A laser-induced graphene-based ultrathin flexible strain sensor and a preparation method thereof

Abstract

The application provides a kind of ultra-thin flexible strain sensor based on laser-induced graphene and preparation method, belongs to flexible sensor technical field, sensor includes: the flexible substrate of spin-coated polyimide film formation;Laser ablation polyimide film surface layer in situ formation of laser-induced graphene sensing layer, bottom remains un-ablated polyimide bottom layer, so that recess microstructure is formed between sensing layer and substrate;And polydimethylsiloxane encapsulation layer is coated on the outside of sensing layer;The application uses CO2 laser processing substrate surface layer to prepare sensing layer, avoids the problems such as structural damage, delamination and other problems caused by graphene transfer;Ultra-thin substrate improves the ability of curved surface fitting, reduces laser thermal damage;Sensing layer is three-dimensional porous honeycomb structure, and the conductive strip is perpendicular to the strain direction, which effectively improves the sensitivity in combination with the recess microstructure.The sensor has fast response and recovery speed, stable performance in multi-frequency cyclic strain and 10,000 times bending test, and excellent mechanical durability.

Description

Technical Field

[0001] This invention belongs to the field of flexible sensor technology, specifically an ultrathin flexible strain sensor based on laser-induced graphene and its preparation method. Background Technology

[0002] Flexible strain sensors have broad application prospects in wearable electronic devices, human motion monitoring, soft robotics, and human-computer interaction. Traditional strain sensors are usually based on metal or semiconductor materials, such as metal strain gauges and silicon-based piezoresistive sensors. They have high sensitivity, but their rigid substrates and packaging structures make them difficult to fit complex curved surfaces, and they are prone to fatigue failure under repeated bending and stretching conditions. Therefore, developing flexible strain sensors with good flexibility, high sensitivity, and stability has become a current research hotspot.

[0003] However, existing commercially available polyimide (PI) films are typically thicker than 50 μm, exhibiting high rigidity and poor conformal bonding capabilities, making it difficult to meet the high flexibility requirements of applications such as human skin and complex curved surfaces. Furthermore, when using thicker commercial PI films for laser ablation, laser thermal damage easily leads to warping, wrinkling, and other deformations, severely affecting the uniformity of laser-induced graphene (LIG) formation and conductivity consistency, thereby limiting the sensitivity and reliability of the sensor.

[0004] In addition, some existing technologies employ a method of laser-induced graphene peeling off from polyimide film and then transferring it to other flexible substrates. However, the transfer process is prone to causing damage to the graphene structure and a decrease in electrical properties. Furthermore, the bonding force between the multilayer interfaces is weak, and delamination or detachment is likely to occur under long-term cyclic strain. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention develops an ultrathin flexible strain sensor based on laser-induced graphene and its fabrication method, as detailed below: An ultrathin flexible strain sensor based on laser-induced graphene, comprising: Flexible substrate: The flexible substrate is a spin-coated polyimide film; Sensing layer: Located on the flexible substrate, the sensing layer is a laser-induced graphene layer formed by CO2 laser ablation of the surface of the polyimide film; the laser-induced graphene layer is formed in situ on the upper surface of the flexible substrate and is an integral structure with the flexible substrate; Encapsulation layer: Located above the sensing layer, the encapsulation layer is composed of polydimethylsiloxane (PDMS), covering the outside of the sensing layer and bonded to the edge of the flexible substrate; The laser-induced graphene layer has an unablated polyimide underlayer, making the surface of the laser-induced graphene layer lower than the original surface of the flexible substrate, thereby forming a grooved microstructure.

[0006] Furthermore, the laser-induced graphene layer has a three-dimensional porous honeycomb structure; the sensing layer is composed of multiple laser-induced graphene conductive strips; and the arrangement direction of the laser-induced graphene conductive strips is perpendicular to the strain direction of the sensor.

[0007] Furthermore, the thickness of the flexible substrate is 10-20 μm; the thickness of the unablated polyimide underlayer is 1-3 μm.

[0008] Moreover, the encapsulation layer is formed by curing a mixture of polydimethylsiloxane prepolymer (polydimethyl-methylvinylsiloxane) and crosslinking agent (polydimethyl-methylhydrosiloxane) at a mass ratio of 10:0.5-1.5.

[0009] Furthermore, the sensor method includes the following steps: (1) A polyimide film is prepared by spin-coating a polyimide solution onto a support substrate and then heated and cured to form a flexible substrate; (2) The polyimide film is ablated by laser to transform the surface of the polyimide film into a laser-induced graphene layer, i.e., a sensing layer; the bottom of the laser-induced graphene layer retains a 1-3 μm unablated polyimide underlayer, so that the surface of the laser-induced graphene layer is lower than the original surface of the flexible substrate, thereby forming a groove microstructure. (3) Mix the polydimethylsiloxane prepolymer and crosslinking agent at a mass ratio of 10:0.5-1.5, stir evenly, spin coat it onto the outside of the sensing layer and the edge of the flexible substrate, and then cure it to form an encapsulation layer; (4) The overall structure is peeled off from the support substrate to obtain an ultrathin flexible strain sensor.

[0010] Furthermore, the spin coating time is 100-140s; the heat curing includes pre-imidization at 100-140℃ for 100-140s, followed by curing at 250-350℃ for 1.5-2.5h to form a polyimide film with a thickness of 10-20μm.

[0011] Moreover, in step (2), the laser is a CO2 laser with a wavelength of 10.6 μm, a laser power of 2.4 W, and a scanning speed of 100 mm / s.

[0012] Moreover, in step (2), the laser beam is controlled by computer software to move on the polyimide film according to a preset pattern to prepare patterned laser-induced graphene; the scanning direction of the laser beam is perpendicular to the direction of the strain experienced by the sensor in use.

[0013] Moreover, in step (3), the spin coating speed is 800-1200 r / min, the time is 50-70 s, the curing temperature is 70-90 ℃, and the curing time is 0.8-1.2 h.

[0014] Moreover, in step (4), the entire structure is removed from the silicon wafer by non-destructive peeling.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses a spin coating process to prepare 10-20μm ultrathin polyimide films as flexible substrates, which are much thinner than conventional commercial polyimide films on the market. The substrate has lower stiffness and excellent conformal ability to curved surfaces, which can closely fit human skin and complex curved surfaces, making it suitable for wearable devices, soft robots and other scenarios. At the same time, it effectively reduces laser thermal damage during laser ablation, avoids film warping and wrinkling, and ensures uniform laser-induced graphene forming and consistent conductivity.

[0016] 2. The present invention provides an ultrathin flexible strain sensor based on laser-induced graphene, wherein the sensing layer is a laser-induced graphene layer formed by laser ablation of the surface of a polyimide film. It is prepared directly on the surface of the polyimide film by in-situ laser ablation, forming an integrated structure with the substrate. It does not require peeling and transfer, thus solving the problems of graphene structure damage and electrical performance degradation caused by traditional transfer processes. The interlayer bonding force is strong, and no delamination or shedding will occur under long-term cyclic strain, resulting in higher device reliability.

[0017] 3. The present invention provides an ultrathin flexible strain sensor based on laser-induced graphene. After laser ablation, a 1-3 μm unablated polyimide underlayer is retained, forming a groove microstructure between the sensing layer and the substrate. The groove microstructure provides a buffer space for the deformation of the LIG conductive strip, amplifies the microstructural changes caused by strain, and further improves the sensor's response sensitivity to strain.

[0018] 4. The present invention provides an ultrathin flexible strain sensor based on laser-induced graphene. The laser-induced graphene layer has a three-dimensional porous honeycomb structure. The conductive strips are arranged and the laser scanning direction is perpendicular to the strain direction. During deformation, the conductive strips are prone to relative displacement and rapid change in contact area. The conductive network reconstruction effect is outstanding. The average sensitivity of the sensor can reach 140.73, which is far superior to the parallel arrangement scheme and the strain recognition capability is stronger.

[0019] 5. The ultrathin flexible strain sensor based on laser-induced graphene provided by this invention is flexible and can be bent and rolled up. It has a response time of 0.38s and a recovery time of 0.52s, and has the ability to provide a millisecond-level transient response. It can track dynamic deformation in real time. The signal is stable under multi-frequency cyclic strain of 0.2-1Hz. After 10,000 consecutive bending cycles, the signal has no obvious attenuation or baseline drift. The structure has no delamination or cracking, and it exhibits long-term reliable mechanical durability and electrical stability.

[0020] 6. The present invention provides an ultrathin flexible strain sensor based on laser-induced graphene. The laser-induced graphene layer has a high degree of graphitization and few defects, and has a complete sp² conjugated carbon structure and a through-porous network, which ensures both excellent conductivity and good fatigue resistance. When combined with a polydimethylsiloxane encapsulation layer, it can effectively buffer mechanical stress, protect the internal functional layer, and assist in achieving non-destructive peeling, further improving the yield and service life of the finished product.

[0021] 7. The present invention provides a method for preparing an ultrathin flexible strain sensor based on laser-induced graphene. It does not require a mask or a special atmosphere. It uses a conventional CO2 laser to prepare patterned graphene in one step in air. Combined with spin coating and exfoliation processes, the process is simple, the equipment requirements are low, and it is easy to achieve mass production. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the manufacturing process of an ultrathin flexible strain sensor. Figure 2 This is a cross-sectional SEM image of an ultrathin flexible strain sensor. Figure 3 This is a physical image of an ultrathin flexible strain sensor; Figure 4 SEM image of the polyimide film before laser ablation; Figure 5 SEM image of the polyimide film after laser ablation; Figure 6 This is a schematic diagram of the laser ablation process of polyimide thin films. Figure 7 SEM image of the groove microstructure; Figure 8 This is a schematic diagram of the sensing principle of an ultrathin flexible strain sensor. Figure 9 This is a schematic diagram of the strain detection principle of the sensor. Figure 10 A comparison of the sensitivity of sensors in different scanning directions for laser-induced graphene. Figure 11 The dynamic response test diagram of the ultrathin flexible strain sensor under stepped strain loading is shown. Figure 12Cyclic strain test diagrams of the ultrathin flexible strain sensor provided by the present invention at different strain frequencies; Figure 13 The graph shows the response time versus recovery time of an ultrathin flexible strain sensor. Figure 14 This is a stability test diagram of an ultrathin flexible strain sensor under 10,000 cyclic bending tests. Figure 15 SEM images of the surface morphology of laser-induced graphene at different magnifications are shown, where A represents the low-magnification morphology and B represents the high-magnification morphology. Figure 16 XRD patterns comparing polyimide film and laser-induced graphene; Figure 17 XPS full spectrum comparison of polyimide film and laser-induced graphene; Figure 18 The images show the C1s spectra of polyimide and laser-induced graphene, where A is the C1s spectrum of polyimide and B is the C1s spectrum of laser-induced graphene. Figure 19 This is the Raman spectrum of laser-induced graphene. Detailed Implementation

[0023] Example 1: Fabrication of an ultrathin flexible strain sensor A method for fabricating an ultrathin flexible strain sensor based on laser-induced graphene, such as... Figure 1 As shown, a laser-induced graphene (LIG) sensing layer was fabricated using CO2 laser processing parameters of 2.4W power and 100mm / s scanning speed. Spin-coated polydimethylsiloxane (PDMS, Dow Corning Corporation, Sylgard 184 silicone rubber) served as the sensor's encapsulation layer and assisted in the non-destructive peeling of the sensor from the supporting substrate, as detailed below: (1) Spin-coat the surface of a silicon wafer with a liquid polyimide (PI, Polyimide (Suzhou) Technology Co., Ltd.) solution at a speed of 1000 r / min for 120 s. After spin coating, place the silicon wafer in an oven for heating. First, set the oven temperature to 120℃ and heat for 120 s for pre-imidization. Then, heat at 300℃ for 2 h to form an ultra-thin PI film (thickness of about 16 μm).

[0024] (2) Laser ablation was performed using a CO2 laser with a wavelength of 10.6 μm. The processing state was set to focused mode, the laser power was set to 8% (2.4 W), and the scanning speed was set to 100 mm / s. No special gas atmosphere or mask was required during the experiment. During the ablation process, the PI film absorbed sufficient energy and decomposed into LIG, but a 1-3 μm thick layer of unablated PI was retained at the bottom, making the surface of the LIG layer lower than the surface of the original PI film, thus forming a groove microstructure. The laser beam was controlled by the EZCAD control software of the SK-C30 CO2 laser to move on the PI film according to the preset pattern, and patterned LIG could be quickly prepared.

[0025] (3) Mix the PDMS prepolymer and crosslinking agent evenly. The PDMS was purchased from Dow Corning Corporation, USA, model Sylgard 184. This product is a two-component kit: component A is the prepolymer (polydimethyl-methylvinylsiloxane) and component B is the crosslinking agent (polydimethyl-methylhydrosiloxane). Mix component A and component B at a mass ratio of 10:1, stir evenly, and then degas in a vacuum chamber for 3 minutes. Then spin coat it onto a PI film with LIG at a speed of 1000 r / min for 60 s. Place it in an oven at 80°C for 1 hour to cure as the encapsulation layer of the flexible strain sensor.

[0026] (4) Lift-off the entire structure from the silicon wafer and trim off the excess area to obtain an ultrathin flexible strain sensor.

[0027] Example 2: Characterization of sensor cross-sectional morphology and structural integrity To verify the structural integrity of the ultrathin flexible strain sensor prepared in Example 1, its cross-section was observed using a scanning electron microscope (SEM). Since polyimide (PI) films are tough at room temperature, the fracture process is mainly characterized by plastic deformation, easily producing tensile and tearing marks, making it difficult to obtain a flat cross-section. In this example, the liquid nitrogen brittle fracture method was used to prepare the sample. The ultrathin flexible strain sensor was immersed in liquid nitrogen. Once the temperature of the PI film dropped below its glass transition temperature, the polymer molecular chain motion was frozen, and the material changed from tough to brittle. At this point, fracture yielded a flat cross-section with a complete morphology and clear interface, providing a reliable sample for SEM observation.

[0028] SEM images of the ultrathin flexible strain sensor prepared in Example 1 are shown below. Figure 2As shown in the figure, the sensor cross-section clearly exhibits a three-layer functional structure: the encapsulation layer, located on top of the sensor, is a uniform, dense, and continuous transparent elastomer with uniform thickness and no obvious pores, cracks, or delamination. PDMS, as a flexible encapsulation material, not only provides physical protection for the internal sensing layer and the flexible substrate but also plays a crucial role in assisting peeling during sensor fabrication. Its high elasticity and mechanical toughness provide support for the ultrathin PI film, preventing it from breaking during silicon wafer peeling. The sensing layer, located between the encapsulation layer and the flexible substrate, exhibits a typical three-dimensional porous honeycomb graphene microstructure. The LIG layer has uniform thickness, is tightly bonded to the upper and lower layers without gaps, and shows no signs of detachment, breakage, or interruption of conductive pathways. This demonstrates that the laser ablation process successfully generated a continuous and complete laser-induced graphene layer in situ on the ultrathin PI film. Its three-dimensional porous structure ensures high conductivity and enables efficient reconstruction of the conductive network under strain, making it the core functional layer for achieving high-sensitivity strain response in the sensor. The flexible substrate, located at the bottom of the sensor, is a flat, continuous, ultra-thin PI film with uniform thickness and a clear, strong interface with the LIG layer. This ultra-thin flexible substrate, prepared by spin coating and high-temperature curing, provides a stable precursor for the LIG and, due to its ultra-thin characteristics, endows the sensor with excellent conformal bonding capabilities to curved surfaces. It can adhere tightly to non-planar surfaces such as human skin, ensuring that external strain can be efficiently transmitted to the sensing layer.

[0029] The ultrathin flexible strain sensor based on laser-induced graphene prepared in this embodiment possesses excellent mechanical flexibility, enabling bending, curling, and twisting deformation at any angle, and adapting to various deformation conditions caused by human joint and limb movements. The sensor is lightweight, thin, and flexible, offering a comfortable wearing experience. Its physical properties are as follows... Figure 3 As shown in the figure, the sensor is well-formed, has a complete overall structure, and has no obvious cracks or damage on the surface, which further confirms the excellent reliability of the process scheme of the present invention and that the sensor produced has excellent molding quality and mechanical flexibility.

[0030] Example 3: Effects of laser ablation on PI thin films and verification of groove microstructure To investigate the effect of CO2 laser on polyimide (PI) films and to verify the formation of the groove microstructure between the sensing layer and the flexible substrate, this embodiment uses scanning electron microscopy (SEM) to observe and analyze the cross-section and surface morphology of the PI film before and after laser ablation.

[0031] This embodiment uses the process parameters from Example 1, employing a 10.6 μm wavelength CO2 laser with a laser power of 2.4 W and a scanning speed of 100 mm / s to ablate the spin-coated and cured PI film. The observation results are as follows: Figure 4 , 5 As shown, Figure 4 The image shows a SEM image of the PI film before ablation. As can be seen from the image, the PI film without laser ablation has a uniform overall thickness, with a measured thickness of 16 μm. Figure 5 The image shows a SEM image of the PI film after ablation. It can be seen that after laser ablation, the surface layer of the PI film is transformed into a laser-induced graphene (LIG) sensing layer, while a 2μm thick unablated PI underlayer remains as a flexible substrate. This demonstrates that the laser only achieves graphitization of the PI film surface layer, without completely burning through the film. The retained PI underlayer ensures the integrity and mechanical stability of the substrate structure.

[0032] Further morphological observation of the laser-processed area was conducted, and the principle of the laser ablation process was as follows: Figure 6 As shown. Under preset process parameters, the PI film in the laser-acted area undergoes pyrolysis and transforms into LIG, reducing the overall film thickness and thus forming a groove microstructure with a certain depth between the sensing layer and the underlying PI substrate. Figure 7 The SEM images of the microstructure of the PI film grooves show that the surface of the film in the laser-ablated area is rough and the thickness is significantly reduced; the surface of the area not affected by the laser is flat and the thickness remains unchanged. The two areas form a clear height difference, which directly confirms the successful preparation of the microstructure of the grooves on the surface of the PI film after laser ablation.

[0033] This grooved microstructure further enhances the sensor's deformation response capability. When the sensor is subjected to external strains such as bending and stretching, the grooved microstructure provides deformation space for the displacement of the LIG conductive strips, strengthening the reconstruction effect of the conductive network and synergistically improving the sensor's strain sensitivity. Simultaneously, this structure is integrally formed based on laser in-situ processing, eliminating the need for additional ablation and assembly, simplifying the fabrication process and ensuring the overall structural stability of the device.

[0034] Example 4: Sensor Sensing Principle and Strain Detection Principle After the fabricated ultrathin flexible strain sensor was non-destructively peeled from the silicon substrate, its sensing performance was tested. To facilitate the analysis of the test results, this embodiment explains the sensor's sensing principle and strain detection principle. Since the strain tolerance range of polyimide (PI) films is limited and direct stretching easily leads to breakage, this experiment uses a bending test to evaluate the sensor's sensing performance.

[0035] The sensing principle of the sensor under bending conditions is as follows: Figure 8 As shown, the sensing element of this sensor consists of multiple LIG conductive strips. In the initial state without strain, adjacent LIG strips maintain a stable spacing W, and the strip edges fully contact and overlap, forming a continuous conductive path. At this time, the sensor exhibits a stable initial resistance. Electrons can be efficiently transferred between adjacent strips, the overall conductive network is complete, and the device outputs a stable basic electrical signal.

[0036] When the sensor is subjected to external bending strain, the flexible substrate and sensing layer deform together, causing displacement of the patterned LIG strips. Under stress, adjacent LIG strips separate, and the strip spacing increases from the initial W to W + ΔW (ΔW is the increment of the strip spacing). This microstructural change directly leads to a reduction in the contact area and an increase in the contact resistance between adjacent LIG strips. Simultaneously, some conductive pathways break due to strip separation, resulting in a significant decrease in the overall conductive network's transmission efficiency. Macroscopically, this manifests as a significant increase in the overall sensor resistance, and the rate of resistance change is positively correlated with the magnitude of the external strain; that is, the greater the strain, the more pronounced the strip separation, and the larger ΔW, the greater the resistance change. Therefore, we can reflect the magnitude of external strain by measuring the resistance change of the LIG.

[0037] In the bending experiment, the sensor is attached to the center of the support substrate surface. Different degrees of bending of the support substrate cause the sensor to experience different strains. The detection principle is as follows: Figure 9 In the diagram, the yellow dashed line represents the neutral layer of the sensor and the supporting substrate. r is the bending radius of the supporting substrate after bending, in mm. s is the bending arc length of the supporting substrate, in mm. d is the bending chord length of the supporting substrate, in mm. The geometric relationship between these three is as follows:

[0038] In the formula, both the bending chord length d and the bending arc length s can be directly measured. Substituting the measured d and s into the above formula, the bending radius r can be obtained after calculation.

[0039] According to the neutral layer strain theory, the strain of the sensor under bending conditions can be calculated by the following formula:

[0040] In the formula, ε is the bending strain of the sensor, Z is the total thickness of the sensor and the supporting substrate (400 μm in this experiment), and r is the bending radius of the supporting substrate. With a fixed total thickness Z, the smaller the bending radius r, the greater the bending strain experienced by the sensor; this trend is consistent with actual deformation characteristics.

[0041] In the sensor performance study, based on the above detection principle, a bending experiment was conducted to control the bending chord length d of the support substrate to produce different bending strains in the sensor. At the same time, a digital source meter was used to collect the resistance data of the LIG in real time to analyze the sensor's sensing performance.

[0042] Example 5: Sensor Sensitivity and Dynamic Response Performance Test This embodiment, based on the sensing and detection principle provided in Embodiment 4, explores the influence of laser-induced graphene scanning direction on sensor sensitivity, and tests the dynamic response characteristics of the device under stepped strain and different loading frequencies, clarifying the optimal structural design and dynamic working performance of the sensor.

[0043] 5.1 Sensitivity Comparison Test A CO2 laser with a wavelength of 10.6 μm was used, with a laser power of 2.4 W and a scanning speed of 100 mm / s. Two sets of sensor samples were prepared, one with the LIG scanning direction parallel to the strain direction and the other with the LIG scanning direction perpendicular to the strain direction, with three parallel samples in each set. The encapsulation layer was prepared and cured according to a 10:1 mass ratio of prepolymer to crosslinking agent, and all samples maintained the same thickness and dimensions. The two sets of sensors were respectively glued and fixed to the center of the support substrate, ensuring a bubble-free and slip-free fit. A universal tensile and compressive testing machine was used to perform graded bending tests on the support substrate, and the bending strain ε was calculated using the geometric relationship of the arc and the neutral layer strain formula. The initial resistance R0 of the sensor under no strain state and the real-time resistance under each strain state were recorded using a digital source meter, and the resistance change ΔR was calculated. Then, the sensitivity formula was applied... To calculate the sensitivity, the average value was taken after multiple tests for each group of samples.

[0044] Test results are as follows Figure 10 As shown in the figure, the horizontal axis represents the bending strain ε experienced by the sensor, and the vertical axis represents the LIG resistance change rate (ΔR / R0). The two curves represent the sensing responses corresponding to the LIG scanning directions perpendicular and parallel to the strain direction, respectively. The slope of the curves represents the sensor sensitivity. When the LIG scanning direction is parallel to the sensor strain direction, the average sensor sensitivity is 58.64; when the LIG scanning direction is perpendicular to the sensor strain direction, the average sensor sensitivity can reach 140.73, showing a significant improvement in sensing performance.

[0045] The core mechanism behind this difference is that LIG is composed of multiple conductive strips. When the strip arrangement direction is perpendicular to the strain direction, external bending and tensile deformation will directly force adjacent conductive strips to separate laterally. The strip spacing increases continuously, the contact area between them decreases rapidly, a large number of conductive paths are broken, and the contact resistance rises sharply. Ultimately, this manifests as a high sensitivity of the resistance change rate to strain. However, when the scanning direction is parallel to the strain direction, the deformation is only transmitted along the axial direction of the conductive strips. There is almost no relative displacement between the strips, the conductive network structure remains basically stable, the resistance change is minimal, and the sensor sensitivity is poor.

[0046] Based on experimental results, it was determined that the laser scanning direction of this invention is perpendicular to the strain direction during actual use of the sensor, which can maximize the strain response characteristics of the LIG conductive network and prepare a high-sensitivity sensor.

[0047] 5.2 Stepped Strain Dynamic Response Test To evaluate the dynamic response characteristics of the sensor under different external strains, a stepped strain loading was applied to the LIG flexible strain sensor. A high-sensitivity sensor with the LIG scanning direction perpendicular to the strain direction was selected for testing. The support substrate carrying the sensor was fixed between the upper and lower clamps of a universal tensile and compressive testing machine, with the initial clamp spacing set to a fixed value L. The upper clamp was controlled to move downwards step by step, with the movement amount ΔL set successively to 5mm, 10mm, 15mm, 20mm, 25mm, and 30mm, applying a continuously increasing stepped bending strain to the sensor. The resistance signal was continuously acquired using a digital source meter throughout the test, and the change curve of the resistance rate over the test time was recorded.

[0048] The results are as follows Figure 11 As shown in the figure, the horizontal axis represents the test time (Time), and the vertical axis represents the sensor resistance change rate (ΔR / R0). As ΔL gradually increases from 5 mm to 30 mm, the bending strain of the sensor increases stepwise, and the resistance change rate shows a synchronous and stable upward trend. Furthermore, the response signals corresponding to different bending strains exhibit clearly distinguishable amplitude differences. Under small bending strains, the sensor outputs a weak but stable resistance change signal; as the bending strain increases, the resistance change rate increases rapidly, and the response signal waveforms under each bending strain are complete, without significant distortion or hysteresis. This indicates that the sensor prepared in this invention possesses excellent resolution and linear response capabilities to gradient strain changes.

[0049] 5.3 Cyclic Strain Testing at Different Frequencies To further verify the signal stability of the sensor at different operating frequencies, the clamp downward displacement ΔL = 10 mm was kept constant, and the strain amplitude was fixed. Cyclic bending experiments were conducted with cyclic bending frequencies of 0.2 Hz, 0.5 Hz, and 1 Hz. The sensor's resistance response signal at different frequencies was acquired in real time using a digital source meter, and the signal waveform, amplitude, and stability were analyzed. The results are as follows: Figure 12As shown in the figure, the horizontal axis represents the test time (Time), and the vertical axis represents the sensor resistance change rate (ΔR / R0). The three curves correspond to strain loading frequencies of 0.2Hz, 0.5Hz, and 1Hz, respectively. Under periodic bending strain at different frequencies, the sensor can output signals with complete waveforms, clear peaks and valleys, and good repeatability. The response signal fluctuates rapidly and synchronously with the bending strain, without significant distortion or attenuation. As the strain frequency increases from 0.2Hz to 1Hz, the sensor maintains a stable and consistent response amplitude, without showing obvious frequency dependence. This indicates that the sensor is suitable for low-to-medium frequency dynamic deformation scenarios and has excellent dynamic tracking capabilities over a wide frequency range. It can meet the dynamic monitoring needs of reciprocating movements of human limbs, laying the foundation for its application in dynamic motion monitoring scenarios.

[0050] In summary, the key to improving sensor sensitivity is that the LIG scanning direction is perpendicular to the strain direction. This sensor can not only identify static strain with gradient changes, but also output signals stably under dynamic cyclic strain at different frequencies. It has excellent overall dynamic response performance and is suitable for applications such as wearable devices and human motion monitoring.

[0051] Example 6: Sensor Response Time and Recovery Time Test Response time refers to the time required for a sensor's output signal to change from an initial steady state and reach a final stable value after being subjected to external strain. Recovery time refers to the time required for the sensor's output signal to recover from its changed stable value to near its initial value and finally stabilize after the external strain is removed. In practical wearable applications, physiological signals such as human joint movements and voice vibrations are dynamically changing strain signals, requiring sensors with millisecond-level response / recovery speeds to achieve high-fidelity acquisition of dynamic strain signals.

[0052] The sensor was fixed flat to the surface of the support substrate. Under a fixed strain magnitude (ΔL=2mm), the sensor resistance change rate was collected in real time using a source meter. To ensure testing accuracy, the fixture's moving speed was set to the maximum (1000mm / min), and the source meter's sampling frequency was set to the maximum (10000Hz). The results are as follows: Figure 13As shown in the figure, the horizontal axis represents the test time (Time), and the vertical axis represents the sensor resistance change rate (ΔR / R0). The curve exhibits a typical pulse-like response characteristic of rapid rise and rapid fall. When bending strain is applied, the sensor electrical signal rises rapidly and reaches a stable response value; when the strain is removed, the signal quickly falls back to its initial state. Statistically, the sensor response time is 0.38s, and the recovery time is 0.52s, reaching the millisecond-level response standard. This excellent transient response characteristic is attributed to the LIG three-dimensional porous honeycomb structure and the substrate groove microstructure. During deformation, the porous graphene network can rapidly undergo structural reconstruction; after the strain is unloaded, under the elastic action of the PI substrate and encapsulation layer, the conductive network can quickly rebound and reset, thus the sensor has the ability to respond and recover quickly. This result shows that the sensor has a high response capability to external strain and can track rapid changes in external deformation in real time, meeting the needs of wearable sensing applications with high response speed requirements, such as micro-movements of the human body and real-time gesture detection.

[0053] Example 7: Sensor Cyclic Stability Test In practical applications, the stability of the electrical performance of flexible strain sensors after prolonged use becomes a key performance parameter. To verify the mechanical durability and electrical stability of the ultrathin flexible strain sensor prepared in this invention under long-term repeated use, the sensor underwent 10,000 consecutive cyclic bending tests, and the sensor resistance change was monitored in real time. The results are as follows: Figure 14 As shown in the figure, the horizontal axis represents the test time (Time), and the vertical axis represents the sensor resistance change rate (ΔR / R0). The main curve is the overall response curve of the sensor during 10,000 bending cycles. The embedded local magnified waveforms above correspond to the local magnified response waveforms in the early stage (approximately 10,000 s), middle stage (approximately 40,000 s), and late stage (approximately 70,000 s) of the cycle, respectively. Under 10,000 bending-recovery cyclic loading, the sensor output signal amplitude is stable, the waveform is complete, and there is no significant attenuation. The response peaks and valleys remain highly consistent, and the signal baseline shows no significant drift. The local response curves in the early, middle, and late stages of the cycle all exhibit the same response amplitude, rising edge, and falling edge characteristics, indicating that the sensor performs stably and reliably under long-term use.

[0054] After the experiment, the sample was disassembled for observation. The encapsulation layer, sensing layer, and flexible substrate were tightly bonded, with an intact appearance and no issues such as cracking, warping, interlayer separation, or graphene shedding. On the one hand, the encapsulation layer possesses excellent elasticity and toughness, which can buffer the mechanical stress generated by cyclic deformation and effectively protect the internal functional layers. On the other hand, the LIG three-dimensional porous structure exhibits outstanding fatigue resistance, and the conductive network will not suffer permanent damage under repeated deformation. Together, these two factors ensure the stability of the sensor during long-term operation.

[0055] Example 8: Characterization of Laser-Induced Graphene (LIG) Materials In this embodiment, laser-induced graphene (LIG) was prepared using a 2.4W laser power and a scanning speed of 100mm / s. The microstructure, crystal structure, chemical composition, functional groups and graphitization degree of LIG were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

[0056] 8.1 Microscopic morphology characterization (SEM) Pure LIG regions on the sensor surface were selected as test samples, and SEM observations were performed in both low-magnification and high-magnification modes. Figure 15 The images show SEM images of the surface morphology of LIG at different magnifications. In Figure A, the surface morphology of LIG at low magnification is shown, where laser-induced graphene sheets are wrinkled and stacked, forming a continuous interconnected network structure. Figure B shows the surface morphology of LIG at high magnification, where the LIG forms a three-dimensional porous honeycomb-like interconnected network structure with uniform pore distribution and consistent pore size. On one hand, the porous network effectively disperses stress and reserves deformation buffer space, giving LIG excellent flexibility and deformation resistance. On the other hand, the overlapping graphene sheets construct complete and continuous conductive pathways, ensuring high conductivity of the material and providing structural support for the sensor to achieve high sensitivity and long-term stable operation.

[0057] 8.2 Crystal structure characterization (XRD) X-ray diffraction was used to compare the crystal structures of the original polyimide (PI) film and LIG to verify the graphitization transformation effect of PI after laser ablation. The test spectra are shown below. Figure 16 As shown, the original PI film exhibits broadened and diffuse diffraction peaks in the 2θ = 10°–30° range, with the peak center located at 2θ ≈ 19°, which is typical of the diffraction characteristics of amorphous polymers, proving that PI has an amorphous structure. The LIG obtained after laser-induced treatment shows a sharp and high-intensity diffraction peak at 2θ ≈ 26.5°, corresponding to the characteristic diffraction of the graphene C(002) crystal plane, confirming that PI has been successfully transformed into a well-crystallized graphene material. The full width at half maximum (FWHM) of this diffraction peak is only 1.5°, indicating that the LIG has a regular internal crystal arrangement, a high degree of graphitization, and a complete sp² conjugated carbon crystal structure, which is the crystallographic basis for the material's excellent electrical conductivity.

[0058] 8.3 Chemical composition and functional group characterization (XPS) X-ray photoelectron spectroscopy was used to analyze the changes in elemental composition and chemical bonds during the conversion of PI to LIG, and the mechanism of laser pyrolysis was explored.

[0059] XPS full spectrum analysis: XPS full spectrum of PI thin film and LIG as follows Figure 17 As shown, the original PI film exhibits C1s, N1s, and O1s characteristic peaks at binding energies of 284.6 eV, 400.0 eV, and 532.0 eV, respectively, corresponding to the carbon, nitrogen, and oxygen elements in the PI molecule, consistent with the chemical composition of polyimide. After laser-induced graphitization, the N1s characteristic peak in the LIG spectrum completely disappeared, and the intensity of the O1s characteristic peak decreased significantly, indicating that under the high-temperature pyrolysis of laser, the nitrogen and oxygen elements in the PI molecule were removed in gaseous form; while the intensity of the C1s peak remained basically stable, indicating that a large amount of carbon was retained.

[0060] C1s analysis: Peak fitting was performed on the C1s characteristic peaks, and the results are as follows: Figure 18 As shown, A is the C1s spectrum of PI, and B is the C1s spectrum of LIG. The comparison reveals that the CO, CN, and C=O bonds in PI break during laser pyrolysis, while the stable C=C bonds are preserved. LIG is mainly composed of sp² hybrid carbon, with a small amount of residual oxygen, primarily due to incomplete pyrolysis or slight oxidation of the graphene surface.

[0061] 8.4 Graphitization degree and defect characterization Raman spectroscopy was used to analyze the carbon lattice structure, defect density, and stacking state of LIG. The test spectra are shown below. Figure 19 As shown. Graphene materials exhibit three typical characteristic peaks: the D peak (1350 cm⁻¹). -1 The peak G (1580 cm⁻¹) reflects material defects and the degree of carbon bond bending. -1 The 2D peak corresponds to the in-plane stretching vibration of sp² hybridized carbon and is used to characterize the level of graphitization; -1 The number of graphene layers and their stacking configuration can be determined. The sample spectrum simultaneously exhibits D, G, and 2D peaks, consistent with standard Raman characteristics of graphene, further confirming the phase structure of LIG. Peak intensity ratio I D / I G The ratio can measure the defect density of carbon materials; the lower the ratio, the higher the degree of graphitization and the fewer the defects. Peak intensity ratio I 2D / I G The ratio can determine the number of graphene layers; the higher the ratio, the fewer the layers. Quantitative analysis of the peak intensity yields the following results: I D / I G The value is approximately 0.85, which is relatively low, indicating that LIG has a low defect density and a high degree of graphitization. I 2D / IG The ratio is approximately 0.45, and combined with the broadening characteristics of the 2D peak, LIG ​​is determined to be a multilayer graphene stacked structure. Raman characterization results further verify the high-quality graphene structure of LIG at the molecular vibrational level.

Claims

1. An ultrathin flexible strain sensor based on laser-induced graphene, characterized in that, include: Flexible substrate: The flexible substrate is a spin-coated polyimide film; Sensing layer: Located on the flexible substrate, the sensing layer is a laser-induced graphene layer formed by CO2 laser ablation of the surface of the polyimide film; the laser-induced graphene layer is formed in situ on the upper surface of the flexible substrate and is an integral structure with the flexible substrate; Encapsulation layer: Located above the sensing layer, the encapsulation layer is made of polydimethylsiloxane, covering the outside of the sensing layer and bonded to the edge of the flexible substrate; The laser-induced graphene layer has an unablated polyimide underlayer, making the surface of the laser-induced graphene layer lower than the original surface of the flexible substrate, thereby forming a grooved microstructure.

2. The ultrathin flexible strain sensor based on laser-induced graphene according to claim 1, characterized in that, The laser-induced graphene layer has a three-dimensional porous honeycomb structure; the sensing layer is composed of multiple laser-induced graphene conductive strips; the arrangement direction of the laser-induced graphene conductive strips is perpendicular to the strain direction of the sensor.

3. The ultrathin flexible strain sensor based on laser-induced graphene according to claim 1, characterized in that, The thickness of the flexible substrate is 10-20 μm; the thickness of the unablated polyimide underlayer is 1-3 μm.

4. The ultrathin flexible strain sensor based on laser-induced graphene according to claim 1, characterized in that, The encapsulation layer is formed by curing a mixture of polydimethylsiloxane prepolymer and crosslinking agent at a mass ratio of 10:0.5-1.

5.

5. A method for preparing the sensor according to claim 4, characterized in that, Includes the following steps: (1) A polyimide film is prepared by spin-coating a polyimide solution onto a support substrate and then heated and cured to form a flexible substrate; (2) The polyimide film is ablated by CO2 laser to transform the surface of the polyimide film into a laser-induced graphene layer, i.e., a sensing layer; the bottom of the laser-induced graphene layer retains a 1-3 μm unablated polyimide underlayer, so that the surface of the laser-induced graphene layer is lower than the original surface of the flexible substrate, thereby forming a groove microstructure. (3) Mix the polydimethylsiloxane prepolymer and crosslinking agent at a mass ratio of 10:0.5-1.5, stir evenly, spin coat it onto the outside of the sensing layer and the edge of the flexible substrate, and then cure it to form an encapsulation layer; (4) The overall structure is peeled off from the support substrate to obtain an ultrathin flexible strain sensor.

6. The method according to claim 5, characterized in that, In step (1), the supporting substrate is a silicon wafer; the spin coating speed is 800-1200 r / min and the time is 100-140 s; the heat curing includes pre-imidization at 100-140℃ for 100-140 s, and then curing at 250-350℃ for 1.5-2.5 h to form a polyimide film with a thickness of 10-20 μm.

7. The method according to claim 5, characterized in that, In step (2), the wavelength of the CO2 laser is 10.6 μm, the laser power is 2.4 W, and the scanning speed is 100 mm / s.

8. The method according to claim 5, characterized in that, In step (2), when the CO2 laser ablates the polyimide film, the laser beam is controlled to move on the polyimide film according to a preset pattern to prepare patterned laser-induced graphene; the scanning direction of the laser beam is perpendicular to the direction of the strain experienced by the sensor in use.

9. The method according to claim 5, characterized in that, In step (3), the spin coating speed is 800-1200 r / min, the time is 50-70 s, the curing temperature is 70-90℃, and the curing time is 0.8-1.2 h.

10. The method according to claim 5, characterized in that, In step (4), the entire structure is removed from the supporting substrate silicon wafer by non-destructive peeling.

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