Multidirectional flexible sensor and preparation method thereof

Abstract

The present invention provides a multidirectional flexible sensor and a method for fabricating the same. The sensor comprises a plurality of conductive pathways extending continuously in a first direction, with the diameter of the conductive pathways gradually increasing along a second direction, which is inclined at a predetermined angle to the first direction. The conductive pathways are radially spaced apart with gaps between adjacent pathways, with the gaps gradually increasing in diameter along the second direction. The flexible substrate fills the gaps, and the flexible substrate and conductive pathways are bonded to form a sensor with an orientation gradient. The multidirectional flexible sensor provided by the present invention possesses a microscopic orientation gradient structure and exhibits excellent multidirectional sensing capabilities, responding to mechanical stimuli in different directions and generating different resistance changes. Furthermore, the flexible substrate material fills the gaps between the conductive pathways, further enhancing the material's flexibility and environmental adaptability.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible multi-directional sensors, and in particular to a multi-directional flexible sensor and a preparation method thereof. Background Art

[0002] With the rapid development of smart devices and wearable technology, flexible sensors, as an essential component of information acquisition, have gradually become a research hotspot for the next generation of sensor technology. Traditional rigid sensors, such as those made of metal or semiconductor materials, are unable to adapt to the complex morphology of flexible surfaces such as human skin due to their high stiffness, poor flexibility, and limited strain range. Therefore, flexible sensors, due to their advantages such as lightness, softness, small size, and good biocompatibility, are widely used in smart wearables, health monitoring, robotic control, and human-computer interaction.

[0003] However, in practical applications, mechanical stimuli (such as pressure, tension, and shear forces) often act simultaneously on sensors, and their mutual coupling complicates the sensor's response. Existing multi-directional sensors face challenges in identifying the direction and motion pattern of applied loads. This is primarily due to the isotropic nature of their structures and materials, which results in similar sensor responses under forces in different directions, making it difficult to effectively distinguish between multiple mechanical stimulation patterns.

[0004] To solve this problem, researchers have proposed multi-directional flexible sensors based on anisotropic design, which decouple multiple stimulation modes by designing special material structures and sensor arrays. However, these methods often involve complex preparation processes and tedious assembly processes, which limit their development in lightweight, functional and large-scale applications. Although integrated molding technologies such as 3D printing have been developed to prepare multi-directional sensors, the assembly of multiple circuits is still required. There is an urgent need for a new and simple preparation method that can realize the spatial distribution of mechanical stimulation and deformation pattern recognition through a single circuit, so as to promote the widespread application of flexible sensors in health monitoring, intelligent auxiliary equipment and other fields. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a multi-directional flexible sensor and a preparation method, which has a microscopic orientation gradient structure and excellent multi-directional sensing function. It has strong flexibility, adaptability and versatility and can be widely used in smart wearables, health monitoring, human-computer interaction and other fields.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

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

[0008] A plurality of conductive paths, wherein the conductive paths extend continuously in a first direction, the diameter of the conductive paths gradually increases along a second direction, the second direction is inclined at a certain angle to the first direction, the plurality of conductive paths are radially distributed with a certain gap between adjacent conductive paths, and the gap gradually increases in diameter along 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 path is any one or a combination of a lamellar, columnar, or porous structure, the conductive path includes a conductive material, the conductive material includes any one or a combination of carbon materials, conductive polymers, ionic liquids, metals, and metal ions, and the flexible matrix includes any one or a combination of hydrogels, aerogels, oil gels, rubbers, and plastics.

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

[0012] Furthermore, the size gradient includes either pore size or particle size or a combination of the two.

[0013] Furthermore, the material gradient includes any one of a concentration gradient and a cross-linking density gradient or a combination of the two, wherein

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

[0015] Furthermore, the conductive path at both sides has a certain angle with the conductive path at the middle portion, and the angle between the conductive path at the outer sides and the conductive path at the middle portion is larger.

[0016] A method for preparing a multi-directional flexible sensor comprises the following steps:

[0017] Prepare conductive material slurry and flexible matrix slurry in a certain proportion;

[0018] Freeze printing: The cold source reaches a set temperature, and the configured conductive material slurry is filled into the extruder. The conductive material slurry is extruded toward the cold source through the extruder while the cold source and the extruder are moved relative to each other in a first direction. During the relative movement of the cold source and the extruder, the conductive material slurry that first contacts the cold source will quickly freeze into ice crystals and grow upward. 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 a conductive path growing in a second direction. The diameter of the ice crystals and the conductive path increases as they move away from the cold source.

[0019] Remove ice crystals formed during freezing;

[0020] The flexible matrix is ​​filled into the position where the original ice crystals are located, and the flexible matrix is ​​combined with the conductive path under certain temperature conditions.

[0021] Furthermore, the extruder uses extrusion freezing printing when extruding the conductive material slurry toward the cold source, and the freezing rate and the cold source movement rate are matched with the viscosity of the conductive material slurry.

[0022] Furthermore, the preparation method includes a mold method and a mold-free method, wherein:

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

[0024] In the mold-free method, the freeze printing process does not require a mold, and 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 is completely radially dispersed.

[0025] Furthermore, the cold source is a point / surface or a combination of the two in space. By adjusting the position and cooling rate of the cold source to control the growth mode of the ice crystals, the radial growth of the ice crystals causes the ice crystals to squeeze 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] The present invention provides a multidirectional flexible sensor with thickness gradients, spacing gradients, and / or orientation gradients, enabling anisotropic responses at the microscopic level. Multidirectional sensing capability is achieved by analyzing the amplitude and waveform of variations, simplifying the sensor's design and manufacturing process. The sensor can effectively detect various mechanical stimuli, such as compression, tension, and bending, and identify the different deformation directions for different stimuli.

[0028] The preparation method provided by the present invention is highly flexible and adaptable, and the properties of the conductive path and flexible substrate material can be adjusted according to different application requirements. Through the configuration of particles with different particle sizes, mold selection, freezing source control and layered design, sensors that meet specific needs can be customized. This variability and adaptability make the present invention widely applicable to smart wearables, health monitoring, human-computer interaction and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings described herein are intended to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0030] Figure 1A schematic diagram of the decomposed structure of a multi-directional flexible sensor provided by an embodiment of the present invention;

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

[0032] Figure 3 A schematic flow chart of a method for preparing a multi-directional flexible sensor provided in Example 2 of the present invention;

[0033] FIG4( a ) is a resistance change curve 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] FIG4( b ) is a resistance change curve of the orientation gradient multi-directional flexible sensor prepared in Example 3 of the present invention under 45° left / right torsional strain;

[0035] FIG4( c ) is a resistance change curve of the orientation gradient multi-directional flexible sensor prepared in Example 3 of the present invention under 45° upward / downward bending strain;

[0036] FIG4( d ) is a resistance change curve of the orientation gradient multi-directional flexible sensor prepared in Example 3 of the present invention under a 45° left / right bending strain;

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

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

[0039] Figure 7 Schematic diagram of a concentration gradient multi-directional flexible sensor provided by an embodiment of the present invention.

[0040] In the figure: 11, conductive path; 12, conductive path formed by deposition of particles of different particle 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. DETAILED DESCRIPTION

[0041] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0042] In the description of the embodiments of the present invention, unless otherwise specified, “ / ” means or, for example, A / B can mean A or B; “and / or” in the text is merely a description of the association relationship of associated objects, indicating that three relationships may exist, for example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, in the description of the embodiments of the present invention, “multiple” refers to two or more than two.

[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 in which they appear in the embodiments of the present invention, or may be executed in parallel. The sequence numbers of the operations are only used to distinguish different operations and do not represent any order of execution. In addition, these processes may include more or fewer operations, and these operations or steps may be executed in sequence or in parallel, and these operations or steps may be combined.

[0044] To make the objectives, technical solutions and advantages of the present invention more clear, 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, such as Figure 1 FIG. 1 is a schematic diagram of the decomposition structure of a multi-directional flexible sensor provided by an embodiment of the present invention. The multi-directional flexible sensor includes:

[0047] A plurality of conductive paths 11, wherein the conductive paths 11 extend continuously 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 dispersed, and there is a certain gap between adjacent conductive paths 11 and / or within the conductive paths 11, and the spacing gradually increases along the second direction;

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

[0049] This embodiment provides a multi-directional flexible sensor, in which the conductive pathways 11 have a cross-sectional orientation gradient along a first direction. The flexible substrate 21 is distributed in the gaps between the conductive pathways 11 to improve flexibility and stress responsiveness. When the multi-directional flexible sensor is deformed, the distance between adjacent conductive pathways 11 or the distance between the conductive materials within a single conductive pathway 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 this part drives the conductive materials on the conductive path 11 away from each other, the original conductive path 11 is interrupted, 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 this part drives the conductive materials on the conductive path 11 closer to each other, constructing more conductive paths 11, and the overall resistance decreases.

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

[0052] In this embodiment, the multi-directional flexible sensor is usually generated by freeze printing, so the first direction is the printing direction, so that the multi-directional flexible sensor has a continuous microscopic conductive path 11 in the longitudinal section, and the second direction is the direction of growth of the conductive path 11. The conductive path 11 is combined with the flexible substrate to make the multi-directional flexible sensor have an orientation gradient in the cross section.

[0053] In this embodiment, the conductive path 11 is any one of a laminar, columnar, and porous structure or a combination of several of them.

[0054] Preferably, lamellar and columnar structures can be achieved by adjusting the printing slurry concentration. When the concentration is below 1%, the material exhibits columnar microscopic conductive pathways after freezing. When the concentration is within the range of 1-20%, the material exhibits lamellar microscopic conductive pathways after freezing. For printing slurries that exhibit high viscosity after dissolution, if the concentration is too high, a larger extrusion device is required.

[0055] In this embodiment, the conductive path 11 includes a conductive material, and the conductive material 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 comprises any one or a combination of hydrogel, aerogel, oleogel, rubber, or plastic. The flexible substrate allows for microscopic interactions with the conductive pathway 11, enhancing mechanical properties and stability. These filler materials not only enhance the sensor's mechanical flexibility but also improve its adaptability to complex surfaces, particularly when applied to human skin or other flexible electronic devices, providing excellent fit and comfort.

[0057] In this embodiment, the flexible substrate 21 can be bonded to the conductive path 11 through microscopic interactions to further enhance mechanical properties and stability. The bonding method is 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 of spacing, size, and material gradients, or a combination of several of them.

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

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

[0061] In this embodiment, different pore size gradient preparation strategies can be used to obtain pore sizes of different scales. The template method can generate pore sizes of 10-200 μm, the foaming method can generate pore sizes of 10-2000 μm, and the 3D printing method can generate macroscopic porous structures with pore sizes of 0.5-10 mm.

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

[0063] Preferably, when preparing 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 height being 500 μm. When printing two layers, the printing interval can be set to 3 seconds; when printing five layers, the printing interval can be set to 1 minute; and when printing ten layers, the printing interval is extended to 3 minutes.

[0064] By controlling the gradient distribution of light, temperature, or crosslinking agents, the degree of crosslinking can be controlled, thereby forming an oriented structure with gradient characteristics. The illumination intensity field can be achieved using a parallel light source, so that the light is irradiated at a specific angle. The higher light intensity near the light source leads to a higher crosslink density. The temperature field can be set by placing a cold source or a heat source. The cold source can be a semiconductor refrigeration plate or a cryogenic liquid such as liquid carbon dioxide, liquid nitrogen, or ice water contact. The heat source can be an electric heating plate or a laser.

[0065] Preferably, in the cross-linking step, an electric 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, thereby forming a temperature gradient and affecting the cross-linking density.

[0066] The layered design allows for the adjustment of the physical and chemical properties of each layer to meet specific needs, optimizing sensor performance by adjusting the conductivity, flexibility, or mechanical strength of each layer. Furthermore, the gradient cross-linking method allows for precise control of the structure of the conductive pathway 11, enabling the sensor to respond differently to forces applied 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, such as Figure 7 , which is a schematic diagram of the concentration gradient multi-directional flexible sensor provided by the present invention.

[0068] In this embodiment, the plurality of conductive paths are arranged axially symmetrically, with the conductive paths on both sides forming a certain angle with the conductive path in the middle, and the angle between the conductive paths on the outer sides and the conductive path 11 in the middle becomes larger. When there is an angle, when the conductive paths are bent in different directions, some parts of the conductive paths 11 may be close to each other and other parts may be partially separated. Figure 1 As shown, when the multi-directional flexible sensor bends downward, the distance between the tops of several conductive paths 11 increases to cause the resistance at the top to increase, and the distance between the bottoms of several conductive paths 11 decreases to cause the resistance at the bottom to decrease; when the multi-directional flexible sensor bends upward, the distance between the bottoms of several conductive paths 11 increases to cause the resistance at the bottom to increase, and the distance between the tops of several conductive paths 11 decreases to cause the resistance at the top to decrease, thereby enhancing the anisotropy, and the larger the angle between the outer wire path and the conductive path 11 in the middle part, the more obvious the anisotropy will be.

[0069] Example 2

[0070] Example 2 provides a method for preparing a multi-directional flexible sensor, such as Figure 3 As shown, the following steps are included:

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

[0072] Step S2: Freeze Printing: The cold source 43 is brought to a set temperature, and the prepared conductive material slurry is filled into the extruder. The conductive material slurry is extruded toward the cold source 43 through the extruder while the cold source 43 is simultaneously moved in the first direction. During the movement of the cold source 43, the conductive material slurry that first contacts the cold source 43 will quickly freeze 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 is squeezed to both sides of the ice crystals to form a conductive path 11 growing in the second direction. The diameter of the ice crystals and the conductive path 11 gradually increases as they move away from the cold source 43.

[0073] Step S3: removing ice crystals generated during the freezing process;

[0074] Step S4: Fill the locations of the original ice crystals with a flexible matrix slurry, and bond the flexible matrix 21 to the conductive pathways 11 under certain temperature conditions. Step S4 introduces the flexible matrix 21 material between adjacent conductive pathways 11 and / or within the conductive pathways 11, replacing the original ice crystals and thereby enhancing the material's flexibility and deformability.

[0075] In this embodiment, in step S1, a conductive material slurry and a flexible substrate slurry are prepared in a certain ratio. The viscosity of the conductive material slurry and the flexible substrate slurry ranges from 500 cP to 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 relies on an extruder, slurries with excessive viscosity will be difficult to extrude due to insufficient extruder power. This embodiment uses mechanical stirring, ultrasound, centrifugation, and other methods to uniformly disperse the solutes in the conductive material slurry and the flexible substrate slurry. The viscosity of the conductive material slurry and the flexible substrate slurry should be adapted to the power of the corresponding extrusion equipment.

[0076] In this embodiment, in step S1, the conductive path 11 is prepared from a solution containing particles of varying sizes. Before printing, a pre-sedimentation method can be used to create a particle size gradient, with the pre-sedimentation time ranging from 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, helping to improve the sensor's sensitivity and multi-directional responsiveness.

[0077] In this embodiment, the orientation gradient arises because, when the temperature field is a point or surface, the radial temperature gradient creates corresponding (i.e., radial) ice crystal growth directions along the circular contour of the cross section, forming a conductive path with an orientation gradient along the cross section. Due to the axisymmetry of the temperature field, the conductivity gradient along the cross section is also symmetrical along the axis of symmetry, resulting in differences in the lamellar orientation of the conductive paths on both sides.

[0078] In this embodiment, in step S2, the temperature of the cold source 43 is set to -100°C to 0°C to ensure that the conductive material slurry that contacts 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 two adjacent conductive pathways 11, which are gaps generated by the ice as a template. Furthermore, when the conductive pathways 11 are porous materials, they can also be filled in the gaps between the conductive material. For example, in an orientation gradient of pore size composition, the flexible substrate 21 can be filled in the micropores.

[0079] In step S2 of this embodiment, the extruder uses extrusion freezing printing when extruding the conductive material slurry toward the cold source 43, and the freezing rate and the moving speed of the cold source 43 match the viscosity of the conductive material slurry.

[0080] In step S3 of this embodiment, freeze drying, ice melting complexation, supercritical drying, room temperature drying, or other methods are used to remove ice crystals generated during the freezing process. 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 immersion, negative pressure immersion, etc., wherein negative pressure immersion is suitable for high-viscosity fillers, and immersion is suitable for low-viscosity materials. The flexible substrate 21 is combined with the conductive path 11 under certain temperature conditions to improve the flexibility and environmental adaptability of the sensor.

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

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

[0084] In the mold-free method, no mold is required in the cryo-printing process. The contact interface between the conductive material slurry and the cold source 43 is a line or a surface, and a certain geometric shape is formed before the conductive material slurry is completely radially dispersed.

[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. The generated structure has four different micro-orientation gradient units, and stresses from four different directions in the plane can be detected.

[0087] In this embodiment, the mold is composed of one or more materials with different thermal conductivities, and the mold shape can be any one of a rectangular, star-shaped, triangular, and circular shape, or a combination thereof. By adjusting the mold's thermal conductivity and shape, the local freezing orientation and rate can be adjusted, thereby precisely controlling the growth of ice crystals and, in turn, regulating the microstructure.

[0088] In this embodiment, the difference in thermal conductivity is obtained by any one or a combination of the 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, a surface, or a spatial combination of the two. By adjusting the position of the cold source 43 and the cooling rate, the growth pattern of the ice crystals can be controlled. The radial growth of the ice crystals helps to displace the material along the printing direction, forming a uniform, continuous conductive path 11. In the specific implementation of this embodiment, the cold source 43 is a cold stage.

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

[0091] In this embodiment, both the mold-based and mold-free methods are adapted to the spacing, size, material gradient design, and preparation method thereof.

[0092] Several specific embodiments are given below to illustrate in detail a multi-directional flexible sensor and a preparation method provided by the present invention.

[0093] Example 3

[0094] Example 3 discloses a method for preparing a multi-directional flexible sensor, which specifically includes:

[0095] Step S1: preparing a conductive material slurry, preparing a 30 mg / mL sodium carboxymethyl cellulose solution, adding multi-walled carbon nanotubes (length 10-20 μm, diameter 5-10 nm) after the solution is fully dissolved, the weight ratio of multi-walled carbon nanotubes to sodium carboxymethyl cellulose being 1:3, and stirring the conductive material slurry for 24 hours before use; preparing a flexible matrix slurry, using water and dimethyl sulfoxide as solvents at a solvent ratio of 2:3, preparing 20 g, then adding 2 g of polyvinyl alcohol to the solvent and stirring at 90°C for 2 hours until fully dissolved to form a hydrogel solution;

[0096] Step S2: extrusion and freezing of the conductive material slurry, the extrusion rate of the extruder is set to 2 mL / min, the moving speed of the cold source 43 is set to 9 m / min, and the temperature of the cold source 43 is set to -70°C;

[0097] Step S3: ice crystal removal, using the ice melting complexation method, which can preserve the internal microstructure in a short time, the frozen sample is placed in a 2 wt% copper nitrate ethanol solution at -20 °C for 2 hours;

[0098] Step S4: Soak the sample after ice melting and complexation in the hydrogel solution prepared in step S1 for 2 hours and then place it at -20°C to complete a freeze-thaw cycle.

[0099] During the cryoprinting process, the conductive material slurry that first contacts the cold source 43 is quickly frozen, forming a cold source 43 in surface contact and freezing upward, forming a radial microscopic orientation gradient. At the bottom closer to the cold source 43, due to the lower temperature, the ice crystals grow faster, the ice crystal diameter is smaller, and the faster growth rate also forms a more obvious radial orientation. At the top of the principle cold source 43, due to the lower thermal conductivity of the conductive material slurry, the ice crystals grow slower and form a larger diameter. At the same time, the ice crystals tend to be more parallel to each other, and the orientation changes, thus there is an orientation gradient from bottom to top. After the flexible substrate 21 replaces the original ice crystal position, different ice crystal diameters generate different conductive path 11 gaps, with a gradient composed of thickness, spacing, and orientation.

[0100] from Figure 4(a)-Figure 4(d) It can be seen that the multi-directional flexible sensor provided in Example 1 of the present invention can effectively identify multiple deformation modes such as stretching, compression, torsion, and bending, as well as bending deformations in different directions. Stretching and compression correspond to different resistance change trends. Compared with compression, stretching can produce greater deformation and corresponding resistance changes. When the sensor is twisted left or right, similar to stretching, tensile stress is applied to the internal conductive path 11, causing the overall resistance to increase. In addition to distinguishing it from stretching by the change in amplitude, stretching and torsion correspond to different microscopic conductive change mechanisms, and therefore can be assisted in distinguishing them by their change waveforms. When the sensor is subjected to bending strain, due to its internal orientation gradient, it responds to different trends of change corresponding to the same deformation mode. When bending upward, the sensor resistance decreases, while when bending downward, 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 upward or downward by the change in amplitude and waveform.

[0101] Example 4

[0102] Example 4 discloses a method for preparing a multi-directional flexible sensor, which specifically includes:

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

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

[0105] Step S2: Extrusion and freezing of the conductive material slurry. The particles were pre-sedimented for 30 seconds before printing to obtain a particle size gradient. The extruder rate was set to 1 mL / min, the movement rate of the cold source 43 was set to 15 m / min, and the temperature of the cold source 43 was set to -50°C. The flexible matrix 21 (21) was evenly dispersed in the gaps between the particles and the layers, improving the mechanical properties and ensuring the stability of the structure under cyclic stress.

[0106] Step S3: ice crystals were removed by freeze drying. The frozen sample was placed in a freeze dryer and dried at -40°C and 30 mtorr for 48 hours to obtain aerogel.

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

[0108] Example 5

[0109] Example 5 discloses a method for preparing a multi-directional flexible sensor, which specifically includes:

[0110] Step S1: Prepare a conductive material slurry, prepare a 20 mg / mL polyvinyl alcohol solution, add graphene oxide powder after the solution is fully dissolved, and make the ratio of graphene oxide to polyvinyl alcohol be 10:1 or 5:1 respectively; Prepare a flexible substrate 21, use polyacrylamide / NN-methylenebisacrylamide hydrogel (AM / MBAA) as the flexible substrate slurry, and the photoinitiator is PI-1733, and its concentration is 15% (w / v) AM, 3‰ (w / v) MBAA, 4% (v / v) PI-1173, respectively. The curing conditions are ultraviolet light irradiation at room temperature for 3 minutes.

[0111] Step S2: Extrusion and freezing of the conductive material slurry. The extrusion rate of the extruder is set to 2 mL / min, the platform moving rate of the cold source 43 is set to 9 m / min, and the temperature of the cold source 43 is set to -70°C.

[0112] Figure 6 Schematic diagram of a layered orientation gradient multi-directional flexible sensor provided in Example 5 of the present invention, wherein the first layered material 31 and the second layered material 32 are generated by printing in batches, and a cold source 43 is provided by a cold source 43 to form an orientation gradient structure in the cross section. This layered design can accurately adjust the physical properties of the conductive path 11. A PDMS wedge mold (high thermal conductivity mold 42) is set at the bottom to generate an orientation gradient structure inside the material. The copper mold molds (high thermal conductivity mold 41) on both sides can generate a microscopic 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: ice crystals were removed by freeze drying. The frozen sample was placed in a freeze dryer and dried at -40°C and 30 mtorr for 48 hours to obtain aerogel.

[0114] Step S4: Use a low-temperature negative pressure immersion method to allow the AM / MBAA hydrogel to enter the gaps between the aerogels, and solidify overnight to obtain the final sensor. The curing conditions are 60°C for 2 hours.

[0115] This embodiment can customize the sensing performance of each area by printing and freezing conductive material slurries with different conductive material concentrations in different regions. The molds composed of different thermal conductivities can regulate the growth orientation of ice crystals during the freezing process, and then regulate the orientation of the conductive channels and the final anisotropic sensing capability. In this embodiment 3, the wedge-shaped, low thermal conductivity mold at the bottom can generate a conductive path with a lamellar structure arranged perpendicular to the freezing platform, and the paired, high thermal conductivity molds at the top can generate a conductive path with a porous structure, thereby further enhancing the difference between the upper and lower layers of materials. At the same time, there are various anisotropies in mechanical properties at both the bottom and the top. This difference enables the material to withstand stress in different directions, thereby improving the overall mechanical stability of the sensor.

[0116] In summary, the present invention provides a multi-directional flexible sensor and a preparation method thereof. A flexible sensor with a microscopic orientation gradient structure is prepared using a simple preparation process, and has excellent multi-directional sensing function. The microscopic orientation gradient structure is composed of a microscopic conductive path 11. The conductive path 11 presents an orientation gradient in the cross section and has a continuous microscopic conductive path 11 in the longitudinal section. It can respond to mechanical stimulation in different directions and produce different resistance changes. At the same time, the flexible matrix 21 material is filled in the gaps between the conductive paths 11, further enhancing the flexibility and environmental adaptability of the material. The preparation method includes generating a conductive path 11 with an orientation gradient through a freezing process, further removing ice crystals in the freezing process, and replacing the ice crystal gaps with a flexible matrix 21 material. The sensor realizes multi-directional sensing function through a single circuit, can effectively detect various mechanical stimuli such as compression, stretching, and bending, and is widely used in smart wearables, health monitoring, human-computer interaction and other fields.

[0117] It should be noted that the serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0118] In the description of the present invention, it should be noted that the terms "upper" and "lower" and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on the present invention. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0119] It should be noted that, in the present invention, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0120] The foregoing description is intended only to provide specific embodiments of the present invention, which will enable those skilled in the art to understand and implement the present 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 present invention. Therefore, the present invention is not intended to be limited to the embodiments shown herein, but is to be construed in the widest possible manner consistent with the principles and novel features disclosed herein.

Claims

1. A multi-directional flexible sensor, characterized in that: include: A plurality of conductive paths, wherein the conductive paths extend continuously in a first direction, the diameter of the conductive paths gradually increases along a second direction, the second direction is inclined at a certain angle to the first direction, the plurality of conductive paths are radially distributed, and a certain gap is formed between adjacent conductive paths, and the gap gradually increases along the second direction; a flexible substrate filled in the gap, wherein the flexible substrate is bonded to the conductive path to form a sensor with an orientation gradient; The conductive path is any one of a columnar structure and a porous structure or a combination of the two. The conductive path comprises a conductive material, and the conductive material comprises any one of a carbon material, a conductive polymer, an ionic liquid, a metal, and a metal ion or a combination thereof. The flexible matrix comprises any one of a hydrogel, an aerogel, an oil gel, a rubber, and a plastic or a combination thereof. The orientation gradient is any one or a combination of spacing gradient, size gradient, and material gradient; the size gradient includes any one or a combination of pore size gradient and particle size gradient; the material gradient includes any one or a combination of concentration gradient and cross-linking density gradient, wherein the concentration gradient is generated by layer-by-layer printing or gradient cross-linking process of materials with different concentrations; the conductive paths on both sides have a certain angle with the conductive path in the middle part, and the angle between the conductive path on the outer side and the conductive path in the middle part is larger.

2. A method for preparing a multi-directional flexible sensor according to claim 1, characterized in that: The following steps are involved: Prepare conductive material slurry and flexible matrix slurry in a certain proportion; Freeze printing: The cold source reaches a set temperature, and the configured conductive material slurry is filled into the extruder. The conductive material slurry is extruded toward the cold source through the extruder while the cold source and the extruder are moved relative to each other in a first direction. During the relative movement of the cold source and the extruder, the conductive material slurry that first contacts the cold source will quickly freeze into ice crystals and grow upward. 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 a conductive path growing in a second direction. The diameter of the ice crystals and the conductive path increases as they move away from the cold source. Remove ice crystals formed during freezing; The flexible matrix is ​​filled into the position where the original ice crystals are located, and the flexible matrix is ​​combined with the conductive path under certain temperature conditions.

3. The method for preparing a multi-directional flexible sensor according to claim 2, wherein: Extrusion freezing printing is used when the extruder extrude the conductive material slurry toward the cold source, and the freezing rate and the cold source movement rate are matched with the viscosity of the conductive material slurry.

4. The method for preparing a multi-directional flexible sensor according to claim 2, wherein: The preparation method includes a mold method and a mold-free method, wherein: There is a mold method, which sets a mold on the cold source. The mold is composed of one or more materials with different thermal conductivity, which is used to control the orientation and rate of local freezing. The conductive material slurry is extruded into the mold on the cold source through an extruder, or In the mold-free method, the freeze printing process does not require a mold, and 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 is completely radially dispersed.

5. The method for preparing a multi-directional flexible sensor according to claim 2, wherein: The cold source is a point / surface or a combination of the two in space. The growth mode of ice crystals is controlled by adjusting the position and cooling rate of the cold source. The radial growth of ice crystals causes the ice crystals to squeeze the conductive path along the second direction to form a continuous conductive path.

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