionic conductivity of the gel. The Li⁺ ions coordinate with the C═O groups of PAM within the gel network, which not only preserves the cross‐linked structure but also creates pathways for ion transport. This dual effect imparts excellent sensing performance to the gel. The synthesized DBS/PAM‐Li organogel functions effectively as a strain sensor for real‐time monitoring of human motions, including joint bending and movement of fingers, wrists, and knees. This work provides a new strategy for developing multifunctional conductive organogels, with potential applications in flexible electronic devices.

2. Results and Discussion

2.1. Preparation and Characterization

In this study, a DBS/PAM‐Li organogel with ionic conductivity and high adhesiveness was fabricated via a one‐step method. Figure 1a illustrates its preparation process. A mixed glycerol solution containing DBS, AM, LiTFSI, MBAA, and 2959 was subjected to UV initiation for 5 min, during which DBS self‐assembled into a nanofiber network [24] that interpenetrated with PAM to form a double‐network structure. The detailed compositions of all organogel formulations are listed in Table S1. By varying the contents of DBS, AM, and LiTFSI and comparing the adhesion strength and stress–strain behavior of different formulations, the optimal composition was determined to be DBS_{2 wt.%}/PAM_5.33‐Li_{30 wt.%} (Figure S1). Throughout the text, this composition is abbreviated as DBS/PAM‐Li for simplicity. To further confirm the successful synthesis of the DBS/PAM‐Li organogel, the spectra of DBS, PAM, DBS/PAM, and DBS/PAM‐Li organogels in the range of 4000–650 cm⁻¹ are presented in Figure 1b. The pure PAM sample exhibits characteristic amide N─H stretching vibration peaks at 3330⁻¹ and 3190 cm⁻¹. The strong absorption band at 1655 cm⁻¹ corresponds to the C═O stretching vibration of the ‐CONH₂ groups, while the peak at 1610 cm⁻¹ is attributed to the N─H bending vibration. After introducing DBS to construct the DBS/PAM double‐network organogel, the N─H and O─H stretching vibration peaks undergo a slight shift, and the C═O peak at 1655 cm⁻¹ becomes broader with decreased intensity. These changes indicate the formation of extensive intermolecular hydrogen bonds between DBS and PAM molecular chains, confirming the successful construction of the double‐network structure. Upon further incorporation of LiTFSI to prepare the DBS/PAM‐Li organogel, the C═O stretching vibration peak at 1655 cm⁻¹ shifts further toward lower wavenumber, demonstrating the coordination interaction between Li⁺ and the carbonyl groups of PAM. Meanwhile, two new characteristic absorption bands appear at around 1130⁻¹ and 1050 cm⁻¹, which are assigned to the ‐SO₂‐ groups of LiTFSI, directly verifying the successful introduction of LiTFSI into the gel network.

FIGURE 1.

(a) Schematic illustration of the fabrication process of DBS/PAM‐Li organogel. (b) FTIR spectra of DBS, PAM, DBS/PAM and DBS/PAM‐Li organogels. (c, d) XPS full spectra and curve‐fitted C 1s XPS spectra of PAM, DBS/PAM and DBS/PAM‐Li organogels. (e, f) TG and DTG curves of PAM, DBS/PAM and DBS/PAM‐Li organogels. (g) Light transmittance of DBS/PAM‐Li organogel and corresponding optical photographs showing its transparency.

XPS was further employed to analyze the elemental composition and chemical states of PAM, DBS/PAM, and DBS/PAM‐Li organogels. As shown in Figure 1c, the spectrum of pure PAM exhibits only three characteristic peaks corresponding to C 1s, N 1s, and O 1s. The DBS/PAM organogel shows a similar elemental signal, but the carbon signal intensity is slightly enhanced due to the introduction of the benzene ring structure from DBS. In the spectrum of DBS/PAM‐Li, new characteristic peaks appear at 689, 168, and 61eV, which are assigned to F 1s, S 2p and Li 1s, respectively, directly confirming the successful doping of LiTFSI. The high‐resolution C 1s spectra (Figure 1d) can be deconvoluted into three typical peaks: C─C/C─H (284.6 eV), C─O/C─N (286.1 eV), and C═O (287.8 eV). Compared with pure PAM, the relative intensity of the C═O peak in DBS/PAM decreases slightly, which is attributed to the influence of hydrogen bonding on the electron cloud of the carbonyl group [25]. After the introduction of Li⁺, the C═O peak shifts distinctly toward lower binding energy, further demonstrating the coordination interaction between Li⁺ and the carbonyl oxygen. These results are consistent with the FTIR analysis, confirming the presence of multiple non‐covalent interactions, including hydrogen bonds and coordination bonds in the DBS/PAM‐Li organogel.

The thermal stability of PAM, DBS/PAM, and DBS/PAM‐Li organogels was characterized by TG and DTG, and the results are shown in Figure 1e,f. The thermal decomposition process of the pure PAM organogel mainly consists of two stages. The first stage is a slight weight loss in the range of 100°C–200°C, corresponding to the evaporation of residual water and small‐molecule solvents. The second stage is a rapid weight loss between 200°C and 400°C, attributed to the thermal decomposition of the PAM backbone, with the maximum decomposition rate occurring at approximately 310°C. After the introduction of DBS, the DBS/PAM double‐network organogel exhibits a slightly higher decomposition onset temperature, and the peak of the maximum decomposition rate shifts toward a higher temperature. This indicates that the physical crosslinking of the DBS nanofiber network restricts the thermal motion of polymer segments, thereby enhancing the thermal stability of the gel. Upon further introduction of LiTFSI to prepare the DBS/PAM‐Li organogel, the thermal stability is further improved: the temperature at the maximum decomposition rate increases to approximately 340°C, and the char yield in the range of 400°C–600°C is significantly higher than that of pure PAM and DBS/PAM. This enhancement is mainly attributed to the coordination crosslinking between Li⁺ and the carbonyl groups of PAM, which strengthens the structural rigidity of the gel network and delays the thermal degradation of polymer chains. Additionally, the hydrogen bonding interactions between the glycerol solvent and the gel network also help suppress solvent evaporation, further contributing to the improved thermal stability of the material. The DBS/PAM‐Li organogel exhibits excellent optical transparency, with a transmittance of 92% at a thickness of 1.45 mm, allowing a landscape photograph placed behind it to be clearly visible. This high transparency enables clear visualization of the internal components when the material is integrated into wearable electronic devices.

2.2. Mechanical Properties and Notch‐Insensitivity

As intuitively demonstrated in Figure 2a, the DBS/PAM‐Li organogel remains intact even under large deformations, including stretching, twisting, stretching after twisting, knotting, and stretching after knotting. To investigate the fatigue resistance of the DBS/PAM‐Li double‐network organogel, cyclic tensile loading‐unloading tests were conducted. Figure 2b displays the consecutive loading‐unloading curves performed at different tensile strains within the range of 100%–600%. The area enclosed by the loading‐unloading curves corresponds to the energy dissipated by the organogel during stretching. The energy dissipation gradually increased with the applied tensile strain. Specifically, as the strain increased from 100% to 600%, the dissipated energy increased from 0.0135 to 0.101 MJ·m⁻³. This indicates that during stretching, partial physical cross‐links within the organogel network are ruptured to prevent permanent deformation of the gel. Upon removal of the external force, the physically cross‐linked network can recover. With increasing tensile strain, the DBS/PAM‐Li organogel could not immediately return to its initial state. This phenomenon suggests that the reformation rate of hydrogen bonds and ionic bonds within the organogel could not keep pace with the reconstruction rate required for the recovery of the double‐network structure [26]. The U/W ratio remains stable at approximately 0.4 with increasing tensile strain, indicating predominantly elastic behavior. Five consecutive tensile cycles were performed at 500% strain, with the corresponding dissipated energy and maximum stress characterized. The organogel exhibited a large hysteresis loop in the first cycle. In the subsequent consecutive cycles, the hysteresis loops diminished, and the dissipated energy decreased from 0.123 to 0.057 MJ·m⁻³, indicating that the physically cross‐linked network could not recover instantaneously. Nevertheless, the DBS/PAM‐Li organogel is capable of recovering to its initial state after a sufficient rest period. To demonstrate the reversibility of the physically cross‐linked network within the DBS/PAM‐Li organogel, cyclic loading‐unloading tests were conducted with a 20‐min rest interval introduced before the third cycle, at a constant strain of 500%. As shown in Figure 2f, in the absence of a rest period, the organogel exhibited a much smaller hysteresis loop in the second cycle compared to the first, with the dissipated energy decreasing from 0.1 to 0.049 MJ·m⁻³. After the 20‐min rest, however, the organogel displayed a significantly larger hysteresis loop in the third cycle, with a corresponding dissipated energy of 0.08 MJ·m⁻³. Concurrently, the residual strain of the organogel was substantially reduced in the third cycle. The recovery observed after the 20‐min interval further substantiated the excellent recovery capability and stability of the organogel [27, 28]. We attribute this phenomenon to the abundant dynamic dissociation and recombination of hydrogen bonds within the organogel. The reversibility of this dynamic process underpins the high mechanical stability of the gel during long‐term cyclic deformation [29]. The DBS/PAM‐Li organogel also exhibits remarkable resistance to crack propagation. Stress–strain curves were recorded for both an unnotched rectangular specimen and a specimen with a 1 mm pre‐fabricated V‐shaped single‐edge notch to calculate the fracture work. The fracture work of unnotched samples was 5.06 × 10⁵ J·m⁻³, and the fracture energy of notched samples reached 1.06 × 10³ J·m⁻². During stretching, pronounced crack blunting was observed around the notch tip. Furthermore, the ratio Γ/W was calculated. This parameter [30], defined as a specific length (fracture cohesive length) related to the size of the inelastic zone around the crack tip, yielded a value of Γ/W = 2.1 mm. The fracture work obtained here is lower than the tensile toughness derived from the stress–strain curve in Figure S1. This difference is attributed to the stress concentration inherent in the stretched rectangular specimen.

FIGURE 2.

Mechanical robustness, fatigue resistance, notch‐insensitivity of the DBS/PAM‐Li organogel. (a) Photographs of the DBS/PAM‐Li organogel: (i) initial state, (ii) stretching, (iii) twisting, (iv) stretching after twisting, (v) knotting, and (vi) stretching after knotting. (b, c) Consecutive loading‐unloading curves of the DBS/PAM‐Li organogel at different strains, with the dissipated energy U and U/W ratio calculated from each cycle. (d, e) Five consecutive loading‐unloading cycles at 500% strain without any rest interval between successive tests, showing the maximum stress and dissipated energy for each cycle. (f, g, h) Cyclic tensile stress–strain curves at 500% strain with different rest intervals, showing the maximum stress, dissipated energy, and recovery ratio for each cycle. (i) Stress–strain curves of notched and unnotched specimens.

2.3. Adhesiveness Performance

Strong adhesion is essential for wearable strain sensors to accurately detect subtle human motions and physiological signals. Robust self‐adhesive capability ensures intimate contact between the sensor and the substrate, which guarantees signal accuracy and device durability. As shown in Figure 3a, the DBS/PAM‐Li organogel exhibits strong adhesion to a wide range of substrate surfaces, including wood, glass, pig skin, aluminum, rubber, copper, plastic, and paper. It can also function as an adhesive to bond and lift a 200 g weight. Furthermore, the DBS/PAM‐Li organogel can also firmly adhere to human skin. Even after the skin sweats, it still maintains tight contact and conforms well to skin deformation (Figure S2). Moreover, the organogel can be easily and completely peeled off from the skin without leaving any residue. Lap‐shear tests were performed with the schematic shown in Figure 3f to measure the adhesion strength of DBS/PAM‐Li on different substrates (Figure 3c,d). The maximum adhesion strengths of DBS/PAM‐Li to wood, glass, paper, PET, and aluminum plate reached 79.58, 68.24, 49.25, 49.65, and 63.85 kPa, respectively.

FIGURE 3.

Adhesive properties of the DBS/PAM‐Li organogel. (a) Photographs of the DBS/PAM‐Li organogel adhering to various substrates. (b) Photographs showing the DBS/PAM‐Li organogel adhered to the surfaces of the human arm, wrist, and knuckles. (c, d) Adhesion strength of the DBS/PAM‐Li organogel to different substrates. (e) Repeated adhesion measurements of the DBS/PAM‐Li organogel on various substrates. (f) Schematic illustration of the lap‐shear test and the proposed adhesion mechanism.

The adhesion mechanism of the DBS/PAM‐Li organogel to different substrates can be attributed to the synergistic effect of interfacial interactions and substrate surface properties [31]. The wood surface is rich in polar groups such as hydroxyl and carboxyl groups, which can form a dense hydrogen bonding network with the amide groups in the gel network, the multiple hydroxyl groups of DBS, and the hydroxyl groups of glycerol, endowing the highest adhesion strength. The silanol groups on the glass surface can form strong hydrogen bonds with the gel. The PET surface contains ester groups, which can act as hydrogen bond acceptors to form weak hydrogen bonds with the gel, resulting in moderate adhesion strength due to their weak polarity. The naturally formed oxide layer (Al₂O₃) on the aluminum surface is rich in aluminum hydroxyl groups (Al‐OH), which can form hydrogen bonds with the gel; meanwhile, Li⁺ ions can form dynamic coordination bonds with Al‐OH or surface oxygen atoms, and also generate electrostatic interactions with negatively charged sites on the oxide layer. Although the paper surface contains hydroxyl groups, its loose fibrous structure and low surface energy make it difficult for the gel to form effective interfacial bonding. The excellent adhesion capability of DBS/PAM‐Li to various substrates originates from the synergistic effect of its multiple functional groups, enabling stable interfacial bonding with different materials through hydrogen bonding, coordination bonds, electrostatic interactions, and hydrophobic interactions [32, 33, 34]. Cyclic lap‐shear tests were conducted on the same sample to investigate the reusability of the organogel. After three cycles of peeling and re‐adhesion, the adhesion strength to various substrates (aluminum, glass, and wood) showed no significant decrease (Figure 3e). This indicates that the organogel possesses good repeatable adhesion, thereby extending the service life of sensors.

2.4. Self‐Healing Performance

For organogel sensors, self‐healing capability after damage can extend their service life [35]. As shown in Figure 4a, in a simple circuit, a DBS/PAM‐Li sample was cut into two pieces, causing a lit LED to extinguish simultaneously. Upon re‐contacting the two pieces, the LED immediately re‐illuminated. Self‐healing experiments were performed on the DBS/PAM‐Li organogel by cutting it with a knife and allowing it to heal at room temperature without external force. With healing time increasing from 15 min to 6 h, the radius of the impedance arc of the gel gradually decreased, approaching its original state. Correspondingly, the electrical conductivity (Figure 4c) gradually recovered from 0.038 to 0.07 mS·cm⁻¹, indicating progressive reconstruction of ion transport pathways during thehealing process, with a conductivity recovery efficiency reaching 40% of the original value. Figure 4e shows typical stress–strain curves of DBS/PAM‐Li before and after healing at ambient temperature for different durations, with the corresponding healing efficiency quantitatively calculated (Figure 4f). The healing efficiency of the organogel was positively correlated with healing time. After 15 min of self‐healing, the tensile stress and tensile strain at break recovered to 0.052 MPa and 182%, respectively, demonstrating that the DBS/PAM‐Li organogel possesses a certain self‐healing capability that can be realized within a short time.

FIGURE 4.

Self‐healing behavior of DBS/PAM‐Li. (a) Photographs showing the change in LED brightness before and after self‐healing. (b–d) Electrochemical impedance spectra, corresponding electrical conductivity, and healing efficiency at different healing times. (e, f) Representative stress–strain curves and the corresponding healing efficiency for initial and healed samples at different healing times. (g) Photographs demonstrating the organogel's self‐healing behavior. (h) Schematic illustration of self‐healing mechanism.

The self‐healing behavior of the DBS/PAM‐Li organogel is synergistically driven by four types of dynamic interactions within the system. The benzyl groups at both ends of DBS are nonpolar hydrophobic moieties, whereas the central sorbitol backbone is a polyhydroxy polar hydrophilic group. The benzyl groups spontaneously approach and aggregate with the hydrophobic groups on the opposite fractured surface via hydrophobic interactions, rapidly forming temporary physical crosslinking points and achieving preliminary adhesion of the fractured interface. This is followed by re‐association through intermolecular hydrogen bonds of DBS. The abundant amide groups (‐CONH₂) on the PAM chains can re‐form intermolecular hydrogen bonds at the interface, further strengthening the interfacial bonding force, which is key to the recovery of mechanical strength of the gel. Li⁺ ions can form dynamic coordination bonds with the carbonyl groups (C═O) of PAM chains and the hydroxyl groups (─OH) of DBS molecules. After fracture, these bonds can re‐coordinate at the interface, reconstructing ion transport pathways and enabling rapid recovery of ionic conductivity. This is the primary reason why the electrical healing efficiency surpasses the mechanical healing efficiency. Moreover, the plasticizing effect of glycerol enhances the mobility of polymer segments, promoting molecular diffusion and rearrangement at the fractured interface. The polyhydroxy structure of glycerol can form dynamic hydrogen bonds with polar groups within the system, participating in network reconstruction while maintaining the wettability of the fractured interface, thereby ensuring intimate contact at the interface. In addition to its superior mechanical properties, robust adhesion, and self‐healing capability, the as‐prepared DBS/PAM‐Li organogel exhibits outstanding environmental stability under varying temperature and humidity conditions (Figures S4 and S5).

2.5. Electrical Conductivity and Strain‐Sensing Performance

The DBS/PAM‐Li organogel exhibits considerable electrical conductivity, which is strongly dependent on the LiTFSI content. As shown in Figure 5a, the electrical conductivity of the organogel increases with LiTFSI concentration due to the increased number of mobile charge carriers. The DBS/PAM‐Li organogel, characterized by high transparency, strong adhesion, and stable strain sensitivity, can serve as a promising candidate for wearable electronics, particularly as a strain sensor for human motion detection. The strain sensor was assembled by attaching copper tabs to both ends of the organogel. During testing, strain (ε) was applied to the organogel sample via a universal tensile testing machine, while the copper tabs were connected to a digital multimeter to record the corresponding resistance change (ΔR).

FIGURE 5.

Electrosensing performance of the DBS/PAM‐Li organogel. (a) Electrochemical impedance spectra of organogels with different mass fractions of LiTFSI. (b) Dynamic tensile sensitivity of the sensor based on the organogel with 30 wt.% LiTFSI. (c) Response and recovery times of the DBS/PAM‐Li organogel under 10% strain (d) Relative resistance change of the organogel sensor under small tensile strains (0.25%‐5%). (e) Relative resistance change of the organogel sensor under large tensile strains (50%‐300%). (f) Stability of the electrosensing performance over 200 cycles at 10% tensile strain. (g) Relative resistance change of the organogel sensor under small compressive strains (1%‐10%). (h) Relative resistance change of the organogel sensor under large compressive strains (10%‐60%). (i) Stability of the electrosensing performance over 200 cycles at 40% compressive strain.

The relative resistance change (ΔR/R ₀) of the DBS/PAM‐Li strain sensor under different strains is shown in Figure 5b. The gauge factor (GF), defined as the rate of relative resistance change per unit strain, reflects the sensitivity of the sensor [36]. When the LiTFSI concentration is low, ion migration channels within the gel are sparse. As the LiTFSI concentration increases, the density of these ion channels reaches an optimal state at 30%. However, a further increase in LiTFSI concentration leads to excessive ion accumulation within the gel, resulting in ion migration saturation and enhanced network rigidity. When the mass fraction of LiTFSI reaches 30 wt.%, the prepared organogel exhibits an ionic conductivity of 0.164 mS·cm⁻¹ and a corresponding volume resistivity of 6097.6 Ω·cm, demonstrating its favorable intrinsic insulation capability. This effectively suppresses current leakage and ensures stable and reliable sensing signals in practical applications. In glycerol solvent, DBS self‐assembles into a nanofiber network via non‐covalent interactions, including hydrogen bonds, π–π stacking, and van der Waals forces. This network interpenetrates with the covalently cross‐linked PAM network. Meanwhile, Li⁺ ions coordinate with the C═O groups of PAM and are uniformly distributed within the organogel matrix, collectively forming a three‐dimensional conductive network. At a LiTFSI content of 30 wt.%, within the first strain stage of 0%–5%, the organogel exhibits a GF of 1.74. During this stage, a small number of hydrogen bonds begin to dissociate, leading to an increase in the distance between conductive lithium ions and consequently a rise in resistance with increasing strain. When the strain exceeds 5% (the second stage), a large number of physical bonds are ruptured. This causes a further increase in the distance between lithium ions, resulting in the GF increasing to 8.72. In addition to high sensitivity, the DBS/PAM‐Li organogel sensor also exhibits a fast response speed. As shown in Figure 5c, the response time and recovery time of the strain sensor are 133 and 143 ms, respectively. The organogel can detect a tiny strain of 0.25% (Figure 5d), indicating its promising potential for sensing fine‐scale human motions. Moreover, the DBS/PAM‐Li sensor demonstrates excellent sensing repeatability under both small strains (0.25%–5%) and large strains (50%–300%). Cyclic tensile tests were conducted to evaluate the durability of the organogel. As shown in Figure 5f, at 10% strain, the resistance signal exhibited periodic changes in synchrony with the stretching/releasing process and showed no significant variation over multiple consecutive cycles. The sensing performance of the DBS/PAM‐Li organogel as a pressure sensor was further investigated through a series of compression tests. Figure 5g,h show the relative resistance change (ΔR/R ₀) of the pressure sensor under different compressive strains. During compression‐unloading cycles, the relative resistance change demonstrated excellent repeatability for both small compressive strains (1%–10%) and large compressive strains (10%‐60%). Furthermore, the value of ΔR/R ₀ gradually decreased as the applied pressure increased. Throughout the cyclic compression tests, the sensing signal also maintained high stability and reproducibility. Even after 200 compression cycles at 40% strain, the ΔR/R ₀ value of the DBS/PAM‐Li pressure sensor retained considerable stability (Figure 5i).

2.6. Multifunctional Applications of the DBS/PAM‐Li Strain Sensor

Owing to its excellent adhesion, good transparency, high sensitivity, and stretchability, the DBS/PAM‐Li organogel was assembled into a skin‐attachable sensor for monitoring human joint motions. Firstly, the sensor was adhered to different body parts to detect human motions. As the finger was gradually bent from 0° to 90° and then gradually straightened back to 0°, a corresponding stepwise increase and decrease in ΔR/R ₀ was observed. The ΔR/R ₀ signal remained essentially constant when the finger was stationary. Moreover, the response exhibited good symmetry, with identical ΔR/R ₀ values recorded at the same bending angles during both flexion and extension. This indicates that the sensor possesses excellent recovery capability and high stability. When the finger, wrist, knee and elbow underwent multiple continuous bending and motion cycles, the ΔR/R ₀ increased sensitively and recovered periodically, demonstrating the sensor's repeatability and stability. Furthermore, the sensor was capable of distinguishing different movement speeds. For instance, the signal period for running is shorter, more abrupt, and exhibits a larger amplitude compared to that for walking. Additionally, irregular oscillations of the elbow during motion can also be accurately identified. Therefore, this sensor can serve as a wearable device for monitoring physical rehabilitation exercises. We further compared the comprehensive performance of our organogel with previously reported gel sensors via the radar plot (Figure 6i). After normalizing all core parameters to 0–100, the comparison proves that the DBS/PAM‐Li organogel simultaneously delivers strong adhesion, high transmittance, excellent sensitivity and great stretchability. Most reference gels only excel at one property with severe deficiencies in other aspects, which confirms the comprehensive competitive edge of our organogel for wearable motion detection.

FIGURE 6.

Monitoring of human motions using the DBS/PAM‐Li strain sensor. (a) Finger bending at different angles (30°, 60°, 90°). (b) Repeated finger bending. (c) Repeated wrist bending. (d) Small‐angle and large‐angle flexion/extension at the wrist joint. (e) Knee joint bending at different angles. (f, g) Walking and running. (h) Elbow flexion during motion. (i) Comparison of comprehensive performance of this work and recently reported gel sensors [37, 38, 39, 40, 41, 42].

3. Conclusion

In summary, we have developed a one‐step method to fabricate a physicochemical double‐network crosslinked DBS/PAM‐Li conductive organogel. The organogel exhibits high and durable adhesion strength to various substrates, providing the basis for real‐time detection and discrimination of various human motions. The excellent adhesion (τ = 68.24 kPa), high stretchability, and considerable self‐healing capability of the DBS/PAM‐Li organogel are attributed to the synergistic effect of its physicochemical dual cross‐linking network. The incorporation of LiTFSI imparts substantial ionic conductivity to the organogel. The strain sensor based on this DBS/PAM‐Li organogel can be employed for real‐time detection and discrimination of various human motions, demonstrating high sensitivity (GF = 8.72), fast response speed (response time of 133 ms), excellent stability (with no significant degradation in electrosensing performance over 200 cycles), and good repeatability. This innovative material provides a novel strategy for developing multifunctional conductive organogels. We believe that the DBS/PAM‐Li organogel holds great promise for broad applications in the field of wearable electronics.

4. Experimental Section

4.1. Materials

1,3:2,4‐Dibenzylidene‐D‐sorbitol (DBS) was purchased from J&K Scientific Ltd.; Acrylamide (AM, 99.0%) and N, N'‐methylenebisacrylamide (MBAA) were obtained from Macklin. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was supplied by Aladdin. Glycerol (AR, 99%) was acquired from Sinopharm Chemical Reagent Co., Ltd. The photoinitiator Irgacure 2959 was used as received. Deionized water was provided by our laboratory.

4.2. Preparation of Organogel

Figure 1 illustrates the preparation process of the organogel via a one‐pot rapid UV‐strategy. Firstly, DBS was dissolved in 8 g of glycerol by magnetic stirring, followed by heating in an oil bath at 170°C until complete dissolution, and then cooling to 150°C. Separately, Irgacure 2959 was dissolved in 3 g of glycerol at 50°C. In another container, LiTFSI, MBAA, and AM were dissolved in 9 g of glycerol at 80°C. Finally, all resulting solutions were then combined and stirred thoroughly to form a homogeneous mixture. This mixture was poured into a polyester mold and subjected to UV irradiation, completing the sol to gel transition within 5 min. The samples are denoted as DBS_x%/PAM_y‐Li_z%, where “x” and “z” represent the mass fractions of DBS and LiTFSI relative to the solvent, “y” represents the molar concentration, and the content of the cross‐linker (MBAA), denoted as “m” is expressed as a percentage relative to the monomers. The specific compositions along with their corresponding adhesive and mechanical properties are presented in Figure S1.

4.3. Characterization

Fourier transform infrared (FTIR) spectra in the range of 700–4000 cm⁻¹ were recorded using a Nicolet 6700 spectrometer in the attenuated total reflection (ATR) mode. X‐ray photoelectron spectroscopy (XPS, ESCALAB 250xi, Thermo Fisher) was employed to characterize the chemical structures of DBS, PAM, DBS/PAM and DBS/PAM‐Li samples, with Al Kα radiation and low‐energy electron flow as the excitation source (225 W, 15 mA, 15 kV). The thermal stability of the as‐prepared organogels was investigated by a thermogravimetric analyzer (SDT Q600, Thermal Advantage, USA). The test was performed from 30°C to 600°C at a heating rate of 20°C·min⁻¹. The transmittance was measured using a UV‐1800 ultraviolet‐visible spectrophotometer with a wavelength range of 200–700 nm, and deionized water was used as the reference sample.

Mechanical Property Tests: The mechanical properties of the organogel were evaluated using a universal tensile testing machine (CMT6103, SANS, Shenzhen, China) at room temperature. The specimens were dumbbell‐shaped with dimensions of 36 mm (length) × 2 mm (width) × 1 mm (thickness). The tests were conducted at a crosshead speed of 100 mm min⁻¹. The load‐displacement curves were recorded and converted into stress–strain curves. Furthermore, the measured strain (ε) and stress (σ) were determined using the following formulas:

$$\epsilon =\left(\mathrm{L}-\mathrm{L}0\right)/\mathrm{L}0\sigma =\mathrm{F}/\mathrm{A}0$$ (1)

where ε represents strain, L and L ₀ represent the stretched length and initial length of the specimen, σ denotes the stress, F is the applied tensile force, and A ₀ is the original cross‐sectional area, respectively.

Defect Sensitivity Testing: The critical defect size can be used to characterize the defect‐sensitive fracture behavior of samples. The tensile work (W) describes the energy required per unit volume to fracture an unnotched dumbbell‐shaped specimen. The specific calculation method is as follows:

$$\mathrm{W}(\lambda \mathrm{c})={\int }_1^{{\lambda }_{\mathrm{c}}}{\sigma }_{\mathrm{un}-\mathrm{notched}}\mathrm{d}{\lambda }_{\mathrm{c}}$$ (2)

Fracture energy (Γ) can be employed to characterize the failure of dumbbell‐shaped specimens in the presence of defects. It represents the energy dissipated per unit area during the fracture of a notched sample. The specific calculation method is as follows:

$$\mathrm{\Gamma }=2\frac{3}{\sqrt{{\lambda }_{\mathrm{c}}}}\mathrm{cW}(\lambda \mathrm{c})$$ (3)

The ratio of these two material parameters (Γ/W) defines the characteristic length of the material, i.e., the critical defect size (in mm). Notched tensile tests were conducted on a universal tensile testing machine. All dumbbell‐shaped specimens were notched, with an initial notch depth of 0.1 times the specimen width. Testing was performed at 25°C with a crosshead speed of 100 mm·min⁻¹. Each test was repeated three times to analyze notch propagation during the tensile deformation of notched samples.

Self‐Healing Performance Test: The organogel was first cut into dumbbell‐shaped specimens and then cut into two halves from the middle. The cut surfaces were brought into close contact without any external force at room temperature to allow self‐healing. The mechanical properties of the self‐healed organogel were tested at a stretching speed of 100 mm·min⁻¹ The self‐healing efficiency (H) based on tensile stress of the organogel was calculated as follows:

$$\mathrm{H}=(\sigma \mathrm{after}/\sigma \mathrm{before})\times 100\%$$ (4)

where H is the self‐healing efficiency of tensile stress (%), σ _after is the tensile stress of the organogel after self‐healing (MPa), and σ _before is the original tensile stress of the organogel before cutting (MPa).

Lap‐Shear Test: The adhesive strength of the organogel was evaluated using a universal mechanical testing machine. DBS/PAM‐Li rectangular organogel specimen (20 mm in length, 20 mm in width, and 1 mm in thickness) was sandwiched between two identical substrates, such as wood, aluminum plates, glass plates, PET and paper. The specimen was then pulled apart at a rate of 100 mm·min⁻¹ until detachment from the substrates occurred. The adhesion strength (τ) was then calculated using the following formula, where F and S represent the maximum load and the original contact area, respectively.

$$\tau =\mathrm{F}/\mathrm{S}$$ (5)

Conductivity Measurement: The ionic conductivity was measured by assembling the organogel (10 mm in length, 10 mm in width, and 1 mm in thickness) into a simulated battery cell, where it was sandwiched between two copper plates. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CS2350H, Wuhan CorrTest Instruments Corp., Ltd.) over a frequency range from 0.01 Hz to 100 kHz with an AC amplitude of 10 mV. The conductivity, κ (in mS·cm⁻¹) of the organogel electrolyte was calculated using the following formula:

$$\kappa =\frac{\mathrm{L}}{\mathrm{S}·{\mathrm{R}}_{\mathrm{a}}}\times 1000$$ (6)

where L (cm) is the thickness of the organogel, S (cm²) is the contact area between the organogel and the copper plate, and R _a (Ω) is the bulk resistance obtained from the electrochemical impedance spectroscopy (EIS) data.

Sensing Performance Tests: The as‐prepared DBS/PAM‐Li organogel served as the primary sensing component to fabricate strain sensors. For tensile strain sensing, the sample was cut into a dumbbell shape. For compressive strain sensing, the sample was cut into a cylinder with a height of 50 mm and a diameter of 10 mm. Conductive copper tapes were attached to both ends of the sensors. A tensile testing machine was used to apply tensile or compressive strain to the sensors. The resistance change of the sensors was monitored in real‐time using a digital multimeter (Keysight 34465A, Keysight Technologies China Co., Ltd.) to characterize their response to the applied strain. The relative resistance change (ΔR/R ₀) was calculated as follows:

$$\mathrm{\Delta }\mathrm{R}/\mathrm{R}0=(\mathrm{R}-\mathrm{R}0)/\mathrm{R}0$$ (7)

where R ₀ (Ω) and R (Ω) represent the electrical resistance of the organogel sensor in the initial state and under strain, respectively. The gauge factor (GF) of the strain sensor is defined as follows:

$$\mathrm{GF}=(\mathrm{\Delta }\mathrm{R}/\mathrm{R}0)/\epsilon$$ (8)

Funding

National Natural Science Foundation of China (Nos. 52073083 and 51603065).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: marc70366‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article.