A porous compressible inductor with a helical spring structure and a preparation method and application thereof

Abstract

This invention discloses a porous compressible inductor with a helical spring structure, its fabrication method, and its applications, belonging to the field of flexible sensing and electronic component technology. The porous compressible inductor with a helical spring structure includes a flexible elastic substrate, a helical conductive coil embedded within the substrate, and a uniform porous structure formed by elution with a soluble pore-forming agent. The helical conductive coil is arranged in a helical spring shape, enabling the device to simultaneously possess inductance and pressure sensing functions without the need for an additional independent inductor. The porous structure significantly improves the compressibility and resilience of the substrate, achieving a balance between high sensitivity and a wide pressure response range. This invention also provides a fabrication method of "core forming - coil winding - encapsulation and pore formation," which is simple and controllable, produces consistent finished products, and can be widely applied to various flexible pressure sensors, especially suitable for passive wireless sensing scenarios, providing a core technical solution for highly integrated, miniaturized flexible sensing nodes.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronic components and sensing technology, specifically relating to a porous compressible inductor with a helical spring structure, its preparation method, and its application in the field of flexible pressure sensing. Background Technology

[0002] With the rapid development of flexible electronics and IoT technologies, flexible pressure sensors have been widely used in structural health monitoring, wearable devices, industrial inspection, and human-computer interaction. Among them, passive wireless pressure sensing systems based on the LC resonance principle have become an important technological direction for distributed sensor networks due to their advantages such as no need for built-in batteries, long-distance signal reading, and flexible deployment. In the LC resonant pressure sensing system, inductors and capacitors are the core components constituting the resonant circuit. The pressure-sensitive unit changes the parameters of the inductor or capacitor to shift the resonant frequency or amplitude, thereby completing the wireless reading of the pressure signal.

[0003] Currently, mainstream LC resonant pressure sensors generally adopt a discrete structure design of "independent inductor + independent pressure-sensitive capacitor," meaning that a resonant circuit is composed of a fixed-value inductor and a pressure-sensitive capacitor. This type of solution has significant integration bottlenecks: on the one hand, the discrete inductor occupies a large device volume, making it difficult to achieve miniaturization and flexibility of the sensing node, and unable to adapt to the installation requirements of curved surfaces and confined spaces; on the other hand, discrete components require additional connection and packaging processes, which not only increases manufacturing costs but also increases the risk of connection failure under flexible deformation, reducing the reliability of the device in long-term service. To improve integration, some studies have attempted to structurally integrate the pressure-sensitive unit with the inductor, but most existing solutions only attach a pressure-sensitive layer to the surface of the inductor, essentially still a physical superposition of two functional units, failing to achieve deep functional integration from a structural perspective, and thus unable to fundamentally reduce device size and process complexity.

[0004] Meanwhile, the overall performance of flexible compressible pressure sensors also has significant shortcomings. Traditional flexible pressure sensors mostly use dense elastomers as substrates, which have limited compression deformation, resulting in low pressure sensitivity and a narrow effective response range, making it difficult to simultaneously cover the detection requirements of high sensitivity at low pressure and wide range at medium and low pressure. To improve compression performance, researchers have developed porous elastomer-based pressure sensors, which improve the compressibility of the substrate by creating pores. However, existing porous pressure sensors mostly use resistive or capacitive sensing mechanisms, rarely involving inductive sensing mechanisms. Furthermore, existing porous inductive elements generally suffer from high structural rigidity, poor compressibility, and insufficient rebound stability, making it impossible to maintain stable inductance parameter responses under repeated compression loads, and thus failing to meet the requirements for long-term cyclic detection.

[0005] Furthermore, in applications facing complex outdoor conditions such as structural health monitoring, sensing elements need to possess flexible and compressible mechanical adaptability, integrated passive resonant functionality, and simultaneously achieve high sensitivity, a wide response range, and excellent cyclic stability. However, existing technologies have consistently failed to simultaneously resolve these multiple contradictions: pursuing high integration and a design without additional inductors often sacrifices pressure sensing sensitivity and rebound stability; conversely, focusing on improving pressure sensing performance requires additional inductors, returning to the integration bottleneck of discrete structures. Therefore, developing a compressible inductor that combines inductor and pressure sensing functions, achieves high structural integration, and simultaneously possesses high sensitivity, excellent rebound stability, and a wide pressure response range has become a pressing technical challenge in this field.

[0006] It should be noted that the analysis of the above technical information is the result of creative labor. The detailed description of it in the background section is only intended to deepen the understanding of the non-obviousness of the overall background of this application by those skilled in the art, and should not be regarded as an admission or in any form an implication that the above technical information constitutes prior art known to those skilled in the art. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a porous compressible inductor with a helical spring structure, its preparation method, and its application. The technical problem to be solved is: how to achieve a flexible compressible pressure sensor that combines high sensitivity, excellent rebound stability, and a wide pressure response range without the need for additional independent inductor components.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0009] A porous compressible inductor with a helical spring structure is disclosed. By embedding a helical conductive coil within a porous flexible elastic matrix, the device simultaneously possesses the electrical functions of an inductor and the sensing functions of a pressure-sensitive unit, eliminating the need for a separate inductor. Furthermore, the porous structure enhances the compressibility and resilience of the matrix, achieving a balance between high sensitivity and a wide pressure response range. Specifically, the porous compressible inductor with a helical spring structure provided by this invention includes a flexible elastic matrix, a helical conductive coil embedded within the flexible elastic matrix, and a porous structure uniformly distributed within the flexible elastic matrix; the porous structure is formed by eluting away a soluble pore-forming agent.

[0010] Its working principle: The spiral conductive coil is embedded in a flexible elastic matrix in the form of a spiral spring, constituting a complete inductor element that can be directly connected to the circuit to participate in resonance. When external pressure is applied to the device, the flexible elastic matrix undergoes compressive deformation, causing the axial dimension of the spiral conductive coil to decrease and the coil shape to change accordingly, thereby changing the inductance. This achieves direct conversion of pressure signal to inductance parameter without the need for an additional pressure-sensitive unit. The uniformly distributed porous structure significantly reduces the compressive modulus of the elastic matrix, allowing the device to produce significant deformation under lower pressure, effectively improving pressure sensitivity. At the same time, the porous structure provides deformation space for elastic deformation, allowing for rapid rebound after pressure is removed using the restoring force of the elastic matrix, ensuring stability and a wide pressure response range for repeated use.

[0011] Furthermore, the material of the flexible elastic matrix is ​​an addition-type silicone rubber elastomer, preferably an Ecoflex series organosilicon elastomer. This type of material has excellent elastic recovery ability, biocompatibility and weather resistance. After curing, it has low Shore hardness and good flexibility, and can maintain stable mechanical properties after repeated compression deformation. It is an ideal material for constructing a compressible flexible matrix, which can provide stable load-bearing and rebound support for helical conductive coils.

[0012] Furthermore, the soluble pore-forming agent is sodium chloride solid particles with a particle size of micron-sized particles that can be uniformly dispersed in the elastomer precursor solution. Sodium chloride, as a water-soluble pore-forming agent, has advantages such as low cost, easy dispersion, and simple, pollution-free elution process. By controlling its addition amount, the porosity and compressibility of the matrix can be precisely controlled, and no harmful impurities remain in the matrix after elution, ensuring the stability of device performance.

[0013] Furthermore, the spiral conductive coil is made of carbon fiber conductive wire, which is uniformly wound in a spiral spring manner; the conductivity of the carbon fiber conductive wire is not less than 5 × 10⁻⁶. 2 S・cm -1 The winding spacing is 1~6mm. Carbon fiber conductive filaments have both excellent conductivity and flexible mechanical properties. They are not easy to break when repeatedly compressed and deformed with the matrix, and their conductivity is stable. By adjusting the winding spacing, the initial inductance value and pressure response sensitivity can be precisely adjusted to meet different detection requirements.

[0014] Furthermore, the pore-forming agent in the porous structure has a mass fraction of 10 wt.% to 70 wt.% of the mass of the flexible elastic matrix precursor, and the pores are interconnected micron-sized porous structures. Within this ratio range, the compressive modulus and porosity of the matrix can be continuously controlled by adjusting the pore-forming agent content, maximizing compressible deformation and resilience while ensuring the mechanical strength of the matrix.

[0015] Furthermore, the porous compressible inductor has an overall cylindrical structure with a diameter of 35~36.5mm and a height adapted to the pressure sensing stroke. The effective sensing area is consistent with the cross-sectional area of ​​the cylinder. The cylindrical structure is subjected to uniform force and deforms regularly under axial pressure, which can ensure the regularity and repeatability of inductance parameters as pressure changes, thereby improving the accuracy and stability of detection.

[0016] The present invention also provides a method for preparing the above-mentioned porous compressible inductor with a helical spring structure, comprising the following steps:

[0017] S1 precursor solution preparation: The two-component precursor of the flexible elastic matrix is ​​mixed evenly in equal mass ratio to obtain the basic precursor solution; a quantitative amount of the basic precursor solution is added to a soluble pore-forming agent, and the mixture is stirred and ultrasonically dispersed to obtain a mixed slurry containing the pore-forming agent.

[0018] S2 Core Molding: A portion of the mixed slurry is injected into the first cylindrical mold and heated to solidify to obtain the elastic core;

[0019] S3 Coil Winding: The conductive wire is evenly wound around the outer wall of the elastic core according to the set spiral spacing, and conductive pins are led out from both ends;

[0020] S4 Encapsulation and Curing: The elastic core with conductive wires wound around it is placed in a second cylindrical mold with a larger inner diameter. The remaining mixed slurry is injected so that the conductive wires and the core are completely wrapped by the slurry. The inductor blank is obtained by heating and curing.

[0021] S5 Pore Forming and Drying: The inductor blank is placed in the elution solvent and repeatedly washed and rubbed to dissolve and remove the soluble pore-forming agent inside. After drying, a porous compressible inductor with a spiral spring structure is obtained.

[0022] The preparation method adopts an integrated molding process of "inner core forming - coil winding - outer layer encapsulation", which can completely embed the spiral conductive coil inside the elastic matrix, ensuring the strong bond between the coil and the matrix and avoiding coil displacement during repeated compression. The porous structure is prepared by the dissolution pore-forming method, which is simple and controllable, with uniform pore distribution, and can be prepared on a large scale without complex equipment.

[0023] Furthermore, in step S1, the ultrasonic dispersion power is 400W, and the dispersion time is 20 minutes. During the dispersion process, a cooling medium is added to control the system temperature and prevent the elastomer precursor from solidifying prematurely. These ultrasonic parameters ensure that the pore-forming agent particles are uniformly dispersed in the precursor liquid, avoiding uneven pore distribution caused by particle agglomeration. The ice bath cooling can offset the heat generated by ultrasound, ensuring that the precursor liquid remains liquid during preparation and guaranteeing the smooth progress of subsequent molding processes.

[0024] Furthermore, in step S2, the curing temperature is 70℃ and the curing time is 60 min; in step S4, the curing temperature is 60℃ and the curing time is 45 min; in step S5, the elution solvent is deionized water, the drying temperature is 60℃, and the drying time is 5 h. This graded curing process ensures the curing quality of the inner core and outer encapsulation layer, reducing interlayer interface defects; deionized water elution is green and pollution-free, and repeated rubbing can thoroughly remove the internal pore-forming agent, forming a well-connected porous structure; constant temperature drying completely removes residual solvent, ensuring stable device performance.

[0025] This invention also provides the application of the aforementioned porous compressible inductor with a helical spring structure in a flexible pressure sensor. This device can be directly connected as an inductor element to an LC resonant circuit, simultaneously performing pressure sensing functions. It is suitable for various passive wireless pressure detection scenarios and can also be used in fields such as flexible wearable pressure detection and micro-load monitoring.

[0026] This invention's device combines the dual functions of an inductor and a pressure-sensitive unit. It can be directly used as the inductor component of a resonant circuit, while simultaneously detecting external pressure. It is suitable for various scenarios such as passive wireless sensing, wearable electronics, and flexible detection systems, and is particularly well-suited for pressure detection applications requiring high integration, flexibility, and long-term stability. Compared to existing technologies, this invention offers the following significant advantages:

[0027] Highly integrated functionality, no need for additional inductors: This invention embeds a spiral conductive coil within a porous elastic matrix, enabling a single device to simultaneously possess the electrical functions of an inductor and the sensing functions of a pressure-sensitive unit. It can be directly connected to a resonant circuit as an inductor, eliminating the need for additional independent inductors and discrete pressure sensors. This significantly improves device integration, reduces size and process complexity, and provides core component support for miniaturized and flexible sensing nodes.

[0028] Porous structure enables both sensitivity and rebound performance: The uniform porous structure formed by elution with a soluble pore-forming agent effectively reduces the compressive modulus of the elastic matrix, allowing the device to undergo significant deformation under low pressure, thus greatly improving pressure sensitivity; at the same time, the porous structure provides ample buffer and recovery space for elastic deformation, which, together with the highly elastic silicone rubber matrix, ensures rapid rebound and performance stability of the device after repeated compression, achieving a balance between a wide pressure response range and high cycle stability.

[0029] With a reasonable structural design and flexible performance control, the initial inductance, pressure sensitivity and compression range of the device can be flexibly adjusted by controlling parameters such as the content of the pore-forming agent and the winding spacing of the conductive wire, to adapt to different application requirements. The helical spring coil structure generates regular inductance changes with the axial compression of the substrate, resulting in good signal response linearity and facilitating subsequent signal reading and processing.

[0030] The preparation process is simple and highly controllable: It adopts a preparation process that combines integral molding and dissolution pore-forming, which does not require complex equipment or stringent process conditions. The preparation process is simple and controllable, the finished product has good consistency, low cost, and is suitable for large-scale production. The water-soluble pore-forming agent elution process is green and environmentally friendly, with no toxic or harmful residues, and the device has good biocompatibility and environmental adaptability. Attached Figure Description

[0031] To more clearly illustrate the embodiments of this technical solution, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this technical solution. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a flowchart of the preparation process according to an embodiment of the present invention.

[0033] Figure 2 Here is a physical diagram of a porous compressible inductor with a helical spring structure according to an embodiment of the present invention:

[0034] These contain different NaCl mass fractions ranging from 10wt.% to 70wt.% and different carbon fiber conductive wire spacings of 1mm, 3mm, and 5mm.

[0035] Figure 3 The following are inductance curves for three different conductive wire spacing devices in this invention, showing how they change with external pressure:

[0036] (a) Spacing 1mm; (b) Spacing 3mm; (c) Spacing 5mm.

[0037] Figure 4 The following are graphs showing the inductance rate of change versus pressure for three different conductive wire spacing devices in this embodiment of the invention:

[0038] (a) Spacing 1mm; (b) Spacing 3mm; (c) Spacing 5mm.

[0039] Figure 5 The following are pressure sensitivity fitting curves for devices with different conductive wire spacings in embodiments of the present invention:

[0040] (a) Spacing 1mm; (b) Spacing 3mm; (c) Spacing 5mm.

[0041] Figure 6 This is a comparison chart of the inductance change rate of devices with different NaCl mass fractions under a fixed pressure in embodiments of the present invention:

[0042] (a) Pressure 0.50 kPa; (b) Pressure 2.37 kPa; (c) Pressure 4.74 kPa; (d) Pressure 9.48 kPa.

[0043] Figure 7 This is a comparison chart of the full-range inductance change rate of devices with different NaCl mass fractions in this invention:

[0044] (a) Spacing 1mm; (b) Spacing 3mm; (c) Spacing 5mm.

[0045] Figure 8 The diagram shows the resonant voltage signal at the transmitter after the device is connected to the LC resonant circuit in an embodiment of the present invention:

[0046] (a) A magnified view of the resonant voltage signal at the transmitting end and (b) a partial magnified view of its resonant voltage signal.

[0047] Figure 9 The diagram shows the resonant voltage signal at the receiving end of the device after it is connected to the LC resonant circuit in an embodiment of the present invention:

[0048] (a) A magnified view of the resonant voltage signal at the receiving end and (b) a partial magnified view of the peak resonant voltage signal.

[0049] Figure 10 The graph shows the rate of change of inductance as a function of pressure for seven devices with different conductor wire spacings. Detailed Implementation

[0050] The technical solutions of this technical solution will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this technical solution. Obviously, the described embodiments are only a preferred embodiment of this technical solution, and not all embodiments. Based on the core concept of this technical solution and the following embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this technical solution.

[0051] It should be noted that these embodiments are provided to make the application thorough and complete, and to fully express the scope of the application to those skilled in the art. It should be observed that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​described in these embodiments should be interpreted as merely exemplary and not as limiting.

[0052] In this technical solution, unless otherwise explicitly specified and limited, the terms installation, connection, linking, fixing, etc., should be interpreted broadly. For example, they can refer to fixed connection, detachable connection, or integral connection; they can refer to mechanical connection or electrical connection; they can refer to direct connection or indirect connection through an intermediate medium; and they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this technical solution can be understood according to the specific circumstances.

[0053] The raw materials, reagents, and instruments used in this series of examples are as follows:

[0054] Experimental raw materials and reagents

[0055] Ecoflex 00-50 two-component addition-cure silicone rubber: Specifications are 4×PintUnits, manufactured by Smooth-on LLC, USA. It consists of two components, A and B, which are mixed in equal mass at room temperature and cured into a flexible elastomer through an addition reaction. After curing, the Shore hardness is 00-50, and it has excellent elastic recovery performance and weather resistance.

[0056] Carbon fiber conductive wire: The specification is 1k, and the conductivity is 5.5×10²S・cm⁻¹. It is produced by Toray Industries, Inc. of Japan. It is made of thousands of carbon fiber monofilaments bundled together. It has both excellent conductivity and flexible bending resistance. The conductivity decreases little after repeated deformation.

[0057] Sodium chloride (NaCl) solid particles: analytical grade, purity 99.0%, produced by Tianjin Zhonglian Chemical Reagent Co., Ltd., used as a water-soluble pore-forming agent, the particle size is in the micron range, can be uniformly dispersed in silicone rubber precursor solution, and can be completely removed by washing with water.

[0058] Deionized water: prepared in the laboratory, resistivity ≥18.2MΩ・cm, used for eluting pore-forming agents and for laboratory cleaning.

[0059] Surface mount capacitor: Ceramic surface mount capacitor, with an accuracy of ±5%, used to form an LC resonant circuit with the inductor of this invention.

[0060] Gas discharge tube (GDT): Breakdown voltage 1600V, used to regulate the output impedance of triboelectric nanogenerator.

[0061] Experimental instruments and equipment

[0062] Electronic balance: Sartorius BP211D, manufactured by Sunny Hengping Instruments, with an accuracy of 0.01mg, used for precise weighing of silicone rubber components and sodium chloride pore-forming agent.

[0063] Ultrasonic Cleaner: GTSONIC-P20 model, manufactured by Guangdong GUTE Ultrasonic Co., Ltd., with a maximum power of 400W, used for defoaming of precursor liquid and dispersion of pore-forming agent.

[0064] Electric heating drying oven: DHG-9070A, manufactured by Shanghai Yiheng Scientific Instruments Co., Ltd., with a temperature control accuracy of ±1℃, used for curing and drying of silicone rubber precursors and samples.

[0065] Laser cutting machine: C-M4060 model, manufactured by Shandong Liaocheng Jingke Laser Co., Ltd., used for processing acrylic molds, with a cutting accuracy of ±0.1mm.

[0066] Linear motor: LinMot type, manufactured by Chuangyue Automation Control Technology (Shanghai) Co., Ltd., used to provide precise and controllable pressure loads with a loading accuracy of ±0.01kPa.

[0067] Pressure gauge: Model SBT673, manufactured by Yishengda Technology Co., Ltd., used to calibrate the applied pressure value.

[0068] Precision LCR meter: Model E4980A, manufactured by Agilent Technologies, with a test frequency range of 20Hz to 2MHz, used for accurate measurement of inductance, capacitance and quality factor.

[0069] Oscilloscopes: Tektronix DPO2012B and Tektronix MDO3034, manufactured by Tektronix LLC, used for acquiring time-domain signals of resonant voltage and frequency-domain analysis.

[0070] Optical flat panel: Manufactured by Beijing Zhongtian Guangzheng Technology Co., Ltd., used as a horizontal reference platform for testing.

[0071] Example 1: Preparation of porous compressible inductors using 20 wt.% NaCl pore-forming agent and 5 mm winding pitch.

[0072] This embodiment is the optimal embodiment of the present invention. The pore-forming agent has a mass fraction of 20 wt.%, and the carbon fiber conductive wire winding spacing is 5 mm. The specific preparation steps are as follows:

[0073] Formulation of two-component silicone rubber-based precursor liquid

[0074] Weigh 50g each of component A and component B of Ecoflex 00-50 using an electronic balance, and pour them into a clean PTFE beaker. Stir slowly in the same direction with a glass rod for 5 minutes to ensure that the two components are thoroughly mixed. Avoid introducing too many air bubbles during stirring. Place the mixed beaker in an ultrasonic cleaner and defoam at room temperature for 3 minutes. Use ultrasonic vibration to break up large air bubbles in the mixture to obtain a clear and transparent basic precursor solution for later use.

[0075] Preparation of mixed slurry containing pore-forming agent

[0076] Weigh 10g of NaCl solid particles (20wt.% of the pore-forming agent mass as the base precursor liquid mass) using an electronic balance, and slowly add them to the aforementioned 50g base precursor liquid while stirring with a glass rod to initially disperse the NaCl particles in the precursor liquid. Place the beaker back into the ultrasonic cleaner, set the ultrasonic power to 400W, and the ultrasonic dispersion time to 20min. To prevent premature curing of the silicone rubber due to increased water temperature during ultrasonication, place ice packs between the outer wall of the beaker and the cleaning tank for cooling, replacing the ice packs every 5min to maintain a system temperature below 25℃. After ultrasonication, a uniformly dispersed milky white slurry is obtained. At this point, the NaCl particles are uniformly suspended in the silicone rubber precursor liquid in micron-sized form, with no obvious agglomeration or sedimentation.

[0077] Elastomer core curing molding

[0078] Take a cylindrical acrylic mold (first cylindrical mold) with a diameter of 35mm and a height of 50mm. Slowly pour the above-mentioned mixed slurry into the mold to a height of 40mm. During the pouring process, the slurry should flow slowly down the inner wall of the mold to avoid generating new air bubbles. After pouring, smoothly transfer the mold into an electric heating drying oven, set the curing temperature to 70℃, and the curing time to 60min. After curing, close the drying oven and wait for the temperature inside the oven to drop naturally to room temperature before removing the mold. Slowly demold to obtain a cylindrical elastic core; the core has a diameter of 35mm, a height of approximately 40mm, a smooth and defect-free surface, and uniformly dispersed NaCl particles inside.

[0079] Spiral conductive coil winding

[0080] The elastic core is vertically fixed on a rotating fixture. The carbon fiber conductive wire is removed, and one end is secured to the bottom face of the core with high-temperature tape. The rotating fixture is then started, rotating the core at a uniform speed while simultaneously controlling the conductive wire to move upwards along the core's axis at a uniform speed, ensuring the winding spacing is strictly controlled at 5mm. After winding to the top of the core, the other end of the conductive wire is secured with tape, leaving 20mm of length at each end as conductive leads for subsequent electrical testing. After winding, the uniformity of the coil arrangement is checked, ensuring that each coil is parallel to the core's end face, with a spacing deviation not exceeding ±0.1mm, resulting in a regular helical spring structure.

[0081] Outer encapsulation and secondary curing

[0082] Take a cylindrical acrylic mold (second cylindrical mold) with a diameter of 36.5 mm and a height of 50 mm. Place the elastic core with the conductive wire wound around it vertically in the center of the mold, ensuring a uniform distance between the core and the mold's inner wall. Slowly pour the remaining mixed slurry into the mold until the liquid level is 5 mm above the top of the core, ensuring the conductive wire and core are completely encapsulated. After pouring, gently agitate the mold to remove any tiny air bubbles introduced during the encapsulation process. Then, transfer it to an electric heating drying oven and set the curing temperature to 60℃ for 45 minutes. Using a lower curing temperature than the core for secondary curing reduces interlayer stress and ensures a strong bond between the core and the encapsulation layer.

[0083] Pore ​​formation, elution, and drying treatment

[0084] After curing, the mold is removed, and a cylindrical inductor blank is obtained by demolding. The blank is placed in a beaker containing deionized water, and the blank is gently rubbed repeatedly by hand to gradually dissolve and diffuse the internal NaCl particles into the water. The deionized water is changed every 30 minutes, and the washing is continued for 12 hours until no chloride ions are detected in the water (no white precipitate is found when titrated with silver nitrate solution), proving that the internal NaCl has been completely washed away. After washing, the porous inductor sample is placed in an electrically heated drying oven, and the drying temperature is set to 60°C for 5 hours to completely remove the residual moisture inside the sample. After drying, it is removed and allowed to cool naturally to room temperature, thus obtaining the porous compressible inductor product with a helical spring structure of this embodiment. Figure 2 As shown.

[0085] Basic morphology and structural characterization of the sample in this embodiment:

[0086] The finished product is a regular cylinder, approximately 36.5 mm in diameter and 45 mm in height, with a soft and elastic texture. The matrix contains a large number of micron-sized pores that are interconnected, forming a three-dimensional porous network structure. The carbon fiber conductive filaments are completely embedded within the elastic matrix, tightly bonded with it without any obvious gaps, and exhibit a regular spiral structure with a uniform number of turns. In its initial state, the sample's initial inductance was measured to be 0.917 μH using an LCR meter at a test frequency of 100 kHz, and the quality factor Q value meets the performance requirements of a flexible inductor.

[0087] Example 2: Comparative Study on the Performance of Different Carbon Fiber Conductive Wire Wrapping Spacing

[0088] To investigate the effect of the winding spacing of the conductive wires on the inductance performance and pressure response characteristics of the device, this embodiment set up four parallel samples, all with a pore-forming agent content of 20 wt.% NaCl. The only difference was the winding spacing of the carbon fiber conductive wires, which were 1 mm, 3 mm, and 5 mm, respectively. The preparation process was completely consistent with Example 1. Systematic pressure-inductance response tests were performed on three groups of samples. The test methods and results are as follows:

[0089] Test methods

[0090] The sample to be tested is placed vertically on an optical plate, with the test fixture of the LCR meter connected to the bottom. The conductive pin is reliably connected to the test terminal of the LCR meter. The pressure head of the linear motor is aligned with the center of the sample, and pressure is gradually applied at a loading rate of 0.5 mm / min. The pressure test points are set sequentially as follows: 0 kPa (initial state), 0.50 kPa, 2.37 kPa, 4.74 kPa, and 9.48 kPa. Each pressure point is held for 30 seconds, and the reading is recorded after the inductance value stabilizes. Each sample is tested three times, and the average value is taken as the final test result.

[0091] The relationship between inductance and pressure

[0092] The inductance values ​​of the three groups of samples under different pressures are as follows:

[0093] Samples with a 1mm pitch: Initial inductance 1.076μH; inductance 1.081μH at 0.50kPa; 1.095μH at 2.37kPa; 1.110μH at 4.74kPa; 1.124μH at 9.48kPa. The overall inductance increases monotonically with increasing pressure, such as... Figure 3 As shown in (a) of the diagram.

[0094] Samples with a 3mm pitch: Initial inductance 0.958μH; 0.965μH at 0.50kPa; 0.982μH at 2.37kPa; 1.005μH at 4.74kPa; 1.036μH at 9.48kPa. The trend is consistent with that of the 1mm pitch, but the initial inductance is lower, and the overall variation is larger. Figure 3 As shown in (b) of the diagram.

[0095] 5mm pitch sample (Example 1): Initial inductance 0.917μH; 0.924μH at 0.50kPa; 0.946μH at 2.37kPa; 0.972μH at 4.74kPa; 1.001μH at 9.48kPa. The variation range was the largest among the four groups, exhibiting a clear step-like variation characteristic, such as... Figure 3 As shown in (c) in the figure.

[0096] To verify the effect of the spacing of the carbon fiber conductive wires wound on the pressure sensor on the rate of change of inductance, sensor devices with different carbon fiber conductive wire winding spacings of 0.5mm, 2mm, 4mm, and 6mm were added to the original sensor. The test results are as follows: Figure 10As shown. Due to sensor size limitations, the maximum winding spacing cannot exceed 6mm. The results show that the rate of change of sensor inductance with different winding spacings initially increases rapidly with increasing pressure, then gradually decreases. As the spacing of the carbon fiber conductive wires wound on the sensor increases, the decreasing trend in the rate of change of inductance becomes less pronounced over high pressure ranges. Simultaneously, the rate of change of inductance also increases with increasing spacing, but the increasing trend gradually diminishes. The curve of the rate of change of inductance with external pressure at a spacing of 6mm is close to that at a spacing of 5mm. Therefore, the optimal choice for the winding spacing of the carbon fiber conductive wires is 5mm.

[0097] Inductance change rate and sensitivity analysis

[0098] Using the initial inductance value L0 as a reference, calculate the rate of change of inductance under different pressures: ΔL / L0 = (L-L0) / L0 × 100%, and plot the rate of change curve as shown below. Figure 4 As shown.

[0099] For samples with a 1mm spacing: the maximum rate of change at 9.48kPa was 4.32%;

[0100] Samples with a 3mm spacing: maximum change rate of 8.14% at 9.48kPa;

[0101] Samples with a 5mm spacing: maximum change rate of 9.08% at 9.48kPa;

[0102] For samples with a 6mm spacing: the maximum change rate was 9.30% at 9.48kPa.

[0103] Piecewise linear fitting is performed on the rate of change curve to calculate the pressure sensitivity (rate of change of inductance per unit pressure), such as... Figure 5 As shown:

[0104] For samples with a 1mm spacing: the sensitivity is 0.851% / kPa in the low-pressure range (0~2.37kPa) and 0.209% / kPa in the high-pressure range (2.37~9.48kPa), with a goodness of fit R0. 2 They are 0.7685 and 0.385 respectively;

[0105] For samples with a 3mm spacing: the sensitivity in the low-pressure section is 1.438% / kPa, and the sensitivity in the high-pressure section is 0.503% / kPa. The goodness of fit R0 2 They are 0.9998 and 0.9443 respectively;

[0106] For samples with a 5mm spacing: the sensitivity in the low-pressure section is 1.506% / kPa, and the sensitivity in the high-pressure section is 0.656% / kPa. The goodness of fit R0 is [value missing]. 2 They are 0.9642 and 0.9654 respectively;

[0107] Results Analysis

[0108] As the spacing between the conductive wire windings increases, the initial inductance of the sample gradually decreases, but the rate of change of inductance and pressure sensitivity significantly increase. The mechanism is that a larger winding spacing results in less mutual inductance between individual coil turns, and a more significant contribution of coil morphology changes during axial compression to the total inductance, thus leading to higher sensitivity. The sensitivity increase is significant when the spacing increases from 1 mm to 5 mm; however, when the spacing exceeds 5 mm and reaches 6 mm, the sensitivity increase becomes gradual, and excessive spacing leads to too few coil turns and a low initial inductance, which is detrimental to subsequent circuit matching. Considering sensitivity, initial inductance, and fabrication feasibility, 5 mm is the optimal winding spacing.

[0109] Example 3: Comparative Study of Performance of Different Pore-Forming Agent Mass Fractions

[0110] To investigate the effect of NaCl pore-forming agent content on the compressibility and pressure response of the device, this embodiment set up 7 parallel samples. The spacing of the conductive wire winding was 5 mm, and only the mass fraction of NaCl was changed to 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%, 60 wt.%, and 70 wt.%, respectively. The preparation process was the same as in Example 1. Comprehensive performance tests were performed on the 7 samples. The test methods and results are as follows:

[0111] Test methods

[0112] The same pressure loading test scheme as in Example 2 was used, with pressure points set at 0 kPa, 0.50 kPa, 2.37 kPa, 4.74 kPa, and 9.48 kPa. The inductance value at each pressure point was recorded, and the rate of change of inductance was calculated. Simultaneously, the compressive mechanical properties of the samples were tested to characterize the compressive modulus and resilience under different pore-forming agent contents.

[0113] Comparison of inductance change rate under different pressures

[0114] The inductance change rate data of the 7 groups of samples under four typical pressure points are as follows: Figure 6 As shown:

[0115] Under 0.50 kPa low pressure conditions: the change rate of the 10 wt.% group was approximately 0.51%, the 20 wt.% group reached a peak of approximately 0.76%, the change rate of the 30 wt.% and above groups gradually decreased, and the 70 wt.% group dropped to approximately 0.42%;

[0116] Under 2.37 kPa conditions: the change rate of the 20 wt.% group was approximately 3.58%, significantly higher than that of other components; the change rate of the 10 wt.% group was approximately 2.10%, and that of the 70 wt.% group was approximately 1.95%.

[0117] Under 4.74 kPa conditions: the change rate was approximately 6.21% for the 20 wt.% group, approximately 3.87% for the 10 wt.% group, and approximately 3.52% for the 70 wt.% group;

[0118] Under high pressure conditions of 9.48 kPa: the change rate of the 20 wt.% group reached the maximum value of 9.08%, the 10 wt.% group was about 5.63%, the 30 wt.% group was about 7.85%, the 40 wt.% group was about 6.92%, the 50 wt.% group was about 6.14%, the 60 wt.% group was about 5.47%, and the 70 wt.% group was about 4.89%.

[0119] Comparison of trends across the entire pressure range

[0120] Plot the inductance rate curves for 7 groups of samples across the entire range of 0–9.48 kPa, as follows: Figure 7 As shown in (c), the rate of change of all components increases monotonically with increasing pressure, with rapid growth at low pressure and slowing growth at high pressure, exhibiting typical compression response characteristics of porous materials. Among them, the 20wt.% component maintains the highest rate of change and the best linearity across the entire pressure range, making it the component with the best overall performance.

[0121] To further verify the interaction between spacing and pore-forming agent content, parallel tests were also conducted on 7 components at 1mm and 3mm spacings, such as... Figure 7 As shown in (a) and (b), the performance variation patterns under the three spacings are highly consistent: the optimal inductance change rate is reached at a 20 wt.% pore-forming agent content, and the larger the spacing, the more prominent the performance peak; proving that the optimality of the 20 wt.% pore-forming agent content is universal and is not significantly affected by the coil spacing.

[0122] Mechanical properties and rebound characteristics analysis

[0123] The compressive stress-strain curves of seven groups of samples were tested using a universal testing machine, and the compressive modulus was calculated as follows: approximately 85 kPa for the 10 wt.% group, approximately 62 kPa for the 20 wt.% group, approximately 51 kPa for the 30 wt.% group, and approximately 38 kPa for the 70 wt.% group. It is evident that the higher the pore-forming agent content, the lower the matrix compressive modulus and the more easily deformation occurs.

[0124] The rebound rate test results showed that the rebound rate was 98.2% for the 10wt.% group, 97.5% for the 20wt.% group, 93.1% for the 30wt.% group, 86.4% for the 50wt.% group, and 79.6% for the 70wt.% group. When the pore-forming agent content exceeded 20wt.%, the rebound rate decreased significantly. This is because excessive porosity leads to insufficient strength of the matrix skeleton, making it prone to plastic deformation after compression and unable to fully rebound.

[0125] Results Analysis

[0126] The content of pore-forming agent has a bidirectional regulatory effect on device performance: when the content is too low, the number of pores is small, the matrix compressibility is poor, the inductance change is small, and the sensitivity is low; when the content is too high, although the compressive modulus is further reduced, the matrix elasticity decreases, the rebound stability becomes worse, the inductance drift is serious after repeated compression, and the insufficient strength of the skeleton leads to deformation saturation in the high-voltage section, resulting in a decrease in sensitivity. A pore-forming agent content of 20 wt.% achieves the best balance between "high sensitivity" and "excellent rebound stability", which is the optimal ratio for overall performance.

[0127] Example 4: Cyclic stability and environmental adaptability testing of porous compressible inductors

[0128] This embodiment uses the optimal sample from Example 1 as the test object to conduct multiple reliability tests, including cyclic compression stability, temperature adaptability, and hysteresis characteristics, to comprehensively verify the long-term service performance of the device.

[0129] Cyclic compression stability test

[0130] A linear motor was used to perform repeated loading-unloading cycle tests on the sample. The loading pressure was 5 kPa and the loading frequency was 1 Hz, and the tests were performed continuously for 1000 cycles. The initial inductance value and the inductance value under 5 kPa pressure were measured after the 1st, 100th, 500th, and 1000th cycles, and the performance retention rate was calculated.

[0131] Test results show that after 100 cycles, the initial inductance drift rate is 0.32%, and the inductance change rate retention rate at 5 kPa is 98.7%; after 500 cycles, the initial inductance drift rate is 0.85%, and the change rate retention rate is 96.2%; after 1000 cycles, the initial inductance drift rate is 1.21%, and the change rate retention rate is 93.5%. The sample showed no obvious damage, and the coil exhibited no breakage or displacement, demonstrating that the device possesses excellent cycle stability and fatigue resistance, meeting the requirements for long-term, repeated testing.

[0132] Temperature adaptability test

[0133] The samples were placed in environmental chambers at 0℃, 10℃, 20℃, 30℃, and 40℃ respectively, and kept at these temperatures for 2 hours. The inductance change rate under a pressure of 5 kPa was then tested. The results showed that within the temperature range of 10~40℃, the fluctuation range of the inductance change rate was less than ±2%, indicating good performance stability. At 0℃, due to a slight increase in the modulus of the silicone rubber, the change rate decreased slightly, but still remained above 90% of the value at room temperature. This demonstrates that the device has stable sensing performance within the normal ambient temperature range and can adapt to complex outdoor temperature environments.

[0134] Hysteresis characteristic test

[0135] Load-unload return tests were performed on the samples, and the inductance values ​​during the boost and buck processes were recorded to calculate the hysteresis error. The results showed that the maximum hysteresis error over the entire range was less than 3%, indicating that the device's pressure response has good reversibility, the return curve and the outgoing curve have high overlap, and the signal repeatability is good.

[0136] Anti-interference performance test

[0137] Simulating a complex outdoor environment, the ambient humidity was adjusted within the range of 30% to 80% relative humidity, and the inductance and pressure response sensitivity of the sample were tested. The results show that within the tested humidity range, the initial inductance and sensitivity fluctuations of the device were both less than 1.5%, with no significant drift. This is attributed to the excellent moisture resistance of the Ecoflex substrate and carbon fiber conductive filaments, as well as the isolation effect of the embedded coil structure on the external environment, ensuring the device's operational stability in high humidity environments.

[0138] Example 5: Integrated Testing of LC Resonant Wireless Pressure Sensing Based on Porous Compressible Inductors

[0139] To verify the application effect of the device of the present invention in a practical sensing system, the porous compressible inductor of Example 1 was used as the core inductor element and connected to an LC resonant circuit to build a self-driven wireless pressure sensing system for system-level performance testing.

[0140] System Composition and Test Plan

[0141] The sensing system consists of two parts: a transmitter and a receiver.

[0142] The transmitter consists of a rotating independent friction layer mode TENG (wind power harvester), a gas discharge tube GDT, the porous compressible inductor (L1) of this invention, and a chip capacitor (C1) connected in series. The TENG rotates at a constant speed of 180 rpm, and the GDT has a breakdown voltage of 1600 V, which is used to achieve impedance matching between the high internal resistance of the TENG and the LC circuit.

[0143] The receiving end consists of a receiving inductor (L2), a receiving capacitor (C2), and an oscilloscope load (R) connected in series. The receiving end and the transmitting end are placed coaxially, with a fixed distance of 0.5m between them.

[0144] During testing, a linear motor was used to apply different pressures to the porous compressible inductor. A dual-channel oscilloscope was used to simultaneously acquire the time-domain signals of the resonant voltage at the transmitting and receiving ends. The signals were then converted into frequency-domain signals using a Fast Fourier Transform (FFT) to analyze the variation of the resonant frequency and peak voltage with pressure.

[0145] Performance characteristics of the TENG power supply

[0146] The TENG system used in this system is a rotating independent friction layer mode. The stator is a copper electrode made of printed circuit board, which is divided into four equal-sized 1 / 4 circular arcs with a radius of 10cm and a spacing of 1cm between adjacent electrodes. The dielectric layer is a 50μm thick PTFE film covering the surface of the copper electrode. The rotor consists of two opposing rabbit hair friction layers with a thickness of 3cm and a radius of 10cm.

[0147] The TENG's output performance was tested at 180 rpm: open-circuit voltage approximately 4200V, short-circuit current approximately 22.5μA, and short-circuit transferred charge approximately 980nC. Peak power testing showed that the maximum power of 29.49mW was achieved with an external load of 100MΩ, matching the TENG's own equivalent internal resistance. With the integrated 1600V breakdown voltage GDT, the output pulse peak value is 1450V, with a half-peak width of approximately 0.3ms, effectively providing high-voltage pulse excitation for the LC resonant circuit. After 450,000 cycles of testing, the TENG's short-circuit current only decreased slightly, demonstrating excellent long-term stability and providing a continuous and stable power supply for the sensing system.

[0148] Transmitter resonant signal test results

[0149] Time-domain signals of the transmitter resonant voltage under different pressures, such as Figure 8 As shown, after the high-voltage pulse generated by TENG is injected into the LC circuit, a damped oscillating resonant waveform is generated. As the pressure increases from 0 kPa to 9.48 kPa, the peak voltage of the resonant wave at the transmitting end gradually increases from 368.08 V to 392.40 V, while the peak frequency of the resonant wave gradually decreases, and the peak of the time-domain waveform shifts to the right.

[0150] This phenomenon is completely consistent with theoretical analysis: increased pressure → compression of porous inductor → increase inductance value L1 → according to the resonant frequency formula The resonant frequency f decreases; at the same time, the change in inductance value alters the circuit impedance matching state, resulting in a corresponding increase in peak voltage.

[0151] Test results of resonant signal at the receiving end

[0152] The receiver receives the signal from the transmitter via magnetic coupling resonance. The voltage signal at the receiver under different pressures is as follows: Figure 9 As shown, the trend of the receiving signal is consistent with that of the transmitting end: when the pressure increases from 0 kPa to 9.48 kPa, the resonant peak voltage at the receiving end increases linearly from 60.00 V to 70.70 V, the peak frequency decreases synchronously, and the peak shifts to the right.

[0153] Linear fitting of the peak voltage at the receiving end with pressure yielded a goodness of fit R² of over 0.98, demonstrating a strong linear relationship between the receiving end signal and the pressure value. This allows for the accurate inverse estimation of the measured pressure value from the receiving end voltage signal, thus enabling passive wireless pressure detection.

[0154] System performance analysis

[0155] This integrated system requires no additional independent inductor components; the single device of this invention can simultaneously achieve both "inductor resonance function" and "pressure sensing function," significantly simplifying the transmitter circuit structure and reducing device size and cost. The system exhibits a fast pressure response, enabling real-time tracking of pressure changes and overcoming the hysteresis problem of traditional energy storage wireless sensors. It provides a high-performance core component for passive wireless sensing in scenarios such as bridge health monitoring and industrial non-destructive testing.

[0156] All aspects not detailed in this invention are conventional technical means known to those skilled in the art.

[0157] The above content shows and describes the basic principles, main features, and beneficial effects of this technical solution. The above description is merely a preferred embodiment of this technical solution and is not intended to limit the scope of this technical solution. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this technical solution should be included within the protection scope of this technical solution.

Claims

1. A porous compressible inductor with a helical spring structure, characterized in that, It includes a flexible elastic matrix, a spiral conductive coil embedded in the flexible elastic matrix, and a porous structure uniformly distributed inside the flexible elastic matrix; the porous structure is formed by eluting away a soluble pore-forming agent.

2. The porous compressible inductor with a helical spring structure according to claim 1, characterized in that, The material of the flexible elastic matrix is ​​an addition-type silicone rubber elastomer.

3. The porous compressible inductor with a helical spring structure according to claim 1 or 2, characterized in that, The soluble pore-forming agent is sodium chloride solid particles with a particle size of micron-sized particles uniformly dispersed in the elastomer precursor liquid.

4. The porous compressible inductor with a helical spring structure according to claim 3, characterized in that, The spiral conductive coil is made of carbon fiber conductive wire, which is uniformly wound in a spiral spring manner; the conductivity of the carbon fiber conductive wire is not less than 5 × 10⁻⁶. 2 S・cm -1 The winding spacing is 1~6mm.

5. The porous compressible inductor with a helical spring structure according to any one of claims 1, 2, and 4, characterized in that, The pore-forming agent in the porous structure has a mass fraction of 10wt.% to 70wt.% of the mass of the flexible elastic matrix precursor, and the pores are interconnected micron-sized porous structures.

6. The porous compressible inductor with a helical spring structure according to claim 5, characterized in that, The porous compressible inductor has an overall cylindrical structure with a diameter of 35~36.5mm, and its height is adapted to the pressure sensing stroke. The effective sensing area is consistent with the cross-sectional area of ​​the cylinder.

7. A method for preparing a porous compressible inductor with a helical spring structure as described in any one of claims 1-6, characterized in that, Includes the following steps: S1 precursor solution preparation: The two-component precursor of the flexible elastic matrix is ​​mixed evenly in equal mass ratio to obtain the basic precursor solution; a quantitative amount of the basic precursor solution is added to a soluble pore-forming agent, and the mixture is stirred and ultrasonically dispersed to obtain a mixed slurry containing the pore-forming agent. S2 Core Molding: A portion of the mixed slurry is injected into the first cylindrical mold and heated to solidify to obtain the elastic core; S3 Coil Winding: The conductive wire is evenly wound around the outer wall of the elastic core according to the set spiral spacing, and conductive pins are led out from both ends; S4 Encapsulation and Curing: The elastic core with conductive wires wound around it is placed in a second cylindrical mold with a larger inner diameter. The remaining mixed slurry is injected so that the conductive wires and the core are completely wrapped by the slurry. The inductor blank is obtained by heating and curing. S5 Pore Forming and Drying: The inductor blank is placed in the elution solvent and repeatedly washed and rubbed to dissolve and remove the soluble pore-forming agent inside. After drying, a porous compressible inductor with a spiral spring structure is obtained.

8. The preparation method according to claim 7, characterized in that, In step S1, the ultrasonic dispersion power is 400W and the dispersion time is 20min; During the dispersion process, a cooling medium is added to control the system temperature and prevent the elastomer precursor from curing prematurely.

9. The preparation method according to claim 8, characterized in that, In step S2, the curing temperature is 70℃ and the curing time is 60 min; in step S4, the curing temperature is 60℃ and the curing time is 45 min; in step S5, the elution solvent is deionized water, the drying temperature is 60℃, and the drying time is 5 h.

10. The application of the porous compressible inductor with a helical spring structure as described in any one of claims 1-6 in a flexible pressure sensor.

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