A multi-modal deformation sensing hydrogel with a nerve-inspired highly anisotropic structure

Abstract

Anisotropic hydrogel sensors have been widely applied in the field of smart devices, but there is still an increasing demand for flexible sensors that can sense three-dimensional mechanical changes and accurately capture complex behavior patterns. As a unique anisotropic biological structure, the peripheral nerve, consisting of bundles of nerve fibers arranged in parallel and enclosed by multiple layers of nerve membranes, exhibits high mechanical strength to resist external forces, rapid responsiveness to external stimuli, and the capability to swiftly transmit bioelectrical signals. Herein, inspired by peripheral nerves, a hydrogel with a multi-layer sandwich structure consisting of oriented fibers–pores–fibers was developed. Such advanced hydrogels exhibit impressive tensile breaking strength of 1.51 MPa and compressive stress of 3.51 MPa, with an ionic conductivity of 0.044 S cm⁻¹. When used as a sensor, the hydrogel possesses a three-dimensional response range, enabling it to simultaneously respond to mechanical changes in orthogonal directions, with good signal repeatability and high response sensitivity. These important capabilities enable the hydrogel to sense different movement patterns, accurately distinguish complex gaits such as walking, running, and jumping, and identify the force area on the sole of the foot. Therefore, the multi-modal deformation sensing hydrogel has great application potential in personalized health care and sports rehabilitation.

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1 Introduction

In recent years, flexible electronic products have attracted widespread attention due to their ability to accurately record mechanical signals and realize real-time monitoring of health and exercise status.^1,2 Hydrogels are considered to be one of the promising materials for the development of smart flexible wearable devices due to their advantages such as simple preparation process, good biocompatibility, environmental friendliness and conductivity.^3–5 However, the isotropic structural properties of hydrogels limit their potential for identifying complex multi-strain environments.^6–8 Nature has provided a lot of inspiration for the development of flexible wearable devices. Numerous soft tissues of organisms, such as skin and muscles, have highly ordered anisotropic microstructures, which can actively sense and manage their own movements, and demonstrate high responsiveness to information and contraction capabilities.^9–11 Consequently, numerous biomimetic structures have been applied to the development of high-performance hydrogel sensors, which have been proven to effectively improve the sensing properties of hydrogels.

Among various anisotropic hydrogel structures, hydrogels with fibrous structure have garnered significant attention due to their unique properties and broad application prospects. Multiple methods have been developed for fabricating fiber-structured hydrogels, including 3D printing,¹⁰ electrospinning,¹² mechanical stretching,^13–15 freeze-casting,^16,17 biotemplating,^18–20 and ion diffusion.²¹ The 3D highly ordered fiber structure enhances the strength of the hydrogels and provides the materials with higher mechanical compliance and flexibility, enabling them to better accommodate a variety of complex mechanical stimuli.²² Additionally, the incorporation of fiber structures significantly increases the surface area and porosity of the materials, thereby offering more sensing sites and facilitating efficient ion mobility.²³ More importantly, the asymmetric sensing characteristics are imparted to the hydrogels by the fiber structure, exhibiting superior wide-range responsiveness and high sensitivity in directions parallel to the fiber orientation, while rapid responses to mechanical changes are also triggered in the perpendicular direction.^3,24 For instance, Zhang et al.²⁵ prepared an anisotropic fibrous structure hydrogel by mechanical stretching, and this structure provided a greater number of ion transport channels along the tensile direction, which was conducive to the rapid movement of ions, leading to a discrepancy in the conductivity along the parallel and perpendicular directions. To a certain extent, this has overcome the limitation of traditional flexible sensors, which can only output a single signal. However, the sensing capabilities of these sensors in the vertical direction are relatively poor, and their response range is confined to two-dimensional mechanical changes. In fact, human movements exhibit complex strain patterns. For instance, during wrist bending or walking, tensile and compressive strains often intertwine or occur simultaneously.^26–28 Therefore, it is essential to develop flexible sensors capable of responding to three-dimensional mechanical changes.

Peripheral nerves, belonging to the nervous system, are capable of receiving external information and transmitting it to the brain and spinal cord for processing.^29–31 They are the dominant regulatory system in the body.³² These special functions of peripheral nerves are due to their unique structure: highly oriented nerve fiber bundles and a multi-layered porous nerve membrane wrapped on the outside.^33,34 When external stimuli are perceived by peripheral nerves, ion exchange occurs, providing the necessary electrochemical environment for the generation and conduction of nerve impulses.³⁵ After that, nerve fibers propagate nerve impulses in the form of bioelectrical signals.³² These interesting properties are very desirable for flexible sensing devices.

Inspired by the multi-layer ordered structure and the ion signal conduction mechanism of peripheral nerves, a hydrogel (AMH) with a multi-layer sandwich structure consisting of oriented fibers–pores–fibers was developed. AMH hydrogel uses calcium alginate (AlgCa) hydrogel with oriented fibrous structure as the skeleton and methacrylated hyaluronic acid (GMHA) as the filler. The anisotropic fibers and pores are arranged alternately in the longitudinal direction to form a multi-scale hierarchical structure. Because of this special structure, the hydrogel exhibits excellent mechanical strength and signal conduction ability, and can sensitively and stably respond to mechanical stimulation from orthogonal directions. AMH can monitor various human sports activities in real time, and accurately identify different types of exercise and force areas. The rare and valuable 3D motion signal recognition ability was exhibited. Here, a simple and feasible approach is provided to prepare a hydrogel that can be used for multiple mechanical modal responses by designing the structure of hydrogels, which has great potential in flexible electronic applications such as sports science and health diagnosis.

2 Materials and methods

2.1 Materials

Sodium alginate (AlgNa), calcium chloride (CaCl₂), hyaluronic acid (HA), glycidyl methacrylate (GM), triethylamine (TEA), tetrabutylammonium bromide (TBAB), ammonium persulfate (APS) and N,N,N′,N′-tetramethyl ethylenediamine (TEMED) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. All reagents in this research were used without purification.

2.2 Preparation of AlgCa hydrogel

Sodium alginate was dissolved in water and stirred overnight to prepare a 4 wt% sodium alginate solution. The 4 wt% sodium alginate solution was poured into the mould, and then a 0.5 M CaCl₂ solution was injected from above. Ca²⁺ gradually diffused in the solution, and calcium alginate gel began to form. After complete gelation, the calcium alginate hydrogel was removed from the mould and the hydrogel was thoroughly washed with deionized water to remove uncrosslinked salts and polymers. The 1.5 mm thick calcium alginate gel was cut into a 10 mm × 100 mm rectangle, and its two long ends were clamped with a bracket and placed in the air (temperature: 25 °C, humidity: 40–60%) to dry for 4 hours.

2.3 Preparation of GMHA

0.5 g HA was dissolved in 50 mL water and stirred at room temperature overnight. 1 mL GM, 1 mL TEA and 1 g TBAB were added to the HA solution and stirred at 55 °C for 1 hour. After cooling to room temperature, the solution was precipitated twice in acetone, and the precipitate was redissolved in 30 mL water and freeze-dried to obtain GMHA with a methacrylation degree (DM) of 4%.^8,36 In order to explore the optimal properties of the hydrogel, GMHA with DMs of 4%, 8% and 12% was prepared. The DM of GMHA was determined by nuclear magnetic resonance using a 500 MHz Agilent VNMR spectrometer.^37,38

2.4 Preparation of AMH hydrogel

0.1 g GMHA was dissolved in 4.38 mL water and stirred overnight. TEMED (25 μL) and 0.1 mL APS solution (0.08 g mL⁻¹) were added to the GMHA solution with a DM of 4%, then the solution was transferred to a rectangular mould, sealed on both sides, and mixed and interacted with calcium alginate hydrogel at 60 °C for 30 minutes to obtain AM₄H. AM₈H and AM₁₂H were prepared following the same procedure. Hydrogels without a directional structure (NS-AMH) are obtained by reacting GMHA (DM of 8%) with calcium alginate hydrogels without fibrous structure.

2.5 AMH hydrogel characterization

The synthesis of AMH hydrogels was confirmed using an infrared spectrometer (Nicolet 6700) with a spectral scanning range of 400 to 4000 cm⁻¹. The sealed samples were heated from 30 °C to 90 °C with an interval of 5 °C to obtain a temperature-dependent infrared spectrum.

2.6 Morphology analysis

The prepared hydrogel samples were frozen at −80 °C for 24 hours. After the hydrogels were completely frozen, they were placed in a freeze dryer (FD-1A-80) until they were completely dry. A small piece of freeze-dried hydrogel sample was cut and treated with gold spraying. The microscopic morphology of the sample was observed using a scanning electron microscope (NERCN-TC-006).

2.7 Mechanical properties

The mechanical properties of hydrogel samples of different components were tested using an electronic universal testing machine (LD23503, Shenzhen, China). Tensile tests were performed on samples with dimensions of 70 × 5 × 1 mm³, and the test speed was 20 mm min⁻¹. Compression tests were performed on cylinders with a diameter of 5 mm and a height of 5 mm, and the test speed was 2 mm min⁻¹. The tensile toughness and compressive toughness were obtained by calculating the area under the stress–strain curve. For tensile loading-unloading cyclic tests, the hydrogel sample was stretched to a preset strain of 10%, then returned to the initial length. The cycle was repeated 20 times. For compressive loading-unloading cyclic tests, the hydrogel sample was compressed to a preset strain of 10%, then returned to the initial length, and the cycle was repeated 20 times. All mechanical tests were performed at room temperature.

2.8 Degradation performance

The degradation performance of AMH was analyzed by in vitro degradation experiments. The hydrogel was immersed in a deionized water solution and placed in a constant temperature shaker at 37 °C and 60 rpm for 30 days of degradation. Every 5 days, the sample was weighed and its mass (w_t) was recorded. The mass loss rate of the hydrogel at different degradation times was calculated according to formula (1):where w₀ is the mass of the hydrogel before degradation; w_t is the mass of the hydrogel at the sampling time point.

2.9 Electrical measurement and sensing performance

The conductivity of the prepared hydrogel was tested using an electrochemical workstation (Biologic VMP-3). The hydrogel (l = 10 mm, r = 5.1 mm) was embedded between two parallel stainless steel washers and connected to the loop. The test frequency range was 1 MHz to 100 MHz with an amplitude of 10 mV. The conductivity (σ) of the hydrogel was calculated according to formula (2):where L is the length, R is the resistance, and S is the cross-sectional area of the hydrogel.

The strain sensing performance of the hydrogel was tested using a Source Measurement Unit (Keithley 2450). At room temperature, the hydrogel was connected to the source measuring unit via a copper wire and fixed on a universal tensile testing machine for testing. At the same time, the hydrogel was attached to the joints of the human body to monitor human movement. The relative resistance change (ΔR/R₀) and sensitivity (GF) during the strain process were calculated according to formulas (3) and (4) based on the measured current–time curve.

where R₀ and R are the initial resistance of the hydrogel and the real-time resistance after strain is applied, respectively, and ε is the strain applied to the hydrogel during the test.

3 Results and discussion

3.1 Preparation and characterization of AMH hydrogel

Inspired by peripheral nerves that have a multi-layered ordered structure and conduct impulses with ionic signals, a simple and effective method was employed to prepare AMH hydrogels with a multi-layered sandwich structure composed of oriented fibers–pores–fibers (Fig. 1a and b). The AMH hydrogel comprises AlgCa and GMHA. Initially, a spontaneous mechanical signal was induced to create a hydrogel with a highly aligned self-templated hierarchical structure. The longer end of the AlgCa hydrogel was fixed to a scaffold (Fig. 1c) and exposed to air for drying. During the drying process, as the water evaporated, the width and thickness of the hydrogel decreased, while the length remained unchanged due to being fixed by the scaffold. Therefore, the mechanical signal sacrificed the ionic interaction of the AlgCa hydrogel, and the polymer chains were arranged along the stress direction to form a hierarchical fibrous structure. Simultaneously, GMHA with DMs of 4%, 8%, and 12% was prepared and characterized using nuclear magnetic resonance hydrogen spectroscopy (¹H NMR). The ¹H NMR spectrum of GMHA exhibited two new proton peaks at 5.22 ppm and 5.50 ppm (Fig. S1†), confirming the successful synthesis of GMHA.³⁶ Subsequently, GMHA and AlgCa were thermally polymerized. Under the action of the APS/TEMED redox initiation system, GMHA molecules were cross-linked through free radical reactions, and the fibrous structure was used as a template to induce the formation of oriented pores. At the same time, GMHA and AlgCa were tightly bound through hydrogen bonds to obtain AMH (Fig. 1d).

Fig. 1 (a) Schematic diagram illustrating the organization of nerve fibers and nerve membranes within peripheral nerves, and a schematic diagram depicting the structure of AMH hydrogel. (b) Cross-sectional morphology of AMH hydrogel. (c) Preparation steps of AMH hydrogel. (d) Schematic diagram of hydrogen bonds in AMH hydrogel and covalent cross-linking between GMHA molecular chains.

Fig. 2a displays the FT-IR spectra of AlgCa, GMHA and AMH hydrogels with various ratios to characterize their compositions. The cross-linking reaction occurs between GMHA molecules, during which the carbon–carbon double bond is destroyed, as evidenced by the disappearance of the absorption bands at 1375 cm⁻¹ and 1153 cm⁻¹ in the spectrum which represent the presence of carbon–carbon double bonds.⁷ Upon further observation, the GMHA spectrum exhibits a broad absorption peak in the 3640–2980 cm⁻¹ region, attributed to the involvement of the –OH and –NH groups in forming intramolecular and intermolecular hydrogen bonds, leading to enhanced stretching vibrations.^37,39 In contrast, AlgCa forms a stable chelating structure with Ca²⁺ due to the –OH and –COOH groups, which diminishes the spectral intensity of these groups, rendering the corresponding absorption peaks difficult to observe.^40,41 AMH also displays a broad peak at the same position, confirming the successful synthesis of the AMH hydrogel. Temperature-dependent FTIR spectroscopy was employed to further investigate the synthesis mechanism of the hydrogel. As the temperature increased from 30 °C to 90 °C, the –OH (3440–34300 cm⁻¹) and –NH stretching bands (2940–2920 cm⁻¹) shifted to higher wavenumbers (Fig. 2b).⁴² At the same time, ν(COOH) shifted to higher wavenumbers, while the ν(C–N) band shifted to lower wavenumbers (Fig. 2c), indicating that the increase in temperature caused the dissociation of hydrogen bonds into free hydroxyl, carboxyl, and amide groups.^43,44

Fig. 2 (a) FT-IR spectra of GMHA, AlgCa, and AMH hydrogels. (b) Temperature-dependent FTIR spectra of AM₈H in the ν(–OH) and ν(–NH) regions when heated from 30 °C to 90 °C. (c) Temperature-dependent FTIR spectra of AM₈H in the ν(COOH) and ν(C–N) regions. (d) Fiber morphology of AMH hydrogel. (e) Pore morphology of AMH hydrogel.

As shown in the results of scanning electron microscopy (SEM), neither the initial AlgCa nor GMHA hydrogels had oriented structures (Fig. S3†). However, axially aligned fibers and pores were observed in the AMH hydrogel, presenting a multi-scale hierarchical structure similar to native nerves (Fig. 1b). By clamping in situ, the AlgCa hydrogel formed a thick fibrous structure arranged along the axial direction (Fig. 2d), with each ultrafine fiber diameter having a submicron diameter (Fig. 2d-iii). GMHA utilizes the oriented fibrous structure as a template. The pores grow along the fibers, and the pore walls are parallel to the fiber arrangement direction, exhibiting an oriented porous structure (Fig. 2e). The AMH hydrogel presents a multi-layer sandwich structure composed of oriented fibers–pores–fibers. Its regular arrangement of fibers and pores provides effective channels for both horizontal and vertical transport of water and ions, thereby facilitating the formation of a large number of hydrogen bonds between molecular chains. Additionally, this special structure significantly improves the mechanical properties of the AMH hydrogel.

3.2 Mechanical properties of AMH hydrogel

In order to verify the enhancement of the mechanical properties of AMH, the mechanical strength of the hydrogel in the tensile and compressive directions was measured respectively. The tensile test revealed that, compared to the synthetic hydrogel without a directional structure (NS-AMH, with an elongation at break of 62% and a breaking strength of 0.26 MPa), the anisotropic structure and the presence of ionic bonds and hydrogen bonds played a toughening role on the AMH hydrogel, while the porous structure provided more deformation space for the material, giving the AMH hydrogel excellent ductility and tensile strength. As shown in Fig. 3a, with the increase of the methacrylation degree of GMHA, the elongation at break gradually increases, and the fracture stress first increases and then decreases. Among them, the elongation at break of AM₈H is 102.24%, and the fracture strength is 1.51 MPa, which is comparable to human nerves. Fig. 3d shows the compressive stress–strain curves of the AMH hydrogel. Under 70% compressive strain, the AMH hydrogels exhibit good compressive properties, among which AM₈H has the maximum compressive stress of 3.51 MPa. The maximum compressive stress of NS-AMH is only 1.45 MPa, and the material yields at 58% compressive strain, and the cracks appear on the surface. The hydrogels with only fibrous structure were subjected to compression tests, and the maximum compressive stress was 1.69 MPa (Fig. 3f). This results show that the aligned hierarchical structure gives the hydrogel good compression properties, and the addition of GMHA further improves the performance. GMHA fills the gaps between layers, thereby buffering pressure and playing a synergistic reinforcement role. The multi-layer sandwich structure, oriented fibers–pores–fibers and the presence of a large number of hydrogen bonds have greatly improved the tensile and compressive strengths of the material. Consequently, the hydrogel exhibits excellent comprehensive mechanical properties (Fig. 3i). AM₈H, which exhibited the best mechanical properties, was selected for cyclic stretching (Fig. 3b) and cyclic compression tests (Fig. 3e) within a strain range of 0–10% to assess the hydrogel's cyclic mechanical properties. In the cyclic stretching and cyclic compression (20 cycles) experiments, the periodic curves overlapped highly, and the hydrogel exhibited good shape recovery ability and cycling stability.

Fig. 3 (a) Tensile stress–strain curves of NS-AMH, AM₄H, AM₈H and AM₁₂H. (b) Tension–relaxation cycles (10% strain, 20 cycles) of the AM₈H hydrogels. (c) Stress–strain curves of the AM₈H hydrogel before and after storage at −20 °C for 48 hours. (d) Compressive stress–strain curves of NS-AMH, AM₄H, AM₈H and AM₁₂H. (e) Compression–relaxation cycles (10% strain, 20 cycles) of the AM₈H hydrogels. (f) Comparison of the compressive properties of the AlgCa hydrogel with fibrous structure and AM₈H. (g) Degradation rate curves of NS-AMH, AM₄H, AM₈H and AM₁₂H. (h) Comparison of mechanical properties of NS-AMH, AM₄H, AM₈H and AM₁₂H. (i) Comparison of mechanical properties of AM₈H and other hydrogels.^46–54

The adaptability of AMH was explored in different usage scenarios. AM₈H was stored at −20 °C for 48 hours and then subjected to a tensile test. The high concentration of Ca²⁺ lowered the freezing point of the aqueous phase.⁴⁵ Under low temperature conditions of −20 °C, the hydrogel still achieved an elongation at break of 88.6% and a breaking stress of 1.1 MPa, demonstrating excellent antifreeze properties and low-temperature stability (Fig. 3c). The degradation ability of AMH in water was tested. After 30 days of continuous observation, the hydrogel's mass decreased by approximately 30% (Fig. 3g), indicating good water stability and environmentally friendly properties.

3.3 Sensing applications of AMH hydrogel

In the human body, electrical signals such as nerve impulses are transmitted through nerve fibers, so conductivity is a basic property of nerves.⁵⁵ The oriented fibers–pores–fibers multilayer sandwich structure of the AMH hydrogel not only brings about good tensile and compressive properties, but also constructs ion transport channels for freely moving conductive ions (Ca²⁺ and Cl⁻) in the hydrogel. AMH was subjected to electrochemical impedance testing (Fig. 4a) and the conductivity was calculated (Fig. S8†). NS-AMH has poor conductivity, σ is about 0.005 S cm⁻¹, while AMH with oriented structure increases the conductivity by about 9 times, among which AM₈H has the best conductivity of 0.044 S cm⁻¹.

Fig. 4 (a) The electrochemical impedance spectroscopic spectra of NS-AMH, AM₄H, AM₈H and AM₁₂H. (b) Relative resistance change of the AM₈H hydrogel when stretched. (c) Relative resistance changes of the AM₈H hydrogel when continuously stretched-released for 500 seconds under 10% tensile strain. (d) Relative resistance changes of the AM₈H hydrogel when compressed. (e) Relative resistance changes of the AM₈H hydrogel when continuously stretched-released for 500 seconds under 10% compressive strain. (f) Relative resistance changes and |GF| of the AM₈H hydrogel under different strains. (g) Dynamic response of the hydrogel under various motions: (i) bending the finger (30°, 60° and 90°), (ii) bending the wrist (45° and 90°). (h) AMH response diagram of the heel and toe ball area during walking: (i) heel area, (ii) toe ball area. (j) Sensing response diagram of the heel area during walking, running, and jumping in different modes of movement.

AM₈H was assembled into a hydrogel sensor, and external strain stimulation was applied to test its mechanical response performance. The relative resistance change (RRC) of the hydrogel under different tensile strains (1%, 2%, 5%, 10%, and 20%) was tested. The image showed that the relative resistance change increased with increasing tensile strain (Fig. 4b) and decreased with the increase of compressive strain (Fig. 4d). These results indicate that AMH hydrogels can convert tensile and compression deformation into stable electrical signals, and exhibit good mechanical response. The sensitivity under different strains was calculated (Fig. 4f). The hydrogel showed good sensitivity to both tensile strain and compressive strain, and |GF| was similar, which revealed that the hydrogel has three-dimensional mechanical response ability. The reason for this analysis is that when the hydrogel is stretched along the orientation direction, the conductive ions move along the fibers and pores, and signal transmission occurs. When the strain is in the orthogonal direction of the orientation structure, the ions can be transmitted through the layered structure and along the pores between the layers. In addition, the layered structure can reduce the contact area during compression, and the final contact area is increased, thereby promoting ion transmission and maintaining high sensitivity. In summary, the special multilayer sandwich structure design of AMH provides an effective channel for the bidirectional transmission of ions, enabling it to sensitively monitor and record small deformations (1%, 2%, 5%) and large strains (10%, 20%). During long-term use, the sensor must maintain stable transmission of electrical signals, and the cycling stability of the hydrogel is tested. Under 10% cyclic strain conditions, the AMH hydrogel was subjected to continuous stretching (Fig. 4c) and compression tests (Fig. 4e) for 500 seconds. The tensile and compressive strain signals recorded by the hydrogel did not show signal attenuation or abnormal deviation, maintaining sensitivity and accuracy.

Based on the fact that the prepared hydrogel not only possesses the advantages of anisotropic structure and mechanical properties similar to peripheral nerves, but also exhibits high conductivity, wide sensing range and high sensitivity, the AMH hydrogel sensor is capable of monitoring human activities with high sensitivity and accuracy. The hydrogel is connected to the fingers and wrists. As shown in Fig. 4g, when the human body moves, the relative resistance change increases with the bending angle and remains stable during repeated movements, indicating that the AMH provides a timely and stable response to bending motion. Continuous gait monitoring is crucial for correcting walking posture and preventing major diseases such as stroke, but gait monitoring is the most complex aspect of health monitoring due to the complex mechanical effects experienced by any area of the sole.⁵⁶ This requires the sensor to possess a three-dimensional sensing range, high sensitivity and long-term stability. To verify the applicability of the AMH hydrogel in gait monitoring, the AMH was fixed to two different pressure points (the heel and the toe ball) on the sole of the left foot to obtain the sensing response. Fig. 4h shows the sensing response (real-time acquisition) from the two pressure areas with the highest biomechanical efficiency of the middle arch when a person walks slowly: the heel (Fig. 4h-i) and the toe ball area (Fig. 4h-ii).^57,58 During walking, when the heel is put down, the pressure increases to a maximum value and then relaxes as the entire foot is placed on the floor. When the toe ball is placed on the floor, the heel is slowly lifted, and the pressure on the ball of the toe increases to a maximum value. Due to the difference in force, there are obvious differences in peak current, peak shape, and interval time between the toe ball and the heel, demonstrating that AMH has a three-dimensional sensing range and the ability to distinguish plantar pressure points. In further experiments, the AMH was fixed to the experimenter's heel, and then walking, running, and jumping activities were performed. High-speed movement produces faster mechanical shocks, resulting in a harsher environment for sensing responses, but the AMH accurately and stably completed the sensing response. As shown in Fig. 4j, the sensing signals of each sports scenario are significantly different. During walking, the peak width is large, showing the characteristics of a wide peak. The movement frequency of running and jumping is faster and the force is more concentrated. Therefore, the peak width continues to decrease and the peak becomes sharper. The AMH was also fixed on the toe ball for walking, running, and jumping movements (Fig. S12†). Although the exact same movements were performed, the peak shapes, width of peak and the intervals between peaks observed at the heel and toe ball were completely different. The special three-dimensional response capability of the AMH hydrogel allows different movement patterns to generate different current signals. Through different electrical signal waveforms, the movement pattern, movement frequency and stress area of the human body can be quickly and accurately determined. In addition, AMH hydrogels have shown promising results in applications such as sensing external roughness (Fig. S10†), handwriting detection (Fig. S11†), and more. AMH hydrogel sensors are promising candidates in areas such as sports science, infant and child health, and rehabilitation training.

4 Conclusions

Inspired by the multilayer ordered structure and the ion signal conduction mechanism of peripheral nerves, the AMH hydrogel was prepared. It was designed through in situ mechanical stretching, covalent crosslinking, and hydrogen bonding. The AMH hydrogel exhibited a unique multilayer sandwich structure composed of oriented fibers–pores–fibers. Benefiting from this rare anisotropic structure, the hydrogel had excellent mechanical strength, with a tensile breaking strength of 1.51 MPa and a maximum compressive stress of 3.51 MPa. Furthermore, excellent electrical conductivity and visual signal transmission were observed. Experimental results demonstrated that the hydrogel exhibits a wide range of detection capabilities, good signal repeatability and excellent response sensitivity to mechanical stimuli. When AMH was applied for gait monitoring, it could quickly and accurately distinguish between intense and complex human movements. The AMH hydrogel has shown great application potential and broad market prospects in the fields of human motion detection, rehabilitation training, biosensors, and so forth.

Data availability

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

Author contributions

Qiuyun Zhang: conceptualization, investigation, methodology, resources, data curation, visualization, software, writing – original draft, writing – review & editing. Yujie Chen: conceptualization, methodology, writing – review & editing, supervision. Sijia Li: methodology, visualization, writing – review & editing. Yuxuan Wu: conceptualization, methodology. Xichen Yang: visualization, writing – review & editing. Yutong Guo: data curation, visualization. Hezhou Liu: methodology, writing – review & editing, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge support from the National Natural Science Foundation of China (NSFC) (No. 52373083 and 52273078).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06639a

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