A multi-directional force sensing flexible sensor based on material orientation regulation and a preparation method thereof

Abstract

The application discloses a multi-directional force sensing flexible sensor based on material orientation regulation and a preparation method thereof. The sensor is formed by stacking a plurality of polyvinyl alcohol oriented hydrogel layers with different microstructure main orientation directions, each layer being provided with a flexible conductive sensing structure, and adjacent hydrogel layers being integrally connected through a polyvinyl alcohol interface layer and a re-freezing and thawing treatment. The different orientation hydrogel layers have different directional mechanical responses, so that external forces acting along different spatial directions produce different force transmission and deformation, and output electrical responses with obviously different amplitudes in corresponding sensing channels, so as to judge the main action direction and size change of the external force. The hydrogel can be added with compatible hydrophilic polymers and water-retaining components to adjust the mechanical properties and use stability. The application can be used for flexible tactile sensing, robot contact interface and wearable mechanical monitoring.

Description

Technical Field

[0001] This invention belongs to the fields of flexible electronics, functional polymer materials, and flexible mechanical sensing technology, specifically relating to a multi-directional force-sensing flexible sensor based on material orientation modulation and its fabrication method. The sensor constructs anisotropic flexible material layers with different microstructural orientations, and ensures that the orientation structure of each material layer and its flexible conductive sensing structure form a synergistic direction-selective mechanical response. This allows external forces applied along different spatial directions to produce differentiated force transmission and deformation in different material layers, thereby achieving a sensing capability sensitive to the direction of force application at the material structure level. Preferably, the anisotropic flexible material is a polyvinyl alcohol-based hydrogel with an oriented microstructure formed by directional freezing. Background Technology

[0002] With the development of robotic tactile sensing, flexible electronics, and human-computer interaction technologies, flexible sensors capable of identifying the direction of external forces are receiving increasing attention. In robotic grasping, dexterous manipulation, rehabilitation assistance, and wearable monitoring, contact loads often do not act in a single direction; they include both compressive components perpendicular to the contact surface and shear components along the contact surface. Therefore, how to maintain the flexibility and integrability of the device while enabling the sensor to produce distinguishable outputs for external forces in different directions is a crucial problem that needs to be solved in the field of flexible mechanical sensing.

[0003] Existing multi-directional force flexible sensors typically obtain direction-dependent outputs through multi-electrode arrangements or special geometric structures. For example, CN103954382A discloses a variable dielectric capacitive flexible tactile sensor that reflects the load direction through the differentiated capacitance changes of multiple sensing electrodes after being subjected to force; CN112539863B discloses a flexible tactile sensor that forms different capacitive responses through a porous elastic layer and multiple electrodes. Such solutions can obtain direction-dependent signals, but their direction sensitivity mainly comes from the spatial arrangement of the electrodes and the macroscopic geometry of the device. The flexible substrate itself has basically the same mechanical properties in all directions and does not have the ability to actively distinguish the load direction.

[0004] Another type of technology obtains multi-directional mechanical responses by altering the spatial orientation of the conductive structures within a flexible substrate. For example, CN110966913B discloses a flexible large strain sensor based on liquid metal, which sets up multiple microfluidic channels in different directions within a flexible substrate and fills them with liquid metal to obtain strain responses in different directions. This type of approach utilizes the geometric orientation of the conductive channels to create a certain degree of directional selectivity, but the flexible substrate still primarily serves as a homogeneous continuous deformation and load transfer medium, and the directional differences depend entirely on the arrangement of the conductive structures.

[0005] Meanwhile, research and patents have been published on the use of directional freezing to construct anisotropic flexible materials. For example, CN114752077B discloses a method for preparing a two-component anisotropic hydrogel. By directional freezing along a predetermined temperature gradient, ice crystals grow in a specific direction, thereby forming directional channels and a polymer skeleton inside the material, and obtaining mechanical properties that differ significantly along different directions. This type of technology shows that directional freezing can effectively control the microstructure orientation inside flexible materials, but its focus is on the anisotropic mechanical properties of the material itself. It does not hierarchically combine multiple material layers with different orientations, nor does it unify the design of material orientation, interlayer force transmission, and the orientation of conductive sensing structures.

[0006] In summary, current flexible sensing technologies primarily rely on electrode arrangement or conductive structure orientation for sensitivity to external force direction, with the material itself often being isotropic. Research on anisotropic materials, however, focuses on the mechanical characterization at the material level. Further research and improvement are needed to develop a multi-directional force-sensing flexible sensing scheme that leverages the synergistic relationship between the material's microstructure orientation, interlayer load transfer paths, and conductive sensing structure orientation to create differentiated mechanical responses within the material before external forces in different directions are converted into electrical signals. Therefore, it is necessary to propose a multi-directional force-sensing flexible sensing scheme that obtains directional sensitivity at the material structure level. Summary of the Invention

[0007] To address the problem that the directional sensitivity of existing flexible mechanical sensors relies on electrode arrangement or macroscopic geometry, and the material itself does not participate in directional differentiation, the present invention aims to provide a multi-directional force-sensing flexible sensor based on material orientation modulation and its fabrication method. By stacking anisotropic flexible material layers with different microstructural orientations, and setting flexible conductive sensing structures matching the directional mechanical response in each material layer, external forces acting along different spatial directions cause differentiated stress transmission and deformation in the material layers with different orientations. This, in turn, generates electrical responses with significantly different amplitudes in each sensing channel, enabling the sensor to identify the direction of external force application.

[0008] To achieve the above objectives, the present invention adopts the following technical solution.

[0009] A multi-directional force-sensing flexible sensor includes three stacked flexible material layers: a first-oriented flexible material layer, a second-oriented flexible material layer, and a third-oriented flexible material layer. Each of the three layers has a different main orientation direction of its microstructure. At least one flexible conductive sensing structure is disposed within or on the surface of each layer. Adjacent flexible material layers are bonded together as a whole through an interface layer formed by homologous or compatible polymers. Under external loads, the differentiated compression, tension, shear, and strain transmission characteristics of the different oriented material layers along different directions cause effective deformation of varying degrees in the layers under loads acting in different spatial directions, further inducing differentiated electrical responses in the corresponding flexible conductive sensing structures.

[0010] Furthermore, the first, second, and third oriented flexible material layers are preferably made of polyvinyl alcohol-based hydrogel material, and are respectively formed into directional channels, sheets, polymer-enriched regions, or combinations thereof through directional freezing in different directions. The main orientation directions of the three material layers are respectively set along the thickness direction of the sensor and two different planar directions, to form a material structure with differentiated responses to loads perpendicular to the contact surface and loads in the two directions along the contact surface; the different orientation directions can be orthogonal or approximately orthogonal to each other, or can be set to other predetermined angles according to the direction of the load to be detected.

[0011] Further, based on the total mass of the hydrogel precursor liquid, the mass fraction of polyvinyl alcohol is 6%–20%, preferably 8%–16%, and more preferably 10%–14%. Different oriented material layers can use the same or different polyvinyl alcohol contents, and the modulus, compressibility, shear properties, and directional mechanical response of each layer can be changed by adjusting the polyvinyl alcohol content, freezing temperature gradient, freezing rate, number of freeze-thaw cycles, and material layer thickness.

[0012] Furthermore, a hydrophilic polymer material compatible with, miscible with, or capable of forming a uniform composite network with polyvinyl alcohol is added to the polyvinyl alcohol-based hydrogel material to adjust the elastic modulus, toughness, compressive strength, and shear strength of each oriented material layer. The hydrophilic polymer material includes one or more of polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, polyacrylamide, polyacrylic acid, gelatin, and water-soluble cellulose derivatives. Based on the total mass of the hydrogel precursor liquid, the mass fraction of the hydrophilic polymer material is 0.2%–8%, preferably 0.5%–5%; or based on 100 parts by mass of polyvinyl alcohol, the hydrophilic polymer material is 2–30 parts, preferably 5–20 parts.

[0013] Furthermore, a water-retaining component is added to the polyvinyl alcohol-based hydrogel material to reduce water evaporation during use and maintain the material's softness. The water-retaining component includes one or more of glycerol, sorbitol, polyethylene glycol, and propylene glycol; the mass fraction of the water-retaining component, based on the total mass of the hydrogel precursor liquid, is 2%–20%, preferably 5%–15%, wherein glycerol is preferably 5%–12%. The amount of water-retaining component added is preferably such that it does not significantly disrupt the oriented channels or lamellar structure formed by directional freezing.

[0014] Furthermore, different material ratios are used for the different oriented material layers to create differences in interlayer modulus. The material layer oriented along the thickness direction has a polyvinyl alcohol mass fraction of 10%–18%, preferably 12%–16%; the material layer oriented along the two planar directions has a polyvinyl alcohol mass fraction of 8%–14%, preferably 9%–12%. In a preferred embodiment, the two planar oriented material layers use the same material composition, and different primary orientations are formed only by changing the directional freezing direction, thereby reducing the influence of material formulation differences on directional response.

[0015] Furthermore, prefabricated trenches, recesses, channels, or cavities are provided in each oriented flexible material layer, and the flexible conductive sensing structure is disposed within the trenches, recesses, channels, or cavities. The flexible conductive sensing structure includes one or more of the following: conductive polymer, conductive hydrogel, ionic conductor, liquid metal, conductive fiber, metal conductor, carbon-based conductive material, or two-dimensional conductive material. The conductive polymer includes polypyrrole, PEDOT:PSS, or polyaniline; the liquid metal includes gallium, gallium-indium alloy, eutectic gallium-indium alloy, gallium-indium-tin alloy, or other metals and alloys that maintain a liquid or deformable conductive state within the sensor's operating temperature range.

[0016] Furthermore, the flexible conductive sensing structures in the material layers with different orientations are respectively configured as linear, serpentine, wavy, zigzag, grid-like, or combinations thereof, and have different extension directions. A predetermined angle is set between the flexible conductive sensing structure and the main orientation direction of the microstructure of the material layer, so that the directional strain formed inside the material preferentially acts on the corresponding conductive structure, thereby forming a synergistic sensing relationship between the orientation of the material microstructure, the strain transmission within the layer, and the orientation of the conductive channel.

[0017] Furthermore, the first, second, and third oriented flexible material layers are stacked after directional freezing and preliminary freeze-thaw crosslinking, respectively. Polyvinyl alcohol or a polyvinyl alcohol-containing interfacial precursor material is placed between adjacent material layers, and the stacked structure is subjected to freeze-thaw treatment again, so that the polyvinyl alcohol segments at the interface diffuse, entangle, hydrogen bond, and physically crystallize and crosslink with the polyvinyl alcohol network in the material layers on both sides, thereby forming a continuous homologous polymer interfacial connection structure.

[0018] Furthermore, the polyvinyl alcohol in the interface bonding layer has a mass fraction of 4%–15%, preferably 6%–10%; 2%–10% glycerol, sorbitol, or polyethylene glycol may also be added to the interface bonding layer as a water-retaining component. The interface bonding layer is used to improve the bonding stability between adjacent oriented material layers, suppress uncontrolled interlayer slippage under load along the contact surface direction, enable external loads to be continuously transferred between different oriented material layers, and maintain the original main microstructure orientation of each material layer.

[0019] Furthermore, the thickness of each oriented flexible material layer is 0.3–5 mm, preferably 0.5–3 mm; different material layers may use the same or different thicknesses to further adjust the response range of loads in different directions. The directional freezing is carried out under conditions below the freezing point of the precursor liquid, with a cold end temperature of -10 to -80°C, preferably -20 to -50°C; the freeze-thaw treatment is performed 1–5 times, preferably 2–4 times, and the specific parameters are adjusted according to the material composition, layer thickness, and required degree of orientation.

[0020] Furthermore, the present invention is not limited to three oriented material layers; two, three, four, or more oriented flexible material layers can be provided depending on the number of load directions to be identified. Different material layers are used to form sensing channels with differentiated responses to compression, shear, torsion, bending, or other mechanical stimuli. Multiple multi-directional force sensing units can also be further arrayed to form a flexible sensing array for distributed mechanical sensing of robot fingers, robot palms, mechanical grippers, robotic arms, soft robots, or other contact interfaces.

[0021] The present invention also provides a method for fabricating the above-mentioned multi-directional force-sensing flexible sensor, comprising the following steps:

[0022] (1) Prepare a polyvinyl alcohol-based hydrogel precursor solution and add water-retaining components and hydrophilic polymer materials that are compatible with, miscible with, or can form a composite network with polyvinyl alcohol as needed.

[0023] (2) The precursor liquid is placed in directional freezing molds with different temperature gradient directions, and first, second and third orientation flexible material layers with different microstructure main orientation directions are formed by directional freezing in different directions, and a preliminary stable polymer network is formed by freeze-thaw treatment.

[0024] (3) Grooves, channels or cavities are formed in each orientation material layer by means of mold prefabrication, template forming, cutting or other methods, and conductive polymers, liquid metals or other flexible conductive materials are placed in them to form corresponding flexible conductive sensing structures;

[0025] (4) Stack flexible material layers with different orientations according to a predetermined orientation relationship, and introduce polyvinyl alcohol or a polyvinyl alcohol-containing interface precursor material between adjacent material layers.

[0026] (5) The stacked structure is subjected to freeze-thaw treatment again to form a continuous polymer network between the interface precursor material and the adjacent material layer, and to combine multiple material layers with different orientations into a whole.

[0027] (6) Set conductive leads and perform flexible encapsulation according to usage requirements to obtain a multi-directional force sensing flexible sensor.

[0028] Furthermore, in one proposed formulation, the thickness-oriented material layer comprises 14% polyvinyl alcohol, 1% polyvinylpyrrolidone, and 6% glycerol by mass; the two planar-oriented material layers each comprise 10% polyvinyl alcohol, 2% polyvinylpyrrolidone, and 8% glycerol by mass, maintaining the same formulation for both layers while changing only the orientation freezing direction; the interface bonding layer comprises 8% polyvinyl alcohol and 5% glycerol by mass. The above proportions are suggested ranges or starting formulations for ease of implementation and optimization, and do not represent the optimal composition determined experimentally.

[0029] Furthermore, this invention also provides a multi-directional force sensing method for the aforementioned flexible sensor. This method involves acquiring the electrical responses of the flexible conductive sensing structures in different oriented material layers, comparing the relative amplitudes and trends of the responses from each channel, and determining the main direction of the external force and its magnitude. The correspondence between each channel and loads in different directions can be pre-calibrated through unidirectional loading.

[0030] The beneficial effects of this invention are as follows: the sensor's directional sensitivity mainly stems from the differences in the microstructure orientation of the materials themselves, rather than solely relying on the electrode arrangement or macroscopic geometry; after being stacked, different oriented material layers are connected through homologous polymer interfaces to form a continuous whole, avoiding abrupt modulus changes and interface slippage caused by rigid adhesives; the material microstructure orientation, interlayer load transfer, and conductive sensing structure orientation are unified in design, enabling external forces in different directions to form distinguishable mechanical responses within the material before being converted into electrical signals. The sensor has a simple structure, good flexibility, and is easy to integrate into an array. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the multi-directional force sensing flexible sensor based on material orientation control according to the present invention.

[0032] Figure 2 The images show actual samples of the oriented hydrogels prepared by directional freezing in different directions according to the present invention.

[0033] Figure 3 This is a microstructure diagram of the hydrogels with different orientations according to the present invention.

[0034] Figure 4 The diagram shows the mechanical properties of the hydrogels with different orientations according to the present invention.

[0035] Figure 5 This is a test diagram of the sensing performance of the multi-directional force sensing flexible sensor of the present invention, including the response of each channel under the action of forces in different directions.

[0036] Figure reference numerals: 1—First polyvinyl alcohol oriented hydrogel layer; 2—Second polyvinyl alcohol oriented hydrogel layer; 3—Third polyvinyl alcohol oriented hydrogel layer; 4—Flexible conductive sensing structure; 5—Polyvinyl alcohol interface connection region; 6—Main orientation direction of material microstructure. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0038] The oriented flexible material layer in this invention refers to a flexible material layer that exhibits differentiated mechanical responses in different directions by forming channels, sheets, polymer-rich regions, or other oriented structures arranged in a predetermined direction through directional freezing or other methods capable of forming directional microstructures. Polyvinyl alcohol-based hydrogel is preferably used as the main material, with the addition of hydrophilic polymers compatible with polyvinyl alcohol and water-retaining components to adjust the material's modulus, toughness, and long-term water retention performance.

[0039] In this invention, the first, second, and third oriented flexible material layers each have different main orientation directions of their microstructures. The first and second oriented flexible material layers form an orientation structure along two intersecting planar directions, while the third oriented flexible material layer forms an orientation structure along its thickness direction. The three oriented material layers are connected as a whole through lamination and homologous polymer interfaces, enabling differentiated force transmission and deformation responses to external forces in different directions within the different material layers.

[0040] Specific Implementation Method 1: A multi-directional force-sensing flexible sensor based on material orientation modulation includes a first-oriented flexible material layer, a second-oriented flexible material layer, and a third-oriented flexible material layer stacked sequentially. Each material layer contains a polypyrrole flexible conductive sensing structure, located in a pre-fabricated trench, channel, or localized sensitive region, and forming a predetermined spatial relationship with the microstructure orientation of its respective material layer. The first and second-oriented flexible material layers are used to selectively respond to loads in two different directions along the contact surface, respectively, while the third-oriented flexible material layer is used to selectively respond to loads perpendicular to the contact surface. The three material layers work together under overall stress, rather than being simply stacked as three independent sensors.

[0041] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the oriented flexible material layer is formed using a directional freezing method. Specifically, polyvinyl alcohol is dissolved in deionized water to form a precursor solution, and one or more of polyvinylpyrrolidone, polyethylene glycol, gelatin, or water-soluble cellulose derivatives are added as needed to adjust the mechanical properties of the material; simultaneously, one or more of glycerol, sorbitol, or propylene glycol are added as water-retaining components. The precursor solution is directionally frozen through temperature gradients in different directions, causing ice crystals to grow along a predetermined direction and forming a directional polymer network after freeze-thaw cycles. The directional freezing temperature is -10 to -40°C, preferably -15 to -30°C; the freezing time is 4 to 16 hours, followed by thawing at 20 to 30°C for 4 to 8 hours, and 1 to 4 freeze-thaw cycles are performed.

[0042] Specific Implementation Method Three: This implementation method differs from Specific Implementation Methods One or Two in that, after the different oriented material layers have undergone preliminary directional freezing and freeze-thaw crosslinking, a new polyvinyl alcohol precursor solution is coated or introduced between adjacent layers, followed by another freeze-thaw treatment. The interfacial polyvinyl alcohol segments diffuse, entangle, form hydrogen bonds, and undergo physical crystallization on the surfaces of adjacent hydrogels, resulting in a continuous connection structure between the different oriented material layers. This connection method can reduce uncontrolled interlayer slippage under load along the contact surface direction, while avoiding significant modulus abrupt changes caused by rigid adhesives and preserving the original orientation structure of the different material layers as much as possible.

[0043] Example 1: Preparation of Oriented Polyvinyl Alcohol Hydrogels. Polyvinyl alcohol with a degree of polymerization of approximately 1700 and a degree of hydrolysis of not less than 98% was selected as the main material. Polyvinyl alcohol was added to deionized water and stirred at approximately 90–95°C for 2 h to obtain a 12% (w / w) polyvinyl alcohol precursor solution. Approximately 1.5% (w / w) of polyvinylpyrrolidone and approximately 6% (w / w) of glycerol were added to the precursor solution, and the mixture was thoroughly mixed and degassed. The precursor solution was added to three directional freezing molds, with the first mold establishing a temperature gradient along the X direction, the second mold along the Y direction, and the third mold along the thickness Z direction. The samples were directionally frozen at approximately -20°C for 8–12 h, followed by thawing at 25°C for approximately 6 h. This freeze-thaw cycle was repeated twice to obtain three polyvinyl alcohol-based hydrogel layers with different main orientation directions of their microstructure. The obtained hydrogels with different orientations could be observed in their physical morphology, and their micro-orientation structures could be characterized using scanning electron microscopy.

[0044] Example 2: Fabrication of a flexible conductive sensing structure and multilayer device using polypyrrole. Grooves with a width of approximately 0.5–1.5 mm and a depth of approximately 0.2–0.8 mm were formed in the predetermined sensing region of the hydrogel layer obtained in Example 1. A pyrrole monomer solution was introduced into the grooves, and a polypyrrole conductive structure was formed in situ using oxidative polymerization. In one embodiment, the pyrrole monomer concentration was 0.05–0.3 mol / L, the oxidant was ferric chloride or ammonium persulfate, the reaction temperature was controlled at 0–10°C, and the reaction time was 1–4 h. After polymerization, the surface was thoroughly cleaned with deionized water to form a continuously distributed flexible conductive sensing structure along the grooves, and flexible wires were provided as signal output terminals.

[0045] Three oriented material layers (X, Y, and Z directions) were stacked in a predetermined order. A polyvinyl alcohol precursor solution with a mass fraction of approximately 8% was coated between adjacent material layers, and approximately 5% glycerol was added to the interface precursor solution. After stacking, a freeze-thaw process was performed again, freezing at -20°C for approximately 8 hours and thawing at 25°C for approximately 6 hours, to allow the three material layers to bond together and form an integrated multi-directional force-sensing flexible sensor. The polypyrrole sensing structures in each material layer were independently extracted to record the resistance changes of each of the three oriented material layers.

[0046] Example 3: Mechanical Property Testing of Material Layers with Different Orientations. Samples of the same size were cut from the X, Y, and Z-direction oriented frozen hydrogels, and tensile, compressive, or shear tests were performed along and perpendicular to the main orientation direction of the microstructure. A constant loading rate, such as 5–20 mm / min, was used during the tests, and stress-strain curves, elastic modulus, and cyclic loading responses were recorded. By comparing the mechanical responses of the same hydrogel under different loading directions, the anisotropy of the material formed by oriented freezing was evaluated, and the correspondence between the oriented freezing direction, the orientation microstructure, and the directional mechanical properties was verified.

[0047] In a further embodiment, modulus matching can be achieved by adjusting the composition of different material layers. For example, the mass fraction of polyvinyl alcohol in the vertical sensing layer can be increased to about 14%, the mass fraction of polyvinylpyrrolidone to about 1%, and the mass fraction of glycerol to about 5%; the two planar sensing layers use about 10%–12% polyvinyl alcohol, 1%–2% polyvinylpyrrolidone, and 6%–10% glycerol. Through this method, the compression and shear response range of each material layer can be further adjusted while maintaining the material orientation structure.

[0048] Example 4: Multi-directional force sensing performance test. The sensor prepared in Example 2 was fixed on the test platform, and known loads were applied along the X, Y, and Z directions, respectively, while the resistance changes of the three polypyrrole sensing channels were recorded simultaneously. The output of each channel is represented by the relative resistance change ΔR / R0, where R0 is the initial resistance in the unloaded state. The unidirectional loading range is determined according to the device size and material modulus; for example, it is initially calibrated in the range of 0–5 N, and the test range is further expanded or narrowed based on the actual response.

[0049] When a load is applied along the X-direction, the response amplitudes of the three sensing channels are compared; the same testing method is used when loading along the Y and Z directions. Since the three material layers have different dominant microstructure orientations, the orientation of the material layer matching the loading direction is expected to produce a more pronounced response, while the responses of the other two channels are relatively weaker. By comparing the response relationships of the three channels under the three loading directions, the selectivity of the sensor to the direction of external force is evaluated.

[0050] In practical applications, the response relationship between each sensing channel and loads in different directions can be established based on the calibration results of unidirectional loading, and the main direction of the external force can be determined based on the relative amplitudes of the three channel outputs. The differential response of the sensor to external forces in different directions mainly comes from the directional mechanical properties of the material layers with different orientations and the synergistic effect between the material orientation and the polypyrrole sensing structure, without relying on machine learning models trained iteratively based on a large number of samples.

[0051] The above embodiments illustrate that by controlling the orientation of the internal microstructure of flexible materials and further combining the synergistic design of material composition, modulus, interlayer connections and conductive sensing structures, external forces in different spatial directions can form differentiated mechanical responses before entering the electrical sensing structure, thereby providing a material structure-level realization approach for flexible multi-directional force sensing.

Claims

1. A multi-directional force-sensing flexible sensor based on material orientation modulation, characterized in that, It includes a first polyvinyl alcohol oriented hydrogel layer, a second polyvinyl alcohol oriented hydrogel layer and a third polyvinyl alcohol oriented hydrogel layer stacked together; The first polyvinyl alcohol oriented hydrogel layer, the second polyvinyl alcohol oriented hydrogel layer and the third polyvinyl alcohol oriented hydrogel layer are formed with different material microstructure main orientation directions by directional freezing in different directions; The main orientation directions of the first polyvinyl alcohol oriented hydrogel layer and the second polyvinyl alcohol oriented hydrogel layer are respectively set along two intersecting planar directions, and the main orientation direction of the third polyvinyl alcohol oriented hydrogel layer is set along the thickness direction of the sensor. Flexible conductive sensing structures are respectively disposed in three polyvinyl alcohol oriented hydrogel layers. The flexible conductive sensing structures are coupled with the directional deformation of the corresponding polyvinyl alcohol oriented hydrogel layers to convert the directional deformation into an electrical response. Adjacent polyvinyl alcohol oriented hydrogel layers are connected by a polyvinyl alcohol interface bonding layer and a freeze-thaw cycle to form an integral connection structure. Different main orientations cause external forces acting along different spatial directions to produce differentiated force transmission and deformation responses in the three polyvinyl alcohol oriented hydrogel layers, and cause the corresponding flexible conductive sensing structures to produce differentiated electrical responses.

2. The multi-directional force-sensing flexible sensor according to claim 1, characterized in that, The included angle between the main orientation directions of the first polyvinyl alcohol oriented hydrogel layer and the second polyvinyl alcohol oriented hydrogel layer is 80° to 100°, and the main orientation direction of the third polyvinyl alcohol oriented hydrogel layer forms a spatial intersection with the aforementioned two planar directions.

3. The multi-directional force-sensing flexible sensor according to claim 1, characterized in that, The polyvinyl alcohol oriented hydrogel layer is formed by directional freezing at -10 to -40°C to form an oriented microstructure, and then subjected to freeze-thaw physical cross-linking after thawing at 20 to 30°C.

4. The multi-directional force-sensing flexible sensor according to claim 1, characterized in that, The polyvinyl alcohol oriented hydrogel layer further includes a hydrophilic polymer and / or a water-retaining component that are compatible with polyvinyl alcohol or can form a homogeneous blend system; the hydrophilic polymer is used to adjust the elastic modulus, toughness, compression properties or shear deformation properties of the material layer, and the water-retaining component is used to reduce water evaporation and maintain the flexibility of the material.

5. The multi-directional force-sensing flexible sensor according to claim 4, characterized in that, The hydrophilic polymer includes one or more of polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, polyacrylamide, polyacrylic acid, gelatin, carboxymethyl cellulose, or hydroxyethyl cellulose; the water-retaining component includes one or more of glycerol, sorbitol, propylene glycol, or polyethylene glycol; based on the total mass of the hydrogel precursor solution, the mass fraction of polyvinyl alcohol is 8%–18%, the mass fraction of the hydrophilic polymer is 0.2%–8%, and the mass fraction of the water-retaining component is 2%–20%.

6. The multi-directional force-sensing flexible sensor according to claim 1, characterized in that, The flexible conductive sensing structure is disposed in the grooves, channels or sensitive regions inside or on the surface of the corresponding polyvinyl alcohol oriented hydrogel layer; the flexible conductive sensing structure includes at least one of conductive polymer, conductive hydrogel, ionic conductor, liquid metal, carbon-based conductive material, two-dimensional conductive material or flexible metal conductive structure.

7. The multi-directional force-sensing flexible sensor according to claim 6, characterized in that, The flexible conductive sensing structure is a conductive polymer sensing structure, and the conductive polymer includes at least one of polypyrrole, PEDOT:PSS or polyaniline; the extension direction of the flexible conductive sensing structure has a predetermined spatial relationship with the main orientation direction of the polyvinyl alcohol oriented hydrogel layer, so that the target direction deformation of the corresponding hydrogel layer preferentially acts on the flexible conductive sensing structure.

8. The multi-directional force-sensing flexible sensor according to claim 1, characterized in that, The polyvinyl alcohol interface bonding layer is formed by introducing a polyvinyl alcohol precursor liquid between two polyvinyl alcohol oriented hydrogel layers that have undergone directional freezing and preliminary freeze-thaw, and then freezing and thawing the laminated structure again, so that a continuous polyvinyl alcohol physical cross-linking network is formed between adjacent hydrogel layers, while maintaining the main orientation difference of the material microstructure of each oriented hydrogel layer.

9. A method for fabricating a multi-directional force-sensing flexible sensor according to any one of claims 1 to 8, characterized in that, Includes the following steps: Step 1: Prepare a polyvinyl alcohol-based hydrogel precursor solution, and add hydrophilic polymers and / or water-retaining components that are compatible with polyvinyl alcohol or can form a homogeneous blend system as needed. Step 2: Establish temperature gradients along the first plane direction, the second plane direction, and the thickness direction respectively, and perform directional freezing and freeze-thaw treatment on the precursor liquid to obtain three polyvinyl alcohol oriented hydrogel layers with different main orientation directions. Step 3: Grooves, channels or sensitive regions are formed in the three polyvinyl alcohol oriented hydrogel layers, and a flexible conductive sensing structure is formed by in-situ polymerization, coating, filling, injection, transfer or deposition. Step 4: Stack the three polyvinyl alcohol oriented hydrogel layers according to the predetermined orientation relationship, and introduce polyvinyl alcohol precursor liquid between adjacent material layers. Step 5: Perform freeze-thaw treatment on the stacked structure again to form an integral connection structure of the three polyvinyl alcohol oriented hydrogel layers.

10. The preparation method according to claim 9, characterized in that, The resulting multi-directional force sensing flexible sensor detects changes in resistance, voltage, current, capacitance, or impedance of three flexible conductive sensing structures, and compares the relative amplitudes of the responses of each sensing structure to determine the main direction of the external force and its magnitude.

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