A flexible strain sensor and a preparation method and application thereof

Abstract

The application discloses a flexible strain sensor and a preparation method and application thereof. The application is based on a laser direct writing technology, realizes in-situ conversion of lignin slurry to patterned graphene by using high aromatic content and rich water-soluble groups of lignin, and then solidifies the unconverted lignin slurry, and integrally prepares a lignin carbon-based hydrogel sensor. The integrated preparation method of the flexible strain sensor provided by the application solves the functional layer assembly mismatch problem, and improves the stability of the sensor.

Description

Technical Field

[0001] This invention belongs to the field of sensing technology, specifically relating to a flexible strain sensor, its fabrication method, and its application. Background Technology

[0002] Flexible sensors can convert various external stimuli into electrical signals to simulate the flexibility and sensory properties of human skin, thus finding wide application in fields such as medical monitoring and human-computer interaction. Compared to traditional silicon-based rigid sensors, flexible sensors have advantages such as low Young's modulus and stretchability, making it easier to achieve close adhesion to complex curved surfaces such as the human body or robots, improving the accuracy and precision of monitoring mechanical signals such as strain or stress. With the increasing applications and market demand, the fabrication methods of flexible sensors are also receiving more attention.

[0003] Flexible sensors are generally composed of elastic polymers and conductive fillers. The elastic polymer acts as the support layer, dominating mechanical properties, while the conductive filler serves as the sensing layer, primarily responsible for electrical conductivity. The support layer and sensing layer often need to be fabricated separately before being assembled into a complete device. Patent document CN 112414294 A discloses a highly sensitive sandwich-layer strain sensor based on hydrogel and carbon materials and its fabrication method: first, a carbon material deposition layer obtained by drying a carbon material dispersion is used as the sensing layer; second, a polymer prepolymer is cured to prepare the support layer; finally, the sensing layer and support layer are assembled into a flexible sensor. Academician Yang Huayong's team reported a method for fabricating a flexible sensor: first, conductive material (LIG) is prepared by laser-induced polyimide carbonization; second, a hydrogel is prepared as the support layer; finally, the sensing layer and support layer are assembled into a complete flexible sensor device {Nat Electron, 2024, 7, 51–65}. Clearly, processing the support layer and sensing layer independently involves numerous steps and the differences in their physicochemical and mechanical properties can easily lead to mismatch between functional layers, resulting in sensor failure. Therefore, the existing technology requires further improvement and development. Summary of the Invention

[0004] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for fabricating a flexible strain sensor, thereby resolving the mismatch problem between the sensor's functional layers and improving the sensor's stability.

[0005] Another object of the present invention is to provide a flexible strain sensor prepared by the above-described preparation method.

[0006] Another object of the present invention is to provide an application of the above-mentioned flexible strain sensor.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for fabricating a flexible strain sensor includes the following steps:

[0009] (1) The lignin pulp is subjected to laser radiation treatment to obtain a lignin pulp with a lignin carbon pattern on the surface;

[0010] (2) The lignin slurry with lignin carbon pattern formed on the surface of step (1) is cured to obtain a flexible conductive hydrogel.

[0011] (3) A layer of lignin slurry from step (1) is coated on the carbon pattern surface of the lignin slurry from step (1), and the curing treatment in step (2) is performed to obtain a sandwich structure flexible conductive hydrogel.

[0012] (4) A layer of lignin slurry from step (1) is coated on the carbon pattern surface of the lignin slurry in step (1) and laser radiation treatment is performed in step (1). Then, a layer of lignin slurry from step (1) is coated on the newly formed carbon pattern surface of the lignin slurry. Finally, the curing treatment in step (2) is performed to obtain a double carbon sandwich flexible conductive hydrogel.

[0013] (5) Coat the surface of the carbon pattern of the lignin slurry in step (1) with a layer of lignin slurry in step (1) and perform laser radiation treatment in step (1). Repeat the operation to coat the surface of the newly formed carbon pattern with a layer of lignin slurry in step (1) and perform laser radiation treatment in step (1) several times. Then coat the surface of the newly formed carbon pattern with a layer of lignin slurry in step (1) and finally perform curing treatment in step (2) to obtain a multi-carbon sandwich flexible conductive hydrogel.

[0014] (6) Connect wires to the two ends of the carbon pattern of the flexible conductive hydrogel in step (2), the sandwich structure flexible conductive hydrogel in step (3), the double carbon sandwich flexible conductive hydrogel in step (4), or the multi-carbon sandwich flexible conductive hydrogel in step (5) to obtain a flexible sensor.

[0015] Preferably, the lignin pulp described in step (1) is processed at 25°C for 0.1 seconds. -1 The shear viscosity is 4–63 Pa·s; more preferably 33–38 Pa·s.

[0016] Preferably, the lignin pulp in step (1) is composed of lignin, polymer and water; the polymer has at least one of hydroxyl, carboxyl and amino groups, and the polymer is a type of polymer or a mixture of two or more types of polymers.

[0017] More preferably, the mass ratio of lignin, polymer and water is 3.75 to 11.25: 5 to 9: 50, even more preferably 5 to 7.5: 5 to 9: 50 or 5: 2: 50; most preferably 7.5: 7.5: 50 or 5: 2: 50.

[0018] More preferably, the lignin and polymer are mixed evenly by stirring at 65-95°C for 0.5-2.5 hours after adding water.

[0019] More preferably, the lignin is at least one of solvent lignin, enzymatically hydrolyzed lignin, alkali lignin, and lignin sulfonate; more preferably, it is sodium lignin sulfonate.

[0020] More preferably, the polymer is at least one selected from gelatin, agar, sodium alginate, sodium carboxymethyl cellulose, hyaluronic acid, polyvinyl alcohol, and carboxymethyl chitosan; even more preferably, it is at least one selected from polyvinyl alcohol and carboxymethyl chitosan.

[0021] Preferably, the laser radiation treatment method in step (1) is as follows: the laser focal plane is raised 0-3 cm away from the surface of the lignin pulp, the laser radiation parameters are set, the laser is run, and a carbon pattern layer is induced to be generated on the surface of the lignin pulp.

[0022] More preferably, the laser is a 10.6 μm CO2 laser.

[0023] More preferably, the laser radiation parameters are: power of 50-100W, laser energy of 40-100%, frequency of 10-90kHz, line spacing of 0.01-0.3mm, and scanning speed of 10-300mm / s; even more preferably: power of 75W, laser energy of 40-100%, frequency of 10-90kHz, line spacing of 0.01-0.3mm, and scanning speed of 10-300mm / s; most preferably: power of 75W, laser energy of 40-100%, frequency of 10-50kHz, line spacing of 0.05-0.09mm, and scanning speed of 30-120mm / s.

[0024] Preferably, the curing treatment method in step (2) is as follows: freezing the lignin slurry at -100 to -10°C for 1 to 12 hours, and then thawing it at 10 to 35°C for 1 to 5 hours, repeating the freezing at -100 to -10°C for 1 to 12 hours and thawing at 10 to 35°C for 1 to 5 hours several times; or immersing the lignin slurry in a coagulation bath for 1 to 5 hours after solidification at room temperature.

[0025] More preferably, the repeated operation of freezing at -100 to -10°C for 1 to 12 hours and thawing at 10 to 35°C for 1 to 5 hours is performed 1 to 5 times.

[0026] More preferably, the coagulation bath contains at least one of aluminum ions, calcium ions, iron ions, epichlorohydrin, glutaraldehyde, tannic acid, and phytic acid.

[0027] Preferably, the number of times in step (5) is 1 to 5 times, more preferably 3 times.

[0028] A flexible strain sensor was prepared by the above method.

[0029] The above-mentioned flexible strain sensor is used in strain sensing; the application does not involve the diagnosis and treatment of diseases, that is, it is an application for non-disease diagnosis and treatment purposes.

[0030] Its preferred application is in flexible strain sensing.

[0031] In current technologies, flexible sensors can be divided into functional layers and support layers, generally following a method of independent fabrication followed by assembly and encapsulation. In this invention, lignin pulp is first selectively transformed into a conductive carbon layer under laser irradiation. This layer not only provides resistance signals through crack expansion and contraction but also serves as a hydrogel-carbon layer interface to provide ionic capacitance signals, or provides dielectric capacitance signals through a carbon layer-hydrogel-carbon layer sandwich structure. The second step involves a curing process, first using a freeze-thaw method to solidify the lignin pulp into a gel shape, and then further enhancing its mechanical properties, such as toughness, through a coagulation bath. In other words, the lignin pulp is transformed into a functional layer in the first step and into a hydrogel support layer in the second step, where the carbon functional layer and the hydrogel support layer are tightly bonded to form a complete sensor front end. Rapid fabrication from lignin pulp to sensor can be achieved with only two consecutive in-situ transformation steps. Furthermore, by continuously repeating the in-situ transformation process, complex flexible sensors with multiple functional layers can also be fabricated.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] 1. Lignin is a natural aromatic compound with a high carbon content. Patterned sensing layers can be directly prepared by laser irradiation of lignin slurry. Unconverted lignin, due to its special three-dimensional network structure and interaction with polymer functional groups, endows the gel with excellent mechanical properties.

[0034] 2. Lignin is rich in functional groups such as hydroxyl, carboxyl, and sulfonic acid groups, which endow it with good water solubility. The liquid nature of lignin slurry allows it to penetrate into carbon pores. After the slurry solidifies, the gel network and the carbon network form an interpenetrating structure, thus solving the interfacial mismatch problem.

[0035] 3. Compared with the step-by-step fabrication method of processing each functional layer of flexible sensor separately and then assembling them, this fabrication method adopts the integrated fabrication of the sensing layer and the support layer, which solves the mismatch problem between the functional layers of the sensor and improves the stability of the sensor; and improves the production process of constructing flexible devices in situ from biomass carbon sources.

[0036] 4. Since the lignin slurry used to coat the sensing layer has the same material system, the slurry has similar elastic modulus and mechanical properties after curing. This solves the problem of sensor failure caused by mechanical mismatch under complex mechanical conditions, and improves the mechanical performance matching of the sensor as well as the stability and reliability of the sensing. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating the preparation process of an embodiment of the present invention;

[0038] Figure 2 Schematic diagram of carbonization of lignin pulp with different viscosities;

[0039] Figure 3 Schematic diagram of carbonization of lignin pulp under different laser energy parameters;

[0040] Figure 4 The graph shows the carbonization behavior of lignin pulps of different viscosities under different laser energy conditions.

[0041] Figure 5 This is a SEM image of carbonization in Example 1;

[0042] Figure 6 The Raman spectrum of carbon in Example 1;

[0043] Figure 7 The stress-strain curve of the lignin-carbon-based gel sensor prepared in Example 1;

[0044] Figure 8 The sensitivity curve of the lignin-carbon-based gel sensor prepared in Example 1;

[0045] Figure 9 The graph shows the cycling performance of the lignin-carbon-based gel sensor prepared in Example 1.

[0046] Figure 10 This is a demonstration of the sensing capabilities of the lignin-carbon-based gel sensor prepared in Example 1;

[0047] Figure 11 Optical photograph of the lignin carbon-based gel prepared in Example 2;

[0048] Figure 12 This demonstrates the multilayer carbon interlayer hydrogel sensor prepared in Example 3;

[0049] Figure 13 This is a comparison diagram of the gel interfaces obtained from the integrated preparation in Example 1 and the stepwise preparation in Comparative Example 1;

[0050] Figure 14 The image shows a comparison of gel stretching between the integrated preparation in Example 1 and the stepwise preparation in Comparative Example 1.

[0051] Figure 15 This is a breakdown diagram of the gel prepared in one piece in Example 1;

[0052] Figure 16 This is a comparison chart of low-viscosity and high-viscosity carbon formation in Comparative Example 2;

[0053] Figure 17 The viscosity of the slurry corresponds to different concentrations of PVA. Detailed Implementation

[0054] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0055] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0056] Experiment 1

[0057] Step 1: Add 5g of sodium lignosulfonate (LS) and different masses of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare LS-PVA carbon source slurries with different viscosities. In parallel experiments, the amounts of polyvinyl alcohol (PVA) were 3.76g, 4.95g, 6.18g, 7.47g, and 8.82g, corresponding to PVA concentrations of 7%, 9%, 11%, 13%, and 15% of the total PVA and water mass, respectively. The slurries with different PVA concentrations were stirred at 25℃ for 0.1s... -1 The shear viscosities under the given conditions were 1 Pa·s, 4 Pa·s, 13 Pa·s, 26 Pa·s, and 63 Pa·s, respectively. (See...) Figure 17 ;

[0058] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern was individually designed. The laser scanning parameters were adjusted to a single-pass processing mode with a 3mm defocus, 100% laser energy (75W), a frequency of 20kHz, a line spacing of 0.05mm, and a scanning speed of 120mm / s, to carbonize the lignin pulp and prepare patterned graphene. The carbonization results are shown in [Figure number missing]. Figure 2 .

[0059] Experiment 2

[0060] Step 1: Add 5g of sodium lignosulfonate (LS) and 6.18g of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. The resulting slurry is then subjected to a stirring time of 0.1s at 25℃. -1 The shear viscosity under the given conditions was 13 Pa·s, and the concentration of PVA in the total mass of PVA and water was 11%.

[0061] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern was individually designed. The laser scanning parameters were adjusted, with a 3mm defocus, different laser energies, a frequency of 20kHz, a line spacing of 0.05mm, and a scanning speed of 120mm / s in a single-pass processing mode to carbonize the lignin pulp and prepare patterned graphene. The parallel experiments used laser energies of 1% (75W), 5% (75W), 10% (75W), 15% (75W), 20% (75W), 40% (75W), 60% (75W), 80% (75W), and 100% (75W). The carbonization results are shown in [Figure number missing]. Figure 3 .

[0062] Experiment 3

[0063] Step 1: Add 5g of sodium lignosulfonate (LS) and different masses of polyvinyl alcohol (PVA) to 50g of water, stir at 95℃ and 250rpm for 2.5h to prepare LS-PVA carbon source slurries with different viscosities. The amounts of polyvinyl alcohol (PVA) in the parallel experiments were 2.63g, 3.76g, 4.95g, 6.18g, 7.47g and 8.82g, respectively, corresponding to PVA concentrations of 5%, 7%, 9%, 11%, 13% and 15% of the total mass of PVA and water.

[0064] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern was individually designed. The laser scanning parameters were adjusted, with a 3mm defocus, different laser energies, a frequency of 20kHz, a line spacing of 0.05mm, and a scanning speed of 120mm / s in a single processing mode to carbonize the lignin pulp and prepare patterned graphene. In parallel experiments, the laser energies were 15% (75W), 20% (75W), 40% (75W), 60% (75W), 80% (75W), and 100% (75W). The carbonization pattern is shown in [the table below]. Figure 4 Blue represents carbon that can be formed, but the carbon is discontinuous (diffusion occurs during carbon formation, resulting in a discontinuous carbon circuit), while red represents a stable and continuous carbon circuit that can be formed.

[0065] Example 1

[0066] Step 1: Add 7.5g of sodium lignosulfonate (LS) and 7.5g of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. This slurry can be stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 33 Pa·s;

[0067] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern is personalized. The laser scanning parameters are adjusted to a single processing mode of 3mm defocus, 100% laser energy (75W), 20kHz frequency, 0.05mm line spacing, and 120mm / s scanning speed to carbonize the lignin pulp and prepare patterned graphene.

[0068] Step 3: On the surface of the prepared patterned graphene, another layer of LS-PVA slurry from Step 1 is coated to form a sandwich structure with a layer of patterned graphene coated on both sides. Then, the surface is placed in a -30°C freezer for three freeze-thaw cycles, each cycle consisting of 8 hours of freezing followed by 3 hours of thawing at room temperature. This prepares a lignin-carbon-based hydrogel with conductive pathways.

[0069] Step 4: Connect guide lines to both ends of the carbon pattern to fabricate a flexible sensor.

[0070] Example 2

[0071] Step 1: Add 5g of sodium lignosulfonate (LS) and 2g of carboxymethyl chitosan (CMCS) to 50g of water, and stir at 65℃ and 20rpm for 0.5h to prepare an LS-CMCS carbon source slurry with viscosity. This slurry is stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 38 Pa·s;

[0072] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern is personalized. The laser scanning parameters are adjusted to carbonize the carbon source slurry in a single processing mode with a defocus of 3mm, laser energy of 50% (75W), frequency of 10kHz, line spacing of 0.06mm, and scanning speed of 60mm / s, thus preparing patterned graphene.

[0073] Step 3: After standing at room temperature for 2 hours, the slurry forms a gel due to the hydrogen bonding between LS and CMCS. Then, 0.1 mol / L AlCl3 solution is added dropwise, and the slurry is immersed in the coagulation bath for 1 hour to promote internal cross-linking of the slurry and improve the mechanical properties of the gel, thereby preparing a lignin carbon-based hydrogel with conductive pathways.

[0074] Example 3

[0075] Step 1: Add 7.5g of sodium lignosulfonate (LS) and 7.5g of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. This slurry can be stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 33 Pa·s;

[0076] Step 2: Using a 10.6μm CO2 laser, the laser-induced pattern is personalized. The laser scanning parameters are adjusted to a single processing mode with a defocus of 5mm, laser energy of 80% (75W), frequency of 50kHz, line spacing of 0.09mm, and scanning speed of 30mm / s to carbonize the lignin pulp and prepare patterned graphene.

[0077] Step 3: On the surface of the prepared patterned graphene, coat another layer of the LS-PVA slurry from Step 1, and then perform laser irradiation treatment as in Step 2 to prepare patterned graphene. Repeat the following operation 3 times: coat the newly formed patterned graphene surface with another layer of the LS-PVA slurry from Step 1, then perform laser irradiation treatment as in Step 2; finally, coat the newly formed patterned graphene surface with another layer of the LS-PVA slurry from Step 1, and then place it in a -30℃ freezer for 3 freeze-thaw cycles, freezing for 8 hours each time followed by thawing at room temperature for 3 hours. A multi-carbon sandwich conductive hydrogel is thus prepared.

[0078] Comparative Example 1: Curing followed by laser carbonization

[0079] Step 1: Add 7.5g of sodium lignosulfonate (LS) and 7.5g of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. This slurry can be stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 33 Pa·s;

[0080] Step 2: Pour the slurry into the mold, then place it in a -30℃ freezer for 3 freeze-thaw cycles, freezing for 8 hours each time and thawing at room temperature for 3 hours.

[0081] Step 3: Using a 10.6μm CO2 laser, the laser-induced pattern is customized. The laser scanning parameters are adjusted to a single processing mode with a defocus of 3mm, laser energy of 100% (75W), frequency of 20kHz, line spacing of 0.05mm, and scanning speed of 120mm / s to carbonize the lignin gel, prepare patterned graphene, and prepare a lignin carbon-based hydrogel with conductive pathways.

[0082] Comparative Example 2

[0083] Comparison sample:

[0084] Step 1: Add 7.5g of sodium lignosulfonate (LS) and 7.5g of polyvinyl alcohol (PVA) to 200g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. This slurry can be stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 0.36 Pa·s;

[0085] Step 2: Using a 10.6μm CO2 laser, the laser scanning parameters are adjusted to be 3mm defocus, 100% laser energy (75W), 10kHz frequency, 0.05mm line spacing, and 120mm / s scanning speed in a single processing mode to induce laser processing on the lignin pulp from Step 1.

[0086] Sample of this invention:

[0087] Step 1: Add 7.5g of sodium lignosulfonate (LS) and 7.5g of polyvinyl alcohol (PVA) to 50g of water, and stir at 95℃ and 250rpm for 2.5h to prepare a viscous LS-PVA carbon source slurry. This slurry can be stirred at 25℃ for 0.1s... -1 Under these conditions, the shear viscosity is 33 Pa·s;

[0088] Step 2: Using a 10.6μm CO2 laser, the laser scanning parameters are adjusted to be 3mm defocus, 100% laser energy (75W), 10kHz frequency, 0.05mm line spacing, and 120mm / s scanning speed in a single processing mode to induce laser processing on the lignin pulp from Step 1.

[0089] Figure 4 This diagram illustrates the carbonization process of lignin pulp. Viscosity restricts the movement of lignin molecules, allowing them to absorb sufficient energy for carbonization. Appropriate viscosity and energy irradiation contribute to obtaining continuous and stable carbon patterns.

[0090] SEM images of the laser-induced graphene material prepared in Example 1 are shown below. Figure 5 As shown, laser-induced carbonization of LS-PVA slurry forms porous carbon structures.

[0091] The Raman spectrum of the laser-induced graphene material prepared in Example 1 is as follows: Figure 6 As shown, laser-induced carbonization of LS-PVA slurry forms graphene.

[0092] The stress-strain curve of the lignin-carbon-based gel sensor prepared in Example 1 is shown below. Figure 7As shown, the addition of lignin enhances the mechanical properties of the gel. The PVA:LS = 1:0.5 and 1:1.5 samples are based on Example 1, with only the amount of LS changed to meet the ratio, and the other conditions are exactly the same as in Example 1.

[0093] The sensitivity curve of the lignin-carbon-based gel sensor prepared in Example 1 is shown below. Figure 8 As shown in the figure, it can be seen that the prepared sensor can achieve sensing.

[0094] The lignin-carbon-based gel sensor prepared in Example 1 maintained stable characteristic signals after 1200 cycles, such as... Figure 9 As shown, this indicates that the sensing layer and the support layer are tightly bonded and there is no mismatch.

[0095] A schematic diagram of the lignin-carbon-based gel sensor prepared in Example 1 controlling the grasping and releasing process of the robotic arm is shown below. Figure 10 As shown in the figure, it can be seen that the prepared sensor can be applied in the field of human-computer interaction.

[0096] Optical photographs of the lignin-carbon-based gel prepared in Example 2, such as... Figure 11 As shown.

[0097] The multilayer carbon-interlayer hydrogel sensor prepared in Example 3 is shown, as follows: Figure 12 As shown.

[0098] Comparative Example 1 demonstrated the advantages of integrated fabrication by altering the order of laser-induced carbonization and gel solidification. Solidification followed by laser induction leads to interfacial separation between the sensing layer and the support layer. Figure 13 As shown, Figure 13 The top image shows the sample of Comparative Example 1, and the bottom image shows the sample of Example 1.

[0099] In Comparative Example 1, the order of laser-induced carbonization and gel solidification was changed, and the tensile properties of the gels prepared by the two methods were compared. Clearly, the gel prepared in one piece can stably undergo stretching and recovery; while the control sample exhibits interfacial mismatch (interfacial separation), such as... Figure 14 As shown.

[0100] Disassembly of the gel prepared in Example 1 showed that the sensing layer and the support layer were tightly bonded, ensuring the stability of the device. Figure 15 As shown.

[0101] Comparative Example 2 compared the effect of viscosity on carbonization of the slurry. At lower viscosity, the slurry had high fluidity, and during laser irradiation, lignin molecules could not absorb enough energy to convert into carbon. At higher viscosity, the slurry had low fluidity, and during laser irradiation, lignin molecules could absorb enough energy to convert into carbon. Figure 16 As shown.

[0102] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for fabricating a flexible strain sensor, characterized in that, Includes the following steps: (1) The lignin pulp is subjected to laser radiation treatment to obtain a lignin pulp with a lignin carbon pattern on the surface; (2) The lignin slurry with lignin carbon pattern formed on the surface of step (1) is cured to obtain a flexible conductive hydrogel. (3) A layer of lignin slurry from step (1) is coated on the carbon pattern surface of the lignin slurry from step (1), and the curing treatment in step (2) is performed to obtain a sandwich structure flexible conductive hydrogel. (4) A layer of lignin slurry from step (1) is coated on the carbon pattern surface of the lignin slurry in step (1) and laser radiation treatment is performed in step (1). Then, a layer of lignin slurry from step (1) is coated on the newly formed carbon pattern surface of the lignin slurry. Finally, the curing treatment in step (2) is performed to obtain a double carbon sandwich flexible conductive hydrogel. (5) Coat the surface of the carbon pattern of the lignin slurry in step (1) with a layer of lignin slurry in step (1) and perform laser radiation treatment in step (1). Repeat the operation to coat the surface of the newly formed carbon pattern with a layer of lignin slurry in step (1) and perform laser radiation treatment in step (1) several times. Then coat the surface of the newly formed carbon pattern with a layer of lignin slurry in step (1) and finally perform curing treatment in step (2) to obtain a multi-carbon sandwich flexible conductive hydrogel. (6) Connect wires to the two ends of the carbon pattern of the flexible conductive hydrogel in step (2), the sandwich structure flexible conductive hydrogel in step (3), the double carbon sandwich flexible conductive hydrogel in step (4), or the multi-carbon sandwich flexible conductive hydrogel in step (5) to obtain a flexible sensor.

2. The method for fabricating a flexible strain sensor according to claim 1, characterized in that, The lignin pulp described in step (1) is subjected to a temperature of 25°C and a loading time of 0.1s. -1 Below, the shear viscosity is 4–63 Pa·s; The lignin pulp in step (1) is composed of lignin, polymer and water; the polymer has at least one of hydroxyl, carboxyl and amino groups.

3. The method for fabricating a flexible strain sensor according to claim 2, characterized in that, The mass ratio of lignin, polymer and water is 3.75 to 11.25: 5 to 9:

50.

4. The method for fabricating a flexible strain sensor according to claim 2, characterized in that, The lignin is at least one of solvent lignin, enzymatically hydrolyzed lignin, alkali lignin, and lignin sulfonate; the polymer is at least one of gelatin, agar, sodium alginate, sodium carboxymethyl cellulose, hyaluronic acid, polyvinyl alcohol, and carboxymethyl chitosan. Alternatively, the lignin may be sodium lignin sulfonate; and the polymer may be at least one of polyvinyl alcohol and carboxymethyl chitosan.

5. The method for fabricating a flexible strain sensor according to claim 1, characterized in that, The laser radiation treatment method described in step (1) is as follows: the laser focal plane is raised 0-3 cm away from the surface of the lignin pulp, the laser radiation parameters are set, the laser is run, and a carbon pattern layer is induced to be generated on the surface of the lignin pulp. The laser is a 10.6μm CO2 laser; The laser radiation parameters are: power of 50-100W, laser energy of 40-100%, frequency of 10-90kHz, line spacing of 0.01-0.3mm, and scanning speed of 10-300mm / s. The number of times mentioned in step (5) is 1 to 5 times.

6. The method for fabricating a flexible strain sensor according to claim 1, characterized in that, The curing treatment method described in step (2) is as follows: freeze the lignin slurry at -100 to -10°C for 1 to 12 hours, and then thaw it at 10 to 35°C for 1 to 5 hours. Repeat the freezing at -100 to -10°C for 1 to 12 hours and thawing at 10 to 35°C for 1 to 5 hours several times; or immerse the lignin slurry in a coagulation bath for 1 to 5 hours after solidifying it at room temperature. The repeated operation of freezing at -100 to -10°C for 1 to 12 hours and thawing at 10 to 35°C for 1 to 5 hours is performed 1 to 5 times. The coagulation bath contains at least one of aluminum ions, calcium ions, iron ions, epichlorohydrin, glutaraldehyde, tannic acid, and phytic acid.

7. The method for fabricating a flexible strain sensor according to claim 1, characterized in that, The lignin pulp in step (1) is composed of lignin, polymer and water; the polymer has at least one of hydroxyl, carboxyl and amino groups, and the mass ratio of lignin, polymer and water is 5-7.5:5-9:50 or 5:2:50; The laser radiation treatment method in step (1) is as follows: the laser focal plane is raised 0-3 cm away from the surface of the lignin pulp, the laser radiation parameters are set, the laser is run, and a carbon pattern layer is induced on the surface of the lignin pulp; the laser is a 10.6 μm CO2 laser; the laser radiation parameters are: power of 75 W, laser energy of 40-100%, frequency of 10-90 kHz, line spacing of 0.01-0.3 mm, and scanning speed of 10-300 mm / s.

8. The method for fabricating a flexible strain sensor according to claim 2, characterized in that, The lignin pulp described in step (1) is subjected to a temperature of 25°C and a loading time of 0.1s. -1 Below, the shear viscosity is 33–38 Pa·s; The lignin and polymer are mixed evenly by adding water and stirring at 65–95°C for 0.5–2.5 h.

9. A flexible strain sensor prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the flexible strain sensor of claim 9 for strain sensing purposes other than disease diagnosis and treatment.

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