An electrostatic flexible pressure sensor and a preparation method and application thereof

Abstract

This invention discloses an ionized flexible pressure sensor, its fabrication method, and its application. The sensor includes a first electrode layer, a second electrode layer, a PDMS ion-dielectric elastomer, and metal leads. Each electrode layer consists of a PI thin film and surface electrodes. The PDMS ion-dielectric elastomer layer is located between the first and second electrode layers, and is composed of a porous PDMS elastic framework. A composite formed by an ionic liquid and polyvinylidene fluoride (PVDF) is loaded within the pores of this framework. This invention utilizes an in-situ composite process to load the ionic liquid-PVDF composite within the porous PDMS elastic framework, achieving a stable bond between the ionic liquid and the porous PDMS elastic framework. This avoids migration or leakage of the ionic liquid during use, thus providing an ionized flexible sensor with high sensitivity, a wide pressure response range, and strong stability.

Description

Technical Field

[0001] This invention belongs to the field of flexible sensing technology, specifically an ionized flexible pressure sensor, its preparation method, and its application. Background Technology

[0002] With the rapid development of wearable health monitoring, electronic skin, human-computer interfaces, and soft robotics technologies, the demand for flexible pressure sensors is becoming increasingly urgent. Among various sensing mechanisms, capacitive pressure sensors are favored due to their simple structure, rapid response, low power consumption, and good fatigue resistance. Ionized pressure sensors are an advanced form of capacitive sensors. By introducing electrolytic materials such as ionic liquids into the dielectric layer, they utilize the double-layer effect of ions at the electrode / electrolyte interface to generate a capacitance per unit area several orders of magnitude higher than that of traditional capacitive sensors, thereby significantly improving the sensor's sensitivity.

[0003] Polydimethylsiloxane (PDMS) is widely used as a substrate or dielectric layer material for flexible sensors due to its excellent flexibility, chemical stability, ease of micro / nano fabrication, and good biocompatibility. However, combining ionic liquids with PDMS to fabricate high-performance ionized sensors faces a significant technical challenge: PDMS is inherently a hydrophobic organosilicon elastomer, while most ionic liquids are strongly polar, resulting in inherent thermodynamic incompatibility. Current techniques typically employ a method of directly physical blending ionic liquids with PDMS prepolymers followed by curing. This method has the following inherent drawbacks: (1) Severe phase separation: Ionic liquids are difficult to disperse uniformly in PDMS matrix and are prone to migration and aggregation during the curing process, forming an uneven composite structure, resulting in large fluctuations in sensor performance and poor repeatability.

[0004] (2) Ionic liquid leakage: Ionic liquids physically embedded in the PDMS network have weak binding forces. Under continuous or cyclic pressure and during long-term use, ionic liquids will gradually seep out from the PDMS. This not only leads to sensor sensitivity decay, signal drift, and shortened device life, but may also contaminate the objects it comes into contact with or human skin.

[0005] (3) Severe performance degradation: The addition of ionic liquids will interfere with the cross-linking and curing reaction of PDMS, which may lead to problems such as incomplete curing and decreased mechanical properties (such as becoming brittle or too soft).

[0006] Therefore, there is an urgent need in this field for an innovative strategy and structural design that can achieve stable, durable and uniform composite of ionic liquids and PDMS elastomers to solve the leakage problem, while simultaneously achieving excellent sensing performance with high sensitivity and wide response range. Summary of the Invention

[0007] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides an ionized flexible pressure sensor, comprising a first electrode layer, a second electrode layer, a PDMS ion-dielectric elastomer, and metal leads; the first electrode layer is composed of a first PI film and a first surface electrode; the second electrode layer is composed of a second PI film and a second surface electrode; the first PI film, the first surface electrode, the PDMS ion-dielectric elastomer, the second surface electrode, and the second PI film are sequentially bonded from top to bottom; the PDMS ion-dielectric elastomer is made of PVDF, ionic liquid, and PDMS; the metal leads lead out the first surface electrode and the second surface electrode respectively.

[0008] Furthermore, in order to improve the dielectric properties and pressure response sensitivity of the dielectric layer, the ionic liquid is one of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium hexafluorophosphate.

[0009] Furthermore, to improve the stability of the sensor, the PDMS ion-dielectric elastomer has a length of 5–20 mm, a width of 5–20 mm, and a thickness of 1–5 mm. This size limitation allows the constructed porous PDMS elastic framework to undergo appropriate compressive deformation under external force, thereby obtaining a larger equivalent dielectric constant and achieving high sensitivity and fast response pressure detection performance.

[0010] On the other hand, the present invention provides a method for fabricating an ionized flexible pressure sensor, which is used to fabricate the ionized flexible pressure sensor as described above, comprising the following steps: Step 1: Mix PDMS prepolymer and curing agent thoroughly at a mass ratio of 10:1, then mix with water-soluble porogen, pour into a mold, cure at 70 ℃, and then soak in deionized water until the water-soluble porogen is completely removed to obtain a porous PDMS elastic skeleton.

[0011] Furthermore, the water-soluble pore-forming agent is either sucrose particles or sodium chloride particles, with an average particle size of 50~500 μm.

[0012] Furthermore, the mass ratio of the PDMS prepolymer to the water-soluble porogen is 1:3~6.

[0013] Step 2: Dissolve PVDF powder fully in an organic solvent and mix it evenly with an ionic liquid to prepare a composite solution. The mass ratio of the ionic liquid to the PVDF powder is 0.5~2:1.

[0014] Furthermore, the molecular weight of the PVDF powder is 8 × 10⁻⁶. 5~ 1×10 6 .

[0015] Furthermore, the mass fraction of PVDF in the composite solution is 5% to 15%.

[0016] Furthermore, the organic solvent is one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

[0017] Step 3: Immerse the porous PDMS elastic skeleton in the composite solution for 2-6 hours, remove it, squeeze it thoroughly to remove excess composite solution, and then dry it at 40-70 ℃ for 6-12 hours.

[0018] Step four: Repeat step three 1-2 times to obtain the PDMS ion dielectric elastomer.

[0019] The main purpose of this step is to ensure that the composite solution can be deeply, uniformly and strongly loaded into all the pores of the porous PDMS elastic framework.

[0020] Furthermore, the mass ratio of the composite solution to the porous PDMS elastic framework is 0.6~0.9:1.

[0021] Step 5: Print conductive silver paste of the same size, not larger than the area of ​​PDMS ion-dielectric elastomer, onto the first PI film and the second PI film respectively, and dry them to obtain the first electrode layer and the second electrode layer.

[0022] Step six: Attach the first electrode layer and the second electrode layer to the upper and lower surfaces of the PDMS ion-dielectric elastomer, respectively, and connect them with metal leads to obtain the ion-displacement flexible pressure sensor. This bonding method can efficiently convert the double-layer change caused by ion migration under pressure into a capacitance signal recognizable by external circuitry.

[0023] The beneficial effects of this invention are as follows: (1) Stable material system. An in-situ composite process is adopted to stably embed ionic liquid into a porous PDMS elastic framework under the dual effects of physical confinement and chemical fixation. The ionic liquid is uniformly distributed in the composite material, avoiding migration and aggregation, forming a composite system with consistent structure, which significantly improves the repeatability and performance stability of the sensor.

[0024] (2) Good stability. Through the dual mechanism of chemical fixation by PVDF and physical confinement formed by the retraction of the PDMS network, leakage of ionic liquid during use is effectively suppressed. Even under long-term and repeated pressure loading cycles, the ionic liquid remains firmly in place, the sensor signal does not drift significantly, the service life is greatly extended, and the risk of environmental pollution is avoided.

[0025] (3) The process is simple and the cost is low. The preparation method only requires simple soaking and drying steps, without the need for complex or high-cost equipment. It is easy to operate, has good repeatability, and is suitable for large-scale production. At the same time, after the PDMS-PVDF composite network structure is formed, the material has both good flexibility and mechanical strength, overcoming the problem of decreased mechanical properties in traditional processes. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the ionized flexible pressure sensor prepared in Example 1; Figure 2 The image shows the elemental distribution of the PDMS ion-dielectric elastomer prepared in Example 1, obtained by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Figure 3 The sensitivity curve of the ionized flexible pressure sensor prepared in Example 1 is shown. Figure 4 The image shows the durability and stability test results of the ionized flexible pressure sensor prepared in Example 1. Figure 5 The response time diagram is shown for the ionized flexible pressure sensor prepared in Example 1. Figure 6 The minimum response test diagram of the ionized flexible pressure sensor prepared in Example 1; Figure 7 Sensitivity curves of the ionized flexible pressure sensors prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3; Figure 8 Sensitivity curves of the ionized flexible pressure sensors prepared in Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6. Figure 9 The durability and stability test results of the ionized flexible pressure sensor prepared for Comparative Example 7 are shown in the figure. Figure 10 This is an application diagram of the ionized flexible pressure sensor prepared in Example 1 for respiratory detection; Figure 11 The image shows the application test of the ionized flexible pressure sensor prepared in Example 1 under high pressure conditions.

[0027] Explanation of reference numerals in the attached figures: 1 First electrode layer, 2 Second electrode layer, 3 PDMS ion-dielectric elastomer, 4 Metal lead, 11 First PI film, 12 First surface electrode, 21 Second surface electrode, 22 Second PI film. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0030] like Figure 1 As shown, this invention provides an ionized flexible pressure sensor, comprising a first electrode layer 1, a second electrode layer 2, a PDMS ion-dielectric elastomer 3, and metal leads 4. The first electrode layer 1 is composed of a first PI film 11 and a first surface electrode 12; the second electrode layer 2 is composed of a second PI film 22 and a second surface electrode 21; the first PI film 11, the first surface electrode 12, the PDMS ion-dielectric elastomer 3, the second surface electrode 21, and the second PI film 22 are sequentially bonded from top to bottom; the PDMS ion-dielectric elastomer 3 is made of PVDF, ionic liquid, and PDMS; the metal leads 4 lead out the first surface electrode 12 and the second surface electrode 21 respectively.

[0031] The sensor of this invention is based on an ionized dielectric layer design using a porous PDMS elastic framework loaded with a PVDF / ionic liquid composite solution. This design achieves several technical advantages, including leak-proof component materials, significantly improved dielectric constant of the dielectric layer, enhanced sensor pressure response sensitivity, and improved device stability. The reasons are as follows: (1) Ionic liquids provide a large number of freely moving cations and anions in the system. When the sensor is subjected to pressure, ions migrate to the electrode interface under the action of electric field or extrusion, forming a nanoscale ion-electron double layer. Due to the extremely small spacing between the double layers, the resulting interfacial capacitance is far greater than that of traditional ordinary dielectrics, thus giving the sensor extremely high sensitivity.

[0032] (2) The porous PDMS elastic framework provides a low elastic modulus and a large compressible space. Under pressure, the porous structure compresses rapidly, which significantly increases the effective contact area between the framework loaded with ionic liquid and the electrode layer. The increase in contact area directly leads to a dramatic increase in the interfacial double layer capacitance, realizing the transformation of small mechanical deformation into amplified electrical signal changes.

[0033] (3) Simple PDMS is hydrophobic and cannot directly adsorb hydrophilic ionic liquids. In the structure of the ionized flexible pressure sensor, PVDF is introduced as a "bridge". By utilizing the good film-forming properties and polarity of PVDF, the ionic liquid is uniformly anchored and wrapped on the surface and inside the pores of the hydrophobic porous PDMS elastic skeleton. This not only prevents the leakage of ionic liquids, but also ensures the stability of the ion conduction pathway under flexible deformation.

[0034] Furthermore, to improve the dielectric properties and pressure response sensitivity of the dielectric layer, the ionic liquid is one of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium tetrafluoroborate, or 1-butyl-3-methylimidazolium hexafluorophosphate. The reason is: This type of ionic liquid belongs to the imidazole class and possesses high polarity and excellent ionic conductivity. It can co-form a composite dielectric layer with PVDF within a porous PDMS elastic framework, exhibiting high polarizability and high ion mobility. This results in a more significant dielectric change in the composite dielectric layer under external compression, thereby significantly improving the sensor's capacitive response amplitude and sensitivity. Furthermore, this type of ionic liquid also possesses strong ionic polarizability, good fluidity and thermochemical stability, and excellent compatibility with PVDF. It can form a uniform and stable ionic composite system within the PDMS pores, effectively improving the dielectric constant and maintaining long-term stable output performance.

[0035] Furthermore, to improve the stability of the sensor, the PDMS ion-dielectric elastomer 3 has a length of 5-20 mm, a width of 5-20 mm, and a thickness of 1-5 mm. This dimensional limitation allows the constructed porous PDMS elastic framework to undergo appropriate compressive deformation under external force, thereby obtaining a larger equivalent dielectric constant and achieving high sensitivity and fast response pressure detection performance. The reason is: (1) When the surface area is too large, the deformation of the sensor under the same pressure will decrease. At this time, the pressure is distributed on an excessively large force-bearing area, thereby weakening the thickness change corresponding to unit pressure and resulting in a significant decrease in sensitivity. This size can provide enough effective area to improve capacitance and signal strength, while maintaining the miniaturization, lightweight and wearability of flexible sensors.

[0036] (2) The thickness of the PDMS ion-dielectric elastomer 3 is limited to 1~5 mm because the equivalent capacitance of the ion-dielectric layer is inversely proportional to the thickness. If the thickness is greater than 5 mm, the thickness change of the dielectric layer in response to pressure is relatively small, which reduces the change in the sensor output signal and decreases the sensitivity; at the same time, the thick material requires a longer ion migration path, which will lead to insufficient ion polarization and reduce the dielectric enhancement effect. If the thickness is less than 1 mm, the pore structure is easily crushed, the mechanical stability is poor, permanent deformation is easy to occur, and it is easy to reach saturation response under small pressure, resulting in a narrower detection range and increased reconstruction error.

[0037] To obtain the aforementioned ionized flexible pressure sensor, this invention provides a method for fabricating an ionized flexible pressure sensor, comprising the following steps: Step 1: Mix PDMS prepolymer and curing agent thoroughly at a mass ratio of 10:1, then mix with water-soluble porogen, pour into a mold, cure at 70°C, and then soak in deionized water until the water-soluble porogen is completely removed to obtain a porous PDMS elastic skeleton.

[0038] Furthermore, the water-soluble pore-forming agent is either sucrose particles or sodium chloride particles, with an average particle size of 50–500 μm. This particle size range can improve the sensitivity and stability of the sensor. The reason is: (1) Particle size determines the pore size range, thereby controlling the compressibility and sensitivity of porous PDMS. When the pore size is in the range of 50~500 μm, the elastic skeleton of porous PDMS can produce significant reversible deformation under pressure. This macroscopic compression behavior is the key to realizing pressure-electric signal conversion. If the particle size is too small (<50 μm), the pores will be too fine and the structure will be too dense, making it difficult to generate effective compression and resulting in decreased sensitivity. If the particle size is too large (>500 μm), the pore structure will not be supported enough, the mechanical stability will be reduced, and the sensor will be prone to collapse or fatigue damage.

[0039] (2) Medium to large pore size provides an effective permeation channel for composite solution, so that composite dielectric material can be uniformly filled and stably confined inside the PDMS pore wall; if the pore size is too small, insufficient permeation will produce a local solidification layer, affecting dielectric uniformity; if the pore size is too large, it is not conducive to the formation of a stable micro-nano confined structure, thereby reducing the fixation efficiency of ionic liquid.

[0040] Furthermore, the mass ratio of PDMS prepolymer to water-soluble porogen is 1:3~6.

[0041] Step 2: Dissolve PVDF powder fully in an organic solvent and mix it evenly with an ionic liquid to prepare a composite solution. The mass ratio of ionic liquid to PVDF powder is 0.5~2:1.

[0042] The content of the ionic liquid in the sensor of this invention directly affects the sensor's sensing performance. Preferably, the mass ratio of ionic liquid to PVDF is 0.5~2:1, which can maintain the mechanical strength of the composite dielectric layer and prevent ionic liquid leakage while ensuring a high signal-to-noise ratio capacitive signal output. The reason is: Ionic liquids in composite materials provide high electrical conductivity and good ionic conductivity. PVDF not only enhances the insulation and mechanical strength of the material but also ensures the uniform mixing and firm adhesion of the ionic liquid to the hydrophobic PDMS framework surface. This is due to PVDF's excellent film-forming properties; during drying, it crystallizes to form a continuous solid polymer network, physically encapsulating and confining the ionic liquid within its microstructure. If the ionic liquid content is too high (>2:1), the PVDF network cannot completely bind the excess liquid, leading to a decrease in the mechanical strength and flexibility of the composite material and increasing the likelihood of ionic liquid precipitation from the polymer network. Conversely, if the ionic liquid content is too low (<0.5:1), insufficient conductive ions will reduce the sensor's electrical response capability.

[0043] Furthermore, the molecular weight of the PVDF powder is 8 × 10⁻⁶. 5 ~ 1×10 6 PVDF in this molecular weight range possesses sufficiently long molecular chains, enabling it to form a dense and robust physically entangled network within the pores of the PDMS framework after solvent evaporation. This network not only bonds tightly to the PDMS framework but also efficiently encapsulates and anchors the ionic liquid within it. This achieves stable and uniform loading of the ionic liquid during in-situ composite processes ("immersion-extrusion-drying"), which is crucial for addressing the issues of ionic liquid migration and leakage, ensuring high sensitivity and excellent long-term stability of the sensor. If the molecular weight is too low, the resulting polymer network is loose, lacking sufficient encapsulation and fixation capabilities; if the molecular weight is too high, the solution viscosity is too high, making it difficult to penetrate deep into the pores.

[0044] Furthermore, the mass fraction of PVDF in the composite solution is 5% to 15%.

[0045] Furthermore, the organic solvent is one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

[0046] Step 3: Immerse the porous PDMS elastic framework in the composite solution for 2-6 hours, then remove it, squeeze it thoroughly to remove excess composite solution, and then dry it at 40-70℃ for 6-12 hours.

[0047] Controlling the temperature and time within this range helps to achieve uniformity and stable electrical properties in the composite dielectric layer structure because: (1) In the composite solution, the ionic liquid and PVDF are dissolved in organic solvents (such as N,N-dimethylformamide, N-methylpyrrolidone, etc.). By controlling the drying temperature (40~70 ℃), it can be ensured that the organic solvent evaporates within an appropriate time, so that the ionic liquid and PVDF in the composite material are uniformly fixed in the porous PDMS elastic framework, while avoiding material inhomogeneity or excessive crystallization of PVDF caused by excessively fast drying. Too high a temperature (>70 ℃) will cause premature crystallization of PVDF or excessively rapid evaporation of organic solvent, forming an uneven film structure and affecting the electrical properties of the composite dielectric layer; too low a temperature (<40 ℃) will cause the organic solvent to evaporate too slowly, prolonging the drying time, resulting in incomplete evaporation of organic solvent inside the composite layer, affecting the stability of the material.

[0048] (2) The drying time of the composite dielectric layer is limited to 6-12 hours because this duration ensures that the organic solvent in the composite solution evaporates completely and that PVDF forms a film, thereby creating a stable and uniform dielectric network structure. When the drying time is less than 6 hours, the organic solvent often cannot completely dissipate, leaving solvent residue inside the composite layer. This results in insufficient crystallization of PVDF and uneven distribution of ionic liquid within the porous framework, ultimately leading to a loose structure, performance drift, and poor repeatability of the dielectric layer. If the drying time exceeds 12 hours, especially under heating conditions of 40-70 °C, PVDF will undergo excessive crystallization due to continuous heat, causing the composite layer to harden, reduce its flexibility, and may even form a dense hard film on the surface, significantly weakening the compressibility of the dielectric layer and thus reducing the sensor sensitivity. At the same time, excessive drying may also cause interfacial shrinkage between PDMS and the composite dielectric layer, resulting in the risk of microcracks or delamination. Therefore, controlling the drying time to 6-12 hours can simultaneously satisfy the complete evaporation of organic solvents, the stable construction of PVDF networks, and the sufficient confinement of ionic liquids, making it the optimal process to ensure the stability and electrical properties of the composite dielectric layer.

[0049] Step four, repeat step three 1-2 times to obtain PDMS ionic dielectric elastomer 3.

[0050] This step ensures that the composite solution is deeply, uniformly, and robustly loaded throughout the entire pore structure of the porous PDMS elastic framework, thereby improving the sensor's sensitivity and stability. The specific reason is: (1) The porous PDMS elastic framework has a three-dimensional interconnected and tortuous pore structure. A single soaking may only wet the surface and shallow macropores of the framework with the composite solution, while the deeper and finer pores are difficult to be fully filled. By repeating the "soak-squeeze-dry" cycle, the composite solution can penetrate into deeper pores one by one under the drive of capillary action, thereby ensuring that the composite solution is evenly distributed inside the entire porous PDMS elastic framework and avoiding the formation of local blank areas or areas with uneven concentration.

[0051] (2) Each drying process causes PVDF to solidify within the pores, forming a polymer gel or film that encapsulates and anchors the ionic liquid to the pore wall surface. Repeating this process is equivalent to reinforcing and thickening the already formed composite layer multiple times. After multiple cycles, PVDF can form a continuous and robust three-dimensional network throughout the pores, more thoroughly confining the ionic liquid within it, greatly improving the structural integrity of the composite dielectric layer and the leakage resistance of the ionic liquid. This operation ensures sufficient ionic liquid to provide high ionic conductivity and capacitive response, while avoiding the risks of pore blockage, decreased elasticity, or phase separation caused by single overfilling. Multiple cycles help achieve an ideal thin layer and uniform loading state, which is a key process step to obtain high sensitivity, wide response range, and excellent cycle stability.

[0052] Furthermore, after repeated soaking and drying operations in the composite solution, the mass ratio of the composite solution to the porous PDMS elastic framework is 0.6~0.9:1. This ratio ensures uniform penetration and thin-layer loading of the composite solution in the deep pores of the porous PDMS elastic framework, while avoiding performance degradation due to overfilling or underloading. The reason is as follows: (1) The porous PDMS elastic framework has a connected and tortuous pore structure inside, and the immersion-driven composite liquid penetrates into the deep pores; the mass ratio of the composite solution to the porous PDMS elastic framework is within 0.9:1, which aims to remove excess solution accumulated in the pore gaps and ensure that the composite gel film is formed only on the surface of the pore wall, avoiding the pore closure caused by the solution filling and blocking the pores. If the ratio is higher than 0.9:1, the pores of the porous PDMS elastic framework will be over-filled by gel, causing the sensor to lose low-pressure sensitivity due to the increase in modulus, and easily causing phase separation and leakage risks.

[0053] (2) This process controls the mass ratio of the composite solution to the porous PDMS elastic framework to be above 0.6:1, ensuring the basis for the ionization response. If the ratio is below 0.6:1, the number of mobile free ions in the system will be insufficient. When forming an electric double layer under pressure, there will not be enough ion concentration to accumulate charge, resulting in a weak change in capacitance per unit area, which significantly reduces the sensor's ability to detect small pressures. Therefore, the process parameters ensure that the sensor has excellent mechanical compressibility, electrical sensitivity, and structural stability.

[0054] Step 5: Print conductive silver paste of the same size and no larger than the area of ​​PDMS ion-dielectric elastomer 3 onto the first PI film 11 and the second PI film 22 respectively, and dry them to obtain the first electrode layer 1 and the second electrode layer 2.

[0055] Step six: The first electrode layer and the second electrode layer are respectively attached to the upper and lower surfaces of the PDMS ion-dielectric elastomer 3, and connected with metal leads 4 to obtain an ion-dielectric flexible pressure sensor. This bonding method can efficiently convert the double-layer change caused by ion migration under pressure into a capacitance signal that can be recognized by external circuitry.

[0056] Preferably, the first electrode layer 1 and the second electrode layer 2 are prepared by printing conductive silver paste onto the surface of a PI film using a flexible electronic printer. Electrodes prepared by this method have the advantages of high pattern precision, strong process flexibility, and outstanding preparation efficiency. The electrodes formed after drying are firmly bonded to the PI substrate and have good flexibility, maintaining stable conductivity under bending or stretching conditions. Furthermore, the silver paste itself has excellent conductivity, which is beneficial for achieving high sensitivity and rapid response in sensors, making it suitable for large-scale production of flexible electronic devices with high consistency requirements.

[0057] Example 1: This Example 1 provides a method for fabricating an ionized flexible pressure sensor, including the following steps: Step 1: Mix 5 g of PDMS prepolymer with 0.5 g of curing agent until homogeneous. Add 20 g of sucrose particles with an average particle size of 300 μm to the mixture and stir thoroughly. Pour the mixture into a 10 mm × 10 mm × 2 mm mold and heat at 70 °C to cure, forming a PDMS / sucrose particle composite solid. Then, immerse the cured composite solid in deionized water until the sucrose particles in the water are completely dissolved, obtaining a porous PDMS elastic framework.

[0058] Step 2: Dissolve 2 g of PVDF powder in 15 g of DMF solvent. While stirring, slowly add 1.5 g of [EMIM][TFSI] to the solution. Continue stirring for 2 hours to prepare the composite solution.

[0059] Step 3: After immersing the porous PDMS elastic skeleton completely in the composite solution prepared in Step 2 for 3 hours, remove it, squeeze it thoroughly to remove excess composite solution, and then dry it at 70 ℃ for 6 hours.

[0060] Step four, repeat step three twice, with the mass ratio of the composite solution to the porous PDMS elastic framework being 0.8:1, to obtain PDMS ionic dielectric elastomer 3.

[0061] Step 5: Using a flexible electronic printer, print the first surface electrode 12 and the second surface electrode 21 with the same size and not larger than the area of ​​the PDMS ion-dielectric elastomer 3 on the surfaces of the first PI film 11 and the second PI film 22, respectively, and dry them at 80°C for 2 hours to obtain the first electrode layer 1 and the second electrode layer 3.

[0062] Step six: The first electrode layer 1 and the second electrode layer 3 containing the surface electrode are tightly bonded to the surface of the PDMS ion-dielectric elastomer 3, and the metal lead wire 4 is led out to obtain the ion-electrode flexible pressure sensor.

[0063] Example 2: This example uses the fabrication method of an ionized flexible pressure sensor provided in Example 1. The difference from Example 1 is that [EMIM][TFSI] in step two is replaced with 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) or 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). Other preparation methods are the same as in Example 1.

[0064] Example 3: This example uses the fabrication method of an ionized flexible pressure sensor provided in Example 1. The difference from Example 1 is that the sucrose particles in step one are replaced with sodium chloride particles. The other preparation methods are the same as in Example 1.

[0065] Example 4: This example uses the fabrication method of an ionized flexible pressure sensor provided in Example 1. The difference from Example 1 is that the DMF in step two is replaced with N-methylpyrrolidone (NMP) or dimethyl sulfoxide (DMSO). Other preparation methods are the same as in Example 1.

[0066] Comparative Example 1: This comparative example was prepared using the method for preparing an ionized flexible pressure sensor provided in Example 1. The difference between this example and Example 1 is that the 1.5 g [EMIM][TFSI] in step two was replaced with 0.5 g [EMIM][TFSI]. The other preparation methods were the same as in Example 1.

[0067] Comparative Example 2: This comparative example was prepared using the method for preparing an ionized flexible pressure sensor provided in Example 1. The difference between this example and Example 1 is that the 1.5 g [EMIM][TFSI] in step two was replaced with 1 g [EMIM][TFSI]. The other preparation methods were the same as in Example 1.

[0068] Comparative Example 3: This comparative example was prepared using the method for preparing an ionized flexible pressure sensor provided in Example 1. The difference between this example and Example 1 is that the 1.5 g [EMIM][TFSI] in step two was replaced with 2 g [EMIM][TFSI]. The other preparation methods were the same as in Example 1.

[0069] Comparative Example 4: This comparative example provides a method for fabricating an ionized flexible pressure sensor, comprising the following steps: Step 1: Mix 5 g of PDMS prepolymer with 0.5 g of curing agent until homogeneous. Add 20 g of sucrose particles with an average particle size of 300 μm to the mixture and stir thoroughly. Pour the mixture into a 10 mm × 10 mm × 2 mm mold and heat at 70 °C to cure, forming a PDMS / sucrose particle composite solid. Then, immerse the cured composite solid in deionized water until the sucrose particles in the water are completely dissolved, obtaining a porous PDMS elastic framework.

[0070] Step 2: Dissolve 2 g of PVDF powder in 15 g of DMF solvent. While stirring, slowly add 1.5 g of [EMIM][TFSI] to the solution. Continue stirring for 2 hours to prepare the composite solution.

[0071] Step 3: After completely immersing the porous PDMS elastic framework in the composite solution prepared in Step 2 for 3 hours, remove it, squeeze it thoroughly to remove excess composite solution, and then dry it at 70℃ for 6 hours. The mass ratio of composite solution to porous PDMS elastic framework is 0.4:1, thus obtaining PDMS ionic dielectric elastomer.

[0072] Step 4: Using a flexible electronic printer, print a first surface electrode and a second surface electrode with the same size and not larger than the area of ​​the PDMS ion-dielectric elastomer on the surfaces of the first PI film and the second PI film, respectively, and dry them at 80°C for 2 hours to obtain the first electrode layer and the second electrode layer.

[0073] Step 5: The first electrode layer and the second electrode layer containing the surface electrode are tightly bonded to the surface of the PDMS ion-dielectric elastomer, and metal leads are drawn out to obtain an ion-electrode flexible pressure sensor.

[0074] Comparative Example 5: This comparative example was prepared using the method for preparing an ionized flexible pressure sensor provided in Example 1. The difference between Example 1 and Example 1 is that in step four, the mass ratio of the composite solution to the porous PDMS elastic skeleton of step three repeated twice and 0.8:1 was replaced by the mass ratio of the composite solution to the porous PDMS elastic skeleton of step three repeated once and 0.6:1. The other preparation methods are the same as in Example 1.

[0075] Comparative Example 6: This comparative example was prepared using the method for preparing an ionized flexible pressure sensor provided in Example 1. The difference between this example and Example 1 is that in step four, repeating step three twice with a mass ratio of 0.8:1 for the composite solution to the porous PDMS elastic skeleton is replaced with repeating step three three times with a mass ratio of 1:1 for the composite solution to the porous PDMS elastic skeleton. The other preparation methods are the same as in Example 1.

[0076] Comparative Example 7: This comparative example aims to verify the preparation of an ionized pressure sensor by directly mixing and solidifying a porous PDMS elastic framework with an ionic liquid instead of using the preparation process of this invention. The preparation method includes the following steps: Step 1: Mix 5 g of PDMS prepolymer with 0.5 g of curing agent until homogeneous. Add 20 g of sucrose particles with an average particle size of 300 μm to the mixture and stir thoroughly. Pour the mixture into a 10 mm × 10 mm × 2 mm mold and heat at 70 ℃ to cure, forming a PDMS / sucrose particle composite solid. Subsequently, immerse the cured composite solid in deionized water until the sucrose particles in the water are completely dissolved, obtaining a porous PDMS elastic framework.

[0077] Step 2: The porous PDMS elastic framework prepared in Step 1 is completely immersed in the [EMIM][TFSI] solution for 3 hours. After soaking, the porous PDMS elastic framework is removed and squeezed thoroughly to remove excess solution. Then, the porous PDMS elastic framework loaded with [EMIM][TFSI] solution is dried at 70 °C for 6 hours.

[0078] Step 3: Repeat step 2 twice. The mass ratio of [EMIM][TFSI] solution to porous PDMS elastic framework is 0.8:1 to obtain PDMS ionic dielectric elastomer.

[0079] Step 4: Using a flexible electronic printer, conductive silver paste of the same size and no larger than the area of ​​PDMS ion-dielectric elastomer is printed on the surface of the PI film, and dried at 80 °C for 2 hours to obtain the first and second electrode layers.

[0080] Step 5: The first electrode layer and the second electrode layer containing the surface electrode are tightly bonded to the surface of the PDMS ion-dielectric elastomer, and metal leads are drawn out to obtain an ion-electrode flexible pressure sensor.

[0081] Comparative Example 8: This comparative example aims to verify that if the preparation process of this invention is not used, and instead the ionic liquid, water-soluble porogen, and PDMS prepolymer are directly blended and cured, an effective PDMS ionic dielectric elastomer cannot be formed. The specific preparation steps are as follows: 5 g of PDMS prepolymer, 0.5 g of curing agent, and 1.5 g of [EMIM][TFSI] solution were mixed and stirred until homogeneous. 20 g of sucrose particles with an average particle size of 300 μm were added to the mixture and stirred thoroughly. The mixture was poured into a 10 mm × 10 mm × 2 mm mold and cured at 70 °C to form a PDMS / sucrose particle composite solid. The cured composite solid was immersed in deionized water to dissolve the sucrose pore-forming agent. During immersion, the edges of the solid rapidly crumbled, and the aqueous solution became extremely turbid. After 6 hours of immersion, the material was removed; it had lost its complete skeletal structure and become a soft, loose gel-like substance with an oily exudate on the surface, making subsequent electrode attachment and performance testing impossible.

[0082] like Figure 2 The images show the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) elemental distribution of the PDMS ion-dielectric elastomer 3 prepared in Example 1. It can be observed that the ionic liquid is uniformly distributed on the surface of the foam skeleton, and the sulfur element originates solely from [EMIM][TFSI], verifying that the ionic liquid was successfully and uniformly loaded onto the surface of the foam skeleton.

[0083] The working mechanism of the ionized flexible sensor prepared in this invention is based on the parallel plate capacitance formula C=ε0ε r A / d, where C is the capacitance of the sensor and ε0 is the vacuum permittivity, approximately 8.854 × 10⁻⁶. -12 F / m, ε r ε is the relative permittivity of the PDMS ion-dielectric elastomer 3, A is the area of ​​the two electrodes facing each other, and d is the distance between the two electrodes. The excitation is externally applied pressure. The core process of converting the force signal into capacitance change is as follows: when pressure is applied to the sensor, the PDMS ion-dielectric elastomer 3 in the middle is compressed, the distance d between the two electrodes decreases, and the effective area A of the PDMS ion-dielectric elastomer 3 facing each other with the electrode layer increases. Furthermore, the compression action causes the ionic liquid within the pores of the PDMS ion-dielectric elastomer 3 to migrate towards the electrode interface, significantly enhancing the double-layer effect. This increases the equivalent relative permittivity ε of the PDMS ion-dielectric elastomer 3. r This also leads to an increase. Therefore, pressure, through synergy, causes d to decrease, A to increase, and ε to increase. r Increasing the capacitance C of the sensor causes a rapid and sensitive change.

[0084] The operating range and sensitivity of the ionized flexible pressure sensor prepared in Example 1 were tested using a signal acquisition device integrating a universal testing machine (CMT4103) and an LCR (TH2839). The results are as follows: Figure 3As shown, the sensor's sensitivity varies within different pressure ranges. The sensitivity S value is calculated, and the S value is the slope of the curve in the graph. That is, S value = capacitance change rate / pressure change, where capacitance change rate = capacitance change ΔC / initial capacitance C0. The graph shows that the sensitivity of Example 1 is divided into three stages: the first stage is the low-pressure stage (0-3 kPa), with S1 = 1.28 kPa. -1 The second stage, the moderate pressure stage (3-110 kPa), has an S² = 2.62 kPa. -1 The third stage, the high-pressure stage (110-500 kPa), has an S3 value of 0.91 kPa. -1 The results show that Example 1 balances high sensitivity with a wide response range, exhibiting high sensitivity throughout the entire operating range.

[0085] like Figure 4 As shown, this is a durability and stability test of the ionized flexible pressure sensor prepared in Example 1. During 3000 load-unload cycles under a relative pressure of 15 kPa, the sensor's signal output curve remained essentially consistent, with no significant signal drift, thus verifying its good durability and signal output stability.

[0086] like Figure 5 The image shows the response time test of the ionized flexible pressure sensor prepared in Example 1. The sensor obtained in Example 1 was subjected to a 15 kPa pressure test, and the response time and recovery time were 51 ms and 98 ms, respectively, demonstrating rapid response and recovery capabilities, further confirming its reliability in applications.

[0087] like Figure 6 As shown, the dynamic response performance of the ionized flexible pressure sensor prepared in Example 1 to weak stimuli is demonstrated. It shows that it can respond quickly and accurately to the capacitance signal generated when a water droplet (approximately 10 Pa) falls, indicating that the sensor can accurately sense weak signal changes at high resolution.

[0088] The inventors discovered through research that, in the preparation process of Example 1, step two, the content of the ionic liquid has a significant impact on the sensor sensitivity. Figure 7 The sensitivity curves of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 show that the sensitivity of Example 1 is much higher than that of Comparative Examples 1, 2, and 3. The sensor sensitivity increases significantly with increasing [EMIM][TFSI] content. The sensor sensitivity also increases significantly with increasing ionic liquid content, but when the [EMIM][TFSI] content is 2 g, the sensitivity shows a decreasing trend. This is because the excessively high ionic liquid content leads to an excessively large initial capacitance of the sensor, thereby reducing the sensor sensitivity.

[0089] The inventors discovered through research that in the preparation process of Example 1, step four, the number of times step three is repeated, has a significant impact on the sensor sensitivity. Figure 8 As shown in the sensitivity curves of Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6, the sensitivity of Example 1 is much higher than that of Comparative Examples 4, 5, and 6. The sensor sensitivity increases significantly with the number of repetitions of step three, but decreases after three repetitions. This is because the composite solution inside the PDMS fills and saturates the pores of the porous PDMS elastic framework, and the initial capacitance of the sensor is also too large at this time, thus reducing the sensor sensitivity.

[0090] This invention utilizes an in-situ composite process to achieve deep, uniform, and controllable loading of the composite solution within a porous PDMS elastic framework through 2-3 cycles of "immersion-extrusion-drying." This not only allows the composite solution to fully penetrate the internal pores and avoid local voids, but also forms a continuous and stable three-dimensional polymer network through the layer-by-layer solidification of PVDF during each drying process. This firmly anchors the ionic liquid to the pore wall surface, effectively suppressing ionic liquid migration and leakage under the dual effects of physical confinement and chemical fixation. Simultaneously, by coordinating the number of cycles and the extrusion operation, this process controls the mass ratio of the composite solution to the framework within the ideal range of 0.6-0.9:1. This ensures sufficient ion concentration to maintain a high capacitive response while avoiding pore blockage and decreased elasticity due to overfilling, thus achieving a balance between structural integrity and mechanical compressibility. Ultimately, this results in a sensor with high sensitivity, a wide response range, excellent cycling stability, and good process reproducibility.

[0091] like Figure 9 The image shows the durability and stability test of the ionized flexible pressure sensor prepared in Comparative Example 7. During 1000 load-unload cycles at a relative pressure of 15 kPa, the sensor's signal output curve showed significant signal drift, and an oily, gel-like substance was observed on its surface. This indicates that directly mixing and solidifying the porous PDMS elastic framework with the ionic liquid, without PVDF as a molecular "bridge," prevents the ionic liquid from firmly attaching to the surface of the porous PMDS elastic framework. This leads to phase separation between the ionic liquid and the porous PMDS elastic framework during use, causing leakage of the ionic liquid from the sensor.

[0092] like Figure 10As shown, the ionized flexible pressure sensor prepared in Example 1 was attached to the inner wrist of a mask. The results of respiratory signal detection show that the sensor can clearly identify different breathing states (normal breathing, rapid breathing, and deep breathing). As the breathing rate increases, the curve becomes denser compared to normal breathing, while the capacitance change is greater during deep breathing.

[0093] To demonstrate that the sensor obtained in Example 1 still possesses relatively stable signal detection capability under high pressure, such as Figure 11 As shown, the ionized flexible pressure sensor prepared in Example 1 was installed at the heel of the insole and subjected to high-intensity exercises such as walking, running, and jumping. The results showed that it can accurately capture the changes in electrical signals caused by the sensor contact point when the heel hits the ground, and can be used to analyze gait and walking posture.

[0094] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An ionized flexible pressure sensor, characterized in that, The device includes a first electrode layer, a second electrode layer, a PDMS ion-dielectric elastomer, and metal leads. The first electrode layer is composed of a first PI film and a first surface electrode. The second electrode layer is composed of a second PI film and a second surface electrode. The first PI film, the first surface electrode, the PDMS ion-dielectric elastomer, the second surface electrode, and the second PI film are sequentially bonded from top to bottom. The PDMS ion-dielectric elastomer is made of PVDF, ionic liquid, and PDMS. The metal leads bring out the first surface electrode and the second surface electrode respectively.

2. The ionized flexible pressure sensor according to claim 1, characterized in that, The ionic liquid is one of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium hexafluorophosphate.

3. The ionized flexible pressure sensor according to claim 1, characterized in that, The PDMS ion-dielectric elastomer has a length of 5-20 mm, a width of 5-20 mm, and a thickness of 1-5 mm.

4. The method for preparing the ionized flexible pressure sensor according to any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1: Mix PDMS prepolymer and curing agent thoroughly at a mass ratio of 10:1, then mix with water-soluble porogen, pour into a mold, cure at 70 °C, and then soak in deionized water until the water-soluble porogen is completely removed to obtain a porous PDMS elastic skeleton. Step 2: Dissolve PVDF powder fully in an organic solvent and mix it evenly with an ionic liquid to prepare a composite solution. The mass ratio of the ionic liquid to the PVDF powder is 0.5~2:

1. Step 3: Immerse the porous PDMS elastic skeleton in the composite solution for 2-6 hours, remove it, squeeze it thoroughly to remove excess composite solution, and then dry it at 40-70°C for 6-12 hours. Step four, repeat step three 1-2 times to obtain the PDMS ion dielectric elastomer; Step 5: Print conductive silver paste of the same size, not larger than the area of ​​PDMS ion-dielectric elastomer, onto the first PI film and the second PI film respectively, and dry them to obtain the first electrode layer and the second electrode layer. Step six: Attach the first electrode layer and the second electrode layer to the upper and lower surfaces of the PDMS ion-dielectric elastomer, respectively, and connect them with metal leads to obtain the ionized flexible pressure sensor.

5. The method for fabricating the ionized flexible pressure sensor according to claim 4, characterized in that, The water-soluble porogen mentioned in step one is either sucrose particles or sodium chloride particles, with an average particle size of 50~500 μm.

6. The method for fabricating an ionized flexible pressure sensor according to claim 4, characterized in that, The mass ratio of the PDMS prepolymer to the water-soluble porogen in step one is 1:3~6.

7. The method for fabricating the ionized flexible pressure sensor according to claim 4, characterized in that, The molecular weight of the PVDF powder mentioned in step two is 8 × 10⁻⁶. 5 ~ 1×10 6 .

8. The method for fabricating an ionized flexible pressure sensor according to claim 4, characterized in that, The mass fraction of PVDF in the composite solution described in step two is 5% to 15%.

9. The method for fabricating an ionized flexible pressure sensor according to claim 4, characterized in that, The organic solvent mentioned in step two is one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

10. The method for fabricating an ionized flexible pressure sensor according to claim 4, characterized in that, In the PDMS ionic dielectric elastomer obtained in step four, the mass ratio of the composite solution to the porous PDMS elastic framework is 0.6~0.9:1.

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