A soft inner core-hard shell coaxial structure piezoelectric sensor and a 3D printing preparation method

Abstract

This invention belongs to the field of 3D printing technology for sensors, and relates to a piezoelectric sensor with a soft core and a hard shell coaxial structure and its 3D printing preparation method. The inner core of the piezoelectric sensor is a hydrogel material formed by 3D printing, and the outer shell is PVDF or its copolymer material. The inner core and the outer shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The output voltage sensitivity of the soft core-hard shell coaxial structure piezoelectric sensor is 0.5~2.0 V / kPa. During preparation, PVDF or its copolymer solution and hydrogel precursor solution are prepared separately, and then simultaneously extruded through a 3D printing nozzle with a coaxial flow channel. After printing, a three-dimensional cross-linked network is formed by solvent replacement. Finally, electrode material is formed on the outer surface of the shell by coating or printing to obtain the soft core-hard shell coaxial structure piezoelectric sensor. The piezoelectric sensor of this invention has high piezoelectric response sensitivity; the indexing method is simple.

Description

Technical Field

[0001] This invention belongs to the field of sensor 3D printing technology, and relates to a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure and a 3D printing preparation method. Background Technology

[0002] With the development of flexible electronics, wearable devices, and human-computer interaction technologies, higher requirements are being placed on flexible pressure and strain sensors in terms of sensitivity, flexibility, and structural stability. Piezoelectric sensors, which can directly convert mechanical stimuli into electrical signals and offer advantages such as no external power supply and fast response speed, have attracted widespread attention.

[0003] Existing flexible piezoelectric sensors mostly use PVDF and its copolymers as the core material, fabricated through methods such as thin-film molding, spinning, or 3D printing. However, due to the high modulus of piezoelectric materials, their deformation is limited under small load conditions, resulting in weak piezoelectric output signals and limited sensitivity. To improve performance, existing technologies typically introduce complex microstructure designs or post-processing techniques such as high-field polarization, leading to complex process flows and difficulties in ensuring device consistency and stability.

[0004] Hydrogel materials possess low modulus, high compressibility, and good flexibility, exhibiting excellent deformation response capabilities in the field of flexible sensing. However, existing hydrogel sensors are mostly based on resistive or capacitive mechanisms, suffering from problems such as high environmental sensitivity and insufficient signal stability. Furthermore, hydrogels are difficult to directly integrate with traditional high-temperature melt-processed piezoelectric materials. Therefore, there is an urgent need for a novel flexible sensor structure and fabrication method that can achieve the synergistic integration of hydrogels and piezoelectric materials under low-temperature conditions.

[0005] For example, the paper (Flexible ZnO nanorod / hydrogel hybrid nanogenerator for self-powered sensing, ACS Applied Materials & Interfaces, 2018, 10(18):15700–15708, DOI: 10.1021 / acsami.8b02123) uses a nanostructure with ZnO as the outer piezoelectric material and a thermoresponsive hydrogel (poly-N-vinylcaprolactam-co-di(ethylene glycol)divinyl ether) as the core. The mechanism is that the hydrogel core deforms under changes in external humidity and temperature, applying mechanical stress to the piezoelectric zinc oxide outer shell, thereby generating a measurable piezoelectric signal. This paper does not directly present the output voltage sensitivity in the form of V / kPa. The output voltage of this type of nanostructure device is usually in the range of millivolts to hundreds of millivolts, roughly in the range of 0.01V to 0.1V. According to the output response curve, its equivalent voltage sensitivity is less than 0.2 V / kPa. Its main drawbacks are: (1) its structure is micro-nano scale, making it difficult to form stable macroscopic devices; (2) its output signal is weak, making it difficult to meet the requirements of high-sensitivity detection; (3) it depends on environmental stimuli (temperature or humidity), resulting in poor stability; and (4) its preparation process is complex, making it difficult to achieve large-scale manufacturing.

[0006] Therefore, it is of great significance to study a coaxial piezoelectric sensor with a soft core and a hard shell and its 3D printing preparation method to solve the problems existing in the prior art. Summary of the Invention

[0007] The purpose of this invention is to solve the problems existing in the prior art and provide a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure and a 3D printing preparation method.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, wherein the inner core is a hydrogel material formed by 3D printing and the outer shell is PVDF or its copolymer material; the inner core and the outer shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure;

[0010] The ratio of the Young's modulus of the hydrogel material to that of PVDF or its copolymer is not greater than 0.001;

[0011] The output voltage sensitivity of the coaxial piezoelectric sensor with a soft inner core and a hard outer shell is 0.5~2.0 V / kPa.

[0012] Piezoelectric enhancement mechanism: The elastic modulus of the hydrogel flexible inner core is significantly lower than that of the PVDF piezoelectric outer shell. Under external load, it preferentially undergoes larger deformation, which concentrates and amplifies the stress on the outer PVDF piezoelectric material, thereby producing greater piezoelectric deformation and higher electrical signal output under the same external force conditions.

[0013] Existing technologies have disclosed nanostructures with ZnO as the outer piezoelectric material and a thermally responsive hydrogel (poly-N-vinylcaprolactam-co-di(ethylene glycol)divinyl ether) as the inner core. However, these existing technologies differ substantially from the present invention in terms of driving mechanism, material system, and mode of action.

[0014] (1) Different driving methods;

[0015] In existing technologies, hydrogels are typically temperature- or humidity-responsive materials. Their volume changes mainly originate from expansion or contraction caused by environmental stimuli (such as changes in temperature or humidity), which in turn apply stress to the ZnO shell and generate piezoelectric signals. Therefore, this type of structure mainly belongs to environmentally responsive piezoelectric structures.

[0016] In this invention, the hydrogel core does not change volume due to temperature or humidity variations. Instead, it acts as a low-modulus mechanical amplification structure, undergoing significant deformation under external pressure or strain, and concentrating mechanical stress to transfer it to the PVDF shell, thereby enhancing the piezoelectric response. This invention belongs to the category of externally mechanically loaded piezoelectric sensing structures.

[0017] (2) Different piezoelectric material systems;

[0018] The ZnO used in the prior art is an inorganic piezoelectric semiconductor material, whose piezoelectric response originates from the polarization changes in its crystal structure; while the present invention uses PVDF or its copolymers as piezoelectric materials, whose piezoelectric properties originate from the orientation polarization of dipoles in the polymer chain (β-phase crystal form). The two materials differ significantly in their piezoelectric response mechanisms and mechanical properties.

[0019] (3) The core materials have different functional positioning;

[0020] In ZnO-hydrogel structures, hydrogels mainly function as active response units, generating volume changes through environmental stimuli. In this invention, however, hydrogels mainly function as flexible mechanical control units, achieving stress concentration and strain amplification through their low modulus characteristics, thereby improving the output signal of piezoelectric materials.

[0021] In summary, this invention utilizes the significant modulus difference between hydrogel and PVDF to amplify mechanical stress and applies it to pressure or strain detection scenarios. Its technical mechanism differs significantly from the scheme of driving ZnO piezoelectric structures with environmentally responsive hydrogels.

[0022] Furthermore, the soft-core-hard-shell coaxial piezoelectric sensor of this invention is a macroscopic continuous structure device directly constructed using coaxial 3D printing technology. This structure can form stable coaxial channels at the millimeter to centimeter scale, achieving integrated molding of the flexible hydrogel core and the PVDF piezoelectric shell. Moreover, the macroscopically scaled flexible hydrogel core can undergo greater volume deformation under external pressure, resulting in a more pronounced stress concentration effect on the outer piezoelectric material. In contrast, most existing core-shell piezoelectric materials are prepared using methods such as electrospinning, solution casting, template methods, or in-situ growth. The resulting structures are typically micro / nanoscale fibers, nanotubes, or thin films, making it difficult to form stable device structures. Mechanistically, existing micro / nano core-shell structures typically rely on material micro-interface effects or nanostructure orientation effects to enhance piezoelectric performance, such as through nanofiber orientation, interface polarization, or crystal structure modulation. The piezoelectric enhancement mechanism of this invention mainly originates from the mechanical amplification effect at the macroscopic structural level: the flexible hydrogel core has a significantly lower elastic modulus than the PVDF shell. Under external pressure or strain, the core first undergoes significant deformation, concentrating the stress and transferring it to the outer PVDF piezoelectric shell. This amplifies the mechanical strain of the PVDF material on a macroscopic scale, thereby improving the piezoelectric output signal. This structural mechanical amplification mechanism based on the difference in modulus between soft and hard materials is difficult to achieve in traditional micro / nano core-shell fiber systems.

[0023] As a preferred technical solution:

[0024] As described above, in a coaxial piezoelectric sensor with a soft inner core and a hard outer shell, the hydrogel material is a polymer-based hydrogel, and the polymer matrix is ​​selected from at least one of polyacrylamide and polyacrylic acid.

[0025] As described above, in a coaxial piezoelectric sensor with a soft inner core and a hard outer shell, the Young's modulus of the hydrogel material is 1~300 kPa, and the Young's modulus of the PVDF or its copolymer material is not less than 1 GPa.

[0026] As described above, the piezoelectric sensor with a soft inner core and a hard outer shell has a core diameter to outer shell diameter ratio of 0.3 to 0.8. The larger the ratio (the thicker the inner diameter / the thinner the outer shell), the higher the sensitivity. The outer shell thickness is 20 to 200 μm. If the outer shell is too thin, the mechanical strength is low and it is easily broken down. If it is too thick, the sensitivity will decrease.

[0027] As described above, a soft core-hard shell coaxial structure piezoelectric sensor further includes an electrode layer disposed on the outer surface of the shell. The function of the electrode layer is to collect the charge generated by the deformation of the shell and to electrically connect it to the external measurement circuit. The thickness of the electrode layer is 1~50 μm. The thickness of the electrode layer should ensure good conductivity while being as thin as possible to reduce the impact on the flexibility of the structure.

[0028] The present invention also provides a method for fabricating a coaxial piezoelectric sensor with a soft inner core and a hard outer shell as described above, comprising the following steps:

[0029] (1) Prepare PVDF or its copolymer solution and hydrogel precursor solution respectively. The hydrogel precursor solution contains monomer, crosslinking agent and initiator;

[0030] (2) The inner core and outer shell are coaxially coated and printed by simultaneously extruding PVDF or its copolymer solution and hydrogel precursor solution through a 3D printing nozzle with a coaxial flow channel.

[0031] (3) After printing, the obtained product is placed in a water or water vapor environment for solvent replacement, so that the monomers in the hydrogel precursor solution undergo free radical polymerization under the action of the initiator and form a three-dimensional cross-linked network under the action of the cross-linking agent; when the solvent (such as DMSO) is gradually replaced by water, the solution environment changes, and the initiator decomposes in the water environment to generate free radicals, thereby initiating the free radical polymerization reaction of the monomers and forming a three-dimensional cross-linked network structure under the action of the cross-linking agent, and finally forming a stable hydrogel core; in some embodiments, the free radical polymerization reaction can also be accelerated by mild heating (e.g., 40~70 ℃) or by adding an accelerator (such as TEMED), thereby improving the cross-linking efficiency of the hydrogel network; in addition, the solvent replacement process will change the dissolution environment of the PVDF molecular chains, causing them to rearrange and form a β-phase crystal structure with high polarity;

[0032] During the 3D printing extrusion process, the PVDF solution is subjected to a certain shear force and flow stretching in the nozzle, which causes the PVDF molecular chains to align to a certain extent, which is also conducive to the formation of the β phase crystal structure.

[0033] In some embodiments, the PVDF crystal structure can be further promoted to transform into the β phase by moderate low-temperature annealing or polarization treatment (such as electric field polarization) to enhance the piezoelectric properties of the material.

[0034] Through the synergistic effects of solvent replacement, shear orientation, and post-treatment, PVDF piezoelectric shells with high β-phase content can be obtained without high-temperature melting processing.

[0035] (4) Electrode material is formed on the outer surface of the housing by coating or printing to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0036] As a preferred technical solution:

[0037] In the preparation method of the soft inner core-hard outer shell coaxial structure piezoelectric sensor described above, the mass fraction of PVDF or its copolymer solution is 10~30%, and the mass fraction of hydrogel precursor solution is 5~30%.

[0038] The method for preparing a coaxial piezoelectric sensor with a soft inner core and a hard outer shell as described above uses acrylamide (AM) and / or acrylic acid as monomers, N,N'-methylenebisacrylamide (MBA) as crosslinking agent, and ammonium persulfate (APS) as initiator; the mass ratio of monomer, crosslinking agent and initiator is 100:0.5~1.5:0.2~0.8.

[0039] The method for fabricating a coaxial piezoelectric sensor with a soft inner core and a hard outer shell, as described above, uses conductive silver paste or conductive ink as the electrode material.

[0040] Beneficial effects:

[0041] (1) The present invention provides a 3D printing method for a soft inner core-hard outer shell coaxial structure piezoelectric sensor. The method uses printable hydrogel as a flexible inner core and takes advantage of its ability to generate large deformation under external pressure or strain to effectively transfer and concentrate mechanical stress to the outer PVDF piezoelectric shell. At the same time, the PVDF shell is formed by solution 3D printing and solidified at low temperature, avoiding damage to the hydrogel structure and performance caused by high temperature melting.

[0042] (2) The piezoelectric sensor of the present invention has a soft inner core-hard outer shell coaxial structure. By introducing a coaxial composite structure of a hydrogel flexible inner core and a PVDF piezoelectric outer shell, the piezoelectric sensor achieves the amplification effect of the deformation of the piezoelectric material through the synergistic structural design of the soft inner core-hard outer shell. This enables the sensor to generate obvious piezoelectric signals under small pressure or strain, thereby obtaining high-sensitivity pressure / strain sensing performance. While ensuring structural stability, the piezoelectric response sensitivity is significantly improved. Attached Figure Description

[0043] Figure 1 This is a cross-sectional schematic diagram of the piezoelectric sensor of the present invention; the red arrow indicates that the pressure is applied from the outside to the inside;

[0044] Figure 2 This is the infrared spectrum of the polyvinylidene fluoride (PVDF) shell material of Embodiment 1 of the present invention. The dashed line indicates that the α crystal of PVDF is located at 764 cm⁻¹. -1 And β crystals are located at 840 cm -1Characteristic peaks;

[0045] Among them, 1-hydrogel flexible inner core, 2-PVDF piezoelectric outer shell, 3-electrode layer. Detailed Implementation

[0046] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0047] The test methods involved in the performance indicators in the embodiments and comparative examples of this invention are as follows:

[0048] Young's modulus: A thin film sample with the same composition as the piezoelectric sensor was prepared and treated under the same solvent displacement and post-treatment conditions. Its modulus was then measured as a reference value. Testing was performed using an Instron 5943 universal testing machine according to ASTM D412 standard. The tensile rate was 50 mm / min, and the sample dimensions were 25 mm in length, 4 mm in width, and 2 mm in thickness. The Young's modulus was calculated by taking the slope of the stress-strain curve within the strain range of 0–10%.

[0049] β-phase content: determined using a Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet iS50) with a scanning range of 400–1400 cm⁻¹. -1 4cm resolution -1 The number of scans was 32, based on 840 cm. -1 (β phase) and 764 cm -1 Calculation of β phase content from (α phase) absorption peak: F(β) = A 840 / (A 840 +1.26A 764 ); where A 764 The α phase is at 764 cm. -1 The absorption peak intensity at point A 840 For β phase at 840 cm -1 The absorption peak intensity at that location.

[0050] Output voltage sensitivity: Using a standard pressure loading device (Instron E1000), pressure was applied in the range of 0~10 kPa, and the output voltage was recorded using an oscilloscope (Keysight DSOX2024A). The sensitivity (unit: V / kPa) was calculated using the slope of the voltage-pressure curve.

[0051] Example 1

[0052] A method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, the specific steps of which are as follows:

[0053] (1) Dissolve PVDF (manufacturer: Arkema, brand: Kynar 761) in DMSO to obtain a PVDF solution with a mass fraction of 10%;

[0054] (2) Acrylamide, N,N'-methylenebisacrylamide and ammonium persulfate were dissolved in DMSO to prepare a 5% (w / w) hydrogel precursor solution;

[0055] The mass ratio of acrylamide, N,N'-methylenebisacrylamide, and ammonium persulfate is 100:0.5:0.2.

[0056] (3) By simultaneously extruding PVDF solution and hydrogel precursor solution through a 3D printing nozzle with coaxial flow channels, the inner core and outer shell are coaxially coated and printed.

[0057] The 3D printing process parameters are as follows: extrusion pressure is 0.05 MPa, printing speed is 2 mm / s, nozzle diameter is 150 μm, printing temperature is 25 ℃; coaxial nozzle inner and outer diameters are 150 μm and 400 μm respectively; inner and outer flow channel flow ratio is 1:1; extrusion pressure difference is controlled at 0.02 MPa; printing path spacing is 0.2 mm.

[0058] (4) After printing, the product is placed in a water environment at 25 °C for solvent replacement, so that the acrylamide in the hydrogel precursor solution undergoes free radical polymerization under the action of ammonium persulfate, and forms a three-dimensional cross-linked network under the action of N,N'-methylenebisacrylamide.

[0059] (5) Conductive silver paste (manufacturer: Heraeus, grade: C8829D) is formed on the outer surface of the shell by coating to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0060] The final fabricated soft-core-hard-shell coaxial piezoelectric sensor features a core made of 3D-printed polyacrylamide hydrogel and a shell made of PVDF. The core and shell are continuously distributed axially and coaxially arranged, forming an integrated composite structure. The ratio of the core diameter to the shell diameter is 0.3, the shell thickness is 20 μm, and the electrode layer thickness is 5 μm. The Young's modulus of the polyacrylamide hydrogel is 10 kPa, and the Young's modulus of the PVDF is 1.2 GPa. Figure 2 As shown, the β phase content of the PVDF material in the outer shell is 72%; the output voltage sensitivity of the piezoelectric sensor with a soft inner core and hard outer shell coaxial structure is 0.5 V / kPa.

[0061] Comparative Example 1

[0062] A method for preparing a piezoelectric sensor is basically the same as in Example 1, except that: in step (2), the mass fraction of the hydrogel precursor solution is 25%; the mass ratio of acrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 100:1.5:0.8; in step (4), during the solvent replacement process, the ambient temperature is adjusted to 60°C to promote a more complete crosslinking reaction.

[0063] The final polyacrylamide hydrogel material has a Young's modulus of 2000 kPa, and the Young's modulus ratio of the hydrogel material to the PVDF material is 0.017; the output voltage sensitivity of the piezoelectric sensor is 0.3 V / kPa.

[0064] Comparing Example 1 and Comparative Example 1, it can be found that when the Young's modulus of the hydrogel core is too high (i.e., the core becomes harder) and the difference between its modulus and that of the outer shell narrows (ratio > 0.001), the core, under pressure, cannot generate sufficient deformation compensation to effectively concentrate and amplify stress on the outer shell due to its decreased deformation capacity. The mechanical strain borne by the PVDF outer shell decreases, and the amount of piezoelectric charge generated also decreases, resulting in a decrease in sensitivity, significantly lower than the sensitivity index of 0.5~2.0 V / kPa of this invention. This indicates that controlling the ratio of the Young's modulus of the hydrogel core to that of the PVDF outer shell to be no greater than 0.001 is a key condition for achieving the mechanical amplification effect and high-sensitivity piezoelectric response of this invention.

[0065] Example 2

[0066] A method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, the specific steps of which are as follows:

[0067] (1) Dissolve PVDF (manufacturer: Arkema, brand: Kynar 761) in DMSO to obtain a PVDF solution with a mass fraction of 20%;

[0068] (2) Dissolve acrylic acid, N,N'-methylenebisacrylamide and ammonium persulfate in DMSO to prepare a 10% (w / w) hydrogel precursor solution;

[0069] The mass ratio of acrylic acid, N,N'-methylenebisacrylamide, and ammonium persulfate is 100:0.8:0.3.

[0070] (3) By simultaneously extruding PVDF solution and hydrogel precursor solution through a 3D printing nozzle with coaxial flow channels, the inner core and outer shell are coaxially coated and printed.

[0071] The 3D printing process parameters are as follows: extrusion pressure is 0.1 MPa, printing speed is 5 mm / s, nozzle diameter is 200 μm, printing temperature is 30 ℃; coaxial nozzle inner and outer diameters are 150 μm and 400 μm respectively; inner and outer flow channel flow ratio is 1:1.2; extrusion pressure difference is controlled at 0.04 MPa; printing path spacing is 0.3 mm.

[0072] (4) After printing, the obtained product is placed in a water vapor environment at 30 °C for solvent replacement and heated to 50 °C so that the acrylic acid in the hydrogel precursor solution undergoes free radical polymerization under the action of ammonium persulfate and forms a three-dimensional cross-linked network under the action of N,N'-methylenebisacrylamide.

[0073] (5) Conductive ink (manufacturer: DuPont, grade: 5025) is formed on the outer surface of the housing by coating to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0074] The final soft-core-hard-shell coaxial piezoelectric sensor has a core made of 3D-printed polyacrylic acid hydrogel material and a shell made of PVDF material. The core and shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The ratio of the core diameter to the shell diameter is 0.4, the shell thickness is 50 μm, and the electrode layer thickness is 10 μm. The Young's modulus of the polyacrylic acid hydrogel material is 30 kPa, and the Young's modulus of the PVDF material is 2.5 GPa. The β phase content of the PVDF material in the shell is 78%. The output voltage sensitivity of the soft-core-hard-shell coaxial piezoelectric sensor is 1.0 V / kPa.

[0075] Example 3

[0076] A method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, the specific steps of which are as follows:

[0077] (1) Dissolve PVDF (manufacturer: Arkema, brand: Kynar 761) in DMSO to obtain a PVDF solution with a mass fraction of 30%;

[0078] (2) Acrylamide, N,N'-methylenebisacrylamide and ammonium persulfate were dissolved in DMSO to prepare a hydrogel precursor solution with a mass fraction of 15%.

[0079] The mass ratio of acrylamide, N,N'-methylenebisacrylamide, and ammonium persulfate is 100:1.0:0.5.

[0080] (3) By simultaneously extruding PVDF solution and hydrogel precursor solution through a 3D printing nozzle with coaxial flow channels, the inner core and outer shell are coaxially coated and printed.

[0081] The 3D printing process parameters are as follows: extrusion pressure of 0.15 MPa, printing speed of 10 mm / s, nozzle diameter of 300 μm, printing temperature of 30 ℃; coaxial nozzle inner and outer diameters of 150 μm and 400 μm, respectively; inner and outer flow channel flow ratio of 1:1.3; extrusion pressure difference controlled at 0.05 MPa; and printing path spacing of 0.35 mm.

[0082] (4) After printing, the obtained product is placed in a water environment at 25 °C for solvent replacement, and then N,N,N',N'-tetramethylethylenediamine (TEMED) is added to allow the acrylamide in the hydrogel precursor solution to undergo free radical polymerization under the action of ammonium persulfate and form a three-dimensional cross-linked network under the action of N,N'-methylenebisacrylamide.

[0083] The mass ratio of TEMED to ammonium persulfate is 1:1.

[0084] (5) Conductive silver paste (manufacturer: Heraeus, grade: C8829D) is formed on the outer surface of the shell by printing to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0085] The final soft-core-hard-shell coaxial piezoelectric sensor has a core made of polyacrylamide hydrogel material formed by 3D printing, and a shell made of PVDF material. The core and shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The ratio of the core diameter to the shell diameter is 0.5, the shell thickness is 80 μm, and the electrode layer thickness is 20 μm. The Young's modulus of the polyacrylamide hydrogel material is 80 kPa, and the Young's modulus of the PVDF material is 2 GPa. The β phase content of the PVDF material in the shell is 85%. The output voltage sensitivity of the soft-core-hard-shell coaxial piezoelectric sensor is 1.7 V / kPa.

[0086] Example 4

[0087] A method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, the specific steps of which are as follows:

[0088] (1) Dissolve PVDF copolymer (manufacturer: Arkema, brand: Kynar Flex® 2801) in DMSO to obtain a PVDF copolymer solution with a mass fraction of 15%;

[0089] (2) Dissolve acrylic acid, N,N'-methylenebisacrylamide and ammonium persulfate in DMSO to prepare a 20% (w / w) hydrogel precursor solution;

[0090] The mass ratio of acrylic acid, N,N'-methylenebisacrylamide, and ammonium persulfate is 100:1.2:0.6.

[0091] (3) The inner core and outer shell are coaxially coated and printed by simultaneously extruding PVDF copolymer solution and hydrogel precursor solution through a 3D printing nozzle with coaxial flow channel.

[0092] The 3D printing process parameters are as follows: extrusion pressure is 0.2 MPa, printing speed is 12 mm / s, nozzle diameter is 350 μm, printing temperature is 35 ℃; coaxial nozzle inner and outer diameters are 150 μm and 400 μm respectively; inner and outer flow channel flow ratio is 1:1.5; extrusion pressure difference is controlled at 0.07 MPa; printing path spacing is 0.4 mm.

[0093] (4) After printing, the obtained product is placed in a water vapor environment at 30 °C for solvent replacement, and at the same time, it is subjected to low-temperature annealing at 80 °C so that the acrylic acid in the hydrogel precursor solution undergoes free radical polymerization under the action of ammonium persulfate and forms a three-dimensional cross-linked network under the action of N,N'-methylenebisacrylamide.

[0094] (5) Conductive ink (manufacturer: DuPont, grade: 5025) is formed on the outer surface of the housing by printing to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0095] The final soft-core-hard-shell coaxial piezoelectric sensor has a core made of 3D-printed polyacrylic acid hydrogel material and a shell made of PVDF copolymer material. The core and shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The ratio of the core diameter to the shell diameter is 0.6, the shell thickness is 120 μm, and the electrode layer thickness is 30 μm. The Young's modulus of the polyacrylic acid hydrogel material is 150 kPa, and the Young's modulus of the PVDF copolymer material is 2.5 GPa. The β phase content of the PVDF copolymer material in the shell is 88%. The output voltage sensitivity of the soft-core-hard-shell coaxial piezoelectric sensor is 1.3 V / kPa.

[0096] Example 5

[0097] A method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure, the specific steps of which are as follows:

[0098] (1) Dissolve PVDF copolymer (manufacturer: Arkema, brand: Kynar Flex® 2801) in DMSO to obtain a PVDF copolymer solution with a mass fraction of 25%;

[0099] (2) A 30% hydrogel precursor solution was prepared by dissolving a mixture of acrylamide and acrylic acid monomers in a mass ratio of 1:1, N,N'-methylenebisacrylamide and ammonium persulfate in DMSO.

[0100] The mass ratio of the acrylamide and acrylic acid mixed monomers, N,N'-methylenebisacrylamide and ammonium persulfate is 100:1.5:0.8.

[0101] (3) The inner core and outer shell are coaxially coated and printed by simultaneously extruding PVDF copolymer solution and hydrogel precursor solution through a 3D printing nozzle with coaxial flow channel.

[0102] The 3D printing process parameters are as follows: extrusion pressure of 0.25 MPa, printing speed of 15 mm / s, nozzle diameter of 400 μm, printing temperature of 40 ℃; coaxial nozzle inner and outer diameters of 150 μm and 400 μm, respectively; inner and outer flow channel flow ratio of 1:1.8; extrusion pressure difference controlled at 0.09 MPa; and printing path spacing of 0.5 mm.

[0103] (4) After printing, the obtained product is placed in a water environment at 25 °C for solvent replacement. At the same time, electrode polarization is performed under an electric field strength of 50~100MV / m so that acrylamide and acrylic acid in the hydrogel precursor solution undergo free radical polymerization under the action of ammonium persulfate and form a three-dimensional cross-linked network under the action of N,N'-methylenebisacrylamide.

[0104] (5) Conductive silver paste (manufacturer: Heraeus, grade: C8829D) is formed on the outer surface of the shell by coating to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

[0105] The final soft-core-hard-shell coaxial piezoelectric sensor has a core made of polyacrylamide and polyacrylic acid hydrogel material formed by 3D printing, and a shell made of PVDF copolymer material. The core and shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The ratio of the core diameter to the shell diameter is 0.5, the shell thickness is 200 μm, and the electrode layer thickness is 50 μm. The Young's modulus of the polyacrylamide and polyacrylic acid hydrogel material is 250 kPa, and the Young's modulus of the PVDF copolymer material is 3 GPa. The β phase content of the PVDF copolymer material in the shell is 90%. The output voltage sensitivity of the soft-core-hard-shell coaxial piezoelectric sensor is 2 V / kPa.

[0106] like Figure 1As shown, the piezoelectric sensor with a coaxial soft core-hard shell structure has a circular hydrogel flexible core 1 at its center, which is covered by an annular PVDF piezoelectric shell 2. The two are coaxially distributed along the axial direction to form an integrated structure. An electrode layer 3 is disposed on the outside of the PVDF piezoelectric shell 2. In use, pressure is applied from the outside in, causing the hydrogel flexible core 1 to deform first, and then the stress is transferred to the PVDF shell 2.

[0107] Comparing Examples 1 with Examples 2-5 shows that, based on solvent replacement, gentle heating, annealing, or polarization treatment can significantly increase the content of β-phase crystals in the PVDF shell. This is because the post-processing further promotes the orientation and alignment of dipoles in the PVDF molecular chain. Experimental data shows that the sensor sensitivity after electric field polarization (Example 5) is 3 times higher than that of Example 1 without polarization treatment, demonstrating that the post-processing has a significant synergistic effect on enhancing the piezoelectric response performance of the sensor.

Claims

1. A soft inner core - hard outer shell coaxial structure piezoelectric sensor, characterized by: The inner core is a hydrogel material formed by 3D printing, and the outer shell is PVDF or its copolymer material; the inner core and the outer shell are continuously distributed along the axial direction and coaxially arranged to form an integrated composite structure. The ratio of the Young's modulus of the hydrogel material to that of PVDF or its copolymer is not greater than 0.001; The output voltage sensitivity of the coaxial piezoelectric sensor with a soft inner core and a hard outer shell is 0.5~2.0 V / kPa.

2. A soft inner core - hard outer shell coaxial structure piezoelectric sensor according to claim 1, characterized in that, The hydrogel material is a polymer-based hydrogel, and its polymer matrix is ​​selected from at least one of polyacrylamide and polyacrylic acid.

3. A soft inner core - hard outer shell coaxial structure piezoelectric sensor according to claim 2, characterized in that, The Young's modulus of hydrogel materials is 1~300 kPa, and the Young's modulus of PVDF or its copolymer materials is not less than 1 GPa.

4. A soft inner core - hard outer shell coaxial structure piezoelectric sensor according to claim 1, characterized in that, The ratio of the inner core diameter to the outer shell diameter is 0.3 to 0.8, and the thickness of the outer shell is 20 to 200 μm.

5. A piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure according to claim 4, characterized in that, The soft-core-hard-shell coaxial piezoelectric sensor also includes an electrode layer disposed on the outer surface of the shell; The thickness of the electrode layer is 1~50μm.

6. The method for fabricating a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure as described in claim 5, characterized in that... Includes the following steps: (1) Prepare PVDF or its copolymer solution and hydrogel precursor solution respectively. The hydrogel precursor solution contains monomer, crosslinking agent and initiator; (2) The inner core and outer shell are coaxially coated and printed by simultaneously extruding PVDF or its copolymer solution and hydrogel precursor solution through a 3D printing nozzle with a coaxial flow channel. (3) After printing, place the obtained product in a water or water vapor environment to perform solvent replacement, so that the monomers in the hydrogel precursor solution undergo free radical polymerization under the action of the initiator and form a three-dimensional cross-linked network under the action of the cross-linking agent; (4) Electrode material is formed on the outer surface of the housing by coating or printing to obtain a piezoelectric sensor with a soft inner core and a hard outer shell coaxial structure.

7. The method of claim 6, wherein the method further comprises: The mass fraction of the PVDF or its copolymer solution is 10-30%, and the mass fraction of the hydrogel precursor solution is 5-30%.

8. The method of claim 6, wherein the piezoelectric sensor is a soft inner core- hard outer shell coaxial structure piezoelectric sensor. The monomer is acrylamide and / or acrylic acid, the crosslinking agent is N,N'-methylenebisacrylamide, and the initiator is ammonium persulfate; the mass ratio of monomer, crosslinking agent and initiator is 100:0.5~1.5:0.2~0.

8.

9. The method of claim 6, wherein the piezoelectric sensor is a soft inner core- hard outer shell coaxial structure piezoelectric sensor. The electrode material is conductive silver paste or conductive ink.

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