A crack-based flexible strain sensor and its fabrication method

Abstract

This invention discloses a crack-based flexible strain sensor and its fabrication method, belonging to the field of strain sensor technology. From bottom to top, it comprises a flexible substrate, a sensitive layer, and a flexible encapsulation layer. On the surface of the flexible substrate near the sensitive layer, multiple morphological regions are sequentially formed along the direction perpendicular to the stretching direction of the flexible substrate. These multiple morphological regions form various microstructures with different roughnesses and thicknesses. Along the stretching direction of the flexible substrate, the thickness of the sensitive layer gradually increases from the middle to both ends. The flexible strain sensor of this invention exhibits a sensitivity of 256.92 within a 150% strain range, with a linear fitting degree as high as 0.983, indicating that the flexible strain sensor of this invention achieves synergistic optimization of sensitivity and linear strain range.

Description

Technical Field

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

[0002] As a crucial core component in wearable electronic devices, flexible strain sensors are fundamental for sensing the external environment, information interaction, and human-machine collaboration, and have broad application prospects in fields such as electronic skin, health monitoring, and human-computer interaction. Crack-based strain sensors have attracted widespread attention from researchers due to their ultra-high sensitivity.

[0003] However, due to the uncontrollable cracking behavior of the sensing material, crack-based strain sensors often exhibit varying sensitivities across different sensing ranges. This nonlinear resistance-strain response necessitates additional signal calibration processing during practical applications, increasing the sensor's program complexity and leading to high energy consumption. Furthermore, widening the linear detection range of a strain sensor often reduces its sensitivity; sensitivity and linear detection range exhibit an inverse relationship. All of these factors hinder the practical application of crack-based strain sensors. Therefore, how to control the cracking behavior of metal thin films to achieve a synergy between the sensitivity and linear detection range of strain sensors remains a significant challenge. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a crack-based flexible strain sensor and its fabrication method. The invention employs a masking method to prepare a metal thin film with a thickness gradient along the sample length direction on the surface of a substrate with different microstructure morphology characteristics, thereby achieving synergistic optimization of sensitivity and linear strain range.

[0005] This invention is achieved through the following technical solution:

[0006] A crack-based flexible strain sensor comprises, from bottom to top, a flexible substrate, a sensing layer, and a flexible encapsulation layer;

[0007] The flexible substrate near the surface of the sensitive layer has multiple morphological regions formed sequentially on the surface along the direction perpendicular to the stretching direction of the flexible substrate. These multiple morphological regions form various tissue morphologies with different roughness and different thicknesses corresponding to the morphological regions.

[0008] The thickness of the sensitive layer gradually increases from the middle to both ends along the stretching direction of the flexible substrate.

[0009] Preferably, the flexible substrate has a second morphology region in the middle, and a first morphology region and a third morphology region on both sides respectively;

[0010] The surface roughness (R) of the first morphological region, the second morphological region, and the third morphological region a (Increase sequentially.)

[0011] Preferably, the thickness of the surfaces of the first morphological region, the second morphological region, and the third morphological region decreases sequentially.

[0012] The thickness difference between the first and second morphological regions is 40-50 μm, and the thickness difference between the first and third morphological regions is 110-120 μm.

[0013] Preferably, the roughness R of the first morphological region a The roughness R of the second morphology region is 3.0-3.5 μm. a The roughness R of the third morphology region is 3.0-3.5 μm. a Less than 0.25μm.

[0014] Preferably, the sensitive layer is a metal thin film.

[0015] Preferably, electrodes are provided at both ends of the sensitive layer.

[0016] Preferably, the materials of the flexible substrate and the flexible encapsulation layer are any one of polydimethylsiloxane, thermoplastic elastomer, and organic platinum silicone.

[0017] A method for fabricating a crack-based flexible strain sensor includes the following steps:

[0018] Step 1: Separately splice multiple templates along the non-stretching direction of the flexible substrate to form a flexible substrate template, with different surface roughness between adjacent templates;

[0019] Step 2: Coat the surface of the flexible substrate template with a flexible substrate material, and obtain a flexible substrate after curing;

[0020] Step 3: Using magnetron sputtering, a sensitive layer is deposited on the surface of the flexible substrate, and the thickness of the sensitive layer gradually increases from the middle to both ends along the stretching direction of the flexible substrate.

[0021] Step 4: Prepare a flexible encapsulation layer on the surface of the sensitive layer to obtain a crack-based flexible strain sensor.

[0022] Preferably, in step 3, a mask plate is first covered on the surface of the flexible substrate along the arrangement direction of the template, and then a sensitive layer is formed by DC magnetron sputtering.

[0023] A wearable electronic device, including the aforementioned crack-based flexible strain sensor.

[0024] Compared with the prior art, the present invention has the following beneficial technical effects:

[0025] This application discloses a crack-based flexible strain sensor. A template method is used to construct a surface morphology on the surface of a flexible substrate, consisting of alternating microstructures arranged along the width direction. A mask method is then used to prepare a metal thin film with a thickness gradient along the sample length on the microstructured substrate surface, achieving synergistic optimization of sensitivity and linear strain range. The surface morphology along the sample width direction effectively controls the stress / strain distribution of the thin film deformation, promoting the formation of a mixed crack morphology composed of locally penetrating cracks in a third morphology region and network cracks in the first and second morphology regions. The thickness difference between different morphology regions along the sample width direction induces the third morphology region to experience minimal local stress / strain concentration, becoming a strain isolation zone, while the first and second morphology regions experience greater local stress / strain concentration. The combined effect of surface morphology and thickness difference prevents the formation of continuous penetrating cracks along the sample width direction, making crack propagation in each region more controllable and dispersed, thereby achieving a wide linear detection range. The thin film thickness gradient along the length direction causes a gradient stress distribution on the thin film surface, inducing multi-stage fracture of the metal thin film and achieving high sensitivity. This flexible strain sensor achieves synergistic optimization of sensitivity and linear strain range by effectively coupling crack control strategies and the inherent properties of polymer elastic materials. The flexible strain sensor of this invention exhibits a sensitivity of 256.92 within a 150% strain range, with a linear fit as high as 0.983, demonstrating that the flexible strain sensor of this invention achieves synergistic optimization of sensitivity and linear strain range.

[0026] Furthermore, the sensitive layer of the flexible strain sensor of the present invention is made of a highly conductive metal thin film. The thickness gradient design induces changes in the fracture toughness of the metal thin film, thereby inducing the metal thin film to exhibit multi-stage fracture behavior, thus further improving the sensor sensitivity. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the structural composition of a crack-based flexible strain sensor according to Embodiment 1 of the present invention;

[0029] Figure 2 Electron micrographs of the surface morphology of the three template materials used in Embodiment 1 of the present invention;

[0030] Figure 3This is an electron microscope image of the surface morphology of the flexible strain sensor described in Embodiment 1 of the present invention along its width direction;

[0031] Figure 4 The image shows the grain size transmission pattern and statistical results of the flexible strain sensor described in Embodiment 1 of the present invention along its length.

[0032] Figure 5 This is an electron microscope image of the microcrack morphology formed by the flexible strain sensor described in Embodiment 1 of the present invention under 60% strain.

[0033] Figure 6 The graph shows the sensitivity and linear strain range performance of the flexible strain sensor described in Embodiment 1 of the present invention under tensile conditions. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0035] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0036] A crack-based flexible strain sensor comprises, from bottom to top, a flexible substrate, a sensitive layer, an electrode, and a flexible encapsulation layer;

[0037] The flexible substrate has multiple morphological regions formed sequentially on its surface near the sensitive layer, along the direction perpendicular to the stretching direction of the flexible substrate. These multiple morphological regions form various tissue morphologies, each with different roughness and different thickness.

[0038] Along the stretching direction of the flexible substrate, the thickness of the sensitive layer gradually increases from the middle to both ends.

[0039] Structurally, the flexible strain sensor of this invention combines surface morphology features and a thickness gradient structure that can effectively modulate the cracking behavior of thin films. The surface morphology features along the width direction are prepared using a template method; the template can be reused, thus enabling repeatable and uniform preparation of the surface morphology. Under tension, the alternating distribution of different morphology regions along the sample width direction induces a mixed crack morphology in the metal thin film. The height difference between different regions generates an effective strain modulation effect, inducing controlled cracking of the metal thin film to achieve a wide linear strain range.

[0040] In this application, roughness is used to characterize the microstructure of the flexible substrate surface, that is, each microstructure has a different roughness, and the roughness of multiple microstructures gradually increases.

[0041] It should be noted that the stretching direction of the flexible substrate is the length direction, and the direction perpendicular to the stretching direction is the width direction of the flexible substrate. Multiple morphological regions are sequentially formed on the surface along the direction perpendicular to the stretching direction of the flexible substrate, that is, multiple morphological regions are sequentially formed along the width direction of the flexible substrate.

[0042] It should be noted again that the number of multiple morphological regions is greater than or equal to the number of tissue morphologies, that is, there are multiple identical morphological regions. For example, eight morphological regions are formed on the surface of a flexible substrate. These eight morphological regions exhibit three different tissue morphologies, that is, three different roughness surfaces. It can also be understood that three morphological regions are the first tissue morphology, three morphological regions are the second tissue morphology, and two morphological regions are the third tissue morphology.

[0043] The thickness of the surfaces of the first morphological region, the second morphological region, and the third morphological region decreases sequentially; the thickness difference between the first morphological region and the second morphological region is 40-50 μm, and the thickness difference between the first morphological region and the third morphological region is 110-120 μm.

[0044] R of the first topographic region a The roughness R of the second morphology region is 3.0-3.5 μm. a The roughness R of the third morphology region is 3.0-3.5 μm. a Less than 0.25μm.

[0045] The tissue morphology of two adjacent morphological regions is different.

[0046] The flexible substrate is prepared by molding, that is, the flexible substrate is prepared on the surface of the template, and regions with different roughness are formed on the surface of the template, and the thickness of each region is different.

[0047] In this application, three templates with different roughnesses are spliced ​​together to form an integral template. The first template is a flat template with a completely smooth surface and a plastic film with a thickness of 80 μm. The second template is a rough template with a rougher microstructure and commercial sandpaper with a thickness of 195 μm. The third template is a microstructure template and commercial sandpaper with a thickness of 130 μm.

[0048] See Figure 1 The flexible substrate of the crack-based flexible strain sensor has five morphological regions along its width direction. The middle region is the second morphological region with a second tissue morphology on its surface. The two sides of the middle morphological region are the first morphological regions with a first tissue morphology on their surfaces. The two sides of the flexible substrate are the third morphological regions with a third tissue morphology on their surfaces.

[0049] The roughness of the first, second, and third tissue morphologies increases sequentially.

[0050] The thickness of the first, second, and third morphological regions decreases sequentially, and a height difference is formed between adjacent morphological regions.

[0051] The flexible matrix is ​​made of any one of polydimethylsiloxane (PDMS), thermoplastic elastomers (styrene-butadiene-styrene block copolymer SBS, styrene-ethylene-butene-styrene block copolymer SEBS, thermoplastic polyurethane elastomer TPU), and organic platinum silicone (Ecoflex).

[0052] The flexible encapsulation layer is made of any one of polydimethylsiloxane (PDMS), thermoplastic elastomers (styrene-butadiene-styrene block copolymer SBS, styrene-ethylene-butene-styrene block copolymer SEBS, thermoplastic polyurethane elastomer TPU), and organic platinum silicone (Ecoflex).

[0053] The sensitive layer is made of a thickness gradient metal film with high conductivity. The thickness gradually increases along the stretching direction of the flexible substrate, that is, the thickness in the middle region of the sensitive layer is thin, and it gradually thickens towards both ends of the stretching direction.

[0054] The metal thin film can be made of gold, silver, copper, or any metal with conductive properties, with silver being the preferred material. The sensitive layer is prepared using a masking method and a magnetron sputtering method.

[0055] Electrodes are provided at both ends of the sensitive layer, and the electrodes are closely attached to the surface of the sensitive layer.

[0056] The flexible encapsulation layer is tightly bonded to the surfaces of the sensitive layer and the electrode material. The flexible encapsulation layer is prepared using a spin-coating method.

[0057] Correspondingly, this application also provides a method for fabricating the above-mentioned crack-based flexible strain sensor, including the following steps:

[0058] Step 1: Assemble multiple templates sequentially along the non-stretching direction of the flexible substrate, ensuring that the surface roughness of adjacent templates is different.

[0059] In this embodiment, three templates are used: the first template is a flat template, the second template is a rough template, and the third template is a microstructure template. Then, each template is a rectangular strip with a size of 50mm×2mm, which is ultrasonically cleaned in anhydrous ethanol for 15 minutes. After cleaning, it is dried with a hair dryer.

[0060] During splicing, the rough template, flat template, microstructure template, flat template, and rough template are arranged closely in sequence along the width direction of the flexible substrate to form a 50mm×10mm flexible substrate template.

[0061] For example, the completely flat plastic film in the template material was purchased from 3M and the different grades of commercial sandpaper with a certain roughness and thickness were purchased from Warrior GmbH, Germany (brands P800 and P2500, respectively).

[0062] Step 2: The liquid material of the degassed flexible substrate is uniformly coated onto the surface of the flexible substrate template. After vacuum standing, curing and peeling, a flexible substrate with surface topology is obtained.

[0063] The defoaming treatment method is as follows:

[0064] The liquid material of the flexible matrix, which is fully and uniformly mixed according to the formula, is placed in a vacuum drying oven for degassing treatment for 2 hours.

[0065] Step 3: Using magnetron sputtering, a sensitive layer is deposited on the surface of the flexible substrate, and the thickness of the sensitive layer gradually increases from the middle to both ends along the stretching direction of the flexible substrate.

[0066] The flexible substrate prepared in step 2 above is placed in a magnetron sputtering instrument, and a mask of a specific size is covered along the width direction of the flexible substrate surface. Then, a metal thin film is prepared by DC magnetron sputtering to obtain a heterostructure metal thin film composed of the surface morphology features of the substrate in the width direction and the thickness gradient structure in the length direction.

[0067] During magnetron sputtering, the pre-deposition chamber pressure is 2.4 × 10⁻⁶. -4 Pa. The sputtering power of the metal target is 120W, the argon gas pressure during deposition is 0.5Pa, and the fixed disk does not rotate.

[0068] Step 4: Use conductive silver paste to fix the metallic silver electrodes to both ends of the sensitive layer to complete the circuit connection.

[0069] Step 5: Spin-coating a 50 μm thick polymer layer onto the surface of the material prepared above using a spin-coating method. The spin-coating speed is 1000 rpm and the spin-coating time is 30 seconds. Once curing is complete, the strain sensor can be successfully fabricated.

[0070] Example 1

[0071] A method for fabricating a crack-based flexible strain sensor includes the following steps:

[0072] Step 1: Cut the three types of template materials into 50mm × 2mm rectangular strips using a cutter. Next, clean the cut base templates in an ultrasonic cleaner with anhydrous ethanol solution for 15 minutes. After cleaning, dry them with a hair dryer.

[0073] Along the width of the cut wooden board, arrange the templates in sequence as rough template - flat template - microstructure template - flat template - rough template to form a 50mm×10mm flexible base template.

[0074] Step 2: Mix the prepolymer of PDMS and the curing agent uniformly at a mass ratio of 10:1, and place them in a vacuum drying oven for 2 hours for degassing. Then, pour the PDMS mixture onto the ideal template surface and let it stand for 4 hours to fully replicate the microstructure of the template surface.

[0075] After curing liquid PDMS at 80 degrees Celsius for 2 hours, it was carefully peeled off from the template to form a solid PDMS sheet (500 μm thick). This successfully prepared a PDMS matrix with template antimicrostructure.

[0076] Due to the varying thicknesses of the templates used, there are corresponding height differences between different regions of the prepared PDMS surface. Therefore, the substrate surface topology has two characteristics: (1) alternating regions with different surface morphologies and (2) height differences between different regions, which allows for effective strain modulation effects to influence the cracking behavior of the metal thin film.

[0077] Step 3: Clean the prepared PDMS substrate and mask with anhydrous ethanol using ultrasonic cleaning for 15 minutes, and then dry them with a hair dryer after cleaning.

[0078] The cleaned PDMS substrate was covered along its width using a custom-designed mask to prepare a thickness-gradient metal film along the sample's length. Prior to deposition, the chamber pressure was 2.4 × 10⁻⁶. -4The deposition power was 120 W, the argon pressure was 0.5 Pa, and the disk was fixed without rotation. After deposition, Ag films with thickness gradients along the length direction were successfully prepared on the PDMS substrate with surface morphology characteristics. The thinnest region of the film was 100 nm thick, and the thickest region was 500 nm thick.

[0079] The mask used to prepare thickness gradient metal thin films is an aluminum sheet with a certain arch height whose size can be adjusted as needed.

[0080] Step 4: Use conductive silver paste to fix the metallic silver electrodes to both ends of the heterogeneous metal thin film to complete the circuit connection.

[0081] Step 5: Spin-coating a 50 μm thick PDMS film onto the surface of the prepared material as an encapsulation layer using a spin-coating method. The spin-coating speed is 1000 rpm, and the spin-coating time is 30 seconds. After curing, a product with the desired properties is successfully obtained. Figure 1 A flexible strain sensor with the structure shown was developed, and its morphology was characterized and its performance was tested.

[0082] like Figure 1 The diagram shows a schematic of a crack-based flexible strain sensor according to Example 1. This heterogeneous structure consists of surface morphology features on a flexible substrate along its width and a thickness gradient structure of a metal sensitive layer along its length. The sensor fabrication process includes preparing a flexible substrate with surface morphology features along the sample width using a template method, and preparing a metal thin film sensitive layer with a thickness gradient along the sample length on its surface. The surface morphology of the template material used is shown in the diagram. Figure 2 As shown in the figure, three different surface microstructures were selected to fabricate PDMS substrates with distinctive surface morphology characteristics, aiming to control the cracking behavior of the surface metal films. The surface morphology characteristics of the fabricated flexible strain sensors are shown in the figure. Figure 3 As shown, the results indicate that not only are there alternating regions with different morphologies along the sample width, but there are also height differences between these regions. This allows for an effective strain modulation effect that can influence the cracking behavior of the metal thin film. Figure 4 The image shows the grain size transmission spectrum of the metal thin film with thickness gradient in Example 1. The results indicate that the grain size increases with increasing film thickness, signifying the successful fabrication of a metal thin film with a thickness gradient along the sample length. The fracture toughness of the film is significantly dependent on the thickness variation; the introduction of the thickness gradient structure is beneficial for inducing multi-stage fracture behavior in the metal thin film, thereby further enhancing the sensor's sensitivity. Figure 5The fracture morphology of the heterostructured metal thin film in Example 1 under 60% strain is shown. The results indicate that the heterostructure induces multi-stage fracture characteristics in the metal thin film. The sensitivity and linear strain range performance test results of the flexible strain sensor in Example 1 are as follows: Figure 6 As shown, the sensor exhibits a sensitivity of up to 256.92 over a 150% strain range, and a linear fit of up to 0.983 across the entire range, indicating that the sensor achieves synergistic optimization of sensitivity and linear strain range.

[0083] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A crack-based flexible strain sensor, characterized in that, From bottom to top, it includes a flexible substrate, a sensitive layer, and a flexible encapsulation layer; The flexible substrate near the surface of the sensitive layer has multiple morphological regions formed sequentially on the surface along the direction perpendicular to the stretching direction of the flexible substrate. These multiple morphological regions form various tissue morphologies with different roughness and different thicknesses corresponding to the morphological regions. The thickness of the sensitive layer gradually increases from the middle to both ends along the stretching direction of the flexible substrate.

2. The crack-based flexible strain sensor according to claim 1, characterized in that, The flexible substrate has a second morphological region in the middle, and a first morphological region and a third morphological region on both sides respectively. The surface roughness of the first morphological region, the second morphological region, and the third morphological region increases sequentially.

3. The crack-based flexible strain sensor according to claim 2, characterized in that, The thickness of the surfaces of the first morphological region, the second morphological region, and the third morphological region decreases sequentially. The thickness difference between the first and second morphological regions is 40-50 μm, and the thickness difference between the first and third morphological regions is 110-120 μm.

4. A crack-based flexible strain sensor according to claim 2, characterized in that, R of the first topographic region a The roughness R of the second morphology region is 3.0-3.5 μm. a The roughness R of the third morphology region is 3.0-3.5 μm. a Less than 0.25μm.

5. A crack-based flexible strain sensor according to claim 1, characterized in that, The sensitive layer is a thin metal film.

6. A crack-based flexible strain sensor according to claim 1, characterized in that, Electrodes are provided at both ends of the sensitive layer.

7. A crack-based flexible strain sensor according to claim 1, characterized in that, The materials of the flexible substrate and the flexible encapsulation layer are any one of polydimethylsiloxane, thermoplastic elastomer, and organic platinum silicone.

8. A method for preparing a crack-based flexible strain sensor according to any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Separately splice multiple templates along the non-stretching direction of the flexible substrate to form a flexible substrate template, with different surface roughness between adjacent templates; Step 2: Coat the surface of the flexible substrate template with a flexible substrate material, and obtain a flexible substrate after curing; Step 3: Using magnetron sputtering, a sensitive layer is deposited on the surface of the flexible substrate, and the thickness of the sensitive layer gradually increases from the middle to both ends along the stretching direction of the flexible substrate. Step 4: Prepare a flexible encapsulation layer on the surface of the sensitive layer to obtain a crack-based flexible strain sensor.

9. The method for fabricating a crack-based flexible strain sensor according to claim 8, characterized in that, In step 3, a mask plate is first covered on the surface of the flexible substrate along the arrangement direction of the template, and then a sensitive layer is formed by DC magnetron sputtering.

10. A wearable electronic device, characterized in that, Including the crack-based flexible strain sensor as described in any one of claims 1-7.

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