Crack-based flexible strain sensor and preparation method thereof

Abstract

The invention discloses a crack-based flexible strain sensor and a preparation method thereof, and belongs to the technical field of strain sensors, and the crack-based flexible strain sensor sequentially comprises a flexible substrate, a sensitive layer and a flexible packaging layer from bottom to top; the flexible substrate is close to the surface of the sensitive layer, a plurality of morphology areas are sequentially formed on the surface in the direction perpendicular to the stretching direction of the flexible substrate, the multiple morphology areas form multiple tissue morphologies, the roughness of the tissue morphologies is different, and the thicknesses of the morphology areas corresponding to the multiple tissue morphologies are different; the thickness of the sensitive layer is gradually increased from the middle part to the two ends along the stretching direction of the flexible substrate; the sensitivity of the flexible strain sensor in a 150% strain range is 256.92, the linear fitting degree of the flexible strain sensor reaches up to 0.983, and it is indicated that the flexible strain sensor achieves collaborative optimization of the sensitivity and the linear strain range.

Description

Technical Field

[0001] The present invention relates to the technical field of strain sensors, and in particular to a crack-based flexible strain sensor and a preparation method thereof. Background Art

[0002] As an important core component of wearable electronic devices, flexible strain sensors are the basis for sensing the external environment, information interaction and achieving human-machine integration, and have broad application prospects in the fields of electronic skin, health monitoring, human-machine interaction, etc. 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 show different sensitivities in different sensing ranges. This nonlinear resistance-strain response requires additional signal calibration processing for the strain sensor during actual use, which increases the complexity of the sensor program and leads to high energy consumption. In addition, broadening the linear detection range of the strain sensor tends to reduce its sensitivity, and the sensitivity and linear detection range show an inverted relationship of one increasing while the other decreases. These have hindered the practical application of crack-based strain sensors. Therefore, how to regulate the cracking behavior of metal films and achieve the synergy between the sensitivity and linear detection range of strain sensors remains an important challenge. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention provides a crack-based flexible strain sensor and a preparation method thereof. A metal film with a thickness gradient along the length direction of the sample is prepared on the surface of a substrate with different microstructural morphology characteristics by a mask method, thereby achieving coordinated optimization of sensitivity and linear strain range.

[0005] The present invention is achieved through the following technical solutions:

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

[0007] The surface of the flexible substrate close to the sensitive layer forms a plurality of morphological regions in sequence on the surface in a direction perpendicular to the stretching direction of the flexible substrate, the plurality of morphological regions form a plurality of tissue morphologies, the roughness of each tissue morphology is different, and the thickness of the morphological regions corresponding to the plurality of tissue morphologies is different;

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

[0009] Preferably, the middle of the flexible substrate is the second morphology region, and the two sides are the first morphology region and the third morphology region in sequence;

[0010] The surface roughness (R a ) in increasing order.

[0011] Preferably, the thicknesses of the surfaces of the first morphology region, the second morphology region and the third morphology region decrease in sequence;

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

[0013] Preferably, the roughness R of the first topographical area is a The roughness R of the second topography area is 3.0-3.5 μm. a The roughness R of the third topography area is 3.0-3.5 μm. a Less than 0.25μm.

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

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

[0016] Preferably, the material of the flexible substrate and the flexible packaging layer is any one of polydimethylsiloxane, thermoplastic elastomer and organic platinum silicone.

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

[0018] Step 1: splicing a plurality of templates in sequence along the non-stretching direction of the flexible substrate to form a flexible substrate template, wherein the roughness of the surfaces of two adjacent templates is different;

[0019] Step 2: coating a flexible substrate material on the surface of the flexible substrate template, and curing the flexible substrate to obtain a flexible substrate;

[0020] Step 3: depositing a sensitive layer on the surface of the flexible substrate by a magnetron sputtering method, 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 packaging layer on the surface of the sensitive layer to obtain a crack-based flexible strain sensor.

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

[0023] A wearable electronic device comprises the crack-based flexible strain sensor.

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

[0025] A crack-based flexible strain sensor of the present application uses a template method to construct a surface morphology feature composed of different microstructures arranged alternately along the width direction on the surface of a flexible substrate, and uses a mask method to prepare a metal film with a thickness gradient along the length direction of the sample on the surface of the microstructured substrate, thereby achieving the coordinated optimization of sensitivity and linear strain range. The surface morphology features along the width direction of the sample can effectively regulate the deformation stress / strain distribution of the film to promote the formation of a mixed crack morphology consisting of local through cracks in the third morphology area and mesh cracks in the first and second morphology areas. The thickness difference of different morphology areas along the width direction of the sample induces the third morphology area to experience the smallest local stress / strain concentration and become a strain isolation area, while the first and second morphology areas experience greater local stress / strain concentration. The combined effect of surface morphology and thickness difference prevents the formation of continuous through cracks along the width direction of the sample, making the expansion of cracks in each area more controllable and dispersed, thereby achieving a wide linear detection range. The film thickness gradient along the length direction can cause the film surface to present a gradient stress distribution, thereby inducing multi-stage fracture of the metal film and achieving high sensitivity. The flexible strain sensor can achieve the coordinated optimization of sensitivity and linear strain range based on the crack control strategy and the effective coupling of the inherent properties of polymer elastic materials. The sensitivity of the flexible strain sensor of the present invention is 256.92 within the strain range of 150%, and its linear fit is as high as 0.983, indicating that the flexible strain sensor of the present invention achieves the coordinated 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 film, and the thickness gradient design induces a change in the fracture toughness of the metal film, thereby inducing the metal film to exhibit multi-stage fracture behavior, thereby further improving the sensitivity of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

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

[0029] Figure 2 Electron microscopic images of the surface morphologies of the three template materials used in Example 1 of the present invention;

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

[0031] Figure 4 This is a transmission diagram of the grain size of the flexible strain sensor along its length direction and its statistical results according to Example 1 of the present invention.

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

[0033] Figure 6 This is a performance test diagram of the sensitivity and linear strain range of the flexible strain sensor described in Example 1 of the present invention under tension. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0035] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

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

[0037] The flexible substrate is close to the surface of the sensitive layer, and multiple morphological regions are sequentially formed on the surface along the direction perpendicular to the stretching direction of the flexible substrate. The multiple morphological regions form multiple tissue morphologies, the roughness of each tissue morphology is different, and the thickness of the morphological regions corresponding to the multiple tissue morphologies is different.

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

[0039] Structurally, the flexible strain sensor of the present invention combines surface morphology features and thickness gradient structures that can effectively regulate the cracking behavior of the film. Among them, the surface morphology features along the width direction are prepared by a template method, and the template can be reused, so the surface morphology can be prepared repeatedly and uniformly. Under tension, different morphological regions alternately distributed along the width direction of the sample induce the metal film to present a mixed crack morphology. The height difference between different regions produces an effective strain modulation effect, inducing controllable cracking of the metal film to achieve a wide linear strain range.

[0040] In the present application, roughness is used to characterize the tissue morphology of the surface of the flexible substrate, that is, each tissue morphology has a different roughness, and the roughness of multiple tissue morphologies increases gradually.

[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. A plurality of morphological regions are sequentially formed on the surface in the direction perpendicular to the stretching direction of the flexible substrate, that is, a plurality of morphological regions are sequentially formed in the width direction of the flexible substrate.

[0042] It needs to be explained 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, 8 morphological regions are formed on the surface of the flexible substrate, and the 8 morphological regions present three different tissue morphologies, that is, three different roughness surfaces. It can also be understood that 3 morphological regions are the first tissue morphology, 3 morphological regions are the second tissue morphology, and 2 morphological regions are the third tissue morphology.

[0043] The thicknesses of the surfaces of the first morphology region, the second morphology region and the third morphology region decrease in sequence; the thickness difference between the first morphology region and the second morphology region is 40-50 μm, and the thickness difference between the first morphology region and the third morphology region is 110-120 μm.

[0044] The roughness R of the first topographical area a The roughness R of the second topography area is 3.0-3.5 μm. a The roughness R of the third topography area is 3.0-3.5 μm. a Less than 0.25μm.

[0045] The tissue morphologies of two adjacent morphological areas are different.

[0046] The flexible substrate is prepared by inverting the mold, 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 roughness are spliced ​​to form an overall template. The first template is a flat template, whose surface is completely smooth and is made of a plastic film with a thickness of 80 μm; the second template is a rough template, which has a rougher microstructure and is made of commercial sandpaper with a thickness of 195 μm; the third template is a microstructure template and is made of commercial sandpaper with a thickness of 130 μm.

[0048] See also Figure 1 The flexible substrate of the crack-based flexible strain sensor forms five morphological regions along the width direction, the middle is the second morphological region, and its surface is the second tissue morphology, the two sides of the middle morphological region are the first morphological regions, and its surface is the first tissue morphology, and the two side regions of the flexible substrate are the third morphological regions, and its surface is the third tissue morphology.

[0049] The roughness of the first tissue morphology, the second tissue morphology and the third tissue morphology increases in sequence.

[0050] The thicknesses of the first morphology region, the second morphology region and the third morphology region decrease in sequence, and a height difference is formed between two adjacent morphology regions.

[0051] The material of the flexible substrate is any one of polydimethylsiloxane (PDMS), thermoplastic elastomer (styrene-butadiene-styrene block copolymer SBS, styrene-ethylene-butylene-styrene block copolymer SEBS, thermoplastic polyurethane elastomer TPU) and organic platinum silicone (Ecoflex).

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

[0053] The material of the sensitive layer is a thickness gradient metal film with high conductivity, and the thickness gradually increases along the stretching direction of the flexible substrate, that is, the thickness of the middle area of ​​the sensitive layer is thin, and gradually thickens toward both ends of the stretching direction.

[0054] The material of the metal film is gold, silver, copper, etc., that is, any metal with conductive properties can be used, preferably silver. The sensitive layer is prepared by mask method and magnetron sputtering method.

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

[0056] The material of the flexible encapsulation layer is closely attached to the surface of the sensitive layer and the electrode material. The flexible encapsulation layer is prepared by spin coating.

[0057] Correspondingly, the present application also provides a method for preparing the above-mentioned crack-based flexible strain sensor, comprising the following steps:

[0058] Step 1: splice a plurality of templates in sequence along the non-stretching direction of the flexible substrate, and the roughness of the surfaces of two adjacent templates is different.

[0059] In this embodiment, three types of 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, and is ultrasonically cleaned in anhydrous ethanol for 15 minutes, and then dried with a hair dryer after cleaning.

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

[0061] For example, the completely flat plastic film in the template material is purchased from 3M Company and different types of commercial sandpapers with certain roughness and thickness are purchased from German Warrior Company (the brands are P800 and P2500 respectively).

[0062] Step 2: Evenly scrape the degassed liquid material of the flexible substrate onto the surface of the flexible substrate template, and then place it in a vacuum state, solidify it, and peel it off to obtain a flexible substrate with a surface topology.

[0063] The degassing treatment method is as follows:

[0064] The liquid material of the flexible matrix that is fully and evenly mixed according to the ratio is placed in a vacuum drying oven for a 2-hour degassing treatment.

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

[0066] The flexible substrate prepared in the above step 2 is placed in a magnetron sputtering apparatus, and a mask plate of a specific size is covered along the width direction of the surface of the flexible substrate. Then, a metal film is prepared by a DC magnetron sputtering method to obtain a heterogeneous structure metal film consisting of the substrate surface morphology characteristics in the width direction and a thickness gradient structure in the length direction.

[0067] During magnetron sputtering, the chamber pressure before deposition was 2.4×10 -4 Pa. The sputtering power of the metal target was 120 W, the argon pressure during the deposition process was 0.5 Pa, and the fixed plate did not rotate.

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

[0069] Step 5: Spin-coat a polymer layer with a thickness of 50 μm on the surface of the above-prepared material by spin coating, wherein the spin coating speed is 1000 rpm and the spin coating time is 30 seconds. After curing is completed, the strain sensor can be successfully prepared.

[0070] Example 1

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

[0072] Step 1: Cut the three template materials into 50 mm × 2 mm rectangular strips with a cutter for later use. Next, clean the cut substrate templates with anhydrous ethanol solution in an ultrasonic cleaning machine for 15 minutes, and dry them with a hair dryer after cleaning.

[0073] Along the width direction of the cut wooden board, the templates are closely arranged in the order of rough template-flat template-microstructure template-flat template-rough template to form a 50mm×10mm flexible matrix template.

[0074] Step 2: Evenly mix the prepolymer and curing agent of polymer PDMS at a mass ratio of 10:1, and place 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 the liquid PDMS was cured at 80 degrees Celsius for 2 hours, it was carefully peeled off from the template to form a solid PDMS sheet (with a thickness of 500 μm). In this way, a PDMS substrate with a template inverse microstructure was successfully prepared.

[0076] Due to the difference in thickness of the templates used, there are corresponding height differences between the various 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 enables an effective strain modulation effect to affect the cracking behavior of the metal film.

[0077] Step 3: The prepared PDMS substrate and mask plate with surface morphology features are ultrasonically cleaned with anhydrous ethanol for 15 minutes, and then dried with a hair dryer after cleaning.

[0078] The cleaned PDMS substrate was covered with a customized mask in its width direction to prepare a metal film with a thickness gradient along the length of the sample. Before deposition, the chamber pressure was 2.4×10 -4Pa. The deposition power is 120W, the argon pressure is 0.5Pa, and the plate is fixed without rotating. After the deposition is completed, a Ag film with a thickness gradient along the length direction is successfully prepared on the PDMS substrate with surface morphology characteristics. The film thickness in the thinnest area is 100nm, and the film thickness in the thickest area is 500nm.

[0079] The mask plate used for preparing the thickness gradient metal film is an aluminum thin plate with a certain arch height and a size that can be adjusted as needed.

[0080] Step 4: Use conductive silver glue to fix the metal silver electrodes at both ends of the metal film with the heterostructure to complete the circuit connection.

[0081] Step 5: Spin coat a 50 μm thick PDMS film on the surface of the above-prepared material as an encapsulation layer by spin coating. The spin coating speed is 1000 rpm and the spin coating time is 30 seconds. After curing, a film having the following characteristics can be successfully obtained: Figure 1 A flexible strain sensor with the structure shown is prepared, and its morphology is characterized and performance is tested.

[0082] like Figure 1 The figure shows a schematic diagram of the structure of a crack-based flexible strain sensor of Example 1. The heterostructure consists of a surface morphology feature on a flexible substrate in the width direction and a thickness gradient structure of a metal sensitive layer in the length direction. The preparation process of the sensor includes preparing a flexible substrate with a surface morphology feature along the width direction of the sample by a template method and preparing a metal film sensitive layer with a thickness gradient along the length direction of the sample on its surface. The surface morphology of the template material used is as follows: Figure 2 As shown. Three different surface microstructures were selected to prepare PDMS substrates with surface morphology characteristics, aiming to regulate the cracking behavior of the metal film on its surface. The surface morphology characteristics of the prepared flexible strain sensor are shown in Figure 3 The results show that along the width of the sample, there are not only alternating regions with different morphologies but also height differences between the regions, which enables an effective strain modulation effect to affect the cracking behavior of the metal film. Figure 4 The transmission diagram of the grain size of the metal film with thickness gradient in Example 1. The results show that as the thickness of the film increases, its grain size also increases, which means that a metal film with thickness gradient along the length of the sample has been successfully prepared. The fracture toughness of the film significantly depends on the thickness change of the film. The introduction of the thickness gradient structure is conducive to inducing the metal film to exhibit multi-stage fracture behavior and further improve the sensitivity of the sensor. Figure 5The fracture morphology of the heterostructured metal film under 60% strain in Example 1. The results show that the heterostructure-induced metal film exhibits multi-stage fracture characteristics. The sensitivity and linear strain range performance test results of the flexible strain sensor in Example 1 are shown in Figure 6 The sensor has a sensitivity of up to 256.92 within a strain range of 150%, and its linear fit over the entire range is as high as 0.983, indicating that the sensor achieves synergistic optimization of sensitivity and linear strain range.

[0083] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present 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 packaging layer; The surface of the flexible substrate close to the sensitive layer forms a plurality of morphological regions in sequence on the surface in a direction perpendicular to the stretching direction of the flexible substrate, the plurality of morphological regions form a plurality of tissue morphologies, the roughness of each tissue morphology is different, and the thickness of the morphological regions corresponding to the plurality of tissue morphologies is different; The sensitive layer is along the stretching direction of the flexible substrate, and the thickness of the sensitive layer gradually increases from the middle to both ends.

2. A crack-based flexible strain sensor according to claim 1, characterized in that: The middle of the flexible substrate is a second morphology region, and the two sides are the first morphology region and the third morphology region in sequence; The surface roughness of the first morphology region, the second morphology region and the third morphology region increases in sequence.

3. A crack-based flexible strain sensor according to claim 2, characterized in that: The thicknesses of the surfaces of the first morphology region, the second morphology region and the third morphology region decrease in sequence; The thickness difference between the first morphology region and the second morphology region is 40-50 μm, and the thickness difference between the first morphology region and the third morphology region is 110-120 μm.

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

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

6. The crack-based flexible strain sensor according to claim 1, characterized in that: Electrodes are respectively arranged at two ends of the sensitive layer.

7. The crack-based flexible strain sensor according to claim 1, characterized in that: The material of the flexible substrate and the flexible packaging layer is 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 to 7, characterized in that: The following steps are involved: Step 1: splicing a plurality of templates in sequence along the non-stretching direction of the flexible substrate to form a flexible substrate template, wherein the roughness of the surfaces of two adjacent templates is different; Step 2: coating a flexible substrate material on the surface of the flexible substrate template, and curing the flexible substrate to obtain a flexible substrate; Step 3: depositing a sensitive layer on the surface of the flexible substrate by a magnetron sputtering method, 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 packaging layer on the surface of the sensitive layer to obtain a crack-based flexible strain sensor.

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

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

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