Multi-directional flexible sensor and preparation method

Abstract

A multi-directional flexible sensor, comprising several electrically conductive paths (11) and several flexible substrates (21), wherein the electrically conductive paths (11) extend continuously in a first direction; the diameters of the electrically conductive paths (11) increase gradually in a second direction, the second direction being inclined at a certain angle to the first direction; the several electrically conductive paths (11) spread radially, with a certain gap between adjacent electrically conductive paths (11); the gaps widen gradually as the diameters increase in the second direction; the flexible substrates (21) fill the gaps; and the flexible substrates (21) are bonded to the electrically conductive paths (11) to form a sensor, which has an orientation gradient. The multi-directional flexible sensor has a microscopic orientation gradient structure, thereby achieving an anisotropic response. The present invention further comprises a preparation method for the multi-directional flexible sensor. The preparation method can adjust the properties of the materials of the electrically conductive paths (11) and the flexible substrates (21) on the basis of different application requirements, and has flexibility and adaptability.

Description

A multi-directional flexible sensor and its fabrication method Technical Field

[0001] This invention relates to the field of flexible multidirectional sensor technology, specifically to a multidirectional flexible sensor and its fabrication method. Background Technology

[0002] With the rapid development of smart devices and wearable technologies, flexible sensors, as an important component of information acquisition, have gradually become a research hotspot in next-generation sensor technology. Traditional rigid sensors, such as those made of metal or semiconductor materials, cannot adapt to the complex shapes of flexible surfaces like human skin due to their high stiffness, poor flexibility, and limited strain range. Therefore, flexible sensors, with their advantages of being lightweight, flexible, small in size, and biocompatible, are widely used in fields such as smart wearables, health monitoring, robot control, and human-computer interaction.

[0003] However, in practical applications, mechanical stimuli (such as pressure, tension, and shear force) often act on the sensor simultaneously, and their mutual coupling complicates the sensor's response. Existing multi-directional sensors face certain challenges in identifying the direction of applied load and motion patterns, mainly due to the isotropic nature of their structure and materials, which leads to similar sensor responses under forces in different directions, making it difficult to effectively distinguish between various mechanical stimulus modes.

[0004] To address this issue, researchers have proposed multi-directional flexible sensors based on anisotropic design, which decouple multiple stimulus modes by designing special material structures and sensor arrays. However, these methods often involve complex fabrication processes and cumbersome assembly procedures, limiting their development in lightweight, functionalization, and large-scale applications. Although integrated molding technologies such as 3D printing have been developed to fabricate multi-directional sensors, the assembly of multiple circuits is still required. There is an urgent need for a new, simple fabrication method that can achieve spatial distribution and deformation pattern recognition of mechanical stimuli through a single circuit, thereby promoting the widespread application of flexible sensors in fields such as health monitoring and intelligent assistive devices. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a multi-directional flexible sensor and its fabrication method. This sensor possesses a microscopic orientation gradient structure and excellent multi-directional sensing capabilities, exhibiting strong flexibility, adaptability, and versatility. It can be widely applied in fields such as smart wearables, health monitoring, and human-computer interaction.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A multi-directional flexible sensor, comprising:

[0008] A plurality of conductive paths are provided, the conductive paths extending continuously in a first direction, the diameter of the conductive paths gradually increasing along a second direction, the second direction being inclined at a certain angle to the first direction, the plurality of conductive paths being radially distributed and having a certain gap between adjacent conductive paths, the gap being gradually increasing along the diameter of the second direction.

[0009] A flexible substrate is filled in the gap, and the flexible substrate is bonded to the conductive path to form a sensor with an orientation gradient.

[0010] Furthermore, the conductive pathway is any one or a combination of sheet-like, columnar, and porous structures, and the conductive pathway includes a conductive material, which includes any one or a combination of carbon materials, conductive polymers, ionic liquids, metals, and metal ions. The flexible matrix includes any one or a combination of hydrogels, aerogels, oleogels, rubber, and plastics.

[0011] Furthermore, the orientation gradient is any one or a combination of spacing, size, and material gradient.

[0012] Furthermore, the size gradient includes any one or a combination of pore size and particle size.

[0013] Furthermore, the material gradient includes any one or a combination of two of the following: a concentration gradient and a crosslinking density gradient.

[0014] The concentration gradient is generated through layer-by-layer printing or gradient cross-linking processes of materials with different concentrations.

[0015] Furthermore, the conductive paths on both sides have a certain angle with the conductive path in the middle, and the angle between the outermost conductive path and the conductive path in the middle is larger.

[0016] A method for fabricating a multidirectional flexible sensor includes the following steps:

[0017] The conductive material slurry and the flexible matrix slurry are prepared in a certain proportion;

[0018] Freeze printing: The cold source is brought to a set temperature, and the prepared conductive material slurry is filled into the extruder. The conductive material slurry is extruded into the cold source through the extruder, and at the same time the cold source and the extruder move relative to each other in the first direction. During the relative movement of the cold source and the extruder, the conductive material slurry that first comes into contact with the cold source will freeze into ice crystals and grow upward. During the growth of the ice crystals, the solute in the conductive material slurry will be displaced on both sides of the ice crystals to form conductive paths that grow in the second direction. The diameter of the ice crystals and conductive paths increases in the direction away from the cold source.

[0019] Remove ice crystals formed during the freezing process;

[0020] A flexible substrate is filled into the original location of the ice crystals, and the flexible substrate is combined with the conductive path under certain temperature conditions.

[0021] Furthermore, extrusion-type cryogenic printing is used when the extruder extrudes the conductive material slurry into the cold source, and the freezing rate, cold source movement rate and viscosity of the conductive material slurry are matched.

[0022] Furthermore, the preparation method includes a molded method and a moldless method, wherein:

[0023] In the mold method, a mold is set on a cold source. The mold is composed of one or more materials with different thermal conductivity to control the local freezing orientation and rate. A conductive material slurry is extruded into the mold on the cold source through an extruder.

[0024] In the moldless method, the cryogenic printing process does not require a mold. The contact interface between the conductive material slurry and the cold source is a line or a surface, forming a certain geometric shape before the conductive material slurry spreads out completely radially.

[0025] Furthermore, the cold source can be a point / surface or a combination of both in space. By adjusting the position and cooling rate of the cold source, the growth mode of the ice crystals can be controlled. The radial growth of the ice crystals causes the ice crystals to displace the conductive path along the first direction to form a uniform and continuous conductive path.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0027] This invention provides a multi-directional flexible sensor with thickness gradient, spacing gradient, and / or orientation gradient, enabling the sensor to achieve anisotropic response at the microstructure level. Multi-directional sensing capability is obtained by analyzing the amplitude and waveform of the changes, simplifying the sensor's design and manufacturing process. It can effectively detect various mechanical stimuli such as compression, tension, and bending, and identify different deformation directions under different stimuli.

[0028] The preparation method provided by this invention offers high flexibility and adaptability, allowing adjustment of the conductive pathway and the properties of the flexible matrix material to meet diverse application requirements. By configuring particles of different sizes, selecting molds, controlling the cryogenic source, and employing layered design, sensors tailored to specific needs can be created. This variability and adaptability enable the invention to be widely applied in fields such as smart wearables, health monitoring, and human-computer interaction. Attached Figure Description

[0029] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are intended to explain the invention, but do not constitute an undue limitation thereof. In the drawings:

[0030] Figure 1 is a schematic diagram of the exploded structure of a multi-directional flexible sensor provided in an embodiment of the present invention;

[0031] Figure 2 is a schematic diagram of the thickness and spacing of a multi-directional flexible sensor provided in an embodiment of the present invention;

[0032] Figure 3 is a schematic flowchart of the fabrication method of the multi-directional flexible sensor provided in Embodiment 2 of the present invention;

[0033] Figure 4a shows the resistance change curves of the orientation gradient multi-directional flexible sensor prepared in Example 3 of the present invention under 20% compressive strain and 20% tensile strain.

[0034] Figure 4b shows the resistance change curves of the orientation gradient multi-directional flexible sensor prepared in Embodiment 3 of the present invention under torsional strain of 45° to the left / right.

[0035] Figure 4c shows the resistance change curves of the orientation gradient multi-directional flexible sensor prepared in Embodiment 3 of the present invention under upward / downward 45° bending strain.

[0036] Figure 4d shows the resistance change curve of the orientation gradient multi-directional flexible sensor prepared in Embodiment 3 of the present invention under 45° bending strain to the left / right.

[0037] Figure 5 is a schematic diagram of the multi-directional flexible sensor with particle size orientation gradient provided in Embodiment 4 of the present invention;

[0038] Figure 6 is a schematic diagram of the layered orientation gradient multi-directional flexible sensor provided in Embodiment 5 of the present invention;

[0039] Figure 7 is a schematic diagram of the concentration gradient multidirectional flexible sensor provided in an embodiment of the present invention.

[0040] In the figure: 11, conductive path; 12, conductive path formed by the deposition of particles of different sizes; 21, flexible substrate; 31, first layer material; 32, second layer material; 41, high thermal conductivity mold; 42, low thermal conductivity mold; 43, cold source. Embodiments of the present invention

[0041] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] In the description of the embodiments of the present invention, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. "And / or" in the text is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, in the description of the embodiments of the present invention, "multiple" means two or more.

[0043] In some processes described in the embodiments of the present invention, multiple operations or steps are included in a specific order. However, it should be understood that these operations or steps may not be executed in the order they appear in the embodiments of the present invention, or may be executed in parallel. The sequence number of the operation is only used to distinguish different operations, and the sequence number itself does not represent any execution order. In addition, these processes may include more or fewer operations, and these operations or steps may be executed sequentially or in parallel, and these operations or steps may be combined.

[0044] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0045] Example 1

[0046] Example 1 provides a multi-directional flexible sensor. As shown in Figure 1, it is an exploded structural diagram of a multi-directional flexible sensor provided in this embodiment of the invention. The multi-directional flexible sensor includes:

[0047] A plurality of conductive paths 11 are continuously extended in a first direction. The diameter of the conductive paths 11 gradually increases along a second direction. The second direction is inclined at a certain angle to the first direction. The plurality of conductive paths 11 are radially distributed and there are certain gaps between adjacent conductive paths 11 and / or inside the conductive paths 11. The gaps gradually increase along the diameter of the second direction.

[0048] A flexible substrate 21 is filled in the gap and bonded to the conductive path 11 to form a sensor with an orientation gradient, which has anisotropic response to external mechanical stimuli.

[0049] This embodiment provides a multi-directional flexible sensor. The conductive path 11 has a cross-sectional orientation gradient along a first direction. The flexible substrate 21 is distributed in the gaps of the conductive path 11 to improve flexibility and stress response capability. When the multi-directional flexible sensor deforms, the distance between adjacent conductive paths 11 or the distance of conductive material inside a single conductive path 11 changes, resulting in a corresponding change in resistance.

[0050] Specifically, when a certain part of the multi-directional flexible sensor is subjected to tensile strain, the flexible substrate corresponding to that part causes the conductive materials on the conductive path 11 to move away from each other, the original conductive path 11 is broken, and the overall resistance increases; conversely, when a certain part of the multi-directional flexible sensor is subjected to compressive stress, the flexible substrate corresponding to that part causes the conductive materials on the conductive path 11 to move closer to each other, constructing more conductive paths 11, and the overall resistance decreases.

[0051] As shown in Figure 2, the conductive paths 11 inside the multi-directional flexible sensor are radially distributed, with thickness gradient and spacing gradient along the second direction. Thickness refers to the width of a single conductive path 11 in the cross-section, spacing refers to the distance between adjacent conductive paths 11 in the cross-section, and orientation gradient refers to the gradient of the former two due to changes in their positions.

[0052] In this embodiment, the multi-directional flexible sensor is typically generated by cryogenic printing. Therefore, the first direction is the printing direction, which gives the multi-directional flexible sensor a continuous microscopic conductive path 11 in the longitudinal section. The second direction is the direction in which the conductive path 11 grows. The conductive path 11 combined with the flexible substrate gives the multi-directional flexible sensor an orientation gradient in the cross section.

[0053] In this embodiment, the conductive path 11 is any one or a combination of several of the following structures: sheet-like, columnar, and porous.

[0054] Preferably, the sheet-like and columnar structures can be obtained by adjusting the concentration of the printing slurry. When the concentration is below 1%, the material exhibits columnar micro-conductive pathways after the freezing process. When the concentration is in the range of 1-20%, the material exhibits sheet-like micro-conductive pathways after the freezing process. For printing slurries that exhibit high viscosity after dissolution, a larger extrusion device is required when the concentration is too high.

[0055] In this embodiment, the conductive path 11 includes a conductive material, which includes any one or a combination of carbon materials, conductive polymers, ionic liquids, metals, and metal ions.

[0056] In this embodiment, the flexible substrate 21 includes any one or a combination of several of hydrogels, aerogels, oleogels, rubber, and plastics. By setting a flexible base, it can combine with the conductive pathway 11 through microscopic interactions, enhancing mechanical properties and stability. These filling materials not only enhance the mechanical flexibility of the sensor but also improve its adaptability to complex shaped surfaces, especially when applied to human skin or other flexible electronic devices, providing good fit and comfort.

[0057] In this embodiment, the flexible substrate 21 can combine with the conductive pathway 11 through microscopic interactions, further enhancing its mechanical properties and stability. The combination can be any one or a combination of covalent bonds, ionic bonds, metallic bonds, hydrogen bonds, and van der Waals forces.

[0058] In this embodiment, the orientation gradient is any one or a combination of spacing, size, and material gradient.

[0059] In this embodiment, the size gradient includes any one or a combination of pore size and particle size. Within the size gradient, conductive materials can be assembled from particles of different sizes. After filling a flexible substrate, if the particles are etched away, the original particle locations become pores, and the particle size is converted into pore size. Strategies for generating porous structures are not limited to etching; they can also include foaming, 3D printing, and template methods.

[0060] Preferably, the pore size gradient can be obtained by pre-settling and etching or foaming. The pre-settling time is controlled between 5 seconds and 1 hour.

[0061] In this embodiment, different pore sizes can be obtained by using different pore size gradient preparation strategies. The pore size generated by the template method is 10-200 μm, the pore size generated by the foaming method is 10-2000 μm, and the macroscopic porous structure with a pore size of 0.5-10 mm can be generated by the 3D printing method.

[0062] In this embodiment, the material gradient includes any one or a combination of concentration gradient and crosslinking density gradient. Specifically, the orientation gradient can be achieved through a layered design of different materials, by printing solutions of different monomer or crosslinking agent concentrations layer by layer. The layer height of the layered printing is controlled between 200-2000 μm. A printing interval can be set between two adjacent layers to ensure that the lower layer is fully pre-cooled, allowing for better interface fusion between the upper and lower layers, thereby improving the mechanical and electrical performance of the sensor. The printing interval time is 1 second to 5 minutes, with the printing interval becoming longer as the number of layers increases.

[0063] Preferably, when fabricating a sensor with a concentration gradient, the monomer concentration can be gradually increased from 10% in the bottom layer to 30% in the top layer, with each layer being 500 μm high. When the number of printing layers is 2, the printing interval can be set to 3 seconds; when the number of printing layers is 5, the printing interval can be set to 1 minute; and when the number of printing layers increases to 10, the printing interval is extended to 3 minutes.

[0064] The degree of crosslinking can be controlled by manipulating the gradient distribution of light, temperature, or crosslinking agent, thereby forming an oriented structure with gradient characteristics. The light intensity field can be achieved using a parallel light source, causing the light to strike at a specific angle, with higher light intensity closer to the light source leading to a higher crosslinking density. The temperature field can be set by placing cold and heat sources. Cold sources can be selected from semiconductor cooling devices or cryogenic liquids such as liquid carbon dioxide, liquid nitrogen, or ice water contact; heat sources can be selected from electric heating plates or lasers.

[0065] Preferably, in the crosslinking step, a hot plate is used for heating, and the temperature is gradually reduced from 100°C at the bottom layer to 50°C at the top layer to form a temperature gradient, which in turn affects the crosslinking density.

[0066] The layered design allows for adjustment of the physical and chemical properties of each layer according to actual needs. By adjusting the conductivity, flexibility, or mechanical strength of different layers, the performance of the sensor can be optimized. Furthermore, the gradient cross-linking method can precisely control the structure of the conductive pathway 11, enabling the sensor to produce different responses when subjected to forces in multiple directions.

[0067] In this embodiment, the concentration gradient is generated by layer-by-layer printing or gradient cross-linking of materials with different concentrations, as shown in Figure 7, which is a schematic diagram of the concentration gradient multi-directional flexible sensor provided by the present invention.

[0068] In this embodiment, several conductive paths are arranged axially symmetrically, with the conductive paths on both sides forming a certain angle with the conductive paths in the middle, and the angle between the outermost conductive paths and the conductive paths in the middle is larger. When there is an angle, bending in different directions can cause some parts of the conductive paths 11 to be close together while others are partially far apart. As shown in Figure 1, when the multi-directional flexible sensor bends downwards, the distance between the tops of the conductive paths 11 increases, leading to an increase in resistance at the top, while the distance between the bottoms of the conductive paths 11 decreases, leading to a decrease in resistance at the bottom. When the multi-directional flexible sensor bends upwards, the distance between the bottoms of the conductive paths 11 increases, leading to an increase in resistance at the bottom, while the distance between the tops of the conductive paths 11 decreases, leading to a decrease in resistance at the top. This enhances anisotropy, and the larger the angle between the outermost conductive paths and the conductive paths in the middle, the more pronounced the anisotropy becomes.

[0069] Example 2

[0070] Example 2 provides a method for fabricating a multidirectional flexible sensor, as shown in Figure 3, including the following steps:

[0071] Step S1: Prepare conductive material slurry and flexible matrix slurry in a certain proportion;

[0072] Step S2: Freeze printing: When the cold source 43 reaches the set temperature, the prepared conductive material slurry is filled into the extruder. The conductive material slurry is extruded into the cold source 43 through the extruder while the cold source 43 moves in the first direction. During the movement of the cold source 43, the conductive material slurry that first comes into contact with the cold source 43 will freeze rapidly into ice crystals and grow away from the cold source 43. During the growth of the ice crystals, the solute in the conductive material slurry will be squeezed to both sides of the ice crystals to form conductive pathways 11 that grow along the second direction. The diameter of the ice crystals and conductive pathways 11 gradually increases in the direction away from the cold source 43.

[0073] Step S3: Remove ice crystals formed during the freezing process;

[0074] Step S4: Fill the original ice crystal location with flexible matrix slurry, and bond the flexible matrix 21 with the conductive path 11 under certain temperature conditions. Step S4 introduces the flexible matrix 21 material between adjacent conductive paths 11 and / or inside the conductive path 11 to replace the original ice crystal location, thereby improving the flexibility and deformability of the material.

[0075] In this embodiment, in step S1, a conductive material slurry and a flexible matrix slurry are prepared in a certain proportion. The viscosity range of the conductive material slurry and the flexible matrix slurry is 500 cP-5000 cP. When the viscosity is low, the conductive material slurry will aggregate into droplets after extrusion, rather than forming a uniform straight line. When the viscosity is high, since this method is based on an extruder, slurries with excessively high viscosity will be difficult to extrude due to insufficient extruder power. This embodiment uses mechanical stirring, ultrasound, centrifugation, etc., to uniformly disperse the solutes in the conductive material slurry and the flexible matrix slurry. The viscosity of the conductive material slurry and the flexible matrix slurry should be adapted to the power of the extrusion equipment.

[0076] In this embodiment, in step S1, the conductive path 11 is prepared from a solution of particles of different sizes. Before printing, a pre-sedimentation method can be used to obtain the particle size gradient, with a pre-sedimentation time of 5 seconds to 1 hour. By designing the particle size gradient, the resistance characteristics of the conductive path 11 in different regions can be controlled, which helps to improve the sensitivity and multi-directional response capability of the sensor.

[0077] In this embodiment, the orientation gradient is generated because when the temperature field is conducted from the bottom as a point or surface, the radial temperature gradient will form a corresponding (i.e., radial) ice crystal growth direction on the circular outline of the cross-section, thus creating a conductive path with an orientation gradient on the cross-section. Due to the axial symmetry of the temperature field, the conductive gradient in the cross-sectional direction is also symmetrical along the axis of symmetry, and there is a difference in the orientation of the sheet structure in the conductive path on both sides.

[0078] In this embodiment, in step S2, the set temperature of the cold source 43 is -100℃ to 0℃ to ensure that the conductive material slurry in contact with the cold source 43 will quickly freeze into ice crystals and grow away from the cold source 43. The flexible substrate 21 is located within the gaps between adjacent conductive pathways 11, and these gaps are formed by ice as a template. Furthermore, when the conductive pathways 11 are porous materials, they can also fill the gaps in the conductive material. For example, in an orientation gradient of pore sizes, the flexible substrate 21 can fill the micropores.

[0079] In step S2 of this embodiment, when the extruder extrudes the conductive material slurry to the cold source 43, extrusion-type cryogenic printing is used, and the freezing rate, the moving speed of the cold source 43 and the viscosity of the conductive material slurry are matched.

[0080] In step S3 of this embodiment, ice crystals generated during the freezing process are removed using methods such as freeze-drying, ice-melting complexation, supercritical drying, and room temperature drying. The original ice crystals are replaced by air or liquid, preserving the microstructure and orientation of the sample.

[0081] In step S4 of this embodiment, the flexible substrate 21 is combined with the conductive path 11 by means of soaking, negative pressure immersion, etc. Negative pressure immersion is suitable for high viscosity fillers, and soaking method is suitable for low viscosity materials. Under certain temperature conditions, the flexible substrate 21 is combined with the conductive path 11 to improve the flexibility and environmental adaptability of the sensor.

[0082] In this embodiment, step S2 includes a molded method and a moldless method to form different sensors, wherein:

[0083] In the mold method, a mold is set on the cold source 43. The mold is composed of one or more materials with different thermal conductivity to control the orientation and rate of local freezing. A conductive material slurry is extruded into the mold on the cold source 43 through an extruder.

[0084] The moldless method eliminates the need for molds during the cryogenic printing process. The interface between the conductive material slurry and the cold source 43 is a line or a surface, forming a certain geometric shape before the conductive material slurry spreads out completely radially.

[0085] Among them, the microstructure generated by the mold method has orientation gradient differences, and the number of different orientations is greater than or equal to 1.

[0086] In this embodiment, the mold method can use dual printing nozzles to print layer by layer, and the generated structure has 4 different micro-orientation gradient units, and stress from 4 different directions in the plane can be detected.

[0087] In this embodiment, the mold is composed of one or more materials with different thermal conductivity, and the mold shape is any one or a combination of rectangles, stars, triangles, and circles. By adjusting the thermal conductivity and shape of the mold, the local freezing orientation and rate can be adjusted, thereby precisely controlling the growth mode of ice crystals and thus achieving the regulation of the microstructure.

[0088] In this embodiment, the difference in thermal conductivity is obtained by any one or a combination of several of the following: material surface structure, thickness, density, thermal conductivity, roughness, and wettability.

[0089] In this embodiment, the cold source 43 during the freezing process can be a point / surface or a combination of both in space. By adjusting the position and cooling rate of the cold source 43, the growth pattern of the ice crystals can be controlled. The radial growth of the ice crystals helps to displace material along the printing direction, forming a uniform and continuous conductive path 11. In this specific embodiment, the cold source 43 is a cold stage.

[0090] In this embodiment, the cryogenic printing process does not use a mold. The interface between the printing paste and the cold source 43 is a line or a surface, forming a certain geometric shape before it is fully spread out.

[0091] In this embodiment, both the molded and moldless methods are suitable for spacing, size, material gradient design, and preparation methods.

[0092] The following specific embodiments illustrate in detail the multidirectional flexible sensor and its fabrication method provided by the present invention.

[0093] Example 3

[0094] Example 3 discloses a method for fabricating a multidirectional flexible sensor, specifically including:

[0095] Step S1: Prepare the conductive material slurry. Prepare a 30 mg / mL sodium carboxymethyl cellulose solution. After the solution is fully dissolved, add multi-walled carbon nanotubes (10-20 μm in length and 5-10 nm in diameter). The weight ratio of multi-walled carbon nanotubes to sodium carboxymethyl cellulose is 1:3. Stir the conductive material slurry thoroughly for 24 hours and set aside. Prepare the flexible matrix slurry. Prepare 20 g of water and dimethyl sulfoxide as solvents in a solvent ratio of 2:3. Then add 2 g of polyvinyl alcohol to the solvent and stir at 90°C for 2 hours until fully dissolved to form a hydrogel solution.

[0096] Step S2: Extrusion and freezing of conductive material slurry. Set the extrusion rate of the extruder to 2 mL / min, the moving speed of the cold source 43 to 9 m / min, and the temperature of the cold source 43 to -70℃.

[0097] Step S3: Ice crystal removal. The ice melting and complexation method can preserve the internal microstructure in a short time. The frozen sample is placed in a 2wt% copper nitrate ethanol solution at -20℃ for 2 hours.

[0098] Step S4: Immerse the thawed and complexed sample in the hydrogel solution prepared in step S1 for 2 hours, and then place it at -20°C to complete one freeze-thaw cycle.

[0099] During cryogenic printing, the conductive material slurry that first contacts the cold source 43 is rapidly frozen, forming a surface-contact cold source 43 that freezes upwards, creating a radial microscopic orientation gradient. Closer to the bottom of the cold source 43, due to the lower temperature, ice crystals grow rapidly with smaller diameters, resulting in a more pronounced radial orientation. At the top of the cold source 43, due to the lower thermal conductivity of the conductive material slurry, ice crystals grow more slowly and form larger diameters. Simultaneously, the ice crystals tend to be more parallel, altering the orientation and creating an orientation gradient from bottom to top. After the flexible substrate 21 replaces the original ice crystal positions, different ice crystal diameters generate different conductive pathways 11 with varying gaps, exhibiting a gradient composed of thickness, spacing, and orientation.

[0100] As shown in Figures 4(a)-4(d), the multi-directional flexible sensor provided in Embodiment 1 of this invention can effectively identify various deformation modes, including stretching, compression, torsion, and bending, as well as bending deformations in different directions. Stretching and compression correspond to different resistance change trends; compared to compression, stretching can produce greater deformation and corresponding resistance changes. When the sensor twists to the left or right, similar to stretching, tensile stress is applied to the internal conductive path 11, causing the overall resistance to increase. Besides distinguishing it from stretching by the change in amplitude, stretching and torsion correspond to different microscopic conductive change mechanisms, and therefore can be further distinguished by their changing waveforms. When the sensor is subjected to bending strain, due to its internal orientation gradient, it responds to different trends corresponding to the same deformation mode. When bending upwards, the sensor resistance decreases, while when bending downwards, the overall resistance increases. When bending left or right, due to the axial symmetry of the cold source 43, it responds to the same resistance change trend, which can be distinguished from bending upwards or downwards by the change in amplitude and waveform.

[0101] Example 4

[0102] Example 4 discloses a method for fabricating a multidirectional flexible sensor, specifically including:

[0103] Step S1: Prepare the conductive material slurry. Prepare a 10 mg / mL sodium carboxymethyl cellulose solution. The solvent is polyethylene glycol (PEG200) and aqueous solution, with a mass ratio of 4:1. After the solution is fully dissolved, add gold nanoparticles and stir thoroughly for 24 hours until the particles are completely dispersed. Prepare the flexible substrate 21. Use polydimethylsiloxane (PDMS) as the flexible substrate 21. The ratio of solution A to solution B is 10:1. The curing condition is heating at 90°C for 1 hour.

[0104] Figure 5 is a schematic diagram of the multi-directional flexible sensor with particle size orientation gradient provided in Embodiment 4 of the present invention. In the sensor, the conductive path 12 formed by the deposition of particles of different sizes naturally settles to form a gradient structure. The particle size gradually decreases from bottom to top. Larger particles can provide high conductivity, while smaller particles can provide higher sensitivity, which helps to enhance the anisotropic response capability of the sensor.

[0105] Step S2: Extrusion and freezing of conductive material slurry. Before printing, the particle size gradient of the particles is obtained by pre-settling for 30 seconds. The extrusion rate of the extruder is set to 1 mL / min, the moving speed of the cold source 43 is set to 15 m / min, and the temperature of the cold source 43 is set to -50℃. The flexible matrix 21 (21) is uniformly dispersed in the gaps between the particles and the layers, which improves the mechanical properties and ensures the stability of the structure under cyclic stress.

[0106] Step S3: Removal of ice crystals. Using the freeze-drying method, the frozen sample is placed in a freeze dryer and dried at -40°C and 30 mtorr for 48 hours to obtain an aerogel.

[0107] Step S4: Use a low-temperature negative pressure immersion method to allow PDMS to enter the aerogel gaps, leave it overnight, and then cure it to obtain the final sensor. The curing conditions are 60°C for 2 hours.

[0108] Example 5

[0109] Example 5 discloses a method for fabricating a multidirectional flexible sensor, specifically including:

[0110] Step S1: Prepare the conductive material slurry. Prepare a 20 mg / mL polyvinyl alcohol solution. After the solution is fully dissolved, add graphene oxide powder so that the ratio of graphene oxide to polyvinyl alcohol is 10:1 or 5:1. Prepare the flexible substrate 21. Use polyacrylamide / NN-methylenebisacrylamide hydrogel (AM / MBAA) as the flexible substrate slurry. The photoinitiator is PI-1733, with concentrations of 15% (w / v) AM, 3‰ (w / v) MABAA, and 4% (v / v) PI-1173. The curing condition is UV irradiation at room temperature for 3 minutes.

[0111] Step S2: Extrusion and freezing of conductive material slurry. Set the extrusion rate of the extruder to 2 mL / min, the platform movement speed of the cold source 43 to 9 m / min, and the temperature of the cold source 43 to -70℃.

[0112] Figure 6 is a schematic diagram of the layered orientation gradient multidirectional flexible sensor provided in Embodiment 5 of the present invention. The first layer material 31 and the second layer material 32 are generated through multiple printing processes, with a cold source 43 providing the cold source 43, forming an orientation gradient structure within the cross-section. This layered design allows for precise adjustment of the physical properties of the conductive path 11. A PDMS wedge mold (high thermal conductivity mold 42) is placed at the bottom to generate an orientation gradient structure within the material. The copper molds (high thermal conductivity molds 41) on both sides can generate a micro-orientation in a vertical direction different from the bottom orientation at the top of the material, further enhancing the anisotropy of the sensor.

[0113] Step S3: Removal of ice crystals. Using the freeze-drying method, the frozen sample is placed in a freeze dryer and dried at -40°C and 30 mtorr for 48 hours to obtain an aerogel.

[0114] Step S4: Use a low-temperature negative pressure immersion method to allow the AM / MBAA hydrogel to enter the aerogel gap, leave it overnight and then cure to obtain the final sensor. The curing conditions are 60℃ for 2 hours.

[0115] This embodiment achieves customized sensing performance in each region by printing and freezing conductive material slurries with different concentrations of conductive material in different regions and layers. Molds composed of different thermal conductivityes can control the growth orientation of ice crystals during freezing, thereby controlling the orientation of conductive pathways and the final anisotropic sensing capability. In this embodiment 3, the wedge-shaped, low-thermal-conductivity mold at the bottom generates layered conductive pathways perpendicular to the freezing platform, while the paired, high-thermal-conductivity molds at the top generate porous conductive pathways, further enhancing the difference between the upper and lower material layers. Simultaneously, the anisotropy of mechanical properties at both the bottom and top allows the material to withstand stresses in different directions, improving the overall mechanical stability of the sensor.

[0116] In summary, this invention provides a multi-directional flexible sensor and its fabrication method. A flexible sensor with a micro-orientation gradient structure is fabricated using a simple process, exhibiting excellent multi-directional sensing capabilities. The micro-orientation gradient structure consists of micro-conductive pathways 11. These pathways exhibit an orientation gradient in cross-section and are continuous in longitudinal section, allowing them to respond to mechanical stimuli and generate different resistance changes in different directions. Simultaneously, a flexible substrate 21 fills the gaps between the conductive pathways 11, further enhancing the material's flexibility and environmental adaptability. The fabrication method includes generating the oriented conductive pathways 11 through a freezing process, further removing ice crystals during freezing, and replacing the gaps between ice crystals with the flexible substrate 21. This sensor achieves multi-directional sensing functionality through a single circuit, effectively detecting various mechanical stimuli such as compression, tension, and bending, and can be widely applied in fields such as smart wearables, health monitoring, and human-computer interaction.

[0117] It should be noted that the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0118] In the description of this invention, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0119] It should be noted that, in this invention, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0120] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

Claims

1. A multi-directional flexible sensor, characterized in that, include: A plurality of conductive paths are provided, the conductive paths extending continuously in a first direction, the diameter of the conductive paths gradually increasing along a second direction, the second direction being inclined at a certain angle to the first direction, the plurality of conductive paths being radially distributed and having a certain gap between adjacent conductive paths, the gap being gradually increasing along the diameter of the second direction. A flexible substrate is filled in the gap, and the flexible substrate is bonded to the conductive path to form a sensor with an orientation gradient.

2. The multi-directional flexible sensor according to claim 1, characterized in that: The conductive pathway is any one or a combination of several of the following: sheet-like, columnar, and porous structures. The conductive pathway includes a conductive material, which includes any one or a combination of carbon materials, conductive polymers, ionic liquids, metals, and metal ions. The flexible matrix includes any one or a combination of several of the following: hydrogels, aerogels, oleogels, rubber, and plastics.

3. The multi-directional flexible sensor according to claim 1, characterized in that: The orientation gradient is any one or a combination of spacing, size, and material gradient.

4. The multidirectional flexible sensor according to claim 1, characterized in that: The size gradient includes any one or a combination of pore size and particle size.

5. The multidirectional flexible sensor according to claim 1, characterized in that: The material gradient includes any one or a combination of two of the following: concentration gradient and crosslinking density gradient. The concentration gradient is generated through layer-by-layer printing or gradient cross-linking processes of materials with different concentrations.

6. The multi-directional flexible sensor according to claim 1, characterized in that: The conductive paths on both sides have a certain angle with the conductive paths in the middle, and the angle between the outermost conductive paths and the conductive paths in the middle is larger.

7. A method for fabricating a multi-directional flexible sensor, characterized in that, Includes the following steps: The conductive material slurry and the flexible matrix slurry are prepared in a certain proportion; Freeze printing: The cold source is brought to a set temperature, and the prepared conductive material slurry is filled into the extruder. The conductive material slurry is extruded into the cold source through the extruder, and at the same time the cold source and the extruder move relative to each other in the first direction. During the relative movement of the cold source and the extruder, the conductive material slurry that first comes into contact with the cold source will freeze into ice crystals and grow upward. During the growth of the ice crystals, the solute in the conductive material slurry will be displaced on both sides of the ice crystals to form conductive paths that grow in the second direction. The diameter of the ice crystals and conductive paths increases in the direction away from the cold source. Remove ice crystals formed during the freezing process; A flexible substrate is filled into the original location of the ice crystals, and the flexible substrate is combined with the conductive path under certain temperature conditions.

8. The method for fabricating a multidirectional flexible sensor as described in claim 7, characterized in that, Extrusion-type cryogenic printing is used when the extruder extrudes the conductive material slurry into the cold source. The freezing rate and the cold source movement rate are matched with the viscosity of the conductive material slurry.

9. The method for fabricating a multidirectional flexible sensor as described in claim 7, characterized in that, The preparation method includes a molded method and a moldless method, wherein: In the mold method, a mold is set on a cold source. The mold is composed of one or more materials with different thermal conductivity to control the local freezing orientation and rate. A conductive material slurry is extruded into the mold on the cold source through an extruder. In the moldless method, the cryogenic printing process does not require a mold. The contact interface between the conductive material slurry and the cold source is a line or a surface, forming a certain geometric shape before the conductive material slurry spreads out completely radially.

10. The method for fabricating a multidirectional flexible sensor as described in claim 7, characterized in that: The cold source can be a point, a surface, or a combination of both in space. By adjusting the position and cooling rate of the cold source, the growth mode of the ice crystals can be controlled. The radial growth of the ice crystals causes the ice crystals to displace the conductive path along the first direction to form a uniform and continuous conductive path.

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