PDMS-MWCNTs flexible dielectric material with stepped microstructure and preparation method and application of PDMS-MWCNTs flexible dielectric material

Abstract

The invention discloses a PDMS-MWCNTs (Polydimethylsiloxane-Multiwalled Carbon Nanotubes) flexible dielectric material with a stepped microstructure as well as a preparation method and application of the PDMS-MWCNTs flexible dielectric material. The flexible dielectric material comprises a continuous layer and a plurality of groups of step structures located on the continuous layer, and each group of step structure comprises at least two stages of dielectric layers; the size of the cross section of the dielectric layer of the next stage is larger than that of the cross section of the dielectric layer of the previous stage; the flexible dielectric material contains polydimethylsiloxane and a multi-walled carbon nanotube.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of pressure sensors, and particularly relates to a PDMS-MWCNTs flexible dielectric material with a stepped microstructure, a preparation method thereof and application. BACKGROUND

[0002] The statements herein are provided only to aid in an understanding of the present application and are not necessarily intended to constitute the prior art.

[0003] With the rapid development of flexible electronics, the demand for flexible pressure sensors in the fields of medical health monitoring, human-computer interaction, wearable devices, etc. is growing. Capacitive flexible pressure sensors have become a research hotspot in the field of flexible sensors due to their high sensitivity, wide working range and good stability.

[0004] However, traditional capacitive flexible pressure sensors still have some problems in practical applications, such as limited working range, insufficient sensitivity, slow response speed, etc. The traditional dielectric material, polydimethylsiloxane (PDMS), is prone to permanent deformation after repeated compression, resulting in drift of the capacitance signal.

[0005] These problems limit their widespread application in complex application scenarios. Therefore, it is of great significance to develop a high-performance capacitive flexible pressure sensing material. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a PDMS-MWCNTs flexible dielectric material with a stepped microstructure, a preparation method thereof and application.

[0007] The above-mentioned purpose is achieved by the following technical solutions: In a first aspect, the present application provides a PDMS-MWCNTs flexible dielectric material with a stepped microstructure, comprising a continuous layer and a plurality of groups of stepped structures located on the continuous layer, each group of stepped structures comprising at least two stepped dielectric layers; the cross-sectional size of the lower stepped dielectric layer is greater than that of the upper dielectric layer; the material of the continuous layer and the stepped dielectric layer is a flexible dielectric material containing polydimethylsiloxane (PDMS) and multi-walled carbon nanotubes (MWCNTs).

[0008] In some embodiments, the stepped structure comprises 2-6 stepped dielectric layers.

[0009] In some embodiments, the cross-section of each stepped dielectric layer of the stepped structure is circular.

[0010] In some embodiments, the radius of the uppermost stepped dielectric layer of the stepped structure is 2-5 mm, preferably 3 mm; the radius of the lowermost stepped dielectric layer is 5-7 mm, preferably 5.5 mm.

[0011] The distance between the centers of each set of stepped structures is 13-17 mm, preferably 15 mm.

[0012] In a second aspect, the present application provides a preparation method of a PDMS-MWCNTs flexible dielectric material with stepped microstructures, comprising the following steps: S1, using polytetrafluoroethylene material to prepare a stepped mold; S2, mixing polydimethylsiloxane (PDMS) main agent and multi-walled carbon nanotubes (MWCNTs), stirring while sequentially adding diluent and curing agent, and then degassing, placing in the stepped mold, and then heating and curing; S3, obtaining the PDMS-MWCNTs flexible dielectric material with stepped microstructures after demolding.

[0013] In a third aspect, the present application provides an application of the PDMS-MWCNTs flexible dielectric material with stepped microstructures in pressure detection, in particular for the preparation of a pressure sensor.

[0014] In a fourth aspect, the present application provides a pressure sensor comprising the PDMS-MWCNTs flexible dielectric material with stepped microstructures, comprising the PDMS-MWCNTs flexible dielectric material with stepped microstructures, and an upper electrode layer, an auxiliary dielectric layer, and a lower electrode layer. The auxiliary dielectric layer is a PDMS-MWCNTs film with holes, and the hole position corresponds to the stepped structure; the upper electrode layer and the lower electrode layer are located at the uppermost layer and the lowermost layer, respectively.

[0015] In some embodiments, the diameter of the hole of the auxiliary dielectric layer is not less than the outer diameter size of the lowermost level of the stepped structure, and the hole thickness is equal to the height of the stepped structure. The auxiliary dielectric layer is introduced to avoid changes in foot pressure distribution in actual application.

[0016] In a fifth aspect, the present application provides a foot pressure detection system comprising the PDMS-MWCNTs flexible dielectric material with stepped microstructures or the pressure sensor.

[0017] It further comprises an upper shielding layer above the sensor and a lower shielding layer below the sensor.

[0018] It further comprises a reference capacitor connected to the stepped dielectric film sensor and a measured capacitor.

[0019] PDMS itself is a typical elastomer material, but when the stress exceeds its yield limit or is subjected to large stress for a long time, irreversible deformation (such as molecular chain slipping, breaking, etc.) may still occur. The present application solves the above problems in the following ways: 1. Adding carbon nanotubes as a composite material improves the mechanical strength (elastic modulus, yield limit, etc.) of PDMS; 2. The step structure design optimizes the stress distribution of the sensor, relieves stress concentration, and reduces the stress peak when receiving the same pressure.

[0020] 3. The auxiliary dielectric layer located at the periphery of each step microstructure is designed to avoid the problem of permanent deformation of the step dielectric layer after repeated compression, reducing the defect of capacitance signal drift.

[0021] The beneficial effects achieved by one or more embodiments of the above invention are as follows: 1. The present application provides a step microstructure design, which gradually expands the electrode contact area by using multi-stage step deformation to relieve stress concentration effect; the present application designs an auxiliary dielectric layer located at the periphery of each step microstructure to reduce the defect of capacitance signal drift.

[0022] 2. The step structure provided by the present application increases the working range of the sensor as the number of steps increases, even in the pressure range close to the working limit, the capacitance still maintains a large increase, which is beneficial to reduce the interference of environmental fluctuations on the sensor; the sensitivity is increased; the step structure has stable pressure response, prolonging the service life.

[0023] 3. Cost and process advantages: the present application uses a template method for preparation process, which does not require complex equipment, the mold can be reused, significantly reducing production cost; at the same time, the step-by-step forming design of the step structure and the auxiliary dielectric layer simplifies the assembly process, which is suitable for large-scale production.

[0024] 4. The dielectric material provided by the present application has strong environmental adaptability: through the cooperative design of the step microstructure and the auxiliary dielectric layer, the resistance of the material to uneven pressure distribution and complex environmental interference is significantly enhanced, which is suitable for diversified scenes such as medical monitoring, sports rehabilitation, intelligent robots, etc.

[0025] 5. The dielectric material provided by the present application has high application flexibility: the step number (2-6 steps) can be flexibly adjusted to adapt to different range requirements. BRIEF DESCRIPTION OF DRAWINGS

[0026] The drawings accompanying the specification of the present application form a part of the specification and serve to further illustrate the present application, the illustrative embodiments of the present application and the description thereof serve to explain the present application and do not constitute an improper limitation of the present application.

[0027] Figure 1 is the structure of a dielectric material with 3 steps.

[0028] Figure 2Figure 1 is a schematic diagram of a capacitive flexible pressure sensor structure; wherein, 1 - stepped structure layer, 2 - auxiliary dielectric layer, 3 - upper electrode layer, 4 - lower electrode layer.

[0029] Figure 3 Figure 2 is a photograph of a single electrode unit and electrode array; wherein, (a) electrode unit; (b) electrode array.

[0030] Figure 4 Figure 3 is a pressure-capacitance image of a stepped dielectric film sensor.

[0031] Figure 5 Figure 4 is the effect of stepped dielectric film on sensor sensitivity.

[0032] Figure 6 Figure 5 is the response time and recovery time of a stepped dielectric film sensor; wherein, (a) 2 steps; (b) 3 steps; (c) 4 steps; (d) 5 steps; (e) 6 steps.

[0033] Figure 7 Figure 6 is the overall test of a stepped dielectric film sensor; wherein, (a) hysteresis curve of a sensor with 6 steps under loading and unloading; (b) consistency curve of 3 sensors on a sensor array.

[0034] Figure 8 Figure 7 is a 2000-cycle dynamic durability test of a sensor with 6 steps; wherein, (a) 2000 cycles; (b) first 10 cycles; (c) last 10 cycles.

[0035] Figure 9 Figure 8 is a continuous dynamic test under different parameters; wherein, (a) continuous dynamic test under different frequencies; (b) continuous dynamic test under different transient pressures; (c) continuous dynamic test under different steady-state pressures.

[0036] Figure 10 Figure 9 is a schematic diagram of a foot pressure detection system design; wherein, (a) foot anatomy schematic diagram; (b) flexible pressure sensor point location schematic diagram; (c) sensor array photograph.

[0037] Figure 11 Figure 10 is a schematic diagram of the structure of a foot pressure detection system; wherein, (a) screen printing process based on foot pressure; (b) PET-conductive carbon screen printed electrode; (c) stepped capacitive flexible pressure sensor array for foot pressure detection.

[0038] Figure 12 Figure 11 is a static posture detection; wherein, (a) standing posture schematic diagram and pressure comparison table; (b) sensor relative capacitance value and pressure image when the first tester stands; (c) sensor relative capacitance value and pressure image when the second tester stands; (d) sensor relative capacitance value and pressure image when the third tester stands.

[0039] Figure 13 For walking posture foot pressure detection; wherein (a) the change curve of each sensor capacitance of walking posture; (b) the foot pressure image of the heel touch ground stage; (c) the foot pressure image of the heel touch ground stage; (d) the foot pressure image of the heel off ground stage; (e) the foot pressure image of the toe off ground stage.

[0040] Figure 14 For jumping posture foot pressure detection; wherein (a) the change curve of each sensor capacitance of jumping posture; (b) the foot pressure image of the standing stage; (c) the foot pressure image of the squatting pre-swing stage; (d) the foot pressure image of the take-off stage.

[0041] Figure 15 For foot internal and external rotation pressure detection results; wherein (a) the pressure comparison table; (b) the relative capacitance value and pressure image of each sensor of "external eight"; (c) the relative capacitance value and pressure image of each sensor of "internal eight". DETAILED DESCRIPTION

[0042] It should be noted that the following detailed description is illustrative only, and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0043] In some embodiments of the present application, a PDMS-MWCNTs flexible dielectric material with stepped microstructure is provided, which comprises a continuous layer and a plurality of groups of stepped structures located on the continuous layer, each group of stepped structures comprising at least two stepped dielectric layers; the cross-sectional size of the lower stepped dielectric layer is larger than that of the upper stepped dielectric layer; the material of the continuous layer and the stepped dielectric layers is a flexible dielectric material containing polydimethylsiloxane (PDMS) and multi-walled carbon nanotubes (MWCNTs).

[0044] In some embodiments, the stepped structure comprises 2-6 stepped dielectric layers.

[0045] In some embodiments, the cross-section of each stepped dielectric layer of the stepped structure is circular.

[0046] In some embodiments, the radius of the uppermost stepped dielectric layer of the stepped structure is 2, 3, 4, 5 mm or any value between 2-5 mm; the radius of the lowermost stepped dielectric layer is 5, 6, 7 mm or any value between 5-7 mm.

[0047] The distance between the centers of each group of stepped structures is 13, 14, 15, 16, 17 mm or any value between 13-17 mm.

[0048] The size and spacing of the electrodes are determined according to the design of the resolution and density of the sensing points. In the embodiment of the present application, the radius of the uppermost dielectric layer is less than 5 mm, and cannot be greater than the size of the electrode, and is greater than 2 mm, and if it is too small, the capacitance will be reduced.

[0049] In some embodiments, a method for preparing a PDMS-MWCNTs flexible dielectric material with a stepped microstructure includes the following steps: S1, a stepped mold is prepared using polytetrafluoroethylene material, the stepped mold has a stepped structure cavity, the stepped structure cavity has 2-6 steps, the total thickness is 0.8-1.5 mm, and the designed dielectric material thickness is the sum of the total height of the stepped structure cavity inside the mold and the thickness of the continuous layer; in an embodiment, the total height of the mold stepped structure is about the thickness of the film minus 0.4 mm; S2, mixing polydimethylsiloxane (PDMS) main agent and multi-walled carbon nanotubes (MWCNTs), stirring while sequentially adding diluent and curing agent, and then degassing, placing in the stepped mold, and then heating and curing; S3, after demolding, a PDMS-MWCNTs flexible dielectric material with a stepped microstructure is obtained.

[0050] In some embodiments of the present application, the application of the PDMS-MWCNTs flexible dielectric material with a stepped microstructure in pressure detection is provided, especially for preparing a pressure sensor.

[0051] In some embodiments of the present application, a sensor containing a PDMS-MWCNTs flexible dielectric material with a stepped microstructure is provided, which contains the PDMS-MWCNTs flexible dielectric material with a stepped microstructure, and an upper electrode layer, an auxiliary dielectric layer and a lower electrode layer, the auxiliary dielectric layer is a PDMS-MWCNTs film with holes, the hole position corresponds to the stepped structure described above; the upper electrode layer and the lower electrode layer are located at the uppermost layer and the lowermost layer, respectively.

[0052] In some embodiments, the diameter of the hole of the auxiliary dielectric layer is not less than the outer diameter size of the lowermost step of the stepped structure, and the hole thickness is equal to the height of the stepped structure. The auxiliary dielectric layer is introduced to avoid changes in foot pressure distribution in actual application.

[0053] The auxiliary dielectric layer with hole structure is designed to wrap the stepped structure and partially support the space near the stepped structure. If the thickness of the hole is small, it is equivalent to only the stepped structure. In addition to saturation under smaller pressure and reducing the sensing range, it also reduces the service life of the sensor, making it more prone to deformation and damage.

[0054] In some embodiments of the present application, a foot pressure detection system is provided, comprising the PDMS-MWCNTs flexible dielectric material with stepped microstructure or the sensor.

[0055] Further comprising an upper shielding layer above the sensor and a lower shielding layer below the sensor.

[0056] Further comprising a reference capacitor and a measured capacitor connected to the stepped dielectric film sensor.

[0057] The technology will be described in detail below in conjunction with specific embodiments.

[0058] Materials: In the embodiments of the present application, the PDMS main agent and curing agent used are fixed reagents of the commercial product "Dow Corning DC184".

[0059] In the embodiments of the present application, isopropyl alcohol is used as a solvent for dispersing carbon nanotubes, which will be removed in the subsequent heating process, and os-20 is used as a diluent to improve the viscosity of PDMS.

[0060] Os-20 is commonly used as a cleaning agent, and is used as a diluent in the embodiments of the present application, effectively maintaining the plasticity of the cured composite material.

[0061] Embodiment 1 A PDMS-MWCNTs flexible dielectric material with stepped microstructure Figure 1 A structure of a three-step stepped dielectric material is provided, including a continuous layer 104 and a plurality of groups of stepped structures on the continuous layer 104, each group of stepped structures including a first-step dielectric layer 101, a second-step dielectric layer 102, and a third-step dielectric layer 103. The continuous layer 104 is of the same material as the stepped dielectric layer and is formed when the template method is used to prepare the stepped structure.

[0062] As a typical embodiment, the thickness of the continuous layer 104 is 0.4 mm.

[0063] The cross section of each dielectric layer is circular.

[0064] The radius of the uppermost stepped first-step dielectric layer 101 is 3 mm, and the radius of the lowermost stepped third-step dielectric layer 103 is 5.5 mm.

[0065] The distance between the centers of each group of stepped structures is about 15 mm.

[0066] The preparation method of the stepped microstructure dielectric layer is as follows: First, a stepped mold is prepared using polytetrafluoroethylene (PTFE). The number of steps is 2-6, the total thickness is 0.8-1.5 mm, and the total height of the stepped structure is approximately the film thickness minus 0.4 mm (continuous layer 104).

[0067] Simultaneously, a dielectric layer is prepared: Weigh the PDMS main agent into a container, heat and stir at 100℃ and 900 r / min to reduce the viscosity of the PDMS main agent; mix MWCNTs with isopropanol, the concentration of MWCNTs in isopropanol should be between 0.5-5wt%, in this example it is 2wt%; stir to mix the MWCNTs evenly; then pour it into the PDMS main agent, controlling the mass fraction of MWCNTs in PDMS to be between 0.5%-3wt%, in this example it is 2.5wt%; continue heating and stirring the PDMS-MWCNTs-isopropanol mixed solution until the bubbles in the beaker disappear, at which point the isopropanol has evaporated completely; then sonicate the evaporated solution for at least 30 minutes to break up and disperse the agglomerated MWCNTs, to obtain the PDMS-MWCNTs mixed solution. The ultrasonically treated PDMS-MWCNTs mixture was stirred at 500-1200 r / min, and the diluent and curing agent were added sequentially. The mixture was stirred continuously until fully mixed. The mass ratio of PDMS main agent to curing agent was 5:1 to 20:1, and in this example it was 10:1. The resulting mixture was vacuumed to remove air bubbles, and then injected into the mold surface and cured in an oven at 80°C for 2 hours.

[0068] Obtain as Figure 1 The stepped structure shown.

[0069] Example 2 Capacitive flexible pressure sensor containing PDMS-MWCNTs flexible dielectric material with stepped microstructure like Figure 2 As shown, the structure includes a PDMS-MWCNTs flexible dielectric material with a stepped microstructure provided in Example 1 as a stepped structure layer 1, an upper electrode layer 3, an auxiliary dielectric layer 2, and a lower electrode layer 4. The auxiliary dielectric layer is used to fill the gaps in the stepped structure and retain the degree of freedom of deformation. The auxiliary dielectric layer structure is a PDMS-MWCNTs thin film with holes. The position of the holes corresponds to the stepped structure described above. The diameter of the holes is not less than the outer diameter of the lowest step of the stepped structure, and the thickness of the holes is equal to the height of the stepped structure.

[0070] The method for preparing the auxiliary dielectric layer is as follows: A PDMS-MWCNTs mixed solution with added curing agent, in the same proportion as the dielectric material in Example 1, was injected into the cavity mold. After curing, an auxiliary dielectric layer was formed. This auxiliary dielectric layer ensures that the stepped gaps are evenly distributed after filling by precisely matching the cavity positions with the stepped structure, while avoiding pressure distribution imbalance.

[0071] The electrode layer is prepared by: Polyethylene terephthalate (PET) is used as the electrode substrate, and conductive carbon adhesive is used as the conductive material. The conductive carbon adhesive is screen-printed onto the PET substrate to form a patterned electrode array. The electrode array adopts a 3×3 patterned design, containing several groups of electrode units. Each electrode unit has a circular electrode point diameter of 10 mm, the spacing between electrode units is 15 mm, and the wire width is 1 mm. The wires are connected to the electrode points and then to external signal lines.

[0072] Physical diagrams of a single electrode unit and an electrode array are shown below. Figure 3 As shown. Its preparation method is as follows: 1) Clean the wire mesh and squeegee with alcohol or dish soap to ensure the wire mesh surface is clean.

[0073] 2) Fix the PET substrate on the experimental platform and align the screen printing plate with the designated position on the PET substrate. Since the screen printing plate is prone to slipping and causing smearing during screen printing, double-sided tape is needed to fix the screen printing plate to the electrode substrate.

[0074] 3) Stir the conductive carbon adhesive evenly and pour it onto one side of the screen. Use a squeegee to spread it evenly over the entire screen, ensuring the conductive carbon adhesive is coated evenly. The conductive gel should be coated onto the entire screen in one application; avoid repeated or overlapping applications, otherwise it will cause rough edges or smudged printing.

[0075] 4) Place the printed electrodes in a 120℃ oven and heat for 10-20 minutes to cure the conductive carbon adhesive and form a stable electrode layer.

[0076] During assembly, such as Figure 2 As shown, the holes of the auxiliary dielectric layer are fitted onto the stepped structure of the stepped structure layer. The auxiliary dielectric layer is fixed to the continuous layer with sealant. The upper electrode layer and the lower electrode layer are located on the upper and lower surfaces of the stepped structure elastic modulus dielectric layer, respectively. The assembly method is as follows: Conductive carbon adhesive is used as the electrode layer, with dimensions matching the dielectric layer—larger than the upper stepped dielectric layer and smaller than the lower stepped dielectric layer. In one embodiment, the electrode size is 10mm in diameter, primarily for accuracy considerations and fabrication process precision. The upper and lower electrode layers are aligned with light to ensure accurate electrode positioning. A stepped-structured elastic modulus dielectric layer is placed between the upper and lower electrodes and fixed using silicone adhesive sealant, venting air to form an integrated capacitive flexible pressure sensor. An auxiliary dielectric layer is used internally to fill the gaps. Unprinted portions of the electrodes are clamped or glued to fix the positions of the upper and lower electrodes. Furthermore, the silicone adhesive sealant, after curing, does not affect the sensor's compressibility and flexibility.

[0077] Light alignment: Due to the optical transparency of the PET substrate of the electrode layer and the opaque conductive carbon adhesive layer printed on its surface, optical alignment can be performed based on this characteristic. The specific operation procedure is as follows: First, place the prepared upper and lower electrode layers above a light source. By transmitting the backlight through the PET substrate, fine-tune the relative positions of the two electrode layers under a microscope until their conductive carbon adhesive patterns are strictly aligned. Then, cut the PET substrate along the outer contour of the aligned electrodes to ensure that the upper and lower electrode layers have consistent dimensions. Finally, select three symmetrical positions in the non-functional area of ​​the two PET substrate layers (near the midpoint of two diagonals and one side) and drill a small hole. Place the dielectric layer between the upper and lower electrode layers, and calculate the dimensions so that the center of the electrode point is aligned with the center of each stepped structure. Use the hole as the positioning reference for the two electrode layers. After fixing with silicone adhesive sealant, cut off the edge portion of the lower electrode layer with the hole.

[0078] Experimental Example 1 1. The Influence of Stepped Dielectric Thin Films on the Sensor Operating Range Compression tests were conducted on the films using a WDW-5 microcomputer-controlled electronic universal testing machine manufactured by Jinan Sida Testing Technology Co., Ltd. The compression rate was 1 mm / min, and the compression displacement and force during the compression process were recorded. A digital multimeter (VC8246B) was used to measure and record the capacitance values ​​that changed in real time with pressure, and the data was connected to a computer in real time via a data cable.

[0079] It should be noted that the PDMS film continues to age in air after thermosetting. As the storage time at room temperature increases, the various physical states of the film will continue to change until they stabilize. To improve the reliability of the measurement data, all sensor measurements in this paper were performed 30 days after the film curing was complete.

[0080] Figure 4Images showing the relative and absolute capacitance of a capacitive flexible pressure sensor composed of dielectric films with 2, 3, 4, 5, and 6 steps as a function of pressure are displayed. The operating ranges are 0-380 kPa, 0-460 kPa, 0-510 kPa, 0-580 kPa, and 0-820 kPa, respectively. The relative capacitance increases by 314%, 375%, 420%, 452%, and 526%, respectively, while the absolute capacitance ranges are 10.4-43.1 pF, 10.5-49.9 pF, 10.7-55.7 pF, 10.8-59.7 pF, and 11-68.8 pF.

[0081] The operating range of this stepped dielectric film sensor increases with the number of steps. This is mainly because a dielectric film with a large number of steps has a smaller individual step height and width, making it more prone to deformation. Furthermore, as the dielectric film is gradually compressed, the contact area between the dielectric film and the electrode also gradually increases, leading to interfacial polarization. Even near the operating pressure limit, the capacitance maintains a significant increase, which helps reduce the interference of environmental fluctuations on the sensor. This type of sensor has a large operating range and is well-suited for foot pressure detection.

[0082] 2. The effect of stepped dielectric films on sensor sensitivity The testing equipment and procedures are the same as in section 1.

[0083] Figure 5 The sensitivity of the stepped dielectric thin film sensor is shown. The sensors with 2, 3, 4, 5, and 6 steps can be roughly divided into three sensitivity regions: high, medium, and low. The GF in the high sensitivity region is 2.06 kPa. -1 (0-80 kPa), 2.57 kPa -1 (0-76 kPa), 3.1 kPa -1 (0-74 kPa), 3.87 kPa -1 (0-64 kPa), 5.57 kPa -1 (0-47 kPa); the GF in the medium sensitivity region is 0.51 kPa. -1 (80-280 kPa), 0.55 kPa -1 (76-251 kPa), 0.56 kPa -1 (74-254 kPa), 0.58 kPa -1 (64-210 kPa), 0.77 kPa -1 (47-183 kPa); the GF in the low sensitivity region is 0.23 kPa. -1(280-380 kPa), 0.23 kPa -1 (251-460 kPa), 0.17 kPa -1 (254-510 kPa), 0.23 kPa -1 (210-580 kPa), 0.23 kPa -1 (183-820 kPa).

[0084] In general, as the number of steps in the dielectric film increases, the GF of the capacitive flexible pressure sensor composed of it shows an increasing trend in the high-sensitivity region. This is mainly because dielectric films with more steps are easier to compress, the MWCNTs inside the film aggregate more quickly, resulting in a faster increase in its dielectric constant, and the contact area between the dielectric film and the electrode changes more rapidly, thus exhibiting higher sensitivity. In the medium-sensitivity region, the GF increases more slowly with the increase of the number of steps, while in the low-sensitivity region, the relationship between the GF and the number of steps is not obvious.

[0085] 3. The effect of stepped dielectric films on sensor response time and recovery time The testing equipment and procedures are the same as in section 1.

[0086] Figure 6 The response and recovery times of the stepped dielectric thin film sensor are given. The response times of the capacitive flexible pressure sensor with 2, 3, 4, 5, and 6 steps are 124 ms, 139 ms, 144 ms, 152 ms, and 163 ms, respectively, and the recovery times are 156 ms, 171 ms, 144 ms, 194 ms, and 197 ms, respectively.

[0087] Overall, the response time and recovery time of the sensor increase with the number of steps. When a dielectric film with a larger number of steps is subjected to pressure and release pressure, the force needs to be transmitted step by step. The transmission time of the sensor with a larger number of steps is longer, and the MWCNTs inside the film need more time to displace, so it has a longer response time and recovery time.

[0088] The sensor's recovery time is longer than its response time, which is consistent with the general pattern of thin film compression. Since the thin film recovers without external force, relying solely on intermolecular elasticity, the sensor's recovery time is longer than its response time. The sum of the response and recovery times for each sensor is 280 ms, 310 ms, 329 ms, 346 ms, and 360 ms, respectively.

[0089] The statistical results are shown in Table 1.

[0090] Table 1

[0091] As can be seen from the comparison, the sensor with a 6-step scale has a range of up to 820 kPa and high sensitivity, so the 6-step scale was chosen for subsequent experiments.

[0092] 4. Overall Testing of Stepped Dielectric Thin Film Sensor The testing equipment and procedures are the same as in section 1.

[0093] When testing durability, a fatigue testing machine is used to repeatedly apply loads to the sensor.

[0094] The hysteresis of the sensor is the non-coincidence of the pressure-capacitance curves during loading and unloading, such as... Figure 7 As shown in Figure a, the maximum difference in relative capacitance between the sensor with a step number of 6 during loading and unloading occurs at a pressure of 500 kPa, where the maximum value is 18.5%. According to the national standard formula for calculating the hysteresis error of the sensor, the hysteresis error of this sensor is calculated to be Ɛ = 3.5%.

[0095] The main reason is the tiny slippage of conductive particles inside the dielectric film. The MWCNTs inside the dielectric film are attached to the Si-O main chain of PDMS. After the sensor operates to the pressure limit, some conductive channels inside the film are disconnected. During unloading, due to the creep phenomenon inside PDMS, the MWCNTs cannot return to their original positions in a short time. In practical applications, since the pressure on the sensor generally does not exceed its operating range, the hysteresis error will also become very small.

[0096] Sensor consistency is defined as the degree of performance uniformity among sensors with identical fabrication parameters. Consistency is verified by selecting three sensing points on the diagonal of a 3×3 sensor array, such as... Figure 7 Figure b shows the consistency curves of the three sensors on the sensor array. According to the consistency error calculation formula, the consistency error of the sensor array is calculated to be Ɛc = 2.1%.

[0097] Inconsistency errors in sensors are unavoidable, including both systematic and random errors. Systematic errors include inconsistencies in the dispersion of MWCNTs, inconsistencies in film thickness, and subtle differences in the stepped structure. Random errors include errors caused by inaccurate sensor assembly and improper operation during pressure-capacitance calibration. This sensor array exhibits low consistency error, meeting the requirements for plantar pressure detection.

[0098] To ultimately apply the stepped capacitive flexible pressure sensor to plantar pressure detection, a dynamic durability test was conducted 2000 times on the sensor with 6 steps at a maximum pressure of 300 kPa and a frequency of 0.4 Hz.

[0099] The results are as follows Figure 8 As shown, overall, the sensor's response to pressure is quite stable. After 2000 pressure cycles, the sensor's waveform remained unchanged, indicating that the sensor has a lifespan of at least 2000 cycles. In the dynamic durability test, the lower boundary of the dynamic durability curve is relatively neat, indicating that the sensor's initial capacitance is relatively stable. The upper boundary shows slight fluctuations, which may be due to the sensor's small thickness, causing some fluctuations in the applied force within a certain range, but overall it is relatively stable. Figure 8 Figures b and c show the first and last 10 cycles of the dynamic durability curve within 2000 cycles. It can be seen that the waveform of the sensor hardly changes, further confirming that the sensor has a reliability of at least 2000 cycles.

[0100] like Figure 9 As shown, Figure 9 In Figure 'a', the dynamic images of a sensor with a dielectric film step number of 6 under a pressure of 200 kPa at frequencies of 0.5 Hz, 1.0 Hz, and 2.0 Hz are shown. It can be seen that under the same pressure, the capacitance change of the sensor at different frequencies is basically consistent, with small fluctuations, but within a reasonable range, demonstrating the stability of the sensor at different dynamic frequencies.

[0101] Figure 9 Images b and c show the dynamic response of the sensor to pressures of 20 kPa, 50 kPa, 100 kPa, 200 kPa, 300 kPa, and 400 kPa. It can be seen that when a steady-state pressure is applied to the sensor, the sensor capacitance remains essentially constant, and the capacitance response to the steady-state force is essentially consistent with the capacitance response to the same transient force. The results indicate that the sensor has a stable response to pressures of different loading types, meeting the requirements for environmental changes during plantar pressure detection.

[0102] Example 3 A foot pressure detection system Its structure is as follows Figure 11 As shown, it mainly consists of five layers: an upper shielding layer 5, an upper electrode layer 3, an intermediate stepped dielectric layer 1 with six steps, a lower electrode layer 4, and a lower shielding layer 6. The intermediate stepped dielectric layer 1 is equipped with an auxiliary dielectric layer. The upper electrode layer 3, the intermediate stepped dielectric layer 1 with six steps, and the lower electrode layer 4 are prepared using the method of Example 2.

[0103] The upper and lower shielding layers are made of conductive fabric, which has good conductivity and can effectively isolate interference from external electric fields.

[0104] The stepped dielectric layer with 6 steps has an auxiliary dielectric layer to avoid changes in foot pressure caused by the raised steps. The upper and lower shielding layers are made of conductive fabric with excellent conductivity, which can perfectly isolate the interference of external electric fields.

[0105] Foot pressure detection system design: (1) Sensor point array design Polyethylene terephthalate (PET) was used as the electrode substrate, conductive carbon adhesive as the electrode conductive material, and polydimethylsiloxane (PDMS) and multi-walled carbon nanotubes (MWCNTs) as the dielectric materials. Based on the above research, the dielectric layer was selected with a step number of 6 and an auxiliary layer.

[0106] Based on the typical anatomical features of the human foot and everyday experience, the arrangement of sensor points should be determined.

[0107] As a typical embodiment, 11 sensing points are arranged at the first and third metatarsal bones, the first, third, and fifth metatarsal bones, the forefoot arch, the posterior foot arch, and the heel. The actual point distribution is as follows: Figure 10 As shown.

[0108] The foot pressure detection system is connected to the electrodes of the stepped dielectric film sensor via a signal line. The system processes the sensor data in real time through the built-in chip and hardware and sends it to the host computer.

[0109] Experimental Example 2 1. Static posture Three participants of different heights (170 cm, 174 cm, 180 cm) and weights (75 kg, 65 kg, 80 kg) were numbered 1, 2, and 3. Under the same conditions, the three participants placed their right feet on the pressure sensor array in turn.

[0110] Test results as follows Figure 12 As shown, experiments have proven that this method can be used to analyze the center of pressure on the sole of the foot, whether the foot is flat, in-toeing, or out-toeing.

[0111] Standing foot pressure test: Under the same conditions, three subjects with different heights (170 cm, 174 cm, 180 cm) and weights (75 kg, 65 kg, 80 kg) placed their right feet on a capacitive flexible pressure sensor array in turn, and the foot pressure measurement system was used to test the force on the feet of the three subjects.

[0112] It allows for a direct comparison of the pressure exerted on different parts of the feet of various participants, and the calculation of the center of pressure (CoP) of the feet when the participants are standing.

[0113] Based on the capacitance-pressure calibration curve, the pressure at various points on the feet of the three participants while they were standing was measured. The foot pressure distribution trends were consistent across the three participants: the pressure at point 11 on the heel was the highest (197, 163, 176 kPa), followed by points 9 and 10; the pressure at point 4 on the metatarsal bone was relatively low (148, 125, 138 kPa); and the pressure at point 8 on the arch of the foot was the lowest (0, 0, 25 kPa). Participant 3 experienced a pressure of 25 kPa on their arch, indicating that they had flat feet, suggesting that this array can be used for flat foot detection.

[0114] Calculations showed that the plantar pressure center (CoP) of the three participants was (-3.84, 16.35), (-13.31, 11.26), and (-9.42, 0.17), respectively, all located near the arch of the foot, which is ergonomic. The difference in CoP location reflects body posture and provides data support for determining standing stability and ergonomic research.

[0115] Foot pronation and supination pressure test results are as follows Figure 15 As shown: To verify the sensor array's ability to detect foot pronation (inward toeing) and supination (outward toeing), experimenter #1 simulated force by standing with a slight tilt. Real-time pressure maps visually displayed changes in the center of gravity. The experiment showed that during supination (center of gravity tilting inward), pressure significantly increased on the medial side of the foot (e.g., feet 1, 3, and 9), especially at the metatarsal bones (e.g., foot 3); pressure decreased on the lateral side of the foot (e.g., feet 2, 5, and 10). The medial side of the heel (foot 9) showed minimal pressure change due to the forward shift of the center of gravity offsetting the inward tilt.

[0116] Internal rotation (outward tilt of center of gravity): Pressure on the outer side of the foot (e.g., size 2, 5, 10) increases significantly, with the greatest change at the metatarsal bone (e.g., size 5); pressure on the inner side of the foot (e.g., size 1, 3, 9) decreases. Pressure on the outer side of the heel (size 10) also changes very little.

[0117] Calculations showed that during internal / external rotation, the center of plantar pressure (CoP) shifted not only medially / laterally compared to when standing, but also significantly forward (as shown in the experimental data). This aligns with the theory that the body requires greater foot support to maintain stability. This sensor array can effectively distinguish the plantar pressure distribution characteristics under different static movements, providing data support for the early diagnosis and treatment of bowlegs.

[0118] 2. Dynamic plantar pressure testing like Figure 13 and Figure 14 As shown, the sensor array has the ability to respond to dynamic pressure on the sole of the foot.

[0119] Walking posture foot pressure testing, such as Figure 13 As shown.

[0120] To monitor gait, sensors were placed inside the right shoe to detect dynamic changes in plantar pressure during walking. Walking is divided into four phases: Heel strike: The pressure of the heel sensors (9, 10, 11) increases from low to high value (e.g., 317 kPa for sensor 11), while the foot sensor reading is 0.

[0121] When the foot lands on the ground: the pressure distribution is similar to standing but the value is greater (the center of gravity shifts to the right foot).

[0122] Heel off the ground: The center of gravity shifts to the ball of the foot, and the pressure of the sensors at the front of the ball of the foot (such as No. 3, 4, and 5) reaches its peak (such as No. 3, 333 kPa), while the pressure on the heel gradually returns to zero.

[0123] Toes off the ground: When the foot leaves the ground, all sensor pressures return to zero.

[0124] Experiments have shown that this sensor array can respond to dynamic pressure and plot capacitance curves in real time. Combined with machine learning to collect big data, it can be applied to fall prevention and early warning systems for the elderly, and rehabilitation treatment for patients with cerebral palsy / stroke, showing great promise in the field of smart healthcare.

[0125] Jumping posture foot pressure test, such as Figure 14 As shown.

[0126] To verify the sensing range and dynamic detection capability, this experiment detected jumping posture (in three stages): Standing phase: The pressure distribution is the same as normal standing, the pressure at each point is basically stable, the center of pressure is near the arch of the foot, and the body is balanced.

[0127] During the pre-squat phase (approximately 700ms): the pressure on the forefoot (numbers 1-6) increases significantly (e.g., number 3 reaches 391 kPa), the pressure on the heel (numbers 9-11) decreases, and the overall pressure center shifts forward.

[0128] Takeoff phase: Pressure at each point increases sharply within 200ms, reaching its peak at the moment of takeoff (e.g., 608 kPa for forefoot No. 3); pressure returns to zero rapidly after takeoff.

[0129] The experiment verified the array's ability to detect high dynamic and large-scale local pressure in real time.

[0130] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A flexible dielectric material of PDMS-MWCNTs with a stepped microstructure, characterized in that, It includes a continuous layer and several sets of stepped structures on the continuous layer, each set of stepped structures containing at least two stepped dielectric layers; the cross-sectional size of the next stepped dielectric layer is larger than the cross-sectional size of the previous stepped dielectric layer; the continuous layer and the stepped dielectric layers are both made of flexible dielectric materials containing polydimethylsiloxane and multi-walled carbon nanotubes.

2. The PDMS-MWCNTs flexible dielectric material with a stepped microstructure according to claim 1, characterized in that, The stepped structure comprises 2 to 6 stepped dielectric layers.

3. The PDMS-MWCNTs flexible dielectric material with a stepped microstructure according to claim 1, characterized in that, The cross-section of each dielectric layer in the stepped structure is circular.

4. The PDMS-MWCNTs flexible dielectric material with a stepped microstructure according to claim 1, characterized in that, The radius of the uppermost step dielectric layer of the stepped structure is 2~5mm; the radius of the lowermost step dielectric layer is 5~7mm.

5. The method for preparing the PDMS-MWCNTs flexible dielectric material with a stepped microstructure according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1, a stepped mold is made of polytetrafluoroethylene, with 2-6 steps and a total thickness of 0.6-1.5 mm; S2, mix polydimethylsiloxane and multi-walled carbon nanotubes, add diluent and curing agent in sequence while stirring, then remove air bubbles, place in a stepped mold, and then heat to cure; S3, after demolding, yields PDMS-MWCNTs flexible dielectric material with a stepped microstructure.

6. The preparation method according to claim 5, characterized in that, In step S2, weigh the PDMS main agent and heat and stir; mix MWCNTs with isopropanol and stir to make the MWCNTs evenly mixed; then pour it into the PDMS main agent, controlling the mass fraction of MWCNTs in PDMS to be 0.5%-3%; continue heating and stirring the PDMS-MWCNTs-isopropanol mixed solution until the bubbles in the beaker disappear; then sonicate the solution after evaporating isopropanol for at least 30 minutes to obtain the PDMS-MWCNTs mixed solution; add the diluent and curing agent to the sonicated PDMS-MWCNTs mixed solution in sequence while stirring, and continue stirring to make it fully mixed; the mass ratio of PDMS main agent to curing agent is 5:1~20:1; vacuum the obtained mixture and then inject it into the mold surface and heat to cure.

7. The application of the PDMS-MWCNTs flexible dielectric material with a stepped microstructure as described in any one of claims 1 to 4 in pressure detection.

8. A pressure sensor, characterized in that, The material comprises a flexible dielectric material of PDMS-MWCNTs with a stepped microstructure as described in any one of claims 1 to 5, and an upper electrode layer, an auxiliary dielectric layer and a lower electrode layer, wherein the auxiliary dielectric layer is a PDMS-MWCNTs thin film with pores, the pore positions of which correspond to the stepped structure described above. The upper electrode layer and the lower electrode layer are located at the top and bottom layers, respectively.

9. The pressure sensor according to claim 8, characterized in that, The diameter of the holes in the auxiliary dielectric layer is not less than the outer diameter of the lowest step of the stepped structure, and the thickness of the holes is equal to the height of the stepped structure.

10. A foot pressure detection system, characterized in that, It includes the pressure sensor as described in claim 9.

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