Conformal bi-directional strain sensor and conductive circuit on a curved structure and integrated manufacturing method thereof

Abstract

The application discloses a conformal bidirectional strain sensor and conductive circuit on a curved structure and an integrated manufacturing method thereof, which comprises a transfer medium layer adhered to a surface to be measured, a strain sensing grid with multi-walled carbon nanotubes adhered to the upper surface of the transfer medium layer, and a conductive circuit with a microstructure array of micro-pillar hole structures arranged in the transfer medium layer; the strain sensing grid is provided with fine and dense cracks to sense strain, and the conductive circuit is covered with a packaging layer material; in the preparation, a template is prepared and surface hydrophilic and hydrophobic treatment is performed first, then strain sensing functional ink filling is performed, then the strain sensing grid is pre-stretched to generate micro-cracks, then multi-conductive functional material composite filling is performed in the conductive circuit area, then in-situ transfer printing of the sensing and conductive integrated structure is performed, and finally the packaging layer is covered; the application realizes high-precision sensing measurement and effective transmission of electrical information; the processing of the strain sensing area and the conductive circuit area is simple, and high-efficiency and low-cost manufacturing is realized.

Description

Technical Field

[0001] This invention belongs to the field of electronic micro-nano manufacturing technology of structural surfaces, specifically relating to a conformal bidirectional strain sensor and conductive circuit on a curved structural component and its integrated manufacturing method. Background Technology

[0002] In engineering and science, an increasing number of high-precision devices require surface strain information, which is crucial for material selection for devices of varying strengths, structural design stability, and long-term stable and safe operation. Currently, surface strain detection for equipment mainly involves contact and non-contact methods. Patent application CN110823117A, entitled "A Single-Step Phase-Shifting Electron Speckle Interferometry Measurement Method, System, Device, and Storage Medium," relates to the electronic speckle method, while patent application CN115046857A, entitled "A Rubber Strain Correction Method and Measurement Device Based on Two-Dimensional Digital Image Correlation," relates to the digital image correlation method. Non-contact measurement methods such as the electronic speckle method and digital image correlation method are suitable for measuring strain in extreme environments such as high temperatures; however, their testing systems and digital image data processing are complex, have high requirements for the testing environment, and are susceptible to measurement errors due to factors such as dust and water mist.

[0003] Contact measurements often employ strain gauges, such as the patent application (publication number CN109690233A) entitled "Strain Gauge and Method for Manufacturing a Strain Gauge." This strain gauge consists of three layers: a bottom insulating layer, a resistance sensing unit, and a top insulating layer. During use, the strain gauge needs to be adhered to the surface to be measured using adhesive. According to strain transfer theory, the greater the material thickness between the sensing unit and the measured surface, the lower the strain transfer rate, and consequently, the lower the measurement accuracy. Furthermore, traditional resistance strain gauges are limited by the strain sensing properties of the material itself, resulting in insufficient sensitivity for small strain measurements.

[0004] Beyond sensing and detecting structural curved surfaces, transmitting strain information across these surfaces via wires is also a challenge. Traditional insulated wires often result in wire clutter and occupy significant space. Curved conformal circuits are typically used instead of traditional printed circuit boards or individual wires. This fully utilizes the precious internal space of electronic devices, reduces device size, minimizes fly-by wires, resulting in a cleaner wiring layout and ultimately achieving an integrated connection between structure and circuitry.

[0005] There are three main methods for manufacturing conformal circuits on curved surfaces. The first is direct-write printing, such as the patent application (publication number CN109366976A) entitled "3D printing device and method for integrated manufacturing of conformal antennas and circuits". Methods such as aerosol inkjet printing and electrofluid inkjet printing often require complex path planning. This path-following printing method is often inefficient, and the efficiency will further decrease as the accuracy of the printed circuit increases. Secondly, this method often requires complex multi-axis linkage printing equipment to achieve accurate conformal printing of curved circuits. The second type is metal deposition composite laser processing manufacturing technology, such as the patent application entitled "A method for manufacturing fine circuits on a printed circuit board" (publication number CN116600481A). This method first uniformly deposits metal material on a substrate, then uses laser etching to remove the photoresist layer on the metal material to expose the metal layer, and then chemically etches away the excess metal, or directly uses etching to remove the excess metal and leave conductive lines. This method has two main drawbacks: first, the metal layer deposited by magnetron sputtering, electron beam evaporation or electroplating is relatively thin; second, the laser processing process is similar to the direct writing printing process, both of which require complex path planning and high requirements for processing equipment, and are especially difficult to apply to curved surface processing. The third type is the manufacturing technology of laser direct forming (LDS) composite coating process, such as the patent application entitled "A method for preparing metal circuits on the surface of fused deposition modeled parts" (publication number CN119101892A). First, a three-dimensional circuit pattern is engraved using laser etching technology, and then a three-dimensional metal circuit is formed in the laser-ablated pattern by electroplating or chemical plating. Due to the limitations of the process on the substrate material, this method is often applied to conformal circuits on non-metallic substrates such as plastics.

[0006] In summary, existing manufacturing technologies suffer from low manufacturing efficiency, poor conformal capability, and limited manufacturing materials, making it difficult to achieve integrated and efficient manufacturing of strain sensors and conductive circuits on curved substrates. There is an urgent need to develop new high-efficiency electronic printing manufacturing technology for curved conformal circuits to achieve efficient and low-cost manufacturing. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, this invention aims to propose a conformal bidirectional strain sensor and conductive circuit on curved structural components, as well as an integrated manufacturing method thereof. The strain sensing unit and the highly adhesive conductive circuit are manufactured on the curved surface of the structure simultaneously using a single transfer printing technique, achieving high-precision sensing measurement and effective transmission of electrical information. The processing of the strain sensing unit and the conductive circuit is simple and does not require complex and precise large-scale printing equipment, thus achieving efficient and low-cost manufacturing.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A conformal bidirectional strain sensor and conductive circuit on a curved structural component include a transfer medium layer adhered to the surface to be measured; a strain sensing grid with multi-walled carbon nanotubes is adhered to the upper surface of the transfer medium layer; conductive circuits with a microstructure array of micropillar pores are disposed inside the transfer medium layer; fine cracks are distributed on the strain sensing grid to sense strain; the conductive circuits are composed of two conductive functional materials; and an encapsulation layer material covers the conductive circuits.

[0009] The integrated manufacturing method of a conformal bidirectional strain sensor and conductive circuit on a curved surface structure includes the following steps: 1) Template preparation and surface hydrophilic and hydrophobic treatment: Design sensor and conductive circuit patterns, arrange microstructure array, and prepare patterned grooves on silicon wafer by photolithography to obtain silicon template; obtain secondary flexible concave template by imprinting the silicon template twice; perform hydrophobic surface treatment on silicon template and primary flexible convex template before and after the first imprinting, respectively, and perform hydrophilic surface treatment on secondary flexible concave template after the second imprinting. 2) Strain sensing functional ink filling: The sensing functional material is scraped and coated in the strain sensing grid area of ​​the secondary flexible concave template to fill the constrained microcavity. The scraping, drying and wiping of excess parts are repeated multiple times until the sensing functional material is uniformly and continuously distributed in the grid constrained microcavity. 3) Pre-stretching of the sensing unit to generate microcracks: The secondary flexible concave template, which has been filled with sensing functional material in the strain sensing grid area, is pre-stretched to generate sensing microcracks on the strain sensing grid for sensing micro-strain. 4) Composite filling of conductive circuit areas with multiple conductive functional materials: Conductive functional materials are composite filled at corresponding positions in the conductive circuit areas of the secondary flexible concave template and fully cured under a thermal field; 5) In-situ transfer of integrated sensing and conductive structure: Using a curing agent, the strain sensing mesh and conductive lines in the secondary flexible concave template are conformally transferred to the curved structural component in one step; 6) Covering the encapsulation layer: After peeling off the two flexible secondary concave templates, the encapsulation layer is printed on the overall structure.

[0010] In step 1), the ratio of the size of a single microstructure to the width of the conductive line is less than or equal to 10:1; a bidirectional isolation microstructure array is set in the connection area between the conductive line and the strain sensing grid.

[0011] In step 1), when using polydimethylsiloxane (PDMS) to mold a silicon template, a PDMS with a higher Shore hardness of 40A to 60A is used for molding, and the thickness of the obtained primary flexible convex template is >1.5 mm; when using PDMS to mold a primary flexible convex template, a softer PDMS with a Shore hardness of 0A to 20A is used for spin-coating molding, and the thickness of the obtained secondary flexible concave template is <200 μm.

[0012] In step 1), the wetting angle of the interface of the secondary flexible concave template after hydrophilic treatment is 50° ~ 60°.

[0013] In step 2), use a scraper to repeatedly scrape and coat the material into the strain sensing grid. After drying, use a cotton swab dipped in anhydrous ethanol or acetone to wipe away any excess material outside the grooves of the secondary flexible concave template.

[0014] Step 3) After pre-stretching the filled strain sensing mesh area, use UV-curing adhesive to continue filling the grooves on top of the cured sensing material, but the UV-curing adhesive should not overflow from the grooves.

[0015] In step 4), the corresponding position refers to the position with the widest line width in the pattern and the position where the line bends. When manufacturing conductive lines, conductive material should be continuously dripped into the corresponding position in the conductive line groove until the groove is completely filled. The distance between each corresponding position should be close.

[0016] In step 4), composite filling refers to using two or more conductive functional materials and filling the conductive circuit with two or more filling methods. The conductive functional materials refer to gold, silver, and copper nanoparticle inks and conductive pastes; the filling methods refer to doctor blade coating, ink capillary-driven self-filling, and direct writing printing technology along the groove lines.

[0017] Step 5) Before the transfer, an insulating layer is created on the curved structure to prevent the strain sensing mesh and conductive lines from directly contacting the metal structure sample.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention proposes a conformal bidirectional strain sensor and conductive circuit on a curved surface structure, along with its integrated manufacturing method. The method employs a pre-filling and then transfer process. First, various functional materials are filled step-by-step into a prefabricated double-flexible concave template containing a microstructure array. Then, the integrated sensing structure and conductive circuit structure filled in the double-flexible concave template are transferred to the curved surface of the structure in one step and encapsulated. The designed microstructure not only increases the contact force at the heterogeneous interface but also allows ultraviolet light to cure the underlying photocurable adhesive. This manufacturing method enhances the adhesion and stability of the curved surface circuit while reducing the number and thickness of the dielectric layer between the strain sensing functional area and the substrate, ensuring testing sensitivity.

[0019] Compared to printing, the transfer printing process used in this invention eliminates the need for large printing equipment and complex path design, reducing manufacturing costs. The area-based manufacturing method saves manufacturing time. Furthermore, the pre-filling followed by transfer printing method minimizes contact time with the precision curved surface during manufacturing, reducing potential damage. This method of integrating sensors and conductive circuits on the curved surface of structural components holds promise for wider applications in fields such as intelligent human-computer interaction, medical sports, and automotive electronics. Attached Figure Description

[0020] Figure 1 This is a top view of the overall shape of the conformal bidirectional strain sensor and conductive circuit according to an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram illustrating the fabrication of a flexible convex template using a silicon template in an embodiment of the present invention.

[0022] Figure 3 This is a schematic diagram illustrating the preparation of a secondary flexible concave template using a primary flexible convex template in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of the scraping and filling of the strain sensing grid area according to an embodiment of the present invention.

[0024] Figure 5 This is a schematic diagram of the ink self-flowing filling in the conductive line groove of an embodiment of the present invention.

[0025] Figure 6 This is a two-dimensional schematic diagram illustrating the process of transferring the filled strain sensing mesh and conductive lines onto a structural component according to an embodiment of the present invention.

[0026] Figure 7 This is a three-dimensional schematic diagram illustrating the process of transferring the filled strain sensing mesh and conductive lines onto a structural component according to an embodiment of the present invention.

[0027] Figure 8 This is a two-dimensional schematic diagram illustrating the testing of strain sensing mesh and conductive lines transferred onto a structural component according to an embodiment of the present invention.

[0028] Figure 9 This is a top view of the microscopic electron microscope morphology of the strain-sensitive grid and conductive lines before and after transfer in an embodiment of the present invention.

[0029] Figure 10 The figures represent cross-sections of conductive circuits filled only with silver ink and those after transfer following the filling of silver ink and silver paste, according to embodiments of the present invention.

[0030] Figure 11 This is a physical image of the overall sensor and conductive circuit structure transferred onto the surface of a curved structure of different materials, as an embodiment of the present invention. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the embodiments and accompanying drawings.

[0032] An integrated manufacturing method for a conformal strain sensor and conductive circuit on a curved surface structure includes the following steps: 1) Template preparation and surface hydrophilic and hydrophobic treatment: Design sensor and conductive circuit patterns, arrange microstructure array, and prepare patterned grooves on silicon wafer by photolithography to obtain silicon template; obtain secondary flexible concave template by imprinting the silicon template twice; perform hydrophobic surface treatment on silicon template and primary flexible convex template before and after the first imprinting, respectively, and perform hydrophilic surface treatment on secondary flexible concave template after the second imprinting. 1.1) Design sensor patterns and manufacture rigid silicon templates: such as Figure 1 , Figure 2 As shown, a mask pattern consisting of a strain sensing grid region and conductive lines is designed. Patterned grooves are fabricated on the silicon wafer surface using photolithography and etching processes to obtain a rigid silicon template 1. In the design of the sensor and conductive line patterns, to facilitate the flow of functional inks, transmit ultraviolet light, and increase the contact area between the functional inks and the substrate to increase adhesion, a "capsule-shaped" microstructure array 7-1 is designed in the conductive lines. To prevent different functional inks from contaminating each other and affecting the function, a bidirectional "arrow-shaped" microstructure array region 7-3 is set at the connection between the conductive lines and the strain sensing grid region for isolation and buffering of different types of functional inks. The strain sensing grid region 7-2 is designed as a grid with equal horizontal and vertical spacing. After the grid is filled with strain functional material, the strain in two vertical directions can be obtained from the two opposite sides of the square sensing region. In this embodiment, the grid width is 75μm, the depth is 60μm, the period is 175μm, and the side length of the entire strain sensing region is 7.5mm. To increase the fluidity and contact area of ​​the functional ink during groove filling without affecting conductivity, the size of a single microstructure in the conductive circuit should be ≤ 1 / 10 of the width of the conductive circuit. In this embodiment, the width of the conductive circuit is 500 μm, and the width of a single "capsule-shaped" microstructure array is 50 μm. To enhance the fluidity of the conductive ink during filling and to minimize stress concentration areas during curing that could cause the entire conductive circuit to break, the microstructures are designed to be spaced apart. 1.2) Hydrophobic surface treatment of silicon template: When the untreated silicon template 1 is used for imprinting, polydimethylsiloxane (PDMS) will be embedded in the groove of the silicon wafer structure and cannot be separated. Therefore, after the octafluorocyclobutane (C4F8) gas is excited by a deep dry etching machine and decomposed in a plasma environment, a fluoropolymer film with a thickness of 30~50nm is formed on the silicon wafer surface to separate the two interfaces. This thickness will not affect the 60μm deep structural groove. 1.3) Imprinting and molding the silicon template to prepare a primary flexible convex template for molding: such as... Figure 2 As shown, a 125μm thin lower PET film 5-1 is placed on the lower tempered glass plate 3-1, and a 500μm thick silicon template 1 is placed on the PET film 5-1. PDMS colloid is completely coated on the pattern of the silicon template 1. After vacuuming for 5 minutes in a vacuum chamber to remove air bubbles, an upper PET film 5-2 and an upper tempered glass plate 3-2 are pressed onto the PDMS in sequence. A 3 kg weight or other heavy object is placed on the upper tempered glass plate 3-2 to apply pressure and thin the PDMS. The thickness is controlled by placing a 2mm thick glass sheet 4 between the lower tempered glass plate 3-1 and the upper tempered glass plate 3-2. After curing on an 80℃ baking table for 2 hours, the mold is removed to obtain a primary flexible convex template 2. 1.4) Perform the same surface hydrophobic treatment on the flexible convex template 2 as in step 1.2); 1.5) A secondary flexible concave template is prepared by embossing and casting the primary flexible convex template: To better fit the surface of the curved structure, the secondary flexible concave template 7 should be as thin as possible; therefore, spin coating is used to control the template thickness. Figure 3 As shown, firstly, the PDMS after vacuuming is evenly coated onto the primary flexible convex template 2 to completely cover the pattern. Then, it is transferred to a vacuum chamber and vacuumed for 5 minutes to eliminate air bubbles. Next, the primary flexible convex template 2 is transferred to the spin coater disk 6 and rotated at a low speed of 500 r / min for 15 seconds, followed by a high speed of 1200 r / min for 20 seconds. This spin coats a thin layer of PDMS onto the primary flexible convex template 2. After curing on an 80℃ baking rack for 60 minutes, it is demolded to obtain the secondary flexible concave template 7. 1.6) Surface hydrophilic treatment of the secondary flexible concave template 7: The secondary flexible concave template 7 is placed in an oxygen plasma environment for cleaning treatment for 20 s to make its surface hydrophilic, which is beneficial to increase the wettability of the conductive ink and make it flow and fill the micro cavity better. The PDMS is prepared by mixing PDMS material and its curing agent at a mass ratio of 10:1, stirring for 5 minutes to ensure thorough mixing, and then evacuating in a vacuum chamber for 5 minutes to remove air bubbles once beforehand. When using PDMS to perform the first molding of the silicon template 1, a PDMS with higher hardness is used for molding. The primary flexible convex template 2 should be relatively hard and thick to avoid deformation, thereby achieving higher replication accuracy during molding. In this embodiment, the Shore hardness of the primary flexible convex template 2 after curing is 60A. When molding the primary flexible convex template 2, a PDMS material with lower hardness is used for spin coating molding. The thickness of the secondary flexible concave template 7 is controlled by adjusting the rotation speed. The secondary flexible concave template 7 should be relatively soft and thin to conformally attach to complex curved surfaces. In this embodiment, the Shore hardness of the material used to manufacture the secondary flexible concave template 7 after curing is 10A. In this embodiment, the thickness of the primary flexible convex template 2 obtained by controlling the thickness using glass sheet 4 is 1.5 mm; the thickness of the secondary flexible concave template 7 obtained by spin coating is 150 μm. During the hydrophilic treatment in step 1.6), the oxygen plasma power and time should be moderate to prevent excessive power and time from causing the material to fail to transfer from the groove in step 5), and to prevent insufficient power and time from causing the conductive ink to agglomerate and fail to self-flow and fill the microcavity in step 4). The wetting angle of the secondary flexible concave template 7 should shift between 50° and 60°. In this embodiment, the wetting angle of the secondary flexible concave template 7 after oxygen plasma hydrophilic treatment is approximately 55°. 2) Functional ink filling of strain sensing area: The functional material of the sensing is scraped and coated to fill the constraint microcavity in the strain sensing grid area of ​​the secondary flexible concave template. The scraping, drying and wiping of excess parts are repeated until the functional material of the sensing is uniformly and continuously distributed in the grid constraint microcavity. like Figure 4 As shown, the secondary flexible concave template 7, after surface hydrophilic treatment, is placed on a tempered glass plate 3. A 10% (w / w) multi-walled carbon nanotube aqueous solution is dropped onto the strain-sensing grid area 7-2 of the secondary flexible concave template 7. A scraper 8 is used to scrape and assist in uniform filling. Then, the secondary flexible concave template 7 is placed on an 85°C drying rack for 1 minute to remove the solvent, leaving the dried multi-walled carbon nanotube solid. The dried multi-walled carbon nanotube outside the grid is gently wiped away with anhydrous ethanol using a cotton swab. The above scraping and drying steps are repeated 3 to 4 times until the strain-sensitive material multi-walled carbon nanotube in the constrained microcavity is uniform, dense and continuous, forming a strain-sensing grid 9. When wiping away excess multi-walled carbon nanotubes outside the mesh with a cotton swab in step 2), it is inevitable that multi-walled carbon nanotubes in the mesh grooves will be brought out. Therefore, the mesh grooves can never be completely filled. In this embodiment, the depth of the mesh grooves of the secondary flexible concave template 7 is 60 μm. After 3 to 4 fillings, the thickness is about 50 to 55 μm, and the depth is 5 to 10 μm remaining. 3) Pre-stretching of the sensing unit to generate microcracks: The secondary flexible groove template, which has been filled with sensing functional material in the strain sensing grid area, is pre-stretched to generate sensing microcracks on the strain sensing grid for sensing stress. The pre-stretching process generates microcracks by fixing the two ends of a secondary flexible concave template 7 filled with multi-walled carbon nanotubes to a stretching machine using clamps. By controlling the displacement of the stretching machine by a certain distance, a certain tensile force is applied to the secondary flexible concave template 7. Because there is a difference in elastic modulus between the substrate of the secondary flexible concave template 7 and the strain sensing mesh filled with multi-walled carbon nanotubes, cracks will be generated in the stress concentration area of ​​the strain sensing mesh. When the curved structure 12 experiences strain, these microcracks will expand or contract, resulting in a change in resistance. By detecting the change in resistance, the strain of the curved structure 12 can be obtained. In this embodiment, the strain sensing mesh is pre-stretched by 10%. After pre-stretching the filled strain sensing mesh area 7-2, in order to protect the area from contamination during the manufacturing of conductive lines in the next step 4), UV-curable adhesive should be used to continue filling the remaining 5-10 μm of the groove depth above the cured strain sensing mesh, but the UV-curable adhesive should not overflow from the groove. 4) Composite filling of conductive circuit areas with multiple conductive functional materials: Conductive functional materials are composite filled at corresponding positions in the conductive circuit areas of the secondary flexible concave template and fully cured under a thermal field; In designing the conductive circuit, a "capsule-shaped" microstructure array 7-1 was prefabricated within the conductive circuit, such as... Figure 5 As shown, Figure 5 yes Figure 1 A schematic diagram of the process of filling region A with silver nano-ink 10 shows that, due to surface tension, the silver nano-ink spreads faster along the sidewalls. The microstructure significantly increases the contact area between the silver nano-ink and the sidewalls, thus the presence of the microstructure significantly promotes the flow behavior of the silver nano-ink 10. Therefore, by simply adding silver nano-ink 10 to a few key points in the conductive circuit, it can diffuse to other locations in the circuit. Figure 5 It can also be seen that even when silver nano-ink 10 overflows the pattern area, the silver nano-ink can quickly diffuse to other locations, thus ensuring circuit accuracy. Due to capillary effect, the silver nano-ink 10 will spontaneously flow from the wider linewidth to the narrower linewidth or diffuse towards both ends until it completely fills the groove. The corresponding positions refer to the widest linewidth and the locations where the line bends. Figure 1 As shown, taking the longest outermost conductive line in this embodiment as an example, silver nano ink 10 only needs to be continuously dripped into the four positions A to D in the groove of the conductive line multiple times. The distance between the four positions is close. This method overcomes the disadvantage of low efficiency of point-to-point tracking printing and can greatly improve the circuit manufacturing speed. Silver nano-ink 10 has good fluidity and is suitable for filling microstructures; the silver nanoparticles have a particle size of about 50nm and are very dense after sintering, which can ensure the morphology of the circuit transferred to the curved structure 12 as much as possible; however, the solute content of silver nano-ink 10 is only 30%, and the filling depth is reduced significantly after solvent evaporation. Even after repeated filling, it is still difficult to fill the groove of the conductive line; the solidified silver conductive line is too brittle to achieve the conductive function on the curved surface. Therefore, micron-level bend-resistant silver paste is used to fill the conductive line after the silver nano-ink 10 is filled again by scraping with a scraper 8. Functionalizing the silver nano-ink 10 involves placing the filled equipment or component surface under a multi-physics field for curing and sintering. In this embodiment, after the initial filling of the silver nano-ink 10, the solvent is evaporated by heating on an 80°C baking rack for 10 minutes, and then sintered at 130°C for 20 minutes to solidify the silver nanoparticles. After filling the silver paste, it needs to be sintered at 130°C for 30 minutes. The silver nano-ink 10 used in this embodiment has a mass fraction of 30%, a viscosity of 5-12 cPs, and a silver nanoparticle size of approximately 50 nm; the silver paste used has a mass fraction of 60%, a viscosity of 15000 cPs, and a silver particle size of <5 μm. 5) In-situ transfer of integrated conductive sensing structure: Using a curing agent, the strain sensing grid area 7-2 and conductive lines in the secondary flexible groove template 7 are conformally transferred to the curved structure 12 in one step; like Figure 6 , Figure 7 As shown, before the transfer, a thin insulating layer should be made on the curved structure 12 to prevent the sensor from directly contacting the metal structure sample. The operation method is to first apply a few drops of NOA61 UV-curable adhesive as an insulating layer 13 on the surface to be tested of the curved structure 12, then attach a thin unstructured PDMS film to the UV curing agent and apply slight pressure, then vacuum in a vacuum chamber for 5 minutes to remove air bubbles and reduce the thickness of the insulating layer as much as possible, wipe off the excess adhesive with a cotton swab, and finally peel off the unstructured PDMS layer after curing in a UV curing chamber. The uniform thin insulating layer 13 manufactured in this embodiment is about 30μm. Next, the strain sensing grid area and conductive lines are transferred in situ. Similarly, a small amount of NOA61 UV-curable adhesive 14 is applied to the insulating layer 13. Then, the secondary flexible template 7, which is filled with the strain sensing grid area and conductive lines, is evenly applied to the adhesive and slight pressure is applied. After vacuuming and wiping off excess adhesive, it is cured. Finally, the secondary flexible template 7 is slowly peeled off from one side. According to the principle of adhesion energy, the energy required to separate the strain sensing grid 9, silver nano ink 10, silver paste 11 and the underlying NOA61 UV-curable adhesive 14 is greater than the energy required to separate these three materials from the secondary flexible template 7. Therefore, when peeling off the secondary flexible template 7, the strain sensing grid 9 and the silver nano ink 10 and silver paste 11 are integrally transferred to the insulating layer 13 on the surface of the curved structure 12. The insulating layer 13 and the NOA61 UV-curable adhesive 14 used for transfer are the same material. After curing, the two layers are fused into one layer. 6) Covering and encapsulation layer: After peeling off the two flexible secondary groove templates, a polytetrafluoroethylene heat insulation and anti-oxidation encapsulation layer is printed on the overall structure.

[0033] like Figure 8 As shown, carefully wipe the surface of the curved structure 12, and use a fine printhead to clean the area except for the lead wire connector area. Figure 1 A 30μm thick polytetrafluoroethylene (PTFE) coating 15 is sprayed or printed on the conductive circuit area outside A) to achieve the purpose of heat resistance, moisture insulation and oxidation prevention, and further increase the adhesion and durability between the conductive circuit and the substrate. During testing, the testing instrument 16 is connected to the end of the wire.

[0034] like Figure 9 As shown, Figure 9 This is a top view of the microscopic electron microscope morphology of the strain sensing mesh 9 and the conductive lines before and after the transfer in this embodiment. It can be seen that the microcrack morphology and the uniform thickness of the conductive lines in the strain sensing mesh before and after the transfer are visible.

[0035] like Figure 10 As shown, Figure 10 In order to be in Figure 1 The left image shows a comparative electron microscope (EM) image of the conductive line groove at position E, which is filled only with silver nano-ink 10 and the cross-section of the conductive line transferred after being filled with both silver nano-ink 10 and silver paste 11. As silver nano-ink 10 flows more easily along the sidewalls, it can be seen that after curing, silver nano-ink 10 exhibits an unevenness of "thick on both sides and thin in the middle". Since the solute content of silver nano-ink is only 30%, after the groove is completely filled with silver nano-ink and cured, only a thin layer remains. Even after three repeated fillings, it is still difficult to fill the conductive line groove completely. The right image shows the conductive line transferred after filling with silver nano-ink 10 three times and then scraping silver paste 11 once. It can be seen that compared to filling with only silver nano-ink 10, silver paste 11 can fill the groove more completely.

[0036] A fuller and more uniform filling of conductive material is essential for achieving the basic conductivity function of a circuit. In this embodiment, after using silver nano-ink 10 for capillary self-flowing filling and then silver paste 11 for scraping filling, the filling depth of the 60μm trench can reach 50~55μm, and the sheet resistance of the conductive line is <10 mΩ / sq, which meets the conductivity requirements.

[0037] like Figure 11 As shown, Figure 11 To transfer the integrated structure of strain sensing mesh and conductive circuitry onto the surface of different materials and curvatures, the insulating layer 13 and NOA61 UV-curable adhesive 14 used have good bonding performance with various material interfaces. The minimum line width of the graphic area is designed to be 75μm, while the printing ink needle diameter used in this embodiment is 0.2mm. Traditional point-to-point printing is difficult to guarantee printing accuracy, but this method can ensure that the silver nano-ink 10 does not cause lateral contamination, thereby ensuring circuit accuracy.

[0038] This invention is simple to operate and requires no complex printing equipment. The integrated sensing and conductive structure prepared is suitable for strain measurement of three-dimensional curved surfaces. The proposed structural design and manufacturing method have significant effects on improving sensing sensitivity and the stability of conductive circuitry. Those skilled in the art will understand that appropriate modifications, partial combinations, and substitutions can be made to this invention depending on design requirements and other factors, provided they are within the scope of the claims and their equivalents.

Claims

1. A method for integrated manufacturing of a conformal bidirectional strain sensor on a curved surface structure, characterized in that, A conformal bidirectional strain sensor on a curved structure includes a transfer medium layer adhered to the surface to be measured, a strain sensing grid with multi-walled carbon nanotubes adhered to the upper surface of the transfer medium layer, and conductive lines of a microstructure array with micropillar pore structure disposed inside the transfer medium layer. The strain sensing grid has fine cracks distributed on it to sense strain, and the conductive circuit is composed of two or more conductive functional materials, and the conductive circuit is covered with an encapsulation layer material. The integrated manufacturing method includes the following steps: 1) Template preparation and surface hydrophilic and hydrophobic treatment: Design sensor and conductive circuit patterns, arrange microstructure array, and prepare patterned grooves on silicon wafer by photolithography to obtain silicon template; obtain secondary flexible concave template by imprinting the silicon template twice; perform hydrophobic surface treatment on silicon template and primary flexible convex template before and after the first imprinting, respectively, and perform hydrophilic surface treatment on secondary flexible concave template after the second imprinting. 2) Strain sensing functional ink filling: The sensing functional material is scraped and coated in the strain sensing grid area of ​​the secondary flexible concave template to fill the constrained microcavity. The scraping, drying and wiping of excess parts are repeated multiple times until the sensing functional material is uniformly and continuously distributed in the grid constrained microcavity. 3) Pre-stretching of the sensing unit to generate microcracks: The secondary flexible concave template, which has been filled with sensing functional material in the strain sensing grid area, is pre-stretched to generate sensing microcracks on the strain sensing grid for sensing micro-strain. 4) Composite filling of conductive circuit areas with multiple conductive functional materials: Conductive functional materials are composite filled at corresponding positions in the conductive circuit areas of the secondary flexible concave template and fully cured under a thermal field; 5) In-situ transfer of integrated sensing and conductive structure: Using a curing agent, the strain sensing mesh and conductive lines in the secondary flexible concave template are conformally transferred to the curved structural component in one step; 6) Covering the encapsulation layer: After peeling off the two flexible secondary concave templates, the encapsulation layer is printed on the overall structure; A bidirectional isolation microstructure array is set in the connection area between the conductive line and the strain sensing grid.

2. The integrated manufacturing method according to claim 1, characterized in that: In step 1), the ratio of the size of a single microstructure to the width of the conductive line is less than or equal to 10:

1.

3. The integrated manufacturing method according to claim 1, characterized in that: In step 1), when using polydimethylsiloxane (PDMS) to mold a silicon template, a PDMS with a higher Shore hardness of 40A to 60A is used for molding, and the thickness of the obtained primary flexible convex template is >1.5 mm; when using PDMS to mold a primary flexible convex template, a softer PDMS with a Shore hardness of 0A to 20A is used for spin-coating molding, and the thickness of the obtained secondary flexible concave template is <200 μm.

4. The integrated manufacturing method according to claim 1, characterized in that: In step 1), the wetting angle of the interface of the secondary flexible concave template after hydrophilic treatment is 50° ~ 60°.

5. The integrated manufacturing method according to claim 1, characterized in that: In step 2), use a scraper to repeatedly scrape and coat the material into the strain sensing grid. After drying, use a cotton swab dipped in anhydrous ethanol or acetone to wipe away any excess material outside the grooves of the secondary flexible concave template.

6. The integrated manufacturing method according to claim 1, characterized in that: Step 3) After pre-stretching the filled strain sensing mesh area, use UV-curing adhesive to continue filling the grooves on top of the cured sensing material, but the UV-curing adhesive should not overflow from the grooves.

7. The integrated manufacturing method according to claim 1, characterized in that: In step 4), the corresponding position refers to the widest position in the line width pattern and the position where the line bends. When manufacturing conductive lines, conductive functional material should be continuously dripped into the corresponding position in the conductive line groove until the groove is completely filled. The distance between each corresponding position should be close.

8. The integrated manufacturing method according to claim 1, characterized in that: In step 4), composite filling refers to using two or more conductive functional materials and filling the conductive circuit with two or more filling methods. The conductive functional materials refer to gold, silver, and copper nanoparticle inks and conductive pastes; the filling methods refer to doctor blade coating, ink capillary-driven self-filling, and direct writing printing technology along the groove lines.

9. The integrated manufacturing method according to claim 1, characterized in that: Step 5) Before the transfer, an insulating layer is created on the curved structure to prevent the strain sensing mesh and conductive lines from directly contacting the metal structure sample.

Images