Insole based on negative Poisson's ratio piezoelectric electrets and preparation method and application thereof

Abstract

The invention belongs to the technical field of intelligent wearable equipment, and particularly relates to a negative Poisson's ratio piezoelectric electret-based insole and a preparation method and application thereof, the insole comprises a piezoelectric electret support structure, a conductive electrode layer and a protective layer; the piezoelectric electret supporting structure is a porous matrix layer made of an electret material or a porous matrix layer of which the surface is coated with an electret layer; the piezoelectric electret supporting structure is a negative Poisson's ratio structure; the conductive electrode layer is coated on the surface of the piezoelectric electret supporting structure, and the conductive electrode layer is connected with an electrode lead; and the protective layer is packaged outside the piezoelectric electret supporting structure and the conductive electrode layer. Compared with the prior art, the problem that in the prior art, an intelligent insole can only achieve a single effect and cannot meet the requirements of diabetic feet for multiple functions is solved. According to the scheme, dynamic monitoring, impact resistance and high comfort are integrated, and an innovative solution is provided for early screening of diabetic feet, ulcer risk assessment and intervention treatment early warning.

Description

Technical Field

[0001] This invention belongs to the field of smart wearable device technology, specifically relating to an insole based on a negative Poisson's ratio piezoelectric electret, its preparation method, and its application. Background Technology

[0002] Diabetic foot is a serious complication of diabetes, with diabetic foot ulcers occurring in up to 10% of diabetic patients. These ulcers significantly increase the risk of disability and death, resulting in a substantial socioeconomic burden. The occurrence and development of diabetic foot are closely related to multiple factors, including peripheral neuropathy, vascular disease, infection, and trauma.

[0003] Recent studies have shown that abnormal plantar load and changes in soft tissue biomechanical properties are key contributing factors to diabetic foot ulcers. Monitoring plantar pressure distribution and the stiffness and elasticity of soft tissues can provide important information for ulcer risk assessment. Based on these findings, plantar pressure monitoring systems have been widely used, especially in-shoe pressure monitoring systems, which have demonstrated significant value in the early diagnosis and intervention of diabetic foot ulcers due to their ability to monitor dynamic plantar pressure distribution in real time and conveniently.

[0004] Existing plantar pressure monitoring products mainly include pressure distribution platforms and in-shoe monitoring systems. Pressure distribution platforms are typically used in laboratory research and are suitable for static and dynamic pressure testing, but they are complex to operate, lack portability, and cannot meet the needs of daily use. In contrast, in-shoe monitoring systems can monitor dynamic plantar pressure parameters in a simple and effective way in daily life, and have good portability. However, existing in-shoe systems still have the following shortcomings:

[0005] 1. Limited sensitivity: It is not responsive enough to small changes in plantar pressure and cannot accurately assess key parameters such as peak pressure and pressure-time integral.

[0006] 2. Limited impact resistance: The cushioning effect of high impact is poor in the heel area, which can easily cause damage to foot tissues.

[0007] 3. Low comfort: When worn dynamically, the fit and adaptability are poor, affecting the user experience.

[0008] Chinese patent CN117889994B discloses a fabric sensor and a smart insole incorporating the sensor. Targeting the monitoring of foot health in diabetic foot patients, the fabric sensor involves a first friction layer and a second friction layer with microridge structures, utilizing the triboelectric effect to sense changes in plantar pressure and shear stress. However, this insole has a complex structure and does not consider the high demands of diabetic foot patients for insole comfort and impact resistance due to the fragility of their feet.

[0009] Chinese patent CN115462593A discloses a biodegradable smart insole and its preparation method. This smart insole, based on a biodegradable piezoelectric electret material, features self-powered operation, integration, and multifunctionality. During use, it can simultaneously monitor multiple personalized human body signals, making it widely applicable, including: daily training monitoring for professional runners, daily health monitoring for patients with gait correction and chronic diseases, without causing additional burden on the user. However, this solution does not employ a negative Poisson's ratio structure and only considers monitoring aspects, neglecting the high demands for insole comfort and impact resistance from diabetic foot patients due to the fragility of their feet.

[0010] Chinese patent CN108284595A discloses a method for 3D printing a negative Poisson's ratio structure shock-absorbing shoe sole and insole. This method utilizes 3D printing technology to fabricate computer-designed shoe soles and insoles from polymer elastomers under specific conditions. The finished products possess excellent impact resistance, as well as outstanding elasticity, shear resistance, and shock absorption. However, this insole lacks a sensing device, neglecting the user's need for dynamic monitoring of foot pressure and failing to reflect the user's foot health.

[0011] However, it should be noted that both piezoelectric electret materials and negative Poisson's ratio structures have independent requirements and demands for their respective structural designs. For example, improper design of the porous framework of a negative Poisson's ratio structure can easily lead to uneven charge distribution in the piezoelectric electret material during polarization. Furthermore, when the electret layer composed of piezoelectric electret material is attached to the surface of a complex concave structure, uneven adhesion or signal distortion may occur, ultimately affecting the overall signal stability. Therefore, there are significant conflicts in processing and performance between piezoelectric electret materials and negative Poisson's ratio structures, making direct combination through simple superposition difficult. Targeted design and creative improvements are required. Summary of the Invention

[0012] The purpose of this invention is to address at least one of the aforementioned problems by providing a shoe insole based on a negative Poisson's ratio piezoelectric electret, its preparation method, and its application. This addresses the limitation of existing smart insoles, which can only achieve a single effect such as dynamic monitoring, impact resistance, or high comfort, making it difficult to meet the diverse functional needs of conditions like diabetic foot. This solution integrates dynamic monitoring, impact resistance, and high comfort, providing an innovative approach for early screening, ulcer risk assessment, and early warning of intervention for diabetic foot.

[0013] The objective of this invention is achieved through the following technical solution:

[0014] The first aspect of this invention discloses an insole based on a negative Poisson's ratio piezoelectric electret, comprising a piezoelectric electret support structure, a conductive electrode layer, and a protective layer;

[0015] The piezoelectric electret support structure is a porous matrix layer made of electret material, or a porous matrix layer with an electret layer on its surface; the piezoelectric electret support structure is a negative Poisson's ratio structure.

[0016] The conductive electrode layer is coated on the surface of the piezoelectric electret support structure, and the conductive electrode layer is connected to electrode leads for conducting electrical signals generated by the piezoelectric electret support structure.

[0017] The protective layer is encapsulated outside the piezoelectric electret support structure and the conductive electrode layer.

[0018] Preferably, the porous matrix layer is an electret material, including polypropylene, fluoropolymer, and polylactic acid.

[0019] Preferably, the porous matrix layer is a non-electret material, including polyurethane foam, silicone, and thermoplastic elastomer; the electret layer is an electret material, including polypropylene, fluoropolymer, and polylactic acid.

[0020] Preferably, the negative Poisson's ratio structure is a concave porous structure.

[0021] Preferably, the conductive electrode layer is silver nanowires.

[0022] Preferably, the protective layer is a flexible elastomer material.

[0023] A second aspect of this invention discloses a method for preparing a shoe insole based on a negative Poisson's ratio piezoelectric electret as described above, comprising the following steps:

[0024] S1: Prepare a piezoelectric electret support structure with a negative Poisson's ratio structure, and polarize the piezoelectric electret support structure;

[0025] S2: A conductive electrode layer is prepared on the surface of the piezoelectric electret support structure using a masking method, and electrode leads are drawn out from the conductive electrode layer;

[0026] S3: The prepared piezoelectric electret support structure and conductive electrode layer are encapsulated with a protective layer to form the insole.

[0027] Preferably, when the piezoelectric electret support structure is a porous matrix layer made of electret material, in step S1: a porous matrix layer with negative Poisson bit properties is prepared by thermal compression, template method or 3D printing method.

[0028] When the piezoelectric electret support structure is a porous substrate layer with an electret layer on its surface, in step S1: a porous substrate layer with negative Poisson bit properties is prepared by thermal compression, template method or 3D printing method, and then an electret layer is covered on the surface of the porous substrate layer by impregnation method.

[0029] The third aspect of the present invention discloses an application of a shoe insole based on a negative Poisson's ratio piezoelectric electret as described above, wherein the shoe insole is disposed inside the wearer's shoe for real-time monitoring of the wearer's foot pressure distribution and dynamic changes.

[0030] Preferably, when the wearer walks or exercises, the piezoelectric electret support structure in the insole is compressed and deformed, generating an electrical signal. An external signal processing platform receives the electrical signal and obtains the geometric deformation of the insole through the change of the electrical signal, thereby realizing the monitoring of the force distribution and magnitude of the insole, and thus obtaining the characteristics of foot pressure change and gait characteristics.

[0031] Preferably, the insole undergoes signal processing using the following method:

[0032] A multi-point electrode array, constructed using a piezoelectric electret support structure, synchronously acquires local electrical signals. These signals are then filtered and denoised to eliminate environmental interference. Subsequently, a plantar pressure distribution map is reconstructed based on a pressure-voltage mapping model (refer to the model published in Belli et al., Modeling and Calibration of Pressure-Sensing Insoles via a New Plenum-Based Chamber, Sensors, 2023, 23, 4501, whose data and model code are publicly available at https: / / github.com / ami-iit / paper_belli_2023_sensors_insole-calibration). Furthermore, a time-series pressure distribution is constructed, and feature parameters including gait cycle, peak pressure, and pressure-time integral are extracted to achieve real-time monitoring of gait characteristics and plantar health status.

[0033] The working principle of this invention is as follows:

[0034] (1) The internal structure of the smart insole is a negative Poisson's ratio structure. A negative Poisson's ratio structure is a type of structure with a negative Poisson's ratio effect (e.g., a concave porous structure), meaning that it expands laterally when stretched and contracts laterally when compressed. Therefore, the smart insole of this solution behaves differently from ordinary insoles under pressure. The negative Poisson's ratio effect can significantly enhance the fit between the smart insole and the sole of the foot, making the insole surface more conform to the complex contours of the foot, thereby dispersing pressure and improving comfort.

[0035] (2) The piezoelectric electret support structure of the smart insole includes a piezoelectric electret, which is the sensing component of this invention. A piezoelectric electret is a porous space charge electret with a piezoelectric effect. Its working mechanism is mainly based on the porous structure inside the material and the charge distribution on the electret after polarization. When the piezoelectric electret is subjected to mechanical stress, the internal porous structure deforms. This deformation causes a change in the macroscopic dipole moment inside the material, thereby generating an electric potential or inducing charge flow between the electrets, realizing the conversion of mechanical energy into electrical energy. Therefore, if further connected to an external testing instrument via electrode leads, when the smart insole is subjected to external force, the geometric deformation of the insole causes a change in charge distribution, thereby generating an electrical signal in the testing instrument. By collecting and analyzing these electrical signals, the magnitude and distribution of the external force, as well as the deformation of the object's surface, can be monitored in real time.

[0036] (3) Silver nanowires are coated on the surface of the insole as a conductive electrode layer to conduct the electrical signals generated by the piezoelectric electret material. Silver nanowires have excellent conductivity and antibacterial properties, which can effectively improve signal acquisition efficiency and the hygiene performance of the insole. After the collected signals are processed by the data acquisition device, they can provide plantar pressure distribution maps and gait analysis data, enabling real-time monitoring.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. Strong dynamic monitoring capability: Piezoelectric electret materials have extremely high sensitivity to changes in external pressure or deformation, and can capture minute mechanical signals. Through a highly sensitive piezoelectric electret sensor array constructed from electret materials, real-time data on plantar pressure distribution and dynamic changes can be captured, providing a scientific basis for the early diagnosis and intervention of diabetic foot.

[0039] 2. Superior impact resistance: The piezoelectric electret support structure with negative Poisson's ratio expands laterally when subjected to force, which can significantly reduce pressure concentration in the heel area and reduce the risk of soft tissue injury and ulceration of the foot.

[0040] 3. Enhanced comfort and fit: The negative Poisson's ratio design of the insole enhances the fit to the sole of the foot, making it especially suitable for complex dynamic gait scenarios and improving the user's wearing experience.

[0041] 4. No external power supply required: Piezoelectric electrets, as sensors, can directly convert mechanical energy into electrical energy using the piezoelectric effect, eliminating the need for an external power supply. This self-powered characteristic makes them particularly suitable for portable, wearable, and remote monitoring devices, significantly simplifying system design.

[0042] The porous matrix layer structure and molding method proposed in this invention enable the piezoelectric electret support structure with a negative Poisson's ratio structure to maintain good structural uniformity and polarization adaptability during molding and polarization. Simultaneously, the highly conductive nanoscale conductive electrode layer effectively avoids problems of uneven charge distribution and signal distortion, thus achieving an organic combination of comfort, shock resistance, and real-time monitoring functionality.

[0043] It is worth noting that piezoelectric electrets without a negative Poisson's ratio structure typically exhibit a high longitudinal piezoelectric effect, but a low transverse piezoelectric effect, resulting in the acquisition of only limited longitudinal information. In contrast, piezoelectric electrets with a negative Poisson's ratio structure not only maintain a high longitudinal piezoelectric effect but also significantly enhance the transverse piezoelectric effect. Therefore, the insole based on a negative Poisson's ratio piezoelectric electret proposed in this invention can simultaneously collect more effective information in both the transverse and longitudinal directions, significantly improving the sensitivity of signal acquisition and making the device more sensitive and accurate in various complex application scenarios.

[0044] In summary, the smart insole of this invention breaks through the limitations of existing technologies, integrating dynamic monitoring, impact resistance, and high comfort, and provides an innovative solution for early screening, ulcer risk assessment, and intervention treatment of diabetic foot. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the cross-sectional structure of the smart insole in Examples 1-3;

[0046] Figure 2 This is a schematic diagram of the cross-sectional structure of the smart insole in Example 4;

[0047] Figure 3 A schematic diagram of the surface structure of a smart insole;

[0048] In the figure: 1-porous substrate layer; 2-electret layer; 3-conductive electrode layer; 4-protective layer. Detailed Implementation

[0049] 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.

[0050] The smart insole of this invention consists of a piezoelectric electret support structure, a conductive electrode layer 3, and a protective layer 4, as shown below. Figure 1-3 As shown. The conductive electrode layer 3 is coated on the surface of the piezoelectric electret support structure and electrode leads are connected to the conductive electrode layer 3; the protective layer 4 is encapsulated outside the piezoelectric electret support structure and the conductive electrode layer 3.

[0051] In this invention, the functional component used for electro-mechanical coupling is a porous negative Poisson's ratio piezoelectric electret, the piezoelectric active part of which is the piezoelectric electret support structure. The piezoelectric electret support structure can be the porous substrate layer 1 itself, or it can be a composite structure composed of the porous substrate layer 1 and the electret layer 2 attached to the porous substrate layer 1.

[0052] The porous matrix layer 1 primarily serves as a support for the foot and a key element in regulating pressure distribution. Therefore, the material of the porous matrix layer 1 should be a flexible polymer. Specifically, electret materials are preferred for the porous matrix layer 1, such as polypropylene (PP or IXPP, etc.), fluoropolymers (PEP or PTFE, etc.), or polylactic acid (PLA), but non-electret materials such as polyurethane foam, silicone, and thermoplastic elastomers can also be used. If a non-electret material is used as the porous matrix layer 1, an electret material layer 2 needs to be attached inside the porous matrix layer 1 through processes such as impregnation.

[0053] The preferred material for the conductive electrode layer 3 is silver nanowires; such as Figure 3 As shown, the conductive electrode layer 3 can be composed of several circular electrode regions.

[0054] The material of protective layer 4 is preferably a flexible, biodegradable elastomer material.

[0055] The geometry of the insole can be customized according to actual needs.

[0056] The preparation process of a smart insole based on a negative Poisson's ratio piezoelectric electret proposed in this invention is as follows: (1) Select a flexible electret polymer material (e.g., PLA, IXPP, PP, etc.) according to the requirements; (2) Prepare a porous matrix layer 1 with negative Poisson's ratio by means of hot compression, template molding or 3D printing; (3) Inject charges through polarization process to give the material piezoelectric activity, that is, to make a piezoelectric electret; (4) Cover the upper / lower surface of the porous matrix layer 1 with customized masks respectively, and uniformly coat the upper / lower surface of the porous matrix layer 1 with conductive materials (e.g., silver nanowires, etc.) to form a conductive electrode layer 3, remove the mask, and lead out the electrode leads from the coated conductive material area respectively; (5) Use a flexible elastomer material (e.g., Ecoflex00-30, etc.) to encapsulate the sample to form a protective layer 4, to ensure that the internal electrode layer is not exposed, and finally obtain a smart insole based on a negative Poisson's ratio piezoelectric electret.

[0057] If a non-electret material (such as polyurethane, silicone, etc.) is selected as the porous matrix layer 1 in steps (1) and (2), then after step (2), a process of immersing the porous matrix layer 1 in an electret solution is required to generate an electret layer 2 attached to the porous matrix layer 1. Thus, the subsequent polarization process will endow the composite structure (mainly the electret layer 2) with piezoelectric activity, turning it into a piezoelectric active layer.

[0058] The above process is applicable to a variety of molding methods and materials, and the detailed parameters can be flexibly adjusted according to actual needs.

[0059] Applications of this smart insole: This smart insole conforms to the sole of the foot and monitors the distribution and dynamic changes of foot pressure in real time. When a user walks or exercises, the piezoelectric electret material inside the insole generates electrical signals due to pressure and deformation. The characteristics of its output signal are directly related to foot pressure and gait characteristics. By dispersing pressure and improving fit through a negative Poisson's ratio structure, this insole not only achieves accurate monitoring of dynamic foot biomechanical signals but also effectively relieves foot pressure and reduces the risk of foot injury. This smart insole has broad application prospects in fields such as early screening and intervention for diabetic foot, gait analysis and rehabilitation, intelligent motion monitoring, human-computer interaction, and virtual reality.

[0060] The advantages of this invention are as follows: By combining a negative Poisson's ratio structure with piezoelectric electret materials, this invention fully utilizes the inherent characteristics of piezoelectric electrets, such as the conversion between mechanical and electrical signals and self-powered capabilities, while also achieving complementary advantages between the two. This results in higher comfort, impact resistance, and sensitivity, breaking through the limitations of traditional smart insoles and demonstrating a deep coupling between mechanics and electricity. The specific advantages are described below:

[0061] a) Self-powered capability and high sensitivity: The sensor constructed from a piezoelectric electret can directly convert mechanical energy into electrical energy using the piezoelectric effect, eliminating the need for an external power source. This endows the insole with self-powered capability during walking, making it particularly suitable for data recording and measurement during walking, thus significantly simplifying the insole's structural design. Simultaneously, the lateral expansion effect of the negative Poisson's ratio structure not only enhances the lateral piezoelectric effect of the piezoelectric electret but also enables it to produce uniform deformation under pressure, thereby significantly improving the sensitivity of signal acquisition and ensuring efficient and stable operation of the device in various usage environments.

[0062] b) Dynamic monitoring: The sensor array can monitor the distribution of plantar stress and gait changes in real time and accurately.

[0063] c) Strong fit: The negative Poisson's ratio structure can expand laterally under pressure, significantly improving the fit of the foot.

[0064] d) Pressure relief: Effectively reduces plantar pressure and decreases the probability of heel ulcers, especially suitable for diabetic patients.

[0065] e) Impact resistance: The negative Poisson's ratio structure improves the impact resistance of the insole, making it suitable for high-intensity applications.

[0066] f) Biocompatibility and antibacterial properties: Silver nanowire electrodes are used to ensure safety and hygiene.

[0067] Example 1

[0068] This embodiment takes the three-dimensional thermal compression method as an example, such as Figure 1 , 3 As shown:

[0069] (1) Select 0.1 kg of PLA foam and cut it into multiple 100 mm × 100 mm × 20 mm cubic samples; (2) Place one cubic sample in a triaxial aluminum mold with an internal size of 50 mm × 50 mm × 10 mm, and apply multiaxial pressure through mechanical compression to compress the PLA sample to 50% of its initial size in all directions; (3) Place the mold containing the compressed PLA sample in an oven at 100°C and heat for 15 minutes to change the original structure inside the foam. After heating, remove the mold, stretch the PLA sample in multiple axes to prevent the skeleton from sticking, put it back into the mold, and reheat to 90°C. (3) Keep the sample from cooling during the entire process to prevent early fixation of the internal structure; (4) Repeat the hot pressing procedure three times, then place the sample in a 50°C oven and heat for 1 hour, and then let it cool naturally at room temperature to fix its negative Poisson's ratio structure; (5) Place the sample in a corona polarization device, set the polarization voltage to 15kV and the polarization time to 20 minutes; (6) Cover the sample surface with a polyimide mask with a size of 280mm×105mm, a thickness of 7.5μm, a circular opening shape, a pore diameter of 10mm, and a pore spacing of 20mm; (7) Fix the masked sample on an automatic coating machine and coat it with a nano silver wire solution with a mass fraction of 0.26wt% using wire bars with a spacing of 20μm. During the coating process, keep the sample moving at a speed of 55 mm / s to ensure that the coating evenly covers the entire sample surface. After coating, remove the mask and draw out the electrode leads from the coated silver nanowire area respectively; (8) Cut the foam sample according to the design shape of the insole and cut out an insole sample of size 42; (9) Finally, use Ecoflex 00-30 material to encapsulate the insole sample, and the smart insole is completed.

[0070] Example 2

[0071] This embodiment takes the template method as an example, such as Figure 1 , 3 As shown:

[0072] (1) Select 0.2 kg of IXPP foam, clean, crush and dry it to obtain small particles with uniform particle size; (2) Use a mold with a concave hexagonal structure and negative Poisson's ratio to load the foam particles into the mold; (3) Place the mold in a hot press and heat it to 150°C, while applying a pressure of 10 MPa, and keep it for 30 minutes to melt the particles and fill the mold evenly; (4) After hot pressing, take out the mold and cool it to room temperature, peel it off to form a foam sample with a negative Poisson's ratio structure; (5) Put the sample into a corona polarization device, set the polarization voltage to 13 kV and the polarization time to 15 minutes; (6) Cover the sample surface with a polyimide mask with a size of 250 mm × 90 mm, a thickness of 7.5 μm, a circular opening shape, a pore diameter of 10 mm and a pore spacing of 20 mm; (7) Fix the sample covered with the mask on an automatic coating machine, and use wire bars with a spacing of 30 μm to coat it with a nano silver wire solution with a mass fraction of 0.15 wt%. During the coating process, keep the sample moving at a speed of 60 mm / s to ensure that the coating evenly covers the entire sample surface. After coating, remove the mask and draw out the electrode leads from the coated silver nanowire area respectively; (8) Cut the foam sample according to the design shape of the insole and cut out an insole sample of size 36; (9) Finally, use Ecoflex 00-30 material to encapsulate the insole sample, and the smart insole is completed.

[0073] Example 3

[0074] This embodiment uses 3D printing as an example, such as... Figure 1 , 3 As shown:

[0075] (1) Select 0.5 kg of PLA foam and process it into powder using a grinding equipment. Then dry the powder to ensure that the material is compatible with the printing equipment before feeding it into the 3D printer; (2) Use AutoCAD software to design a shoe insole model with a recessed honeycomb unit structure (negative Poisson's ratio structure) and a size of 39, and convert it into a corresponding control program; (3) Connect the computer to the 3D printer and use the control program to drive the 3D printer, and set the printing parameters: laser power of 30W, scanning speed of 1.2m / s, powder layer thickness of 100μm, and printing ambient temperature of 170℃. During the printing process, the insole sample is manufactured by sintering layer by layer to ensure that the printed foam structure has a precise negative Poisson's ratio characteristic; (4) After printing, the sample is placed in a corona polarization device, and the polarization voltage is set to 20kV and the polarization time is 15 minutes; (5) A polyimide mask with a size of 260mm×95mm, a thickness of 7.5μm, a circular opening shape, a pore diameter of 10mm, and a pore spacing of 20mm is covered on the sample surface; (6) The sample covered with the mask is fixed on an automatic coating machine, and a nano silver wire solution with a mass fraction of 0.2wt% is coated using wire bars with a spacing of 10μm. During the coating process, the sample is kept moving at a speed of 50m / s to ensure that the coating uniformly covers the entire surface of the insole. After the coating is completed, the mask is removed, and electrode leads are led out from the coated silver nanowire area; (7) Finally, the insole sample is encapsulated using Ecoflex00-30 material, and the smart insole is completed.

[0076] Example 4

[0077] In this embodiment, the porous matrix layer does not use electret materials and takes a three-dimensional hot compression method combined with an impregnation process as an example. Figure 2 , 3 As shown:

[0078] (1) Select 0.3 kg of PU foam and cut it into multiple 100 mm × 100 mm × 20 mm cube samples. (2) Place one cube sample in a custom triaxial aluminum mold with an internal size of 50 mm × 50 mm × 10 mm. Apply multiaxial pressure through mechanical compression to compress the PU sample to 50% of its initial size in all directions. (3) Place the mold containing the compressed PU sample in an oven at 210°C and heat for 15 minutes to change the original structure inside the foam. After heating, remove the mold, stretch the PU sample in multiple axes to prevent the skeleton from sticking, put it back into the mold, and reheat to 200°C. (4) Keep the sample from cooling during the entire process to prevent premature fixation of the internal structure; (5) After repeating the above hot pressing procedure three times, place the sample in a 100°C oven and heat for 1 hour, then allow it to cool naturally at room temperature to fix its negative Poisson's ratio structure; (6) Dissolve Teflon AF 1600 powder in FC-43 solvent to prepare a 5wt% solution, and stir magnetically at 70°C for 4 hours until completely dissolved; (7) Completely immerse the porous matrix layer obtained in step (4) in the Teflon AF solution, and maintain it under vacuum (-0.1MPa) for 30 minutes to ensure that the solution fully penetrates to the pore walls. Then fix the impregnated porous matrix layer on the spin coater turntable and spin coat at 500RPM for 1 minute to remove excess solution; (8) Place the sample in a 100°C oven to cure for 3 hours to form a Teflon matrix uniformly attached to the pore walls. (8) Place the sample into a corona polarization device, set the polarization voltage to 18kV and the polarization time to 20 minutes; (9) Cover the sample surface with a polyimide mask with a size of 270mm×100mm, a thickness of 7.5μm, a circular opening shape, a pore diameter of 10mm, and a pore spacing of 20mm; (10) Fix the masked sample on an automatic coating machine and coat it with a 0.2wt% silver nanowire solution using wire rods with a spacing of 20μm. During the coating process, keep the sample moving at a speed of 60mm / s to ensure that the coating evenly covers the entire sample surface. After coating, remove the mask and lead out electrode leads from the coated silver nanowire area; (11) Cut the foam sample according to the design shape of the insole to cut out an insole sample with a size of 40; (12) Finally, encapsulate the insole sample with Ecoflex 00-30 material, and the smart insole is completed.

[0079] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A shoe insole based on a negative Poisson's ratio piezoelectric electret, characterized in that, It includes a piezoelectric electret support structure, a conductive electrode layer (3), and a protective layer (4); The piezoelectric electret support structure is a porous matrix layer (1) made of electret material, or a porous matrix layer (1) with an electret layer (2) on its surface; the piezoelectric electret support structure is a negative Poisson's ratio structure. The conductive electrode layer (3) is coated on the surface of the piezoelectric electret support structure. The conductive electrode layer (3) is connected to electrode leads for conducting electrical signals generated by the piezoelectric electret support structure. The protective layer (4) is encapsulated outside the piezoelectric electret support structure and the conductive electrode layer (3).

2. The insole based on a negative Poisson's ratio piezoelectric electret according to claim 1, characterized in that, The porous matrix layer (1) is an electret material, including polypropylene, fluoropolymer and polylactic acid.

3. The insole based on a negative Poisson's ratio piezoelectric electret according to claim 1, characterized in that, The porous matrix layer (1) is a non-electret material, including polyurethane foam, silicone and thermoplastic elastomer; the electret layer (2) is an electret material, including polypropylene, fluoropolymer and polylactic acid.

4. The insole based on a negative Poisson's ratio piezoelectric electret according to claim 1, characterized in that, The negative Poisson's ratio structure is a concave porous structure.

5. The insole based on a negative Poisson's ratio piezoelectric electret according to claim 1, characterized in that, The conductive electrode layer (3) is silver nanowire.

6. The insole based on a negative Poisson's ratio piezoelectric electret according to claim 1, characterized in that, The protective layer (4) is a flexible elastomer material.

7. A method for preparing a shoe insole based on a negative Poisson's ratio piezoelectric electret as described in any one of claims 1-6, characterized in that, Includes the following steps: S1: Prepare a piezoelectric electret support structure with a negative Poisson's ratio structure, and polarize the piezoelectric electret support structure; S2: A conductive electrode layer (3) is prepared on the surface of the piezoelectric electret support structure by means of a mask method, and electrode leads are drawn out from the conductive electrode layer (3); S3: The piezoelectric electret support structure and conductive electrode layer (3) are encapsulated by a protective layer (4) to form the insole.

8. The method for preparing a shoe insole based on a negative Poisson's ratio piezoelectric electret according to claim 7, characterized in that, When the piezoelectric electret support structure is a porous matrix layer (1) made of electret material, in step S1: a porous matrix layer (1) with negative Poisson bit properties is prepared by thermal compression, template method or 3D printing method. When the piezoelectric electret support structure is a porous matrix layer (1) with an electret layer (2) on its surface, in step S1: a porous matrix layer (1) with negative Poisson bit properties is prepared by thermal compression, template method or 3D printing method, and then an electret layer (2) is covered on the surface of the porous matrix layer (1) by impregnation method.

9. An application of a shoe insole based on a negative Poisson's ratio piezoelectric electret as described in any one of claims 1-6, characterized in that, The insole is installed inside the wearer's shoe to monitor the distribution and dynamic changes of foot pressure in real time.

10. The application of a shoe insole based on a negative Poisson's ratio piezoelectric electret according to claim 9, characterized in that, When the wearer walks or exercises, the piezoelectric electret support structure in the insole is compressed and deformed, generating an electrical signal. An external signal processing platform receives the electrical signal and obtains the geometric deformation of the insole through the changes in the electrical signal, thereby realizing the monitoring of the force distribution and magnitude of the insole, and thus obtaining the characteristics of foot pressure change and gait characteristics.

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