A strain sensor and a method of manufacturing the same

Abstract

This invention relates to a strain sensor and its fabrication method, belonging to the field of strain measurement and sensor technology. The invention constructs a polyimide precursor film on the surface of a flexible support substrate and transforms it into a polyimide film through an imidization process. Utilizing the thermomechanical mismatch between the film and the flexible support substrate during imidization, an in-situ wrinkled structure is induced in the film. The wrinkle morphology is controlled by adjusting the modulus of the flexible support substrate, the imidization conditions, and the thickness of the polyimide film. Laser direct writing technology is used to carbonize the wrinkled polyimide film to form a conductive carbonized layer, thus obtaining the strain sensor. This invention utilizes the residual stress from the imidization process as a driving force for constructing the wrinkled microstructure; combined with laser carbonization, it achieves in-situ patterned fabrication of the piezoresistive sensing element. This method is simple, low-cost, and yields a piezoresistive sensor with high sensitivity.

Description

Technical Field

[0001] This invention relates to the field of strain measurement and sensor technology, specifically to a strain sensor and its fabrication method. Background Technology

[0002] When a thin, elastic film is subjected to sufficiently high compressive stress or strain, it will undergo mechanical instability and form wrinkles. These wrinkles exhibit periodic, fluctuating geometric structures and are widely found in both natural and man-made structures. Due to the enormous potential of wrinkles in controlling material surface / interface properties and constructing multifunctional devices, the application potential of wrinkled structures in materials science, biology, and nanotechnology is increasingly prominent. Utilizing micro / nano fabrication techniques to precisely construct wrinkles, and inducing wrinkle formation through mechanical, thermomechanical, or swelling methods, has become an important approach for the design and fabrication of wrinkled functional devices.

[0003] Polyimide (PI) is a high-performance engineering polymer with excellent thermal stability, mechanical properties, and chemical inertness, making it widely used in microelectronics, flexible electronics, aerospace, and other fields. In the preparation of PI films, the imidization reaction of the precursor is a crucial step. This process, accompanied by solvent evaporation, molecular chain rearrangement, and thermal shrinkage, generates significant residual stress within the film. When the PI film is supported on a rigid substrate (such as a silicon wafer), the coefficient of thermal expansion (CTE) of the polyimide (PI) film is much lower than that of the substrate. This results in the residual stress within the imidized film exhibiting tensile stress, which is generally considered a process defect requiring control, as it can lead to film cracking, delamination, or device failure. How to modulate the residual stress from the imidization process into compressive stress, thereby transforming it into a driving force for the construction of polyimide wrinkle morphology and applying it to the sensing field, remains a pressing technical problem to be solved in this field. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a strain sensor and its fabrication method, particularly relating to a method for constructing a strain sensor by utilizing the spontaneous wrinkling behavior during the imidization process to prepare a polyimide film supported by a flexible substrate, combined with laser direct writing carbonization technology. The aim is to achieve a highly sensitive piezoresistive sensor fabrication method. This sensor involves constructing a polyimide precursor film on the surface of a flexible substrate, and then transforming the precursor film into a polyimide film through an imidization process. The thermomechanical mismatch between the film and the flexible substrate during imidization induces in-situ wrinkle formation in the film. Precise control of the wrinkle morphology is achieved by adjusting the modulus of the flexible substrate, the imidization conditions, and the thickness of the polyimide film. Finally, laser direct writing technology is used to carbonize the wrinkled polyimide film to form a conductive carbonized layer, thus obtaining the strain sensor.

[0005] The core principle of this invention lies in the ingenious use of the thermomechanical property mismatch between the polyimide film and the soft support substrate during the imidization process, which transforms the residual stress of imidization, which is usually regarded as a negative factor, into the driving force for constructing the wrinkled structure. Combined with laser direct writing carbonization technology, it enables the in-situ and controllable preparation of high-performance strain sensors.

[0006] The fundamental principle of the strain sensor fabrication method of this invention stems from a reverse innovation addressing the problem of residual stress in traditional thin films. On traditional rigid substrates (such as silicon wafers), the coefficient of thermal expansion (CTE) of the polyimide (PI) film is much lower than that of the substrate, leading to tensile stress within the film after imidization. This invention, however, uses a flexible material with a high coefficient of thermal expansion as the supporting substrate. Because the coefficient of thermal expansion of the flexible supporting substrate is much greater than that of the polyimide (PI) film, the drastic contraction of the substrate during the imidization cooling process applies significant compressive stress to the upper PI film. This compressive stress drives the film to elastically buckle and form a regular wrinkled structure in situ.

[0007] Furthermore, the coefficient of thermal expansion of PI is negatively correlated with its degree of imidization. This means that as the imidization reaction progresses and the imidization increases, the coefficient of thermal expansion of the PI film gradually decreases, while the mismatch between the coefficient of thermal expansion and the flexible support substrate increases. Therefore, the compressive residual stress induced by the flexible support substrate will continue to increase and be "amplified" during the imidization process, thus reliably inducing wrinkling of the film. These spontaneously generated wrinkled microstructures, due to their geometric undulations, can generate local stress concentration effects under subsequent stress, laying the foundation for significantly improving the piezoresistive sensitivity of the sensor.

[0008] Finally, selective carbonization of the PI film with a predetermined wrinkle morphology is performed using laser direct writing technology. The high energy density of the laser transforms the PI material into a conductive carbon layer with a piezoresistive effect, thereby directly converting the microscopic wrinkle morphology into the sensitive element of the sensor.

[0009] In the process chain combining "flexible support substrate-induced spontaneous wrinkling" and "laser direct writing carbonization" of this invention, the final piezoresistive sensitivity and stability of the sensor are determined by multiple key parameters and their interactions. First, precise control of the wrinkle morphology is the foundation for achieving the stress concentration effect: the elastic modulus of the flexible support substrate determines its constraint strength on the polyimide (PI) film, while the thickness of the PI film directly determines the period and wavelength of the wrinkles; simultaneously, the imidization temperature program dynamically controls the accumulation process of compressive residual stress by influencing the degree of imidization and the evolution of the coefficient of thermal expansion (CTE) of PI, thereby achieving precise control of wrinkle geometry (such as wavelength and amplitude). Building upon this, the laser-direct writing carbonization process transforms a PI film with a predetermined wrinkled morphology into a conductive carbon layer. Its conductivity and piezoresistive effect are influenced by parameters such as laser power and scanning speed—these laser parameters determine the carbonization depth, the microstructure of the carbon material, and the formation of conductive pathways. Simultaneously, the undulations of the wrinkled structure not only amplify the electrical response of mechanical deformation through localized stress concentration but also alter the geometry and interfacial bonding of the carbonized layer, increasing the effective sensing area and potentially inducing microcracks and other structural features to synergistically enhance the piezoresistive effect. Therefore, through a series of adjustments to the characteristics of the soft support substrate, the precursor film thickness, the imidization thermal history, and the laser carbonization parameters, this invention achieves end-to-end control from the construction of micro-wrinkles to the patterning of the conductive functional layer, ultimately obtaining a highly sensitive piezoresistive strain sensor.

[0010] In summary, this invention achieves full-chain regulation from micro-fold construction to conductive functional layer patterning by controlling a series of parameters, including the characteristics of the soft support substrate, precursor film thickness, imidization thermal history, and laser carbonization parameters, ultimately obtaining a highly sensitive piezoresistive strain sensor.

[0011] This invention is achieved through the following technical solution:

[0012] The first objective of this invention is to provide a strain sensor comprising a soft support substrate, a polyimide film with a wrinkled structure, a laser-written carbon material, and an electrical extraction structure.

[0013] The laser-written carbon material is formed on the surface of the polyimide film with a wrinkled structure, and is obtained by laser-written in-situ carbonization of the polyimide with a wrinkled structure.

[0014] Conductive adhesive is used to connect wires to laser-written carbon material to form an electrical lead-out structure.

[0015] In one embodiment of the present invention, the total thickness of the interface transition region between the laser-written carbon material and the underlying soft support substrate is 0~20 nm.

[0016] The laser-written carbon material and the flexible support substrate are connected in one of the following three ways:

[0017] (1) The laser completely penetrates the polyimide film with a wrinkled structure, and the laser-written carbon material is directly connected to the soft support substrate;

[0018] (2) The laser did not completely penetrate the polyimide film with a wrinkled structure, and the laser-written carbon material was connected to the soft substrate through a residual uncarbonized polyimide film layer.

[0019] (3) The laser completely penetrates the polyimide film with a wrinkled structure and carbonizes the surface layer of the soft substrate; the laser-written carbon material is connected to the soft substrate through the surface carbonization layer of the soft substrate.

[0020] In one embodiment of the present invention, the soft support substrate is selected from polydimethylsiloxane, polyisoprene, polyurethane, styrene-ethylene-butene-styrene thermoplastic elastomer, or styrene-butadiene rubber.

[0021] And / or, the modulus of the soft support substrate is 0.5 MPa to 20 MPa;

[0022] And / or, the thickness of the flexible support substrate is 0.5 mm to 5 mm;

[0023] And / or, the coefficient of thermal expansion of the flexible support substrate is 100 × 10⁻⁶. -6 K -1 ~600 × 10 -6 K -1 ;

[0024] And / or, the thermal weight loss rate of the soft support substrate at 250°C for 60 min is less than 5%;

[0025] And / or, the soft support substrate further includes a surface hydrophilic treatment.

[0026] In one embodiment of the present invention, the polyimide film with a wrinkled structure is obtained by heat treatment of a polyimide precursor.

[0027] In one embodiment of the present invention, the polyimide precursor is polyamic acid;

[0028] And / or, the thickness of the polyimide precursor is 50 nm to 1300 nm.

[0029] In one embodiment of the present invention, the heat treatment temperature is 150°C to 300°C and the time is 1 min to 60 min.

[0030] In one embodiment of the present invention, the thickness of the polyimide film with the wrinkled structure is 40 nm to 1000 nm;

[0031] And / or, the modulus of the polyimide film with the wrinkled structure is 100 MPa to 1500 MPa;

[0032] And / or, the coefficient of thermal expansion of the polyimide film with the wrinkled structure is 5 × 10⁻⁶. -6 K -1 ~40×10 -6 K -1 ;

[0033] And / or, the folding period of the polyimide film with the folded structure is 2 μm to 50 μm.

[0034] In one embodiment of the present invention, the laser energy is 10 J·m. -1 ~50 J·m -1 The preferred value is 30 J·m -1 ~35 J·m -1 ;

[0035] And / or, the laser wavelength for laser direct writing is 10 nm to 1 mm;

[0036] And / or, the power of the laser direct writing is 1 W to 1.1 W;

[0037] And / or, the linear velocity of the laser direct writing is 25 mm·s. - ¹~30 mm·s - ¹;

[0038] And / or, the line spacing of the laser direct writing is 40 μm to 50 μm.

[0039] In one embodiment of the present invention, the electrical lead-out structure is a metal wire connected to both ends of a carbon wire by a two-point method or a four-wire method using conductive silver paste, which achieves reliable electrical lead-out while avoiding the introduction of additional stress and thermal mismatch.

[0040] In one embodiment of the present invention, the piezoresistive sensitivity coefficient of the strain sensor is 100-3000.

[0041] A second objective of this invention is to provide a method for fabricating the aforementioned strain sensor, comprising the following steps:

[0042] S1. The soft support substrate is hydrophilized.

[0043] S2. Coat the surface of the hydrophilicized soft substrate obtained in step S1 with a polyimide precursor, and then heat-treat to induce wrinkling to obtain a polyimide film with a wrinkled structure.

[0044] S3. The polyimide film with a wrinkled structure obtained in step S2 is subjected to laser direct writing carbonization to obtain laser direct writing carbon material.

[0045] S4. A conductive adhesive is used to connect the wires to the laser-written carbon material to form an electrical lead-out structure; thus obtaining the strain sensor.

[0046] In one embodiment of the invention, the hydrophilization treatment is plasma treatment, chemical etching, or coating with a hydrophilic coupling agent.

[0047] In one embodiment of the present invention, the strain sensor further includes an external sensing carrier; the laser-written carbon material is connected to the external sensing carrier by an adhesive. The external sensing carrier is one or more of the following: steel sheet, aluminum alloy sheet, stainless steel sheet, titanium alloy sheet, epoxy resin substrate, glass fiber reinforced composite material sheet, and ceramic sheet;

[0048] And / or, the adhesive is an epoxy resin adhesive with a bonding strength of not less than 15 MPa;

[0049] And / or, the connection conditions are: room temperature to 150°C for 30 min - 24 h.

[0050] When a strain sensor is combined with an external sensing carrier for strain testing, the sensor can be prepared first and then integrated with the external sensing carrier. Alternatively, a self-wrinkling polyimide film supported by a soft support substrate can be combined with the external sensing carrier first, and then laser direct writing carbonization and electrical connection can be performed to prepare the strain sensor.

[0051] The technical solution of the present invention has the following advantages compared with the prior art:

[0052] (1) This invention provides a strain sensor and its fabrication method, specifically a strain sensor based on the synergy of spontaneous wrinkling and laser direct writing. The sensor utilizes the imidization process of a polyimide (PI) precursor on a soft substrate to transform residual stress, which is difficult to avoid in traditional processes, into a driving force for constructing microstructures, thus forming regular wrinkles in situ. Combined with laser direct writing carbonization technology, a conductive carbon layer is directly formed on the wrinkled surface, achieving low-cost and high-efficiency fabrication of a high-sensitivity piezoresistive sensor. This invention achieves precise control of the wrinkle morphology by adjusting the soft substrate modulus, PI film thickness, and imidization conditions, thereby optimizing the piezoresistive response characteristics of the sensor and obtaining a strain sensing element with both high sensitivity and good stability.

[0053] (2) This invention ingeniously utilizes a dual mechanism of "stress-driven self-assembly" and "laser selective carbonization" to enhance performance. On the one hand, the local stress concentration effect generated by the wrinkled structure under stress can significantly amplify the piezoresistive response of the carbonized layer, making the sensor highly sensitive to minute deformations. On the other hand, the undulating structure of the wrinkles increases the effective surface area of ​​the carbonized layer and induces the formation of a microcrack network, further enhancing the strain sensing capability. Based on this, this invention achieves flexible and controllable adjustment of the strain sensor sensitivity through a synergistic strategy of "soft substrate selection - film thickness control - imidization process - laser parameter optimization," obtaining a wide range of adjustable strain coefficients (GF) to meet the sensitivity requirements of different application scenarios.

[0054] (3) The fabrication process of this invention has unique technical advantages. Unlike traditional micro-nano fabrication methods that require photolithography, etching, or template assistance, the self-initiated wrinkling behavior used in this invention is a spontaneous and adaptive physical phenomenon that does not require complex patterning equipment or process steps, greatly simplifying the fabrication process. At the same time, laser direct writing technology facilitates the flexible design and rapid prototyping of carbonized patterns. The entire fabrication method is characterized by simple process, strong controllability, good repeatability, and low cost, making it suitable for large-scale industrial production.

[0055] (4) The present invention has wide applicability. The sensor can be directly fabricated on the surface of a soft support substrate to form a flexible strain sensor that can be attached and bent, suitable for wearable electronics, human motion monitoring, soft robots and other scenarios; or it can be integrated with a rigid substrate (such as steel sheet, ceramics, etc.) through the methods of "fabrication before transfer" or "attachment before processing", and applied to traditional structural health monitoring, precision manufacturing, aerospace and other fields. This flexible integration strategy provides great convenience for the sensor to adapt to different application scenarios and expands its application boundaries. Attached Figure Description

[0056] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0057] Figure 1 This is a schematic diagram of the strain sensor of the present invention;

[0058] Figure 2These are the infrared spectra and imidization degree (ID value) results of polyimides with different processing temperatures and thicknesses according to the present invention; wherein, (a) is the infrared spectrum of polyimides obtained at different imidization temperatures in the examples; (b) is the relationship between the imidization temperature and ID value of the polyimide film in (a); (c) is the infrared spectrum of polyimide films with different thicknesses (the imidization heat treatment conditions are fixed at 250℃ for 60 min); (d) is the ID result of polyimide films with different thicknesses in (c);

[0059] Figure 3 These are optical photographs of the surface morphology of polyimide precursor films with different thicknesses supported by a soft substrate in embodiments of the present invention (top row: optical microscope in reflection mode; bottom row: optical microscope in transmission mode).

[0060] Figure 4 This is a graph showing the relationship between the wrinkle morphology and wrinkle wavelength of the polyimide film of the present invention and the modulus of the flexible support substrate; wherein, (a) is the wrinkle morphology of the 75 nm thick polyimide film obtained by atomic force microscopy, scanning electron microscopy, and Fourier transform of optical microscopy; (b) is the change of wrinkle morphology (optical micrograph) of the polyimide film in the example with film thickness (with the modulus of the flexible support substrate fixed at 2.7 MPa); (c) is the change of wrinkle morphology (optical micrograph) of the polyimide film in the example with the modulus of the flexible support substrate (with the PI thickness fixed at 823 nm); (d) is the relationship between wrinkle wavelength and polyimide film thickness in the example; (e) is the relationship between wrinkle wavelength and the modulus of the flexible support substrate in the example.

[0061] Figure 5 The wrinkle morphology and wavelength results of the PI film (75 nm) supported by a soft substrate at different imidization stages in the embodiments of the present invention are shown.

[0062] Figure 6 This is a scanning electron microscope image of a wrinkled PI film after laser carbonization in an embodiment of the present invention;

[0063] Figure 7 The figures show the piezoresistive performance test results in the embodiments of the present invention; where (d1) is the multi-cycle displacement applied to the free end of the cantilever beam; (d2) is the relative resistance change ΔR / R0 synchronously recorded by the laser direct writing in-situ carbonization of the carbon material prepared from the wrinkled PI film; (d3) summarizes the relationship between ΔR / R0 and the cantilever beam displacement; the figures also show the piezoresistive response of the carbon material prepared from the flat PI film supported by the polydimethylsiloxane soft support substrate obtained in Comparative Example 2 by laser direct writing in-situ carbonization;

[0064] Figure 8This is the durability evaluation result of the laser direct-write carbon film prepared from the wrinkled PI film in the embodiments of the present invention based on the piezoresistive response. Detailed Implementation

[0065] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0066] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0067] Example 1

[0068] This example relates to the effect of imidization temperature on the degree of imidization of polyimide. Polyamic acid (purchased from Shanghai Yehe Industry & Trade Co., Ltd., YE1001), a polyimide precursor of pyromellitic dianhydride-4,4'-diaminodiphenyl ether type (PMDA-ODA), was coated onto polydimethylsiloxane (CAS No. 9016-00-6). The imidization temperature was varied to 150℃, 175℃, 200℃, 250℃, and 300℃, with a treatment time of 60 min. The obtained polyimide was characterized by infrared spectroscopy, and the results are shown below. Figure 2 As shown in (a), with increasing imidization temperature, the stretching vibration of the CN bond in polyimide at 1378 cm⁻¹ can be observed. - The intensity of the characteristic infrared absorption peak at ¹ increases accordingly. This indicates that the degree of imidization of the polyimide film is positively correlated with the imidization temperature. Using benzene rings at 1500 cm⁻¹... - The stretching vibration at ¹ is used as an internal standard to quantify the degree of imamidation (ID value) using the following formula:

[0069] (Equation 1)

[0070] Among them, A 1378 and A 1500 1378 cm respectively - ¹ and 1500 cm - The peak intensity at ¹; the subscripts s and r represent the sample and reference, respectively. Figure 2 (b) summarizes the relationship between the imidization temperature and ID value of polyimide films obtained by heat treatment at different temperatures. After quantitative analysis, it can also be seen that as the heat treatment temperature increases from 150℃ to 300℃, the ID value increases from 0.26 to 0.78.

[0071] Example 2

[0072] This embodiment relates to the effect of polyimide film thickness on the degree of imidization and wrinkling behavior. PMDA-ODA type polyimide precursor polyamic acid was coated onto a flexible substrate made of polydimethylsiloxane to obtain precursor films of different thicknesses. The infrared spectra of the polyimide films obtained after heat treatment at 250°C for 60 min are shown. Figure 2 Images (c) and (d) show the infrared spectra and quantized ID results of polyimide films with thicknesses of 75 nm, 182 nm, 313 nm, 443 nm, and 823 nm, respectively. It can be seen that there is a positive correlation between the ID value and the film thickness; that is, the thicker the PI film, the higher its degree of imidization. This is because thinner polyimide precursor films have a higher specific surface area and a shorter solvent diffusion path, which may hinder the solvent plasticization-assisted imidization process, resulting in a lower degree of imidization in the final polyimide film.

[0073] Optical characterization was performed on precursor films of different thicknesses before imidization treatment, from... Figure 3 It can be seen that the precursor films with thicknesses of 78 nm, 297 nm, 420 nm, 620 nm, and 1202 nm exhibit flat surface morphologies. However, after imidization treatment at 250℃, the final polyimide films all formed two-dimensional periodic wrinkled patterns. For example... Figure 4 Figure (a) shows the wrinkle morphology of a 75 nm polyimide film obtained by atomic force microscopy, scanning electron microscopy, optical microscopy, and diffraction pattern obtained by two-dimensional fast Fourier transform of the optical microscope. As can be seen from the figure, the same herringbone or zigzag wrinkle morphology pattern is observed under different characterization methods. This herringbone wrinkle pattern typically occurs when the film is subjected to equal biaxial compressive stress at a high stress value. The turning angle of the zigzag pattern is 90°, indicating that the wrinkled polyimide film is subjected to equal biaxial compressive stress / strain. The wrinkle wavelength is quantified in the diffraction pattern obtained by two-dimensional fast Fourier transform. Figure 4 (b) shows the wrinkle patterns of polyimide films of different thicknesses (75 nm, 182 nm, 313 nm, 443 nm and 823 nm) under the condition of fixed modulus (2.7 MPa) of soft support substrate, where the polyimide films of 75 nm, 182 nm, 313 nm, 443 nm and 823 nm correspond to precursor films of 78 nm, 297 nm, 420 nm, 620 nm and 1202 nm, respectively. Figure 4 (c) shows the wrinkle patterns when the polyimide film thickness (823 nm) is fixed but the polydimethylsiloxane undermodulus is varied (1.5 MPa, 1.9 MPa, 2.7 MPa, 3.3 MPa, and 3.9 MPa). From Figure 4 As can be seen from (b) and (c), all polyimide films exhibit a herringbone wrinkle pattern.

[0074] Further investigation was conducted into the dependence of the folding wavelength on the polyimide film thickness and the elastic modulus of the soft support substrate, through... Figure 4 Images (b) and (c) were used to perform imaging analysis and two-dimensional Fourier transform to quantify the fold wavelengths of all samples. Figure 4 Figures (d) and (e) summarize the relationship between the folding wavelength (λ) and the polyimide film thickness and the modulus of the flexible support substrate, respectively. Figure 4 As can be seen from (d), the wrinkle wavelength is positively correlated with the polyimide film thickness. This is because the thicker the PI film, the higher the degree of imideization, resulting in a higher modulus. From... Figure 4 As can be seen from (e), the fold wavelength is linearly proportional to the {-1 / 3} square of the soft substrate modulus.

[0075] Example 3

[0076] This embodiment investigates the wrinkling behavior of polydimethylsiloxane-supported PI films at relatively low levels of thermal residual strain. A series of polydimethylsiloxane-supported PI films (75 nm) were prepared, and their wrinkling behavior at different stages of the imidization process was examined (Stage 1: 0.5 h at 100 °C; Stage 2: 0.5 h at 100 °C and 1.5 h at 150 °C; Stage 3: 0.5 h at 100 °C, 1.5 h at 150 °C, and 0.5 h at 250 °C). Figure 5 As shown, wrinkling of the PI film occurs not only in stage-3 but also in the intermediate imidization stages (stages-1 and-2), although the thermal residual strain accumulated in the film during these intermediate stages is relatively low. Furthermore, from... Figure 5 As can be seen, the wrinkling wavelength of the PI film gradually decreases as the process progresses from stage-1 to stage-3. This indicates that the thickness of the PI film decreases with increasing imidization degree. This suggests that when the PI film cools down from an intermediate imidization stage, the thermal residual strain accumulated in the film should be lower than that of the film undergoing the complete three-stage imidization process due to the smaller temperature difference and the higher coefficient of thermal expansion of the PI film with a lower degree of imidization. However, it is still sufficient to provide the critical compressive stress for wrinkling.

[0077] Example 4

[0078] This embodiment provides a method for fabricating a strain sensor, the specific steps of which are as follows:

[0079] Polydimethylsiloxane with dimensions of 40×40×2 mm was selected as the substrate and subjected to oxygen plasma treatment for 3 minutes.

[0080] A polyamic acid precursor of pyromellitic dianhydride-4,4'-diaminodiphenyl ether type (PMDA-ODA) with a thickness of 1202 nm was coated on the surface of the hydrophilized soft substrate. After heat treatment at 250℃ for 60 min, wrinkling was spontaneously initiated in situ, resulting in a polyimide film with a thickness of 823 nm and a wrinkled structure.

[0081] Laser-direct writing carbonization was performed on a polyimide film with a wrinkled structure. During the laser-direct writing carbonization process, the laser power, laser beam scanning speed, and line spacing were maintained at 1.05 W and 30 mm·s, respectively. - ¹ and 50 μm.

[0082] Two copper wires were bonded to the two ends of a laser-written carbon material using conductive silver paste to form an electrical lead-out structure, thus obtaining a strain sensor.

[0083] like Figure 6 As shown in the scanning electron microscope (SEM) images, the wrinkled PI film after laser-written carbonization exhibits a three-dimensional fibrous porous network structure. This is because the soft properties and high coefficient of thermal expansion (CTE) of the silicone rubber substrate cause it to undergo significant expansion and contraction during the laser-written carbonization process. This induces high thermal stress on the wrinkled PI film, which in turn affects the pyrolysis and carbonization reaction of the PI film, ultimately resulting in the carbon film exhibiting an uncommon fibrous porous network structure. This wrinkled PI film supported by polydimethylsiloxane was bonded to a steel cantilever beam to fabricate a cantilever beam sensor. Figure 7 (d1) in the figure shows the multi-period displacement applied to the free end of the cantilever beam. The relative resistance change ΔR / R0 of the carbon material obtained by laser direct writing in-situ carbonization of a wrinkled PI film is recorded synchronously as follows: Figure 7 As shown in (d2); Figure 7Figure (d3) summarizes the relationship between ΔR / R0 and the cantilever beam displacement. This figure also shows the piezoresistive response of the carbon material obtained by laser direct writing in-situ carbonization, prepared from a flat PI film supported by a thin polydimethylsiloxane flexible substrate (0.1 mm thick), as shown in Comparative Example 2. The piezoresistive sensitivity of the carbon material obtained by laser direct writing carbonization prepared from the wrinkled PI film is higher when measuring tension (cantilever beam moving downwards, displacement is negative) than when measuring compressive deformation (cantilever beam moving upwards, displacement is positive). In both cases, its piezoresistive sensitivity is significantly higher than that of the carbon material obtained by laser direct writing in-situ carbonization prepared from the flat PI film (supported by a 0.1 mm thick polydimethylsiloxane flexible substrate). Further evaluation of the sensitivity coefficient of carbon materials: For the wrinkled PI film, the GF under tension and compression is 2300 and 120, respectively, which is much higher than the piezoresistive sensitivity coefficient of existing commercial sensors (the sensitivity coefficient of common metal strain gauges is 1-2, and that of silicon is 50-200), indicating that the carbon material obtained by laser direct writing in-situ carbonization prepared from the wrinkled PI film proposed in this invention has high piezoresistive sensitivity.

[0084] Example 5

[0085] The durability of the laser-written carbon film prepared from the wrinkled PI film in Example 4 was evaluated in terms of piezoresistive response, and the results are as follows: Figure 8 As shown, the relative resistance changes recorded in 1000 cantilever beam bending tests demonstrate good long-term stability.

[0086] Comparative Example 1

[0087] This comparative example provides a method for fabricating a strain sensor, which is similar to Example 4, except that a quartz substrate is used as the support substrate, while the other steps are the same as in Example 4.

[0088] A dense and uniform carbon film with a quartz-supported surface morphology can be obtained. Under applied mechanical strain, the resistance of this carbon film changes, but its piezoresistive sensitivity coefficient ((ΔR / R0) / ε, i.e., the relative resistance change per unit strain) is approximately 1.5. The sensitivity coefficient of the sensor obtained in this comparative example is significantly lower than that in Example 4 (the piezoresistive sensitivity coefficients of the wrinkled laser-written carbon material supported by a soft substrate under tension and compression are 2300 and 120, respectively).

[0089] Comparative Example 2

[0090] To investigate the effect of PI film wrinkles on the surface morphology and piezoresistive sensitivity of carbonized products obtained by laser direct writing in situ carbonization, this comparative example provides a method for fabricating a strain sensor, similar to Example 4, except that the wrinkled PI film is replaced with a flat PI film (supported by a 0.1 mm thick polydimethylsiloxane substrate, at which thickness the thermal stress during the imide treatment is insufficient to induce wrinkling of the polyimide film). The remaining steps are consistent with Example 4. The results are shown in […]. Figure 7 (d2) and (d3) are shown in the figures. It can be seen that for a flat PI film, the sensitivity coefficients under stretching and compression are 35 and 30, respectively. Compared to Example 4, the carbon material obtained by laser direct writing in-situ carbonization prepared from a wrinkled PI film exhibits significantly higher piezoresistive sensitivity. This indicates that introducing a wrinkled structure into the precursor PI film is an effective way to enhance the piezoresistive performance of the carbon film obtained by final laser direct writing in-situ carbonization.

[0091] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A strain sensor, characterized in that, The sensor includes a flexible support substrate, a polyimide film with a wrinkled structure, a laser-written carbon material, and an electrical extraction structure. The laser-written carbon material is formed on the surface of the polyimide film with a wrinkled structure, and is obtained by laser-written in-situ carbonization of the polyimide with a wrinkled structure. Conductive adhesive is used to connect wires to laser-written carbon material to form an electrical lead-out structure.

2. The strain sensor according to claim 1, characterized in that, The total thickness of the interface transition region between the laser-written carbon material and the soft support substrate below it is 0~20 nm.

3. The strain sensor according to claim 1, characterized in that, The soft support substrate is selected from polydimethylsiloxane, polyisoprene, polyurethane, styrene-ethylene-butene-styrene thermoplastic elastomer, or styrene-butadiene rubber; And / or, the modulus of the soft support substrate is 0.5 MPa to 20 MPa; And / or, the thickness of the flexible support substrate is 0.5 mm to 5 mm; And / or, the coefficient of thermal expansion of the flexible support substrate is 100 × 10⁻⁶. -6 K -1 ~600 × 10 -6 K -1 ; And / or, the thermal weight loss rate of the soft support substrate at 250°C for 60 min is less than 5%; And / or, the soft support soft substrate further includes a surface hydrophilic treatment.

4. The strain sensor according to claim 1, characterized in that, The polyimide film with a pleated structure is obtained by heat-treating a polyimide precursor supported by a soft support substrate.

5. The strain sensor according to claim 4, characterized in that, The polyimide precursor is polyamic acid; And / or, the thickness of the polyimide precursor is 50 nm to 1300 nm.

6. The strain sensor according to claim 4, characterized in that, The heat treatment temperature is 150℃~300℃, and the time is 1 min~60 min.

7. The strain sensor according to claim 1, characterized in that, The thickness of the polyimide film with the wrinkled structure is 40 nm to 1000 nm; And / or, the modulus of the polyimide film with the wrinkled structure is 100 MPa to 1500 MPa; And / or, the coefficient of thermal expansion of the polyimide film with the wrinkled structure is 5 × 10⁻⁶. -6 K -1 ~40×10 -6 K -1 ; And / or, the folding period of the polyimide film with the folded structure is 2 μm to 50 μm.

8. The strain sensor according to claim 1, characterized in that, The laser energy is 10 J·m -1 ~50 J·m -1 ; And / or, the laser wavelength for laser direct writing is 10 nm to 1 mm; And / or, the power of the laser direct writing is 1 W to 1.1 W; And / or, the linear velocity of the laser direct writing is 25 mm·s. - ¹~30 mm·s - ¹; And / or, the line spacing of the laser direct writing is 40 μm to 50 μm.

9. The strain sensor according to claim 1, characterized in that, The piezoresistive sensitivity coefficient of the strain sensor is 100-3000.

10. A method for preparing a strain sensor according to any one of claims 1-9, characterized in that, Includes the following steps: S1. The soft support substrate is hydrophilized. S2. A polyimide precursor film is coated on the surface of the hydrophilicized soft support substrate obtained in step S1. During the process of converting the precursor film into a polyimide film through imidization heat treatment, wrinkles are spontaneously generated to obtain a polyimide film with a wrinkled structure. S3. The polyimide film with a wrinkled structure obtained in step S2 is subjected to laser direct writing carbonization to obtain laser direct writing carbon material. S4. A conductive adhesive is used to connect the wires to the laser-written carbon material to form an electrical lead-out structure; thus obtaining the strain sensor.

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