A hollow PVDF micro-nano fiber microstructure piezoelectric sensor and its preparation method

Abstract

The present invention provides a hollow PVDF micro-nanofiber microstructure piezoelectric sensor and a preparation method thereof. The preparation method comprises: S1: pouring PVDF and polyvinyl pyrrolidone powders into a mixture of dimethylacetamide and acetone, and heating and stirring to obtain an outer layer spinning solution; dissolving the polyvinyl pyrrolidone powder in anhydrous ethanol and heating and stirring to obtain an inner layer spinning solution; S2: printing a microstructure collector using stereolithography technology, and fixing the microstructure collector on the surface of an electrospinning flat plate collector; S3: preparing a PVDF / PVP composite fiber membrane by coaxial electrospinning the inner and outer layer spinning solutions; S4: removing PVP from the composite fiber membrane and drying it to obtain a PVDF micro-nanofiber membrane; S5: covering electrodes on both sides of the PVDF micro-nanofiber membrane and encapsulating it to complete the preparation. The piezoelectric sensor prepared by the present invention has high sensitivity, excellent mechanical durability and stability, and high commercial value and promotion value.

Description

Technical Field

[0001] The present invention relates to the technical field of piezoelectric sensor preparation, and in particular to a hollow PVDF micro-nano fiber microstructure piezoelectric sensor and a preparation method thereof. Background Art

[0002] Currently, in the fields of human-computer interaction and human activity monitoring, traditional detection equipment faces challenges such as large size, high cost, and lack of portability. Flexible sensors are thin-film electronic devices that maintain high photoelectric efficiency, reliability, and integration even when bent, folded, twisted, compressed, and stretched. Wearable electronic devices made of flexible electronic materials have attracted widespread attention due to their enormous application potential and broad market opportunities.

[0003] Piezoelectric materials stand out among the promising materials for wearable electronic devices due to their high flexibility, light weight, biocompatibility, and remarkable piezoelectric properties. Polyvinylidene fluoride (PVDF), in particular, is widely used due to its portability, piezoelectricity, and other remarkable properties, and has an extremely high piezoelectric coefficient. Electrospinning is a simple method for preparing high-β-phase crystalline PVDF nanofibers through in situ mechanical stretching and poling during the fiber processing process.

[0004] Currently, the main methods for increasing the β-phase content of prepared PVDF fiber membranes include adding salts and nanofillers. However, the disadvantage of these methods is that the steps are relatively complicated. Although they can increase the β-phase content, the addition of other materials exacerbates the instability of the electrospinning process, making it difficult to form a complete fiber membrane without obvious defects. However, the prepared planar fiber membrane has a limited amount of compression when subjected to external force, which limits the detection range of the sensor to a small range. The mechanical durability and stability of the sensor are also relatively general. With the increasing application of flexible sensors, higher requirements are also placed on the performance of the sensors.

[0005] In the related research of existing technologies, changing the shape of PVDF fibers can further improve their piezoelectric properties, such as core-shell nanofibers, high-porosity fibers, multi-channel nanofibers, etc. The output voltage or power has been significantly improved compared with solid fibers after testing. In order to improve the comprehensive performance of flexible sensors, in addition to enhancing the material properties, constructing a microstructure in the functional layer of the sensor is also an effective method. The piezoelectric film with a microstructure has high compressibility and strong resistivity, which improves the piezoelectric properties of the nanofiber membrane. Changing the shape of the fiber and constructing a microstructure on the surface of the fiber membrane can improve the performance of the flexible sensor, but previous studies have almost always discussed these two solutions separately, and research on the combination of the two is currently relatively rare. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a preparation method for a hollow PVDF micro-nanofiber microstructure piezoelectric sensor, so as to solve the problem that the existing preparation method has complex steps, is unstable, and is not easy to form a complete fiber membrane without obvious defects, thereby limiting the detection range of the sensor to a smaller range. A preparation method is provided that can improve the performance of the flexible sensor by changing the shape of the fiber and constructing a microstructure on the surface of the fiber membrane.

[0007] In order to solve the above technical problems, the present invention provides a method for preparing a hollow PVDF micro-nano fiber microstructure piezoelectric sensor, comprising the following steps:

[0008] S1: Pour PVDF and polyvinyl pyrrolidone powders into a mixture of dimethylacetamide and acetone, and heat and stir thoroughly to obtain a PVDF / PVP mixed outer layer spinning solution; dissolve polyvinyl pyrrolidone powder in anhydrous ethanol and heat and stir to obtain an inner layer spinning solution;

[0009] S2: Printing a microstructure collector by stereolithography technology, and fixing the microstructure collector on the surface of the electrospinning flat plate collector;

[0010] S3: using the outer layer spinning solution and the inner layer spinning solution of step S1 as raw materials to prepare a PVDF / PVP composite fiber membrane by coaxial electrospinning;

[0011] S4: removing PVP from the PVDF / PVP composite fiber membrane of step S3 and drying the membrane in a vacuum oven to obtain a PVDF micro-nano fiber membrane with a porous surface, a hollow inner layer, and a microstructure on the fiber membrane surface;

[0012] S5: Covering electrodes on both sides of the PVDF micro-nano fiber membrane obtained in step S4, and then encapsulating it with PDMS and a curing agent to prepare a hollow PVDF micro-nano fiber microstructure piezoelectric sensor.

[0013] Compared with the prior art, the preparation method of a hollow PVDF micro-nano fiber microstructure piezoelectric sensor in the present application has the following advantages: the preparation method of a hollow PVDF micro-nano fiber microstructure piezoelectric sensor in the present invention increases the compressibility of the nanofiber membrane through multiple levels, thereby improving the piezoelectric output of the prepared PVDF nanofiber membrane, so that the sensitivity of the sensor is improved. Through the method of coaxial electrospinning, the concentration of the inner layer spinning solution is further regulated, and finally a nanofiber with a porous surface and a hollow interior is prepared, and a microstructure collector is also used to directly construct a microstructure on the surface of the fiber membrane. The synergistic effect of the multi-level microstructure further increases the compressibility of the fiber membrane and the sensitivity of the final sensor. The experimental measurement shows that the output voltage of the microstructured hollow PVDF nanofiber is about three times that of the solid PVDF nanofiber without microstructure.

[0014] The present invention prepares a hollow PVDF fiber membrane containing a high β phase under the combined action of the electric field force stretching of electrospinning and the inner and outer layer spinning solutions, without the need for subsequent treatment, thus simplifying the preparation process; and using the coaxial electrospinning method, the internal shape of the prepared fiber can be controlled by simply changing the concentration of the inner layer spinning solution, and the prepared fiber morphology is intact without obvious defects, thereby further improving the stability of the final sensor.

[0015] In a possible implementation, in step S1, the polyvinyl pyrrolidone powder in the outer layer spinning solution is polyvinyl pyrrolidone K30; and the polyvinyl pyrrolidone powder in the inner layer spinning solution is polyvinyl pyrrolidone K90.

[0016] In the above embodiment, the selection of polyvinyl pyrrolidone K30 as a porogen is conducive to forming a porous surface structure after removal, and combined with the selection of K90 in the inner layer spinning solution, the performance of the final product is further improved.

[0017] In one possible embodiment, in step S1, the molecular weight of the PVDF is 300±50k, which is conducive to the stretching and orientation of the molecular chains to form bead-free round fibers; the mass ratio of the PVDF and polyvinylpyrrolidone K30 is (1.7±0.2): (0.8±0.2); the mass ratio of the dimethylacetamide and acetone is (1±0.1): (1±0.1).

[0018] In one possible embodiment, in step S1, the concentration of polyvinyl pyrrolidone in the inner layer spinning solution is 5-15%, and the heating and stirring conditions are: stirring by magnetic stirring, and the stirring temperature is 60°C, the stirring time is 6-8h, and the rotation speed is 400-500rpm.

[0019] In the above embodiment, sufficient heating and stirring are beneficial for forming a stable spinning solution, thereby improving the spinning effect.

[0020] In a possible implementation, in step S2, the grid size of the microstructure collector is (2.2±0.4) mm, and the grid spacing is 0.1 mm.

[0021] In the above possible implementation manner, the present invention prepares a collector with a microstructure and assembles it on a flat collector, which can directly prepare a fiber membrane with a microstructure on the surface, greatly reducing the preparation steps and time.

[0022] In a possible embodiment, in step S3, the method for preparing the PVDF / PVP composite fiber membrane includes:

[0023] The inner layer spinning solution and the outer layer spinning solution of step S1 are added to the syringe respectively, connected to the coaxial needle, and two micro-injection pumps are used to accurately control the flow rate of the inner and outer layer solutions to prepare the PVDF / PVP composite fiber membrane by coaxial electrospinning.

[0024] In one possible embodiment, in step S3, the size of the coaxial needle is 23 / 16G, the flow rate of the inner layer spinning solution is 0.55 ml / h, the flow rate of the outer layer spinning solution is 1.3 ml / h, the voltage applied for spinning is 18-22 kV, the spinning distance is 15 cm, and the spinning time is 2 hours.

[0025] In the above possible embodiments, the present invention utilizes a coaxial electrospinning method, and the internal shape of the prepared fiber can be controlled simply by changing the concentration of the inner spinning solution, and the prepared fiber morphology is intact and has no obvious defects;

[0026] In one possible embodiment, in step S4, the method of removing PVP is to remove the PVP in the fiber by immersing the PVDF / PVP composite fiber membrane in anhydrous ethanol and performing ultrasonic treatment, and the ultrasonic treatment time is more than 2 hours; the drying condition is to place the composite fiber membrane in a vacuum oven for drying at a temperature of 60°C for more than 6 hours, which is conducive to sufficient drying and forming a hollow fiber with a full morphology.

[0027] In one possible embodiment, in step S5, the electrode is copper foil, and the mass ratio of PDMS to curing agent is 10:1. The packaging conditions are: after mixing PDMS and curing agent, pouring them on the piezoelectric film, and curing them at 60°C for 2 hours.

[0028] Another technical problem to be solved by the present invention is to provide a hollow PVDF micro-nano fiber microstructure piezoelectric sensor to solve the problems of small detection range and low sensitivity of conventional piezoelectric sensors.

[0029] In order to solve the above technical problems, the present invention provides a hollow PVDF micro-nano fiber microstructure piezoelectric sensor, which is prepared by the preparation method.

[0030] Compared to existing technologies, the hollow PVDF micro-nanofiber piezoelectric sensor of the present invention has a higher β phase and porosity, better mechanical durability and stability, a fast response time and good recovery time, and excellent dynamic performance. It has great potential for human activity monitoring and intelligent human-computer interaction. The hollow PVDF micro-nanofiber piezoelectric sensor of the present invention has good sensitivity, can detect signals under very low pressure, and is suitable for signal detection and acquisition in multiple scenarios, with high universality. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of a method for preparing a hollow PVDF micro-nano fiber microstructure piezoelectric sensor in the present invention;

[0032] Figure 2 Figure 1 is a crystal structure characterization and tensile performance test diagram of the fiber membrane prepared in Example 1;

[0033] Figure 3 The surface and cross-sectional morphologies of nanofibers were observed using scanning electron microscopy (SEM);

[0034] Figure 4 Output voltage signal diagram of PVDF-0, PVDF-5, PVDF-10, and PVDF-15 without microstructure;

[0035] Figure 5 The images are the physical images of the microstructure collector, the physical images of the microstructure fiber membrane and the basic representation images of the microstructure dimensions;

[0036] Figure 6 The cross-sectional size images of three groups of microstructured fiber membranes;

[0037] Figure 7 The sensitivity curves of three groups of microstructured fiber membranes and PVDF-15 fiber membranes;

[0038] Figure 8 This is a 14,400-cycle endurance test diagram of the hollow PVDF micro-nanofiber microstructure piezoelectric sensor;

[0039] Figure 9 The output voltage signal diagram of the sensor in different human activity monitoring;

[0040] Figure 10 Output voltage signal diagram for sensors monitoring various types of pulses and heartbeats of the human body;

[0041] Figure 11 is the image that the sensor uses for gesture recognition. DETAILED DESCRIPTION

[0042] First, those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the embodiments of the present application and are not intended to limit the scope of protection of the embodiments of the present application. Those skilled in the art may adjust them as needed to suit specific application scenarios.

[0043] In the description of the embodiments of this application, it should be noted that, unless otherwise specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium. Those skilled in the art will understand the specific meanings of the above terms in the embodiments of this application based on the specific circumstances.

[0044] The present invention provides a method for preparing a hollow PVDF micro-nano fiber microstructure piezoelectric sensor. The preparation process is as follows: Figure 1 As shown, the following steps are included:

[0045] S1: Pour PVDF and polyvinyl pyrrolidone powders into a mixture of dimethylacetamide and acetone, and heat and stir to obtain a PVDF / PVP mixed outer layer spinning solution; dissolve polyvinyl pyrrolidone powder in anhydrous ethanol and heat and stir to obtain an inner layer spinning solution;

[0046] S2: Printing a microstructure collector by stereolithography technology, and fixing the microstructure collector on the surface of the electrospinning flat plate collector;

[0047] S3: using the outer layer spinning solution and the inner layer spinning solution of step S1 as raw materials to prepare a PVDF / PVP composite fiber membrane by coaxial electrospinning;

[0048] S4: removing PVP from the PVDF / PVP composite fiber membrane of step S3 and drying the membrane to obtain a PVDF micro-nano fiber membrane with a porous surface, a hollow inner layer, and a microstructure on the fiber membrane surface;

[0049] S5: Covering electrodes on both sides of the PVDF micro-nano fiber membrane obtained in step S4, and then encapsulating it with PDMS and a curing agent to prepare a hollow PVDF micro-nano fiber microstructure piezoelectric sensor.

[0050] As a preferred solution, in step S1, the polyvinyl pyrrolidone powder in the outer layer spinning solution is polyvinyl pyrrolidone K30; and the polyvinyl pyrrolidone powder in the inner layer spinning solution is polyvinyl pyrrolidone K90.

[0051] As a preferred solution, in step S1, the molecular weight of the PVDF is 300±50k; the mass ratio of the PVDF and polyvinylpyrrolidone K30 is (1.7±0.2):(0.8±0.2); and the mass ratio of dimethylacetamide and acetone is (1±0.1):(1±0.1).

[0052] As a preferred solution, in step S1, the concentration of polyvinyl pyrrolidone in the inner layer spinning solution is 5-15%, and the heating and stirring conditions are: stirring by magnetic stirring, the stirring temperature is 60°C, the stirring time is 6-8h, and the rotation speed is 400-500rpm.

[0053] As a preferred solution, in step S2, the grid size of the microstructure collector is (2.2±0.4) mm, and the grid spacing is 0.1 mm.

[0054] As a preferred solution, in step S3, the method for preparing the PVDF / PVP composite fiber membrane includes:

[0055] The inner layer spinning solution and the outer layer spinning solution of step S1 are added to the syringe respectively, connected to the coaxial needle, and two micro-injection pumps are used to accurately control the flow rate of the inner and outer layer solutions to prepare the PVDF / PVP composite fiber membrane by spinning.

[0056] As a preferred solution, in step S3, the size of the coaxial needle is 23 / 16G, the flow rate of the inner layer spinning solution is 0.55 ml / h, the flow rate of the outer layer spinning solution is 1.3 ml / h, the voltage applied for spinning is 18-22 kV, the spinning distance is 15 cm, and the spinning time is 2 hours.

[0057] As a preferred solution, in step S4, the method of removing PVP is to remove PVP by immersing the PVDF / PVP composite fiber membrane in anhydrous ethanol and then ultrasonically treating it, and the ultrasonic treatment time is more than 2 hours; the drying condition is to place the composite fiber membrane in a vacuum oven for drying at a temperature of 60°C for more than 6 hours.

[0058] As a preferred solution, in step S5, the electrode is copper foil, and the mass ratio of PDMS to curing agent is 10:1. The packaging conditions are: after mixing PDMS and curing agent, pouring it on the piezoelectric film, and curing at 60°C for 2h.

[0059] The present invention also provides a hollow PVDF micro-nano fiber microstructure piezoelectric sensor, which is prepared by the preparation method.

[0060] The following provides examples that combine specific experimental data and operating methods, and compare the experiments using the inner layer spinning solution as a controlled variable. Of course, the present invention does not only include examples of the following experimental conditions. The experimental parameters, experimental time, and experimental conditions within the scope of the above technical solution of the present invention should be included in the scope of protection of the present invention. The following examples further describe the above technical solution of the present invention by controlling the changes in some data, and provide the results of testing the finished product:

[0061] Example 1:

[0062] In Example 1, three groups of inner layer spinning solution cases with different concentrations are included in step S1, and three groups of micro-lattice cases with different sizes are included in step S2, so as to facilitate further comparison in the subsequent testing stage:

[0063] S1: 1.7 g PVDF powder and 0.8 g PVP were weighed into a screw-cap bottle at room temperature. A mixed solution of DMAC and acetone (10 mL in total) with a volume ratio of 1:1 was added to the bottle and placed on a magnetic stirrer. After stirring at 60°C for 6 h, a uniform and stable outer layer spinning solution was obtained.

[0064] PVP K90 powder was added to a screw-cap bottle at room temperature, anhydrous ethanol was added to the bottle and the bottle was placed on a magnetic stirrer and stirred at 60° C. for 6 h to prepare inner layer spinning solutions with concentrations of 5%, 10%, and 15%, respectively.

[0065] S2: Use SLA to prepare a collection plate with microstructures. The sizes of the micro grids are 1.8mm×1.8mm, 2.2mm×2.2mm, and 2.6mm×2.6mm, respectively. The size between each group of grids is 0.1mm. The prepared microstructured small blocks are assembled onto the flat plate collector of electrospinning.

[0066] S3: The inner and outer layer spinning solutions are added to the syringe respectively and the syringe is connected to the 23 / 16G coaxial needle, and the needle is grounded. The collecting plate is connected to the positive pole of the high-voltage power supply and the voltage is set to 18-22kV. The distance between the needle and the collecting plate is 15cm. Two micro-injection pumps are used to precisely control the flow rate. The flow rate of the inner layer spinning solution is controlled at 0.55ml / h, and the flow rate of the outer layer spinning solution is controlled at 1.3ml / h. The fiber collection time is 2h. A 21G uniaxial needle is used for spinning as a control group. The spinning solution flow rate is 1.5ml / h, the applied voltage is 17KV, the spinning distance is 15cm, and the fiber collection time is 2h.

[0067] S4: After the prepared PVDF / PVP composite fiber membrane is naturally dried, it is transferred to anhydrous ethanol for ultrasonic treatment for 2 hours to remove the PVP inside the fiber, and then transferred to a vacuum oven and dried at 60°C for 6 hours to finally obtain a hollow PVDF micro-nano fiber membrane with a microstructure.

[0068] S5: Copper foil was attached to each side of the piezoelectric film as electrodes. 20g of PDMS liquid and 2g of curing agent were weighed and placed in a beaker, stirred for 5 minutes, poured onto the piezoelectric film, and cured at 60°C for 2 hours. After the PDMS solidified, it was cut into appropriate sizes, resulting in a hollow PVDF micro-nanofiber microstructure piezoelectric sensor.

[0069] Example 2-3:

[0070] Examples 2-3 are similar to Example 1, except that, in step S1, the concentrations of the inner layer spinning solution are different, namely 10% and 15%, respectively, and the fibers prepared in Examples 1-3 are named PVDF-5, PVDF-10, and PVDF-15, respectively, according to the concentrations of the inner layer spinning solution.

[0071] Comparative Example 1:

[0072] Comparative Example 1 is also similar to Example 1, except that in step S1, the concentration of the inner layer spinning solution is different, which is 0%, that is, anhydrous ethanol. The fiber obtained in Comparative Example 1 is named PVDF-0.

[0073] The following relevant tests were performed on the above-mentioned Examples 1-3 and Comparative Example 1 of the present invention:

[0074] like Figure 2 As shown in FIG, the fiber prepared in the above embodiment was subjected to crystal structure characterization and tensile performance testing. Figure 2As shown in Figure a, the melting temperatures of four groups of fiber membranes (PVDF-0, PVDF-5, PVDF-10, and PVDF-15) prepared in Examples 1-3 and Comparative Example 1 were measured using differential scanning calorimetry (DSC). It can be seen that the melting temperatures of all hollow PVDF micro-nanofibers are higher than those of the solid fiber PVDF-0 in Comparative Example 1. Figure 2 As shown in Figures 2b and 2c, the content of β phase in the fiber was obtained by X-ray diffraction analysis and Fourier transform infrared spectroscopy, and as the inner layer PVP solution was gradually introduced, the original non-polar α phase gradually decreased. The crystallinity and β phase content of the four groups of fiber membranes were measured as shown in Figure 2c. Figure 2 As shown in Figure d, the crystallinity of the three groups of hollow PVDF is slightly lower than that of solid fiber PVDF-0, but the β-phase content is significantly improved. When the inner layer spinning solution concentration is 15%, the β-phase content reaches a maximum of 91.31%. Figure 2 As shown in Figure e, the porosity of the four groups of fiber membranes was measured using the n-butanol method. It can be seen that the porosity of the three groups of hollow PVDF F fibers is higher than that of the solid fiber PVDF-0, and the porosity of PVDF-15 is the highest, reaching 88.94. Figure 2 As shown in Figure 5, four groups of fiber membranes were subjected to uniaxial tensile testing. The three hollow PVDF fiber groups all achieved higher tensile strains than the solid fiber PVDF-0, with PVDF-15 achieving the highest, 56.9%. These test results fully demonstrate that the prepared hollow PVDF fibers have a high β-phase content and excellent strain properties. They also prove that the spinning solution prepared by the special preparation method of the present invention has solved the problems existing in the background art after practical application.

[0075] like Figure 3 As shown in the figure, the surface and cross-sectional morphologies of the nanofibers were observed using a scanning electron microscope (SEM). It can be seen that as the concentration of the inner layer PVP solution increases, the fibers gradually transform from a porous interior to a hollow structure, the surface becomes increasingly rough and porous, and the average diameter of the fibers increases accordingly. The main reason for this phenomenon is that as the concentration of the inner layer solution increases, the inner layer solution gradually diffuses outward. When the inner layer solution concentration is 15%, the fibers exhibit a well-defined, completely hollow structure.

[0076] like Figure 4 As shown, Figure 4PVDF-0, PVDF-5, PVDF-10, and PVDF-15 are PVDF micro-nano fiber membranes prepared by removing step S2 and step S4 from Examples 1-3 and Comparative Example 1; four groups of fiber membranes (PVDF-0, PVDF F-5, PVDF-10, and PVDF-15) without microstructures prepared by removing step S2 from Examples 1-3 and Comparative Example 1 were tested, and the packaged sensors were subjected to piezoelectric performance tests. The piezoelectric outputs of the three groups of fibers with hollow structures under the impact of a 5N, 2Hz exciter were higher than those of the solid fiber PVDF-0, and the output voltage generated by PVDF-15 was as high as 7.4V, while the maximum voltage of PVDF-0 was only 3.4V. Moreover, under the same frequency (2Hz) and different external forces (1N, 3N, 5N, 7.5N, 10N, and 15N), PVDF-15 had higher sensitivity and detection range than PVD F-0. This shows that the hollow micro-nano fibers can generate piezoelectric output higher than that of solid fibers, and the piezoelectric output increases with the increase of the hollowness. Since the performance of the prepared PVDF-15 (Example 3, step S2 is removed) is better than that of the other types, PVDF-15 is used in the subsequent experiments in the following tests.

[0077] like Figure 5 As shown, in order to further enhance the piezoelectric performance of the sensor, the present invention further constructs three microstructures of different scales on the surface of the above-mentioned PVDF-15 fiber membrane. In the step S2, the sizes of the micro grids are 1.8mm×1.8mm, 2.2mm×2.2mm, and 2.6mm×2.6mm, respectively, and the size between each group of grids is 0.1mm, respectively named M1.8, M2.2, and M2.6. Further tests are carried out in the following tests. Compared with the above-mentioned PVDF-15 of the present invention, the microstructure processing of step S2 of the present invention is added.

[0078] like Figure 6 As shown, the cross-sectional dimensions of the three groups of microstructured fiber membranes prepared in the above embodiments of the present invention were scanned using a laser confocal microscope (CLSM). It can be seen that the maximum surface undulation difference of the microstructure of M2.6 is 250 μm, while that of the other two groups is only 120 μm and 180 μm.

[0079] like Figure 7As shown, the piezoelectric output of three groups of nanofiber membranes with microstructures was tested under different external forces (1N, 3N, 5N, 7.5N, 10N, and 15N) at the same frequency (2Hz) and plotted as sensitivity curves. As shown in the figure, the introduction of the microstructures increased the output voltage of the nanofiber membranes, with the maximum output voltage reaching 10.1V, approximately 1.6 times higher than that of PVDF-15 (6.2V) without step S2 treatment. Compared to PVDF-0 (3.4V) without step S2 treatment, this increase is approximately 3 times. This further demonstrates that the introduction of the microstructure in step S2 can effectively improve the sensitivity of the sensor, and the synergistic effect of the above-mentioned spinning solution configuration and concentration ratio configuration further enhances the sensitivity. Compared with the other two microstructured sensors, the M2.6 has greater surface fluctuations and greater compression. In the 3-5N range, the maximum sensitivity of the M2.6 is 2.7V / N (1.08V / kPa), more than double that of the PVDF-15 without microstructures.

[0080] like Figure 8 As shown, durability testing of the microstructure sensor prepared in Example 3 revealed that after 14,400 impact cycles, the sensor's output voltage only slightly decreased, and the sensor maintained a stable piezoelectric output signal. This durability test demonstrates that the sensor prepared using the present invention has excellent mechanical durability and stability, resolving the problems encountered in the prior art.

[0081] like Figure 9 As shown, in order to verify the feasibility of the prepared sensor in human activity monitoring, the sensor prepared in Example 3 was attached to various parts of the human body for signal detection. When the sensor is used to monitor the wrist ( Figure 9 a) Elbows Figure 9 b) and knee joints ( Figure 9 c), the sensor has obvious signal changes at the same bending angle but different bending frequencies. Taking knee joint monitoring as an example, the output signals of volunteers in three motion states of walking, jogging and sprinting are shown in 7c. Based on the measured number of voltage cycles and voltage peaks, the running frequency during exercise can be calculated, and the exercise state of the volunteer can also be judged. The sensor is connected to the finger for activity monitoring, such as Figure 9 d. When the finger is bent at different angles (30°, 60°, and 90°) and the activity frequency is the same, the sensor shows significant signal changes. The enlarged images after the finger is bent and released show that the sensor has a fast response time (60.4ms) and good recovery time (157.6ms).

[0082] like Figure 10As shown, we also used the sensor prepared in Example 3 to detect pulse and heartbeat at various locations on volunteers' bodies, including the temporal artery, brachial artery, carotid artery, posterior tibial artery, radial artery, and heart. The waveforms at various locations exhibited distinct characteristic peaks, demonstrating that our sensor is capable of receiving these weak signals from the human body.

[0083] like Figure 11 As shown, the sensor of the present invention can also be used for human gesture recognition and synchronous control of the movement of the robot palm ( Figure 11 b). Design the circuit schematic diagram for controlling the manipulator as shown below: Figure 11 As shown in Figure a: The system consists of five microstructured hollow PVDF nanofiber flexible sensors, an operational amplifier, an Arduino development board, and a robotic palm. When the fingers bend, the electrical signals generated by the sensors are amplified and filtered by the operational amplifier. The output analog signals are converted into digital signals through pulse width modulation, thereby controlling the bending of the corresponding fingers of the robotic palm. Figure 11 As shown in (c), the operator can make the same gestures as the volunteer ("Victory", "OK" and "I Love You") at the same time. Therefore, the prepared sensor has good dynamic performance and has great potential in human activity monitoring and intelligent human-computer interaction.

[0084] The above-mentioned Examples 1-3 and Comparative Example 1 further demonstrate that the present invention provides a hollow PVDF micro-nanofiber microstructure piezoelectric sensor and a preparation method thereof. The sensitivity of the piezoelectric sensor prepared by the preparation method of the present invention is greatly improved compared with conventional technical means, and the response time is short and has a good recovery time, which solves the problems existing in the background technology. The prepared sensor has excellent mechanical durability and stability, and has high commercial value and promotion value.

[0085] In the description of the embodiments of the present application, it should be noted that in the description of the present application, terms such as "inside" and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and does not indicate or imply that the device or component must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation on the present application.

[0086] In the description of the present application, the description with reference to the terms "one embodiment", "some embodiments", "in the present embodiment", "specific example", or "some examples" means that the specific features, mechanisms, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, mechanisms, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and features of different embodiments or examples, unless they are mutually inconsistent.

[0087] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A method for preparing a hollow PVDF micro-nano fiber microstructure piezoelectric sensor, characterized in that: The following steps are involved: S1: Pour PVDF and polyvinyl pyrrolidone powder into a mixture of dimethylacetamide and acetone, and heat and stir to obtain a PVDF / PVP mixed outer layer spinning solution; dissolve the polyvinyl pyrrolidone powder in anhydrous ethanol and heat and stir to obtain an inner layer spinning solution; S2: Print a microstructure collector by stereolithography technology, and fix the microstructure collector on the surface of an electrospinning flat plate collector; S3: Prepare a PVDF / PVP composite fiber membrane by coaxial electrospinning using the outer layer spinning solution and the inner layer spinning solution of the step S1 as raw materials; S4: Remove PVP from the PVDF / PVP composite fiber membrane of the step S3 and dry it to obtain a PVDF micro-nano fiber membrane with a porous surface, a hollow inner layer and a microstructure on the fiber membrane surface; S5: Cover electrodes on both sides of the PVDF micro-nano fiber membrane obtained in the step S4, and then encapsulate it with PDMS and a curing agent to prepare a hollow PVDF micro-nano fiber microstructure piezoelectric sensor; In the step S1, the polyvinyl pyrrolidone powder in the outer layer spinning solution is polyvinyl pyrrolidone K30; the polyvinyl pyrrolidone powder in the inner layer spinning solution is polyvinyl pyrrolidone K90; In step S1, the molecular weight of the PVDF is 300±50k; the mass ratio of the PVDF to polyvinylpyrrolidone K30 is (1.7±0.2):(0.8±0.2); the mass ratio of the dimethylacetamide to acetone is (1±0.1):(1±0.1); In step S1, the concentration of polyvinyl pyrrolidone in the inner layer spinning solution is 5-15%, and the heating and stirring conditions are: stirring by magnetic stirring, and the stirring temperature is 60° C., the stirring time is 6-8 hours, and the rotation speed is 400-500 rpm; In step S3, the method for preparing the PVDF / PVP composite fiber membrane comprises: adding the inner layer spinning solution and the outer layer spinning solution of step S1 into syringes respectively, connecting the syringes to coaxial needles, and using two microinjection pumps to precisely control the flow rates of the inner and outer layer solutions to prepare the PVDF / PVP composite fiber membrane by spinning; In step S3, the size of the coaxial needle is 23 / 16G, the flow rate of the inner layer spinning solution is 0.55 ml / h, the flow rate of the outer layer spinning solution is 1.3 ml / h, the voltage applied during spinning is 18-22 kV, the spinning distance is 15 cm, and the spinning time is 2 hours; In step S4, the method of removing PVP is to soak the PVDF / PVP composite fiber membrane in anhydrous ethanol and then ultrasonically treat the membrane to remove PVP, and the ultrasonic treatment time is more than 2 hours; the drying condition is to place the composite fiber membrane in a vacuum oven for drying at a temperature of 60° C. for more than 6 hours; In step S5, the electrode is copper foil, and the mass ratio of PDMS to curing agent is 10:

1. The packaging conditions are: after mixing PDMS and curing agent, pouring them on the piezoelectric film, and curing them at 60° C. for 2 hours.

2. The method for preparing the hollow PVDF micro-nano fiber microstructure piezoelectric sensor according to claim 1, characterized in that: In step S2, the grid size of the microstructure collector is (2.2±0.4) mm, and the grid spacing is 0.1 mm.

3. A hollow PVDF micro-nano fiber microstructure piezoelectric sensor, characterized in that: The piezoelectric sensor is prepared by the preparation method according to any one of claims 1-2.

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