Programmable negative Poisson's ratio superstructure high-sensitivity sensing flexible sensor and preparation method thereof

Abstract

The invention relates to a programmable negative Poisson's ratio superstructure high-sensitivity sensing flexible sensor and a preparation method, and the sensor comprises a flexible substrate layer which is provided with a star-shaped-concave hexagonal composite negative Poisson's ratio structure. The star-concave hexagonal composite negative poisson ratio structure is formed by compounding star units and concave hexagonal units through connecting rods, and the centers of the star units coincide with the centers of the concave hexagonal units to form a cooperative deformation system. The functional layer and the functional sensing layer are coated on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure of the flexible substrate layer; the electrodes are connected to the two ends of the functional layer; and the packaging layer coats the functional layer and the electrode. High sensitivity, wide working range and low hysteresis are realized; through the cooperative coupling and strain amplification effect of the star-shaped unit and the concave hexagonal unit, the structure can efficiently convert micro axial strain into obvious transverse deformation, thereby greatly improving the sensitivity of the sensor.

Description

Technical Field

[0001] This invention relates to the interdisciplinary field of intelligent structural mechanics, advanced materials and information technology, and in particular to a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor and its fabrication method. Background Technology

[0002] Flexible sensors typically use flexible substrate materials (such as PDMS, Ecoflex, PET, etc.) as their core. Among them, resistive (piezoresistive) sensors are one of the most widely studied types due to their simple structure, high sensitivity, and low cost. To improve performance, existing technologies often construct traditional microstructures such as micro pyramids and micro domes on the surface of flexible substrates to enhance sensitivity through stress concentration effects. In addition, negative Poisson's ratio structures with unique lateral expansion behavior are also being explored for strain sensing, showing high sensitivity application potential. In terms of fabrication processes, although high-precision technologies such as photolithography are used, more traditional spray coating processes or paper-based and fabric-based solutions are also commonly used in practice.

[0003] In terms of materials, the significant modulus difference between conductive functional layers such as carbon nanotubes and graphene and flexible substrates makes them prone to interfacial slippage or conductive network breakage during cyclic deformation, leading to increased sensor hysteresis and signal drift. While liquid metals such as gallium-indium alloys offer excellent flexibility, they possess potential micro-toxicity and complex encapsulation processes, limiting their application in scenarios with high biocompatibility requirements. At the structural design level, traditional structures such as micro-pyramids have limited strain-bearing capacity, restricting the sensor's detection range. Single negative Poisson's ratio structures generally suffer from poor deformation stability, high initial peak stress, and susceptibility to non-uniform buckling or hinge failure, resulting in strong nonlinear electrical signal response and difficulty in simultaneously achieving high sensitivity and stability over a wide strain range. Regarding fabrication processes, photolithography and other techniques are complex, costly, and limited in their applicable materials. Traditional processes such as spraying struggle to achieve controllable construction of three-dimensional irregular structures and uniform integration of functional materials, easily leading to poor product performance consistency. While paper-based or fabric-based sensors possess a degree of flexibility, the materials themselves have poor durability and insufficient environmental stability, making it difficult to meet the requirements for long-term reliable monitoring. These shortcomings collectively limit the application of flexible sensors in real-world scenarios that require high reliability, a wide operating range, and long-term stable operation. Summary of the Invention

[0004] This invention provides a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor to overcome the problems of poor deformation stability, high initial peak stress, and strong nonlinear electrical signal response of single negative Poisson's ratio structures. The sensor utilizes a programmable composite design of star-shaped units and concave hexagonal units to construct a core sensing structure that simultaneously achieves high sensitivity, low hysteresis, and excellent cyclic stability over a wide strain range. Through the synergistic tensile expansion effect and effective strain amplification of the composite structure, the sensor's deformation sensitivity is significantly improved. Furthermore, by adjusting geometric parameters such as the concave angle, unit cell thickness, and star arm length, the mechanical behavior and electromechanical response characteristics of the structure can be flexibly controlled to meet the specific requirements of deformation range and mechanical performance in different application scenarios. This invention also provides a method for fabricating the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor, aiming to solve the problems of complex and costly traditional photolithography processes and the difficulty in achieving uniform integration of three-dimensional structures through processes such as spraying. By employing integrated molding technologies such as sacrificial layer method, laser cutting, screen printing or rigid-flexible coupling 3D printing, the composite negative Poisson's ratio structure is completely integrated into the base layer, functional layer and encapsulation layer of the sensor, thereby effectively eliminating interlayer interface mismatch and improving the structural integrity and performance consistency of the device.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A programmable negative Poisson's ratio superstructure high-sensitivity flexible force-sensing sensor, comprising:

[0007] A flexible substrate layer having a star-shaped-concave hexagonal composite negative Poisson's ratio structure, wherein the star-shaped-concave hexagonal composite negative Poisson's ratio structure is composed of star-shaped units and concave hexagonal units through connecting rods, wherein the center of the star-shaped unit coincides with the center of the concave hexagonal unit, forming a cooperative deformation system;

[0008] Functional layer, the functional layer being coated on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure of the flexible substrate;

[0009] Electrodes, the electrodes being connected to both ends of the functional layer;

[0010] And an encapsulation layer that covers the functional layer and the electrode.

[0011] A further improvement of the present invention is that: the star-shaped unit is a four-cornered star, each corner including two branch arms; the concave hexagonal unit includes two parallel straight sides and four concave sides, each straight side is adjacent to two concave sides, and the two ends of each concave side are respectively adjacent to another concave side and a straight side; the included angle between the concave side and the adjacent straight side is the concave angle, and the connection point of two adjacent concave sides is the concave point; in the star-shaped unit, the junction between two adjacent corners is the concave part of the star-shaped unit; the two opposite concave parts of the star-shaped unit are respectively connected to the two concave points of the concave hexagonal unit through parallel rods, and the other two concave parts of the star-shaped unit are respectively connected to the two straight sides of the adjacent concave hexagonal unit through vertical rods.

[0012] A further improvement of the present invention is that:

[0013] The star-shaped-concave hexagonal composite negative Poisson's ratio structures on the flexible substrate are arranged in a close-packed array, and adjacent star-shaped-concave hexagonal composite negative Poisson's ratio structures share the straight or concave sides of the concave hexagonal unit.

[0014] Alternatively, they can be arranged in an equally spaced array, with adjacent columns of structural units connected by transverse connecting rods; in the same column, adjacent star-shaped-concave hexagonal composite negative Poisson's ratio structures share a straight edge.

[0015] A further improvement of the present invention lies in that, during the fabrication process, the deformation range and mechanical properties of the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor are adjusted by regulating the geometric parameters of the star-shaped-concave hexagonal composite negative Poisson's ratio structure; wherein, the geometric parameters include: the concave angle of the concave hexagonal unit. Length-to-height ratio (L / H) and unit cell thickness (t), branch arm length of star-shaped units and the angle between branches .

[0016] A further improvement of the present invention is that the material of the flexible substrate layer includes one or more combinations of Ecoflex, polydimethylsiloxane, polyurethane, and hydrogel; and the material of the functional layer includes one or more combinations of PEDOT:PSS, silver nanowires, carbon nanotubes, Mxene, or liquid metal.

[0017] This invention also provides a method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor. The method comprises: fabricating a flexible substrate layer with a star-shaped-concave hexagonal composite negative Poisson's ratio structure; the fabrication method of the flexible substrate layer includes: template method, sacrificial layer method, laser cutting, screen printing method, and rigid-flexible coupling multi-material printing method.

[0018] A further improvement of the present invention is that the fabrication method of the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using the template method or the sacrificial layer method includes the following steps:

[0019] A flexible substrate mold is prepared, which has a cavity that complements the star-shaped-concave hexagonal composite negative Poisson's ratio structure; for the template method, the material of the flexible substrate mold includes PLA, and for the sacrificial layer method, the material of the flexible substrate mold includes PVA.

[0020] The liquid flexible substrate material is poured into the cavity of the flexible substrate layer mold and then cured.

[0021] Demolding; for the template method, the flexible substrate mold is directly separated from the flexible substrate; for the sacrificial layer method, the flexible substrate mold together with the flexible substrate is placed in warm water to dissolve the flexible substrate mold.

[0022] The flexible substrate is fixed to a predetermined strain, and a functional layer is prepared on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure; after the functional layer is prepared, the flexible substrate is released.

[0023] The electrodes are connected to both ends of the functional layer, and the electrodes and the functional layer are encapsulated using an encapsulation layer.

[0024] A further improvement of the present invention is that the method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using laser cutting includes the following steps:

[0025] Flexible substrate and functional layers are fabricated through a stacked assembly method;

[0026] The composite integrated sensing unit is directly formed by synchronously cutting a pre-set star-shaped-concave hexagonal composite negative Poisson's ratio structure pattern using a laser cutting machine.

[0027] A further improvement of the present invention is that the fabrication method of the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using screen printing includes the following steps:

[0028] A screen printing plate is prepared, wherein the hollowed-out pattern of the screen printing plate is a star-shaped-concave hexagonal composite negative Poisson's ratio structure. In view of the isolated block characteristics presented by the complementary structure of the hollowed-out part, narrow weak connecting bridges are introduced between the isolated blocks to connect the isolated units with the main body, thereby ensuring the structural integration, precise forming and processability of the screen printing plate during the preparation and printing process.

[0029] The pattern of the composite negative Poisson's ratio structure is formed by screen printing on the flexible substrate.

[0030] After the printing step is completed, the uncovered areas caused by the auxiliary weak connection bridges are locally coated with functional material to ensure the continuity and integrity of the functional layer.

[0031] A further improvement of this invention lies in the following method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using a rigid-flexible coupling multi-material printing method:

[0032] Identify the stress concentration region in the star-shaped-concave hexagonal composite negative Poisson's ratio structure;

[0033] Rigid materials are used for printing in the stress concentration areas, while flexible composite materials containing conductive fillers are used for printing in other areas, directly forming a flexible substrate layer with rigid-flexible coupling. This flexible substrate layer and the functional layer are an integral structure.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) High sensitivity, wide operating range and low hysteresis are achieved simultaneously: Through the synergistic coupling and strain amplification effect of star-shaped units and concave hexagonal units, this structure can efficiently convert small axial strain into significant lateral deformation, thereby greatly improving the sensitivity of the sensor. Experiments have verified that under a certain pre-stretch strain, the resistance change rate (ΔR / R0) exceeds 10,000 and the sensitivity coefficient (GF) is as high as 30,000 or more. At the same time, the composite structure effectively avoids stress concentration and premature failure through multi-stage stable deformation, so that the working strain range of the sensor can reach 40%, and the hysteresis phenomenon caused by interlayer slippage is significantly reduced due to the integrated structural design.

[0036] (2) Programmable mechanical and sensing performance: By flexibly adjusting geometric parameters such as concave angle, unit cell thickness, star arm length and included angle, the deformation behavior, stiffness, fracture strength and electromechanical response curve of the sensor can be precisely programmed. This programmability enables the present invention to overcome the trade-off between traditional sensor performance and flexibly adapt to the demanding requirements of different application scenarios, from capturing tiny physiological signals of the human body to monitoring large deformations of robot joints.

[0037] (3) High structural stability and signal reliability: As a mechanical metamaterial, the composite negative Poisson's ratio structure provides stable and predictable mechanical behavior through its synergistic deformation mechanism, overcoming the shortcomings of single negative Poisson's ratio structures that are prone to non-uniform buckling or hinge failure. This provides a stable and reliable deformation environment for the functional layer, ensuring the consistency of changes in the conductive network, thereby outputting a stable and repeatable electrical signal, which greatly improves the cycle life and measurement reliability of the sensor.

[0038] (4) Flexible fabrication process and good structural integrity: The integrated fabrication process adopted, including sacrificial layer method, laser cutting, screen printing and rigid-flexible coupling 3D printing, can synchronously and accurately construct complex composite superstructures in the base layer, functional layer and encapsulation layer of the sensor. This integrated technology effectively eliminates the interlayer interface mismatch problem inherent in traditional lamination process, and ensures the structural integrity of the sensor under repeated deformation. It is the key guarantee for achieving low hysteresis, high consistency and long-term stable operation.

[0039] (5) Broad application prospects: This invention not only provides a new technical path for improving the performance of flexible sensors and expands the application field of negative Poisson's ratio metamaterials, but its high sensitivity, wide range and high compliance characteristics can also directly meet the urgent needs of cutting-edge fields such as smart health care, soft robot tactile perception, intelligent manufacturing and health monitoring of aerospace equipment for high-performance flexible sensing technology. It has significant practical application value and promotion potential. Attached Figure Description

[0040] Figure 1 It is a star-shaped-concave hexagonal composite negative Poisson's ratio structure;

[0041] Figure 2 Schematic diagram of a star-shaped-concave hexagonal composite negative Poisson's ratio unit cell structure: (a) planar view; (b) three-dimensional model view;

[0042] Figure 3 Schematic diagrams of the concave hexagonal expansion structural unit before and after deformation: (a) before stretching; (b) after stretching;

[0043] Figure 4 Schematic diagrams of the star-shaped negative Poisson's ratio structural unit before and after deformation: (a) before stretching; (b) and (c) after stretching;

[0044] Figure 5 A schematic diagram of the star-shaped-concave hexagonal composite expansion structural unit before and after deformation;

[0045] Figure 6 Stress-strain curves for concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures;

[0046] Figure 7 Young's modulus curves for concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures;

[0047] Figure 8 Fracture stress curves for concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures;

[0048] Figure 9 Force sensing sensitivity curves for concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures;

[0049] Figure 10 Molds for forming concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures;

[0050] Figure 11 This is a flowchart illustrating the fabrication process of a flexible sensor.

[0051] Figure 12 This is a schematic diagram of the flexible sensor structure. Detailed Implementation

[0052] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0053] Example 1: As Figure 1 , 2 As shown, an embodiment of the present invention provides a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor, which is based on a star-shaped-concave hexagonal composite negative Poisson's ratio structure, and its flexible substrate layer is fabricated using a sacrificial layer method. The fabrication process includes:

[0054] S1. Mold 3D Model Design: Design the mold cavity with a star-shaped and concave hexagonal composite negative Poisson's ratio structure, and model the mold using 3D modeling software. Specifically, the star-shaped unit has a branch arm length of 1 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1 mm.

[0055] S2. Mold Printing: An FDM molding 3D printer is used, and the printing material is water-soluble PVA. Considering that PVA is easy to absorb water, the PVA material is kept in the drying oven during the printing process. The nozzle printing temperature is set to 210℃, the printing bed temperature is set to 55℃, the nozzle diameter is 0.2mm, and the printing speed is 30mm / s. A mold with a star-shaped-concave hexagonal composite cavity structure is prepared according to the preset three-dimensional model.

[0056] S3. Preparation of flexible substrate: Prepare Ecoflex 00-30 silicone substrate (A glue: B glue = 1:1 (w / w)), stir evenly and then degas under vacuum for 3 minutes; slowly pour the degassed Ecoflex liquid into the cavity of the PVA mold and gently vibrate to remove air bubbles; let it cure at room temperature to obtain a flexible substrate with a star-shaped-concave hexagonal composite negative Poisson's ratio structure.

[0057] S4. Demolding: Immerse the PVA mold along with the flexible substrate composite in warm water, gently shake until the PVA is completely dissolved, rinse with deionized water, and dry at room temperature to complete the demolding of the flexible substrate.

[0058] S5. Functional layer preparation: The flexible substrate layer is fixed at a predetermined strain by stretching, and PEDOT:PSS conductive ink is coated layer by layer on the surface of the patterned flexible substrate layer one or more times. After drying at room temperature for 2 hours or in an oven at 60°C for 30 minutes, the strain is released to obtain a flexible substrate-functional layer composite integrated sensing unit with a star-shaped-concave hexagonal composite negative Poisson's ratio structure.

[0059] S6. Electrode Connection and Encapsulation: Use silver paste to bond 0.1mm diameter copper wires to both ends of the PEDOT:PSS film as electrodes, and place them in a 70℃ oven to dry for 10 minutes to cure. In situ drop-coating of Ecoflex mixture onto the covering film, and use a coating rod to control the thickness of the encapsulation layer to about 0.5mm. Curing at room temperature for 24 hours, the final flexible sensor product is obtained.

[0060] like Figure 1 and Figure 2 As shown, in step S2, the unit cell of the star-shaped-concave hexagonal composite negative Poisson's ratio structure includes: a star-shaped unit and concave hexagonal units surrounding the star-shaped unit. These two units are combined via connecting rods to form a cooperative deformation system. Specifically, the connecting rods include parallel rods and vertical rods. The star-shaped unit and the concave hexagonal unit are connected by two parallel rods at the concave portion of the star-shaped structure. and two vertical bars The structure is composed of interconnected components; the star-shaped unit branches radially from its center. Simultaneously, two transverse connecting rods extend from the concave corners of the concave hexagonal unit. ( Figure 2 a).

[0061] like Figure 2 (a) Figure 2 As shown in (b), in one specific embodiment, the star-shaped-concave hexagonal composite negative Poisson's ratio structure includes star-shaped units and concave hexagonal units; wherein, the concave hexagonal unit includes two parallel straight edges. , The two left endpoints of the two straight edges , Through two concave edges ( , Connection; the two right endpoints of the two straight edges , Also through two concave edges , The two straight edges and four concave edges are connected to form the concave hexagonal unit. In this embodiment, each straight edge is adjacent to two concave edges, and each concave edge has two ends adjacent to another concave edge and a straight edge, respectively. The connection point of two adjacent concave edges. , The concave point is the angle between the concave side and the adjacent straight side. .

[0062] The star-shaped unit is a four-pointed star, with each point including two star-shaped unit branch arms, and the connection point of two adjacent points serving as the concave part of the star structure. For example... Figure 1 and Figure 2 As shown, the concave angle of the concave hexagonal unit The angle is 60°, and the two straight edges are... , The side lengths of the concave hexagonal units are equal, and the ratio of the side length to the concave side is 2. The side length of the concave side is used as the side length of the concave hexagonal unit, and its value is 2mm. The star-shaped unit branch arms at each corner are of equal length, with a length of 1mm, and the included angle between the two star-shaped unit branch arms at each corner is... The angle is 30°. In the star-shaped-concave hexagonal composite negative Poisson's ratio structure, the concave hexagonal unit surrounds the star-shaped unit, and their centers coincide. The two concave points of the concave hexagonal unit ( , The two opposite star-shaped structures within the star-shaped unit are recessed. Located on the same straight line. The recessed part of the star-shaped structure adjacent to the straight edge in the star-shaped unit. Through vertical rods respectively With adjacent straight edges ( , Connect, the other two star-shaped structures are concave. Passing through parallel rods ( ) and adjacent concave points ( , Connection. In this embodiment, the parallel rod ( The line connecting the two concave points is on the same straight line.

[0063] like Figure 1 As shown, in some embodiments, the star-shaped-concave hexagonal composite negative Poisson's ratio structures on the flexible substrate are arranged in an array in a close-packed manner; in each column of star-shaped-concave hexagonal composite negative Poisson's ratio structures, two connected star-shaped-concave hexagonal composite negative Poisson's ratio structures share the straight edge; two adjacent columns of star-shaped-concave hexagonal composite negative Poisson's ratio structures are staggered, and the star-shaped-concave hexagonal composite negative Poisson's ratio structures share the concave edge with the star-shaped-concave hexagonal composite negative Poisson's ratio structures in the adjacent column.

[0064] like Figure 5 As shown, in some other embodiments, the star-shaped-concave hexagonal composite negative Poisson's ratio structures on the flexible substrate are arranged in an array with equal intervals; in the same column, adjacent star-shaped-concave hexagonal composite negative Poisson's ratio structures share the straight side, and the vertical rods are connected to each other; the parallel rods of the star-shaped-concave hexagonal composite negative Poisson's ratio structures extend outward to form parallel rods. Collinear transverse connecting rods ( It is connected to the transverse connecting rod of the star-shaped-concave hexagonal composite negative Poisson's ratio structure in the adjacent column.

[0065] In this embodiment, each side of the star-shaped-concave hexagonal composite negative Poisson's ratio structure is a raised line or a hollow structure on a flexible substrate, and the functional layer is fabricated on the top surface of the lines. The unit cell thickness refers to the width of each line. The unit cell height H refers to the height of the two straight sides of the star-shaped-concave hexagonal composite negative Poisson's ratio structure. The distance between them.

[0066] In this embodiment, the electrodes are connected to both ends of the functional layer, and the functional layer is fabricated on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure. The two ends of the functional layer refer to the sides adjacent to the two outermost columns of the star-shaped-concave hexagonal composite negative Poisson's ratio structure in the array composed of the star-shaped-concave hexagonal composite negative Poisson's ratio structure. Both of these sides are perpendicular to the straight sides of each star-shaped-concave hexagonal composite negative Poisson's ratio structure.

[0067] During the preparation process, the concave angle of the concave hexagon is adjusted. Length-to-height ratio (L / H), unit cell thickness (t), and branch arm length of star-shaped units. and the angle between branches Isogeometric parameters ( Figure 2 This allows for precise adjustment and control of the sensor's deformation range and mechanical properties.

[0068] Performance Testing: The obtained flexible sensor was clamped at both ends to a tensile testing machine and stretched at a constant speed (1.25 mm / s). The two electrodes were connected to an electrical signal acquisition system, and the sensor's force and electrical signals were recorded simultaneously. The aforementioned stretching speed is only for testing other performance parameters of the sensor and is not intended to limit its actual application. The stretching speed can be adjusted within a reasonable range according to the sensor's performance and testing conditions.

[0069] Performance test results of the flexible sensor: The programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor has a strain detection range of 40%, a resistance change rate (ΔR / R0) as high as 10410, and a sensitivity coefficient (GF) of 31569, demonstrating excellent force sensing performance. When the unit cell thickness is adjusted to 2mm, although the strain detection range decreases to 25%, the resistance change rate decreases to 413, and the sensitivity coefficient decreases to 2513, showing some attenuation compared to the 1mm structure, it still significantly improves the force-electric coupling efficiency through negative Poisson's ratio topology optimization design compared to the single concave hexagonal structure and star structure. While maintaining the improvement of structural stiffness, it effectively controls the performance degradation, providing a new design paradigm that combines the advantages of sensitivity and detection range for the engineering application of flexible sensors in wearable devices and biomechanical monitoring.

[0070] The flexible substrate layer employs a programmable mechanical property design with a star-shaped-concave hexagonal composite negative Poisson's ratio structure. This structure ensures excellent deformation capability of the flexible substrate layer and endows it with excellent compliance through its programmable geometry. This prevents interface problems caused by deformation mismatch between the functional layer and the flexible substrate layer structure during deformation, facilitating tight bonding and stable and reliable detection of the sensor on diverse, irregular, or dynamically changing surfaces. The programmable mechanical property structure of the flexible substrate layer can be constructed using template method or 3D direct printing method.

[0071] The encapsulation layer preferably uses the same or similar material as the flexible substrate layer. Alternatively, it can use highly flexible and highly adhesive materials, temperature-resistant materials, weather-resistant materials, or composite polymer materials that combine the above properties, depending on the application requirements. The encapsulation layer can effectively prevent the conductive polymer material in the functional layer from being damaged by the external environment, and precisely match the programmable mechanical properties of the functional layer and the flexible substrate layer to achieve synchronous and coordinated deformation, ensuring the overall interface bonding and structural integrity of the sensor.

[0072] In this embodiment, the rate of change of resistance ( ) and the axial strain experienced by the sensor ( A significant correlation was observed between them. This relationship can be described as follows: ,in It is a function of strain. The change in resistance under strain is the amount of resistance change. ; The initial resistance before strain occurs; This represents the change in the sensor's length relative to its initial length under strain. Specifically, the resistance changes relatively gradually at low strain levels; however, as strain increases, the rate of resistance change rises sharply, exhibiting an exponential growth trend. This superior response characteristic is mainly attributed to the synergistic strain amplification effect generated by the composite negative Poisson's ratio structure. This effect gradually elongates and increases the spacing of conductive pathways in the conductive network of the functional layer, eventually leading to breakage and separation, a sharp reduction in the number of conductive paths, and thus causing a corresponding change in resistance.

[0073] In this embodiment, the high sensitivity of the sensor originates from the multi-level negative Poisson's ratio effect deformation of the star-shaped-concave hexagonal composite structure. The tensile expansion deformation of the star and concave hexagons achieves a lateral strain amplification effect, causing a significant change in the conductive topology network of the sensing layer, resulting in a noticeable electrical signal response. Simultaneously, the sensitivity of the sensor can be adjusted by modifying geometric structural parameters such as the interior angle of the concave hexagon, the unit cell thickness, and the branch length and angle of the star unit, enabling synchronous programmable control of the force-electrical performance, optimizing the sensor's sensitivity, and meeting customized requirements.

[0074] In this embodiment, the sensor combines two negative Poisson's ratio structures, and its overall deformation performance and force sensing capability are superior to those of a single structure, thus ensuring that the sensor as a whole possesses high flexibility, high elasticity, and high sensitivity.

[0075] Comparative Example 1: This example uses the same process as Example 1 to fabricate a flexible sensor, which is also fabricated using the sacrificial method. The difference is that in this comparative example, the mold cavity only uses... Figure 4 The star-shaped negative Poisson's ratio structure shown is not the star-shaped-concave hexagonal composite negative Poisson's ratio structure in Example 1.

[0076] During the preparation process, the length of the branch arm of the star-shaped unit in the star-shaped negative Poisson's ratio structure is 1 mm, the included angle between the branches is 30°, and the shape parameters are the same as those of the star-shaped negative Poisson's ratio structure in the star-shaped-concave hexagonal composite negative Poisson's ratio structure in Example 1.

[0077] The test results of the flexible sensor prepared in this comparative example are as follows: the strain detection range reaches 14%, the resistance change rate (ΔR / R0) is 104, and the sensitivity coefficient (GF) is 761.

[0078] Comparative Example 2: This example uses the same process as Example 1 to fabricate a flexible sensor, which is also fabricated using the sacrificial method. The difference is that in this comparative example, the mold cavity only uses... Figure 3 The concave hexagonal negative Poisson's ratio structure shown is not the star-shaped-concave hexagonal composite negative Poisson's ratio structure in Example 1.

[0079] During the fabrication process, the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1 mm. Its shape parameters are the same as those of the star-shaped negative Poisson's ratio structure in the star-shaped-concave hexagonal composite negative Poisson's ratio structure of Example 1.

[0080] The test results of the flexible sensor prepared in this comparative example are as follows: the strain detection range reaches 6%, the resistance change rate (ΔR / R0) is 85, and the sensitivity coefficient (GF) is 1537.

[0081] like Figure 3 , Figure 4 , Figure 5 As shown, the shape changes of concave hexagonal, star-shaped, and star-concave hexagonal composite negative Poisson's ratio structures before and after stretching are illustrated. From the perspective of deformation principles, Figure 3 (a) and (b) show that when the concave hexagonal structure is stretched, its concave angles gradually open, causing the structure to expand laterally, which reflects the negative Poisson's ratio effect. However, this single structure is prone to non-uniform buckling during deformation and has poor deformation stability. Figure 4 The results show that when the star-shaped structure is stretched, its star-shaped angles gradually widen, resulting in lateral expansion. However, stress concentration easily occurs at the tip of the star-shaped structure, leading to hinge failure and limited deformation range. In contrast, as... Figure 5 As shown, the star-shaped unit and the concave hexagonal unit are connected to form a cooperative deformation system. Under stress, the mutual constraint between the two structures forms a unique cooperative deformation mechanism, and its deformation process is clearly divided into six stages:

[0082] In the first stage, when an axial load is applied, the stress first passes through the connecting nodes (such as the parallel rods described). and vertical rod Simultaneously, stress is transmitted to the radial branches of the star-shaped unit and the inclined arms of the concave hexagonal unit. At this stage, the structure is in the elastic deformation zone. The composite structure achieves efficient and uniform initial stress distribution through rigid connections, avoiding premature plastic yielding caused by hinge stress concentration in a single concave hexagonal structure, thus laying the foundation for a wide working range. Figure 5 i).

[0083] In the second stage, as the load increases, the branches of the star-shaped element rotate and extend outward around its center. This process causes the star-shaped element itself to expand axially and laterally (perpendicular to the loading direction), exhibiting a negative Poisson's ratio effect. Almost simultaneously, the concave corners of the outer concave hexagon also begin to open. Utilizing the characteristic of the star-shaped element to directly induce multi-directional expansion through axial tension provides a strong initial tensile driving force for the composite structure. Figure 5 ⅱ).

[0084] In the third stage, the connecting rods on both sides of the star-shaped structure gradually rotate to a position nearly perpendicular to the tensile direction, while the expansion of the concave hexagon intensifies. This process manifests axially as continuous elongation of the entire structure, and laterally, due to the synergistic effect of the inner and outer units, the expansion rate significantly accelerates. Through the lever action of the rods, axial tension is efficiently converted into lateral displacement, thus achieving an efficient conversion of axial strain into lateral strain. Figure 5 (iii).

[0085] In the fourth stage, when the key connecting rods of the star-shaped structure and the concave hexagonal unit both reach a specific angle (e.g., perpendicular), the strain amplification effect of the composite structure reaches its peak. At this point, while the structure is significantly stretched axially, its transverse width also reaches a relative maximum, and the negative Poisson's ratio effect is most significant. The two units achieve the most efficient force transmission and motion coupling under a specific geometric configuration, enabling the generation of the maximum transverse strain for a given axial strain. Figure 5 iv).

[0086] In the fifth stage, as strain continues to increase, the star-shaped structure continues to expand, but the hinge region connecting it to the concave hexagon begins to enter the plastic deformation stage. In this stage, the resistance to axial deformation increases, while the rate of lateral expansion begins to slow down. Therefore, energy can be dissipated through the controlled formation of plastic hinges, ensuring that the structure can continue to extend axially while maintaining stability in lateral deformation, thus avoiding instability and failure. Figure 5 (v).

[0087] In the sixth stage, both the star-shaped and concave hexagonal units gradually transform into a shape resembling a regular hexagon. At this point, axial stretching of the structure becomes difficult, and the behavior changes from a negative Poisson's ratio to a positive Poisson's ratio, with the entire structure entering a homogeneous and compacted stage. This controllable morphological transformation greatly extends the compressible stroke of the structure, thereby effectively widening the sensor's working range in the axial direction. Figure 5 (vi).

[0088] From the perspective of experimental data, Figures 6 to 9 The curves further validate the performance advantages of the composite structure. Figure 6 The stress-strain curves show that, under the same 1057% strain, the stress value of the star-shaped-concave hexagonal composite negative Poisson's ratio structure is 0.06 MPa, which is three times that of the concave hexagonal structure and the star-shaped structure (both 0.02 MPa). This proves that it can withstand a greater load under the same deformation. Meanwhile, the stress growth curve of the composite structure is smoother and more continuous, demonstrating that it effectively expands its working range through multi-stage stable deformation and avoids premature fracture due to local failure. Figure 7The initial modulus curves further reveal the superior stiffness of the composite structure. Within the strain range, the Young's modulus of the star-shaped-concave hexagonal composite negative Poisson's ratio structure is significantly higher than that of the two single structures, with an average value approximately three times that of the concave hexagonal and star-shaped structures. This indicates that the composite structure possesses higher initial stiffness and stronger resistance to deformation, while its modulus change curve is flatter, reflecting more controllable deformation behavior, which is beneficial for maintaining stable electromechanical coupling of the sensor over a wide strain range. Figure 8 The fracture stress curve directly reflects the ultimate bearing capacity of the structure. The star-shaped-concave hexagonal composite negative Poisson's ratio structure achieved the highest fracture stress, reaching 0.06 MPa, which is 1.5 times that of the concave hexagonal structure and 3 times that of the star-shaped structure. This data fully demonstrates the effectiveness of the composite design in dispersing stress and improving mechanical strength, giving the sensor higher durability and enabling it to withstand more severe deformation conditions. Figure 9 The force-sensing sensitivity curves show that the resistivity change rate (ΔR / R0) and sensitivity coefficient (GF) of the star-shaped-concave hexagonal composite negative Poisson's ratio structure are significantly higher than those of the single structure: in Example 1, ΔR / R0 reaches 10410 and GF reaches 31569, while in Comparative Examples 1 and 2, ΔR / R0 is only 104 and 85, respectively, and GF is 761 and 1537, respectively. This demonstrates that the composite structure greatly improves the force-to-electricity conversion efficiency through a synergistic tensile effect, achieving high-sensitivity sensing while maintaining low hysteresis and high cycling stability.

[0089] Example 2: The main difference between this example and Example 1 is that the concave angle of the concave hexagonal unit is reduced. To enhance the concavity of the structure. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concavity angle of 45°, and a unit cell thickness of 1 mm. By adjusting these geometric parameters, the star-shaped-concave hexagonal composite negative Poisson's ratio structure can achieve greater lateral expansion than in Example 1 when subjected to the same tensile force.

[0090] Example 3: The main difference between this example and Example 1 is that the concave angle of the concave hexagonal unit is increased. To reduce the degree of concavity in the structure. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concavity angle of 80°, and a unit cell thickness of 1 mm. By adjusting these geometric parameters, the structure achieves less lateral expansion compared to Embodiment 1 under the same tensile force.

[0091] Example 4: The main difference between this example and Example 1 is that the unit cell thickness is reduced. This reduces the structure's stiffness and fracture strength, making it suitable for low-load scenarios. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 0.7 mm.

[0092] Example 5: The main difference between this example and Example 1 is that the unit cell thickness is increased. To enhance the structure's stiffness and fracture strength, making it suitable for high-load scenarios. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1.2 mm.

[0093] Example 6: The main difference between this example and Example 1 is that the length of the branch arm of the star unit is increased. This enhances the coupling strength between the star-shaped unit and the concave hexagonal unit. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1.2 mm and an included angle of 30° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1 mm. This adjustment enhances the efficiency of strain transfer from the axial to the transverse direction.

[0094] Example 7: The main difference between this example and Example 1 is that the length of the star unit branch arm is reduced. This reduces the coupling strength between the star-shaped unit and the concave hexagonal unit. Specifically, in this embodiment, the branch arm length of the star-shaped unit is 0.9 mm, and the included angle between the branches is 30°; the side length of the concave hexagonal unit is 2 mm, the concave angle is 60°, and the unit cell thickness is 1 mm. This adjustment reduces the efficiency of strain transfer from the axial to the transverse direction.

[0095] Example 8: The main difference between this example and Example 1 is that the angle between the branches of the star unit is increased. This reduces the coupling strength between units. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 35° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1 mm. This adjustment reduces the efficiency of strain transfer from the axial to the transverse direction.

[0096] Example 9: The main difference between this example and Example 1 is that the angle between the branches of the star unit is reduced. To enhance the coupling strength between units. Specifically, in this embodiment, the star-shaped unit has a branch arm length of 1 mm and an included angle of 26° between branches; the concave hexagonal unit has a side length of 2 mm, a concave angle of 60°, and a unit cell thickness of 1 mm. This adjustment enhances the efficiency of strain transfer from the axial to the transverse direction.

[0097] As can be seen from Examples 1 to 9, in the process of fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor, the deformation range and mechanical properties of the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor can be adjusted by regulating the geometric parameters of the star-shaped-concave hexagonal composite negative Poisson's ratio structure. This flexible programming of geometric parameters can be extended to the programmability of electrical properties, enabling the present invention to overcome the performance contradictions between sensitivity, operating range, and strength in traditional sensors.

[0098] In practical implementation, precise control of the final sensor's mechanical sensing performance can be achieved by adjusting the geometric parameters of the star-shaped-concave hexagonal composite negative Poisson's ratio structure. This meets the diverse needs of various fields, from human health monitoring to intelligent manufacturing and robotics. For example, for high-sensitivity applications requiring the detection of minute strains, a smaller concave angle can be used. The design amplifies the strain signal; for scenarios requiring large deformation monitoring, a larger concave angle can be used. and optimize arm length The design aims to extend the operating range while maintaining signal linearity.

[0099] Example 10: As Figure 11 As shown, the present invention also provides a method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor. The method in this embodiment is similar to the method in embodiment 1, and the flexible substrate is fabricated using a template method or a sacrificial layer method.

[0100] S1. A flexible substrate mold for PLA (template method) / PVA (sacrificial layer method) is fabricated using 3D printing technology. The cavity of the flexible substrate mold includes a complementary structure of star-shaped-concave hexagonal composite negative Poisson's ratio structure (e.g., Figure 10 (as shown)

[0101] S2. Pour the liquid flexible substrate material into the mold cavity and let it cure at room temperature. The material of the flexible substrate layer is skin-like silicone (Ecoflex), or it can be one of the elastomers or composite materials such as polyurethane (PU), polydimethylsiloxane (PDMS), and hydrogel, to provide a highly flexible and elastic substrate with adjustable and customizable mechanical properties.

[0102] S3. For PLA molds, direct demolding yields a flexible substrate layer with a star-shaped-concave hexagonal composite negative Poisson's ratio structure; for PVA (sacrificial layer method) molds, immersing them in warm water dissolves the mold to obtain a flexible substrate layer with a star-shaped-concave hexagonal composite negative Poisson's ratio structure.

[0103] S4. The solidified substrate layer is fixed under a certain strain, and the sensing functional material is assembled (coated) layer by layer on the patterned substrate surface in one or more steps to form a functional layer and dried. After releasing the strain, a patterned substrate-functional layer composite integrated sensing unit with a preset star-shaped-concave hexagonal composite negative Poisson's ratio structure is obtained. In addition to PEDOT:PSS, the material of the sensor's functional layer can also be one of a variety of conductive functional materials such as silver nanowires (AgNW), multi-walled carbon nanotubes (MWCNT), Mxene, or liquid metal (LM).

[0104] S5. Electrode Connection and Encapsulation: Connect the electrodes to both ends of the functional layer; and coat the encapsulation layer.

[0105] Example 11: The main difference between this example and Example 10 is that this example uses laser cutting to prepare the flexible substrate layer, which includes the following steps:

[0106] S1. A flexible substrate layer and a functional layer are prepared sequentially by a layer-by-layer assembly method, and the two are connected to form an integrated structure. The functional layer can be assembled once or repeatedly as needed.

[0107] S2. Using a laser cutting machine, a pre-defined star-shaped-concave hexagonal composite negative Poisson's ratio structure pattern is cut to simultaneously construct a flexible substrate-functional layer integrated sensing unit with a star-shaped-concave hexagonal composite negative Poisson's ratio structure. Subsequently, electrodes and an encapsulation layer can be fabricated on this flexible substrate-functional layer integrated sensing unit. In Example 11, a flexible substrate layer can be prepared first using laser cutting, and then the functional layer can be prepared by coating.

[0108] Example 12: The main difference between this example and Example 10 is that this example uses screen printing to prepare the flexible substrate layer, which includes the following steps:

[0109] S1. Prepare a screen printing plate. The hollow part of the screen printing plate has a star-shaped-concave hexagonal composite negative Poisson's ratio structure. In view of the isolated block characteristics of the complementary structure of the hollow part, a narrow weak connecting bridge is introduced between the isolated blocks to connect the isolated unit with the main body, so as to ensure the structural integration, precise forming and processability of the screen printing plate in the preparation and printing process.

[0110] S2. The pattern of the composite negative Poisson's ratio structure is formed on the flexible substrate by screen printing.

[0111] S3. After the printing step is completed, apply local functional material to the uncovered areas caused by the auxiliary weak connection bridge to ensure the continuity and integrity of the functional layer.

[0112] S4. Place the repaired substrate-functional layer composite in a 60°C oven for 30 minutes to cure. After curing, a flexible substrate-functional layer composite integrated sensing unit with a star-shaped-concave hexagonal composite negative Poisson's ratio structure is obtained. Electrodes and encapsulation layers can then be fabricated on this flexible substrate-functional layer composite integrated sensing unit.

[0113] Example 13: The main difference between this example and Example 10 is that this example uses a rigid-flexible coupling multi-material printing method to prepare a flexible substrate layer, which includes the following steps:

[0114] S1. Locate the easily tensile and fractured parts in the flexible substrate layer with a star-shaped-concave hexagonal composite negative Poisson's ratio structure, and design the three-dimensional structure of the flexible substrate layer, wherein the easily tensile and fractured parts are set as rigid regions; and the other regions of the flexible substrate layer are conductive regions.

[0115] S2. A flexible substrate layer with a star-shaped-concave hexagonal composite negative Poisson's ratio structure is prepared using multi-nozzle printing technology. This flexible substrate layer and the functional layer are integrated into one structure, and the substrate layer also functions as the functional layer. During the preparation process, rigid materials are used for printing in the rigid areas, while flexible composite materials containing conductive fillers are used for printing in the conductive areas.

[0116] Methods for locating easily tensile fracture sites include finite element simulation or fabricating a sample of a flexible substrate with a star-shaped-concave hexagonal composite negative Poisson's ratio structure, and determining the stress concentration area of ​​the fracture location through tensile testing. Easily tensile fracture sites are typically the tips of the star-shaped units and the hinges connecting the concave hexagons. The conductive areas are printed to form a strain-sensitive network, creating a flexible substrate that is rigidly coupled to the conductive functional layer sensing network. The flexible sensor fabricated using this method eliminates the need for the functional layer and encapsulation layer.

[0117] Example 14: In this example, a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor is fabricated using a multi-iteration method. Each iteration includes the following steps:

[0118] S1. According to the predetermined design of the flexible substrate, a flexible substrate with a star-shaped-concave hexagonal composite negative Poisson's ratio structure is prepared by multi-nozzle printing technology; the flexible substrate includes conductive regions, which are printed using a flexible composite material containing conductive fillers; if the predetermined design includes rigid regions, then rigid materials are used for printing; after printing, the flexible substrate also functions as a functional layer, therefore a separate functional layer is not prepared, and its rigid and conductive regions are an integral structure; initially, the flexible substrate does not include rigid regions;

[0119] S2. Prepare electrodes and encapsulate the flexible substrate by coating it with an encapsulation layer to obtain the sensor for this round;

[0120] S3. Test the deformation behavior and mechanical response of the sensor;

[0121] S4. Conduct a tensile test to determine the parts that are prone to tensile fracture;

[0122] S5. If the deformation behavior and mechanical response detected in step S3 meet the design target, and the tensile test in step S4 does not find any parts that are prone to tensile fracture, then the predetermined design of the flexible substrate used in this iteration is taken as the final design; otherwise, the predetermined design of the flexible substrate is adjusted and the next iteration is entered.

[0123] Specifically, during the adjustment of the predetermined design of the flexible substrate, if the tensile test in step S4 reveals areas prone to tensile fracture, then a rigid region is added to reinforce these areas in the predetermined design. If the deformation behavior and mechanical response detected in S3 do not meet the design target, then the geometric parameters of the star-shaped-concave hexagonal composite negative Poisson's ratio structure in the predetermined design are adjusted according to the deviation between the deformation behavior and the mechanical response. After adjustment, the predetermined design of the flexible substrate for the next iteration can be obtained.

[0124] The deformation behavior and mechanical response of a sensor include its strain range, sensitivity coefficient, and rate of change of resistance. When the strain range exceeds the design target range, it needs to be reduced. Adjusting the geometric parameters can be achieved by reducing the length of the branch arms of the star-shaped element. Increase unit cell thickness Increase the concave angle One or more combinations of these, and vice versa; when the sensitivity coefficient exceeds the design target range, it will cause the sensor signal to saturate prematurely and the linear operating range to be narrow. To overcome this problem, it is necessary to reduce the sensitivity coefficient. Methods for adjusting geometric parameters include: reducing the branch arm length of the star unit. Increase unit cell thickness Increase the concave angle One or more combinations of these, and vice versa. If the rate of change of resistance exceeds the design target range, the causes and solutions are similar to those for excessive sensitivity; therefore, it is necessary to reduce the rate of change of resistance. Adjustments to geometric parameters include increasing the unit cell thickness. Increase the concave angle .

[0125] In Examples 10 to 14, any one of these five fabrication methods can achieve the structural molding and functionalization of the highly sensitive force-sensing flexible sensor with programmable mechanical properties. In the above five embodiments, the encapsulation layer is composited with the substrate layer and functional layer through an in-situ molding method, realizing integrated construction and functional integration from the substrate to the encapsulation layer, ensuring the overall structural integrity and synchronous deformation capability of the sensor.

[0126] The high sensitivity of the sensor in this embodiment can be widely used in robot tactile perception systems to improve the robot's environmental perception capabilities and human-computer interaction performance. At the same time, the sensor can be integrated into a flexible global health monitoring system for human health management and smart healthcare. Preferably, the negative Poisson's ratio structure of the sensor gives it high compliance, which can be effectively applied to the detection and diagnosis of robots, aerospace equipment, and other devices with complex curved surfaces and surface structures.

[0127] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor, characterized in that... include: A flexible substrate layer having a star-shaped-concave hexagonal composite negative Poisson's ratio structure, wherein the star-shaped-concave hexagonal composite negative Poisson's ratio structure is composed of star-shaped units and concave hexagonal units through connecting rods, wherein the center of the star-shaped unit coincides with the center of the concave hexagonal unit, forming a cooperative deformation system; Functional layer, the functional layer being coated on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure of the flexible substrate; Electrodes, the electrodes being connected to both ends of the functional layer; And an encapsulation layer that covers the functional layer and the electrode.

2. The programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 1, characterized in that: In the star-shaped-concave hexagonal composite negative Poisson's ratio structure, the star-shaped unit is a four-cornered star, with each corner including two branch arms; the concave hexagonal unit surrounds the star-shaped unit and includes two parallel straight sides and four concave sides. Each straight side is adjacent to two concave sides, and each concave side has two ends adjacent to another concave side and a straight side, respectively; the angle between a concave side and an adjacent straight side is a concave angle, and the connection point of two adjacent concave sides is a concave point; in the star-shaped unit, the junction between two adjacent corners is the concave part of the star-shaped unit; two opposite concave parts of the star-shaped unit are respectively connected to two concave points of the concave hexagonal unit through parallel rods, and the other two concave parts of the star-shaped unit are respectively connected to two straight sides of the adjacent concave hexagonal unit through vertical rods.

3. The programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 2, characterized in that: The star-shaped-concave hexagonal composite negative Poisson's ratio structures on the flexible substrate are arranged in a close-packed array, and adjacent star-shaped-concave hexagonal composite negative Poisson's ratio structures share the straight or concave sides of the concave hexagonal unit. Alternatively, they can be arranged in an equally spaced array, with adjacent columns of structural units connected by transverse connecting rods; in the same column, adjacent star-shaped-concave hexagonal composite negative Poisson's ratio structures share a straight edge.

4. A programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 2 or 3, characterized in that: During the fabrication process, the deformation range and mechanical properties of the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor are adjusted by regulating the geometric parameters of the star-shaped-concave hexagonal composite negative Poisson's ratio structure; wherein, the geometric parameters include: the concavity angle of the concave hexagonal unit. Length-to-height ratio (L / H) and unit cell thickness (t); branch arm length of star-shaped units. and the angle between branches .

5. The programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 1, characterized in that: The flexible substrate layer is made of one or more of the following materials: Ecoflex, polydimethylsiloxane, polyurethane, and hydrogel; the functional layer is made of one or more of the following materials: PEDOT:PSS, silver nanowires, carbon nanotubes, Mxene, or liquid metal.

6. A method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor, used to fabricate the programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor as described in any one of claims 1 to 5, characterized in that, include: Prepare a flexible substrate with a star-shaped-concave hexagonal composite negative Poisson's ratio structure; The methods for preparing the flexible substrate include: template method, sacrificial layer method, laser cutting, screen printing method, and rigid-flexible coupling multi-material printing method.

7. The method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 6, characterized in that, The fabrication method of a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using the template method or the sacrificial layer method includes the following steps: A flexible substrate mold is prepared, which has a cavity that complements the star-shaped-concave hexagonal composite negative Poisson's ratio structure; for the template method, the material of the flexible substrate mold includes PLA, and for the sacrificial layer method, the material of the flexible substrate mold includes PVA. The liquid flexible substrate material is poured into the cavity of the flexible substrate layer mold and then cured. Demolding; for the template method, the flexible substrate mold is directly separated from the flexible substrate; for the sacrificial layer method, the flexible substrate mold together with the flexible substrate is placed in warm water to dissolve the flexible substrate mold. The flexible substrate is fixed to a predetermined strain, and a functional layer is prepared on the surface of the star-shaped-concave hexagonal composite negative Poisson's ratio structure; after the functional layer is prepared, the strain of the flexible substrate is released. The electrodes are connected to both ends of the functional layer, and the electrodes and the functional layer are encapsulated using an encapsulation layer.

8. The method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 6, characterized in that, The fabrication method of a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using laser cutting includes the following steps: Flexible substrate and functional layers are fabricated through a stacked assembly method; The composite integrated sensing unit is directly formed by synchronously cutting a pre-set star-shaped-concave hexagonal composite negative Poisson's ratio structure pattern using a laser cutting machine.

9. The method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 6, characterized in that, The fabrication method of a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using screen printing includes the following steps: Prepare a screen printing plate, wherein the pattern of the hollow part of the screen printing plate is a star-shaped-concave hexagonal composite negative Poisson's ratio structure; The pattern of the composite negative Poisson's ratio structure is formed by screen printing on the flexible substrate layer using the screen printing plate. After the printing step is completed, the uncovered areas caused by the auxiliary weak connection bridges are locally coated with functional material to ensure the continuity and integrity of the functional layer.

10. The method for fabricating a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor according to claim 6, characterized in that, The fabrication method of a programmable negative Poisson's ratio superstructure high-sensitivity flexible sensor using a rigid-flexible coupling multi-material printing method includes the following steps: Identify the stress concentration region in the star-shaped-concave hexagonal composite negative Poisson's ratio structure; Rigid materials are used for printing in the stress concentration areas, while flexible composite materials containing conductive fillers are used for printing in other areas, directly forming a flexible substrate layer with rigid-flexible coupling. This flexible substrate layer and the functional layer are an integral structure.

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