Asymmetric-structure hydrogel flexible strain sensor and preparation method and application thereof

Abstract

The invention belongs to the technical field of wearable electronic equipment, and particularly relates to an asymmetric-structure hydrogel flexible strain sensor and a preparation method and application thereof. According to the preparation method, carboxylic acid modified polyvinyl alcohol is taken as a raw material, and the asymmetric double-layer structure hydrogel is constructed through the synergistic effect of chemical crosslinking mediated by ferric salt and physical crosslinking driven by freeze-thaw cycle. One layer of the obtained hydrogel has excellent biological adhesion and can be tightly and comfortably attached to the surface of skin; and the other layer is a non-adhesive sensing layer and is directly responsible for strain sensing and signal transmission. The structure can fundamentally avoid the interference of adhesion components on an internal conductive network, ensure the stability and reliability of sensing signals, and effectively prevent environmental pollutants such as sweat and dust from being directly attached to a sensitive area, thereby improving the durability and service life of the sensor, and improving the reliability of the sensor. And the technical defects of signal abrupt change, insufficient sensitivity, weak environmental interference resistance and the like of a single conductive network sensor can be effectively overcome.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of wearable electronic devices, and particularly relates to an asymmetric structure hydrogel flexible strain sensor and a preparation method and application thereof. BACKGROUND

[0002] Wearable electronic devices have become one of the core research directions in the field of health management and artificial intelligence due to their excellent flexibility and expandability, and have shown broad application potential in flexible sensors, electronic skin, personalized medical detection and other scenarios. Among them, the application prospect of flexible strain sensors in the fields of wearable health monitoring and human-computer interaction is particularly outstanding.

[0003] However, traditional flexible strain sensors mostly adopt homogeneous or symmetric structures (such as directly dispersing conductive fillers in an elastic matrix), and such design has obvious defects, resulting in mutual restriction between the two core functions of "sensing performance" and "skin adhesion": on the one hand, the introduction of adhesive components for improving skin adhesion tends to interfere with the stability of the internal conductive network, causing problems such as signal drift and response lag; on the other hand, if the conductive network structure is optimized, the flexibility and skin adhesion comfort of the material are often sacrificed. At the same time, the sensing surface is directly exposed to the external environment, which is easy to attach dust and sweat, further affecting the working performance and service life of the sensor.

[0004] In addition, existing strain sensors mostly adopt a single conductive network mechanism, which can be divided into two categories, and each has its own limitations: one category is the conductive filler type sensor, which is prone to conductive path breakage under large strain conditions, resulting in sensor failure or nonlinear dramatic changes in signal; the other category is the ion conductive sensor (depending on the ion conductivity of the hydrogel electrolyte), which generally has the problems of low sensitivity and easy environmental humidity and electrolyte concentration interference. Overall, the single conductive network mechanism is difficult to achieve stable and high-sensitivity gradient response in a wide strain range.

[0005] Based on the above problems, it is necessary to study an asymmetric double-layer structure flexible strain sensor to realize the unification of stable and high-sensitivity gradient response in a wide strain range and excellent skin adhesion. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the present application provides a preparation method of a flexible strain sensor, and the prepared flexible strain sensor belongs to an asymmetric strain sensor, the functions of the adhesive layer and the sensing layer are separated, the performance is more stable, and the flexible strain sensor has stable strain, high-sensitivity gradient response in a wide range and excellent skin adhesion. In addition, the preparation process of the method is simple, which is conducive to popularization and application.

[0007] In order to achieve the above purpose, the technical scheme adopted by the present application is: The first aspect of the present application provides a preparation method of an asymmetric structure hydrogel flexible strain sensor, which comprises the following steps: S1, preparation of carbon nanotube dispersion: carbon nanotubes (CNT) and water-based dispersant are added to water, and after stirring and pulse ultrasonic treatment, a carbon nanotube dispersion with a concentration of 0.6-1.0 mg / mL is prepared; S2, preparation of polyvinyl alcohol / carbon nanotube dispersion: carboxylic acid modified polyvinyl alcohol (PVA), glycerol and water are mixed, soaked, and then stirred under heating conditions until the PVA is completely dissolved to prepare a carboxylic acid modified polyvinyl alcohol (PVA) solution. Then, the carbon nanotube dispersion of S1 is added to the obtained carboxylic acid modified polyvinyl alcohol (PVA) solution under heating and stirring conditions, and a polyvinyl alcohol / carbon nanotube dispersion (PVA / CNT dispersion) is obtained after mixing. S3, preparation of sensing layer sol: a trivalent iron salt is mixed with water to prepare an ion conductive additive, which is then added to the polyvinyl alcohol / carbon nanotube dispersion, stirred and heated to fully mix, and then treated by ultrasonic treatment and reheated to obtain a uniform sensing layer sol. S4, preparation of sensing layer gel: the sensing layer sol of S3 is poured into a molding mold, and after leveling, the sol in the mold is subjected to freeze-thaw cycle treatment to prepare a "semi-freezing state" gel with a softened surface, which is sticky and not melted. S5, preparation of adhesion layer sol: tannic acid (TA) is mixed with water, stirred until completely dissolved to prepare a tannic acid solution; then the carboxylic acid modified polyvinyl alcohol (PVA) solution in S2 is mixed with the obtained tannic acid solution, stirred and mixed to prepare a polyvinyl alcohol-tannic acid (PVA-TA) mixed solution; then the ion conductive additive in S3 is added to the above mixed solution under stirring and heating conditions, and the adhesion layer sol is obtained after mixing. S6, preparation of asymmetric structure hydrogel: the adhesion layer sol of S5 is poured on the surface of the "semi-freezing state" sensing layer gel of S4, and after leveling, the composite system is subjected to freeze-thaw cycle treatment to finally prepare an asymmetric structure hydrogel with an adhesion layer on the upper layer and a sensing layer on the lower layer. S7, installation of electrodes to prepare strain sensor: the asymmetric structure hydrogel in S6 is taken out of the mold, cut and then has metal sheets attached to both ends, and the metal sheets are fixed as electrodes using insulating tape to obtain an asymmetric structure hydrogel flexible strain sensor.

[0008] Preferably, the water-based dispersant comprises one or more of sodium dodecyl sulfonate, sodium dodecyl benzene sulfonate, polyvinylpyrrolidone (PVP), TNWDIS, and carboxymethyl cellulose; and the mass ratio of the CNT to the water-based dispersant is 1:1-10.

[0009] More preferably, the mass ratio of the CNTs to the aqueous dispersant is 1:3.5; the pulsed ultrasonic treatment is performed using a cell disruptor, and the pulsed ultrasonic treatment lasts for 2-4 hours.

[0010] Preferably, the concentration of the PVA solution is 5-15 wt%, the mass ratio of the PVA, glycerol and water is 1:1-5:4-10; and the mass ratio of the CNT dispersion to the PVA solution is 1-4:10-30.

[0011] More preferably, the concentration of the PVA solution is 10 wt%, the mass ratio of the PVA, glycerol and water is 1:3:6; and the mass ratio of the CNT dispersion to the PVA solution is 3:20.

[0012] Preferably, in S2, after being soaked for 15-30 minutes, the PVA is completely dissolved by magnetic stirring at 500-700 rpm under an oil bath at 80-95°C, and the carbon nanotube dispersion of S1 is added to the obtained PVA solution under the condition of a water bath at 50-70°C and stirring at 500-700 rpm.

[0013] Preferably, the ferric salt includes one or more of ferric chloride, ferric sulfate and ferric nitrate.

[0014] Preferably, the mass ratio of the ferric salt to water is 1:6-15; and the mass ratio of the ion-conducting additive to the PVA / CNT dispersion is 1:8-15.

[0015] More preferably, the mass ratio of the ferric salt to water is 1:9; and the mass ratio of the ion-conducting additive to the PVA / CNT dispersion is 5:46.

[0016] Preferably, the freeze-thaw cycle treatment of S4 is first freezing at -50°C for 6-18 hours, then thawing at room temperature for 6-18 hours, again freezing at -50°C for 10-15 hours, and then thawing at room temperature for 10-60 minutes. The "semi-freezing state" of S4 refers to an intermediate state formed after the gel of the sensing layer is taken out of the frozen environment and thawed at room temperature for 10-60 minutes, in which the outer layer is softened but the inner part is still partially frozen.

[0017] Preferably, the mass concentration of TA in the TA solution is 5%-15%; the mass ratio of the TA solution to the PVA solution is 1:5-15; and the mass ratio of the ion-conducting additive to the PVA-TA mixed solution is 1:20-40.

[0018] More preferably, the mass concentration of TA in the TA solution is 10%; the mass ratio of the TA solution to the PVA solution is 1:10; and the mass ratio of the ion-conducting additive to the PVA-TA mixed solution is 1:22.

[0019] Preferably, in S5, the stirring and heating conditions are a water bath of 70-90℃ and stirring at 200-500 rpm.

[0020] Preferably, the freeze-thaw cycle treatment in S6 is freezing at -50℃ for 6-18 hours, then thawing at room temperature for 6-18 hours, and repeating the freeze-thaw cycle process 5-8 times.

[0021] The second aspect of the present application also provides an asymmetric structure hydrogel flexible strain sensor prepared by the preparation method of the first aspect.

[0022] The third aspect of the present application also provides the use of the asymmetric structure hydrogel flexible strain sensor of the second aspect in the preparation of a wearable electronic device.

[0023] Compared with the prior art, the present application has the following beneficial effects: (1) The present application uses carboxylic acid modified polyvinyl alcohol (PVA) as raw material, and constructs an asymmetric double-layer structure hydrogel through the synergistic effect of trivalent iron salt mediated chemical crosslinking and freeze-thaw cycle driven physical crosslinking. One of the layers of the obtained hydrogel has excellent biological adhesion, can be closely and comfortably attached to the surface of the skin, and avoids the use of external adhesive tape or bandage; the other layer is a non-adhesive sensing layer, which is directly responsible for strain sensing and signal transmission. This structure can fundamentally avoid the interference of adhesive components on the internal conductive network, ensure the stability and reliability of the sensing signal, and effectively prevent environmental pollutants such as sweat and dust from directly adhering to the sensitive area, thereby improving the durability and service life of the sensor.

[0024] (2) The present application introduces iron ions provided by trivalent iron salt as ionic conductive additives in the material system, and cooperates with uniformly dispersed carbon nanotubes to construct a stable double conductive network structure. Among them, the iron ions can give the sensor high sensitivity and signal stability under small strain; the carbon nanotubes can maintain the partial connection of the conductive path under large strain, avoiding the complete rupture of the conductive network. This double network design makes the sensor have high sensitivity and good response linearity in a wide strain range, effectively solving the technical defects of single conductive network sensors such as signal mutation, insufficient sensitivity and weak environmental interference resistance. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 FIG. 1 is a cross-sectional view of an asymmetric structure hydrogel flexible strain sensor; wherein 1 is an adhesive layer containing trivalent iron, tannic acid and polyvinyl alcohol, 2 is a sensing layer containing trivalent iron, carbon nanotubes and polyvinyl alcohol, and 3 is a metal sheet electrode.

[0026] Figure 2A comparison diagram of asymmetric adhesive hydrogel in Example 1 in contact with the skin; wherein the left diagram shows the contact of the hydrogel adhesive layer with the surface of the human skin, and the right diagram shows the effect of the same piece of hydrogel sensing layer in contact with the skin.

[0027] Figure 3 A data diagram of the asymmetric structure hydrogel flexible strain sensor prepared in Example 1 worn on the finger, showing that the resistance change rate after the finger bending is 60%.

[0028] Figure 4 A data diagram of the asymmetric structure hydrogel flexible strain sensor prepared in Example 1 worn on the wrist, showing that the resistance change rate after the wrist bending is 28%.

[0029] Figure 5 A data diagram of the asymmetric structure hydrogel flexible strain sensor prepared in Example 1 worn on the elbow, showing that the resistance change rate after the elbow bending to 90° is 120%. DETAILED DESCRIPTION

[0030] The specific embodiments of the present application are further described below. It should be noted that the description of these embodiments is used to help understand the present application, but does not constitute a limitation on the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0031] The experimental methods in the following examples are all conventional methods unless otherwise specified. The experimental materials used in the following examples are all commercially available unless otherwise specified.

[0032] Example 1: (1) Preparation of carbon nanotube dispersion: carbon nanotubes (CNT) and carbon nanotube aqueous dispersant (sodium dodecyl sulfonate) were added to deionized water at a mass ratio of 1:3.5, and a carbon nanotube mixture solution with a concentration of 0.8 mg / mL was prepared. After magnetic stirring for 2 h, pulsed ultrasonic treatment was performed for 2 h by using an ultrasonic cell crusher, and the power was set to 60 W. Finally, a uniform and stable carbon nanotube dispersion was obtained.

[0033] (2) Preparation of PVA / CNT dispersion: Carboxylic acid modified polyvinyl alcohol (PVA), glycerol, and deionized water were added to a round-bottom flask in a mass ratio of 1:3:6. After soaking for 20 minutes, magnetic stirring was performed at 600 rpm under an oil bath at 90°C until the PVA was completely dissolved, obtaining a PVA solution with a concentration of 10 wt%. Then, 6 g of carbon nanotube dispersion was added dropwise to 40 g of the above PVA solution under stirring at 600 rpm in a 60°C water bath. After the dropwise addition was completed, stirring was continued to obtain a uniform PVA / CNT dispersion.

[0034] (3) Preparation of sensing layer sol: Iron salt and deionized water were mixed in a mass ratio of 1:9 to obtain an ion-conducting additive (ferric chloride solution). Then, the ion-conducting additive and the PVA / CNT dispersion were mixed in a mass ratio of 5:46, and the mixed system was transferred to a 80°C water bath and stirred at 500 rpm for 30 minutes. Subsequently, the mixed solution was subjected to ultrasonic treatment for 30-60 minutes at a power of 40 W, and then was reheated on a 80°C heating table for 60 minutes, obtaining a uniform sensing layer sol.

[0035] (4) Preparation of sensing layer gel: The above sensing layer sol was poured into a mold (a 60 mm diameter petri dish), and after leveling, was placed in a -50°C freezer for 12 hours. Then, it was taken out and thawed at room temperature for 12 hours, which was one complete freeze-thaw cycle. The gel after one freeze-thaw cycle was again placed in a -50°C freezer for 12 hours, and after being taken out, was thawed at room temperature for 30 minutes. At this time, the surface of the gel was partially softened, sticky, and not melted, and was in a “semi-freeze-thaw state”, ready for use.

[0036] (5) Preparation of adhesion layer sol: Tannic acid (TA) and deionized water were mixed in a mass ratio of 1:9 and stirred until completely dissolved, obtaining a 10 wt% tannic acid solution. The 10 wt% PVA solution prepared in step (2) and the tannic acid solution were mixed in a mass ratio of 10:1, and after magnetic stirring, a PVA-TA mixed solution was obtained. Then, under stirring at 300 rpm in an 80°C water bath, 1 mL of the ion-conducting additive solution (i.e., the ferric chloride solution) prepared in step (3) was slowly added to the above PVA-TA mixed solution in a mass ratio of 1:22, and stirring was continued to obtain an adhesion layer sol.

[0037] (6) About 4 g of the adhesion layer sol was poured onto the surface of the sensing layer gel in a “semi-freeze-thaw state” prepared in step (4), and was allowed to level naturally for 5 minutes. Subsequently, the composite system was placed in a -50°C freezer for 12 hours, and then was thawed at room temperature for 12 hours. This freeze-thaw cycle was performed a total of 3 times (i.e., after 2 freeze-thaw cycles of the sensing layer, the adhesion layer was added, and then the two were collectively freeze-thawed for 3 times). Finally, a double-layer composite gel was obtained, in which the upper layer was the adhesion layer and the lower layer was the sensing layer.

[0038] (7) Peel the gel from step (6) from the mold (a petri dish with a diameter of 60 mm) and cut it to the specified size, such as 3 cm × 1 cm. Then attach copper sheets to both ends and use insulating tape to fix the copper sheets as electrodes, thus obtaining an asymmetric double-layer flexible strain sensor. Its structure is as follows: Figure 1 As shown, the sensor has an asymmetric double-layer structure, including an adhesive layer, a sensing layer, and electrodes. The sensing layer is distributed on the adhesive layer, and the electrodes are distributed at both ends of the sensing layer.

[0039] Example 2: The preparation method is basically the same as in Example 1, except that in step (2), 2g of carbon nanotube dispersion is weighed and slowly added dropwise to 40g of the above PVA solution under the conditions of 60℃ water bath and 600 rpm stirring. After the addition is completed, stirring is continued to mix evenly to obtain a uniform PVA / CNT dispersion.

[0040] Example 3: The preparation method is basically the same as in Example 1, except that in step (2), 4g of carbon nanotube dispersion is weighed and slowly added dropwise to 40g of the above PVA solution under the conditions of 60℃ water bath and 600 rpm stirring. After the addition is completed, stirring is continued to mix evenly to obtain a uniform PVA / CNT dispersion.

[0041] Example 4: The preparation method is basically the same as in Example 1, except that in step (2), 8g of carbon nanotube dispersion is weighed and slowly added dropwise to 40g of the above PVA solution under the conditions of 60℃ water bath and 600 rpm stirring. After the addition is completed, stirring is continued to mix evenly to obtain a uniform PVA / CNT dispersion.

[0042] Example 5: The preparation method is basically the same as that in Example 1, except that in step (3), iron salt and deionized water are mixed at a mass ratio of 1:6 to obtain ion-conducting additive (ferric chloride solution).

[0043] Example 6: The preparation method is basically the same as that in Example 1, except that in step (3), iron salt and deionized water are mixed at a mass ratio of 1:12 to obtain ion-conducting additive (ferric chloride solution).

[0044] Example 7: The preparation method is basically the same as that in Example 1, except that in step (3), iron salt and deionized water are mixed at a mass ratio of 1:15 to obtain ion-conducting additive (ferric chloride solution).

[0045] Comparative Example 1: (1) Preparation of the sensing layer sol: Carboxylic acid modified polyvinyl alcohol (PVA), glycerol, and deionized water were added to a round-bottom flask at a mass ratio of 1:3:6. After soaking for 20 minutes, the mixture was magnetically stirred at 600 rpm in an oil bath at 90°C until the PVA was completely dissolved, resulting in a PVA solution with a concentration of 10 wt%. Iron salt and deionized water were mixed at a mass ratio of 1:9 to obtain an ion-conducting additive (ferric chloride solution). The ion-conducting additive (ferric chloride solution) was then mixed with the PVA solution at a mass ratio of 5:46. The mixture was then stirred in an 80°C water bath, followed by ultrasonic treatment, and then reheated on an 80°C heating stage to obtain a uniform sensing layer sol.

[0046] (2) Preparation of the sensing layer gel: The above-mentioned sensing layer sol was poured into a mold (a petri dish with a diameter of 60 mm) and frozen at -50°C for 12 hours. Then it was taken out and thawed at room temperature of 25°C for 12 hours. This is one complete freeze-thaw cycle. The gel that has completed one freeze-thaw cycle was frozen again at -50°C for 12 hours. After taking it out, it was thawed at room temperature of 25°C for 30 minutes. At this time, the surface of the gel was partially softened, sticky and unmelted, in a "semi-freeze-thaw state", and ready for use.

[0047] (3) Preparation of the adhesion layer sol: Tannic acid (TA) and deionized water were mixed at a mass ratio of 1:9 and stirred until completely dissolved to obtain a 10 wt% tannic acid solution. The 10 wt% PVA solution prepared in step (1) was mixed with the tannic acid solution at a mass ratio of 10:1 and magnetically stirred to obtain a PVA-TA mixed solution. Then, under the conditions of 80℃ water bath and 300 rpm stirring, the ion-conductive additive solution (i.e., ferric chloride solution) prepared in step (1) was slowly added dropwise to the above PVA-TA mixed solution at a mass ratio of 1:22 using a 1 mL syringe, and stirring was continued to obtain the adhesion layer sol.

[0048] (4) Pour approximately 4g of the adhesive layer sol onto the surface of the sensing layer gel, which is in a "semi-freeze-thaw state" in step (2), and let it stand for 5 minutes to allow it to level naturally. Then, freeze the composite system at -50°C for 12 hours, and then thaw it at room temperature of 25°C for 12 hours. This freeze-thaw cycle is performed a total of 3 times (i.e., the sensing layer is frozen and thawed twice, then the adhesive layer is added, and then the two layers are frozen and thawed together 3 times). Finally, a bilayer composite gel with an adhesive layer on top and a sensing layer on the bottom is obtained.

[0049] (5) Peel the gel from the mold in step (4) and cut it to the specified size, such as 3cm×1cm. Then attach copper sheets to both ends and use insulating tape to fix the copper sheets as electrodes to obtain the asymmetric double-layer flexible strain sensor.

[0050] Comparative Example 2: (1) Preparation of carbon nanotube dispersion: Carbon nanotubes (CNTs) and carbon nanotube aqueous dispersant were added to deionized water at a mass ratio of 1:3.5 to prepare a carbon nanotube mixture with a concentration of 0.8 mg / mL. After magnetic stirring for 2 hours, the mixture was subjected to pulsed ultrasound for 2 hours using an ultrasonic cell disruptor to finally obtain a uniform and stable carbon nanotube dispersion.

[0051] (2) Preparation of the sensing layer sol: Carboxylic acid-modified polyvinyl alcohol (PVA), glycerol, and deionized water were added to a round-bottom flask in a mass ratio of 1:3:6. After soaking for 20 minutes, the mixture was magnetically stirred at 600 rpm in an oil bath at 90°C until the PVA was completely dissolved, resulting in a PVA solution with a concentration of 10 wt%. Then, 6 g of carbon nanotube dispersion was weighed and slowly added dropwise to 40 g of the above PVA solution under stirring at 600 rpm in a water bath at 60°C. The mixture was then transferred to an 80°C water bath and stirred. The mixture was then sonicated and reheated on an 80°C heating stage to obtain a uniform sensing layer sol.

[0052] (3) Preparation of the sensing layer gel: The above-mentioned sensing layer sol was poured into a mold (a petri dish with a diameter of 60 mm) and frozen at -50°C for 12 hours. Then it was taken out and thawed at room temperature of 25°C for 12 hours. This is a complete freeze-thaw cycle. The gel that has completed one freeze-thaw cycle was frozen again at -50°C for 12 hours. After taking it out, it was thawed at room temperature of 25°C for 30 minutes. At this time, the surface of the gel was partially softened, sticky and unmelted, in a "semi-freeze-thaw state", and ready for use.

[0053] (4) Preparation of the adhesion layer sol: Tannic acid (TA) and deionized water were mixed at a mass ratio of 1:9 and stirred until completely dissolved to obtain a 10 wt% tannic acid solution. The 10 wt% PVA solution prepared in step (2) was mixed with the tannic acid solution at a mass ratio of 10:1 and magnetically stirred to obtain a PVA-TA mixed solution. Iron salt and deionized water were mixed at a mass ratio of 1:9 to obtain an ion-conducting additive (ferric chloride solution). Then, under the conditions of 80℃ water bath and 300 rpm stirring, the prepared ion-conducting additive solution (i.e., ferric chloride solution) was slowly added dropwise to the above PVA-TA mixed solution at a mass ratio of 1:22 using a 1 mL syringe, and stirring was continued to obtain the adhesion layer sol.

[0054] (5) Pour approximately 4g of the adhesive layer sol onto the surface of the sensing layer gel, which is in a "semi-freeze-thaw state" in step (3), and let it stand for 5 minutes to allow it to level naturally. Then, freeze the composite system at -50℃ for 12 hours, and then thaw it at room temperature of 25℃ for 12 hours. This freeze-thaw cycle is performed a total of 3 times (i.e., the sensing layer is frozen and thawed twice, then the adhesive layer is added, and then the two layers are frozen and thawed together 3 times). Finally, a bilayer composite gel with an adhesive layer on top and a sensing layer on the bottom is obtained.

[0055] (6) Peel the gel from the mold in step (5) and cut it to the specified size, such as 3cm×1cm. Then attach copper sheets to both ends and use insulating tape to fix the copper sheets as electrodes to obtain the asymmetric double-layer flexible strain sensor.

[0056] Comparative Example 3: (1) Preparation of carbon nanotube dispersion: Carbon nanotubes (CNTs) and carbon nanotube aqueous dispersant were added to deionized water at a mass ratio of 1:3.5 to prepare a carbon nanotube mixture with a concentration of 0.8 mg / mL. After magnetic stirring for 2 hours, the mixture was subjected to pulsed ultrasound for 2 hours using an ultrasonic cell disruptor to finally obtain a uniform and stable carbon nanotube dispersion.

[0057] (2) Preparation of PVA / CNT dispersion: Carboxylic acid-modified polyvinyl alcohol (PVA), glycerol, and deionized water were added to a round-bottom flask in a mass ratio of 1:3:6. After soaking for 20 minutes, the mixture was magnetically stirred at 600 rpm in an oil bath at 90°C until the PVA was completely dissolved, resulting in a 10 wt% PVA solution. Then, 6 g of carbon nanotube dispersion was weighed and slowly added dropwise to 40 g of the above PVA solution under stirring at 600 rpm in a 60°C water bath. After the addition was complete, the mixture was stirred until homogeneous, resulting in a uniform PVA / CNT dispersion.

[0058] (3) Preparation of the sensing layer sol: Iron salt and deionized water were mixed at a mass ratio of 1:9 to obtain an ion-conducting additive (ferric chloride solution). Then, the ion-conducting additive was mixed with PVA / CNT dispersion at a mass ratio of 5:46, and the mixture was stirred in an 80°C water bath. The mixture was then sonicated and reheated on an 80°C heating stage to obtain a uniform sensing layer sol.

[0059] (4) Preparation of the sensing layer gel: The above-mentioned sensing layer sol was poured into a mold (a petri dish with a diameter of 60 mm) and frozen at -50°C for 12 hours. Then it was taken out and thawed at room temperature of 25°C for 12 hours. This is one complete freeze-thaw cycle. The gel that has completed one freeze-thaw cycle was frozen at -50°C again for 12 hours. This freeze-thaw cycle was performed a total of 3 times. Finally, a bilayer composite gel with an upper layer of adhesion layer and a lower layer of sensing layer was obtained.

[0060] (5) Peel the gel from the mold in step (4) and cut it to the specified size, such as 3cm×1cm. Then attach copper sheets to both ends and use insulating tape to fix the copper sheets as electrodes to obtain a single-layer flexible strain sensor.

[0061] Experimental example: As can be seen from Table 1, under 10% strain, the flexible strain sensor prepared by the present invention, using uniformly dispersed carbon nanotubes and iron ions provided by trivalent iron salts as the sensing layer, exhibits high sensitivity (GF=2.05 in Example 1), and is significantly superior to Comparative Example 1 (Fe only). 3+ Comparative Example 1 (GF=0.57) and Comparative Example 2 (CNT only, GF=1.23); under a large strain of 80%, the resistance change rate of the flexible strain sensor prepared in this invention (e.g., Example 1=248.6%) is also better than Comparative Example 1 (162.4%) and Comparative Example 2 (241.3%). This indicates that the sensor prepared using only iron ions or carbon nanotubes as conductive fillers has low sensitivity over a wide stretching range. Comparative Example 3 is a flexible strain sensor with only a sensing layer. Its sensitivity and resistance change rate are not significantly different from those of Example 1, indicating that adding an adhesion layer has almost no impact on the performance of the sensing layer.

[0062] In summary, the flexible strain sensors prepared using the method of this invention all exhibit improved sensitivity under the same stretching conditions, with Example 1 showing the best sensitivity improvement. This indicates that the dual conductive network constructed by CNTs and iron ions not only ensures high sensitivity and signal stability under small strains but also allows the sensor to remain connected under large strains, preventing the conductive network from completely breaking. This achieves high sensitivity and good linear response, overcoming problems such as signal abrupt changes, insufficient sensitivity, or significant environmental interference that are common in single conductive network sensors.

[0063] Table 1: Sensitivity and resistance change rate of the flexible strain sensors prepared in Examples 1-7 and Comparative Examples 1-3 at 10% and 80% strain. Continued from the previous table: Figure 2The images show a comparison of the asymmetric adhesion hydrogel in Example 1 and its contact with skin. The left image illustrates the contact between the hydrogel adhesion layer and the human skin surface, reflecting that the gel on this side can form a tight, conformal fit with the skin, exhibiting excellent immediate adhesion and long-lasting adhesion stability, sufficient to meet the requirements of wearable devices for fixed use on dynamic skin surfaces. The right image shows the effect of the same hydrogel sensing layer in contact with skin, clearly demonstrating that it has almost no adhesion and cannot adhere to the skin alone. These results indicate that the flexible strain sensor prepared in this invention achieves selective one-sided adhesion. Its asymmetric adhesion structure fundamentally avoids interference from the adhesion components on the internal conductive network, ensuring the stability and reliability of the sensing signal, thereby improving the sensor's durability and lifespan.

[0064] at the same time, Figures 3-5 This indicates that the asymmetric structure hydrogel flexible strain sensor in Example 1 is adaptable to multiple parts of the human body, and the rate of change of resistance is positively correlated with the degree of joint deformation, demonstrating its potential for human motion monitoring.

[0065] In summary, the flexible strain sensor prepared in this invention separates the functions of the adhesion layer and the sensing layer, resulting in more stable performance of the asymmetric strain sensor. Furthermore, the construction of the dual conductive network allows it to simultaneously achieve both high sensitivity and a wide tensile range. Therefore, the flexible strain sensor prepared in this invention has high application value.

[0066] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for fabricating an asymmetric hydrogel flexible strain sensor, characterized in that, Includes the following steps: S1. Preparation of carbon nanotube dispersion: Carbon nanotubes and an aqueous dispersant are added to water, and after stirring and pulsed ultrasound to make them uniform and stable, a carbon nanotube dispersion with a concentration of 0.6-1.0 mg / mL is prepared. S2, Preparation of polyvinyl alcohol / carbon nanotube dispersion: Carboxylic acid modified polyvinyl alcohol, glycerol and water are mixed and soaked. After soaking, the mixture is stirred under heating conditions until PVA is completely dissolved to obtain a carboxylic acid modified polyvinyl alcohol solution. Then, under heating and stirring conditions, the carbon nanotube dispersion of S1 is added to the obtained carboxylic acid modified polyvinyl alcohol solution and mixed to obtain a polyvinyl alcohol / carbon nanotube dispersion. S3. Preparation of the sensing layer sol: Trivalent iron salt is mixed with water to prepare an ion-conducting additive, which is then added to a polyvinyl alcohol / carbon nanotube dispersion, stirred and heated to mix it thoroughly, and then subjected to ultrasonic treatment and reheating to obtain a uniform sensing layer sol. S4. Preparation of the sensing layer gel: The sensing layer sol of S3 is poured into the molding mold. After standing and leveling, the sol in the mold is subjected to freeze-thaw cycle treatment to make it into a "semi-freeze-thaw state" gel with a partially softened, viscous and unmelted surface. S5. Preparation of the adhesive layer sol: Tannic acid is mixed with water and stirred until completely dissolved to obtain a tannic acid solution; then the carboxylic acid modified polyvinyl alcohol solution in S2 is mixed with the obtained tannic acid solution and stirred until homogeneous to obtain a polyvinyl alcohol-tannic acid mixed solution; then, under stirring and heating conditions, the ion-conducting additive in S3 is added to the above mixed solution and mixed until homogeneous to obtain the adhesive layer sol. S6. Preparation of asymmetric structure hydrogel: The adhesive layer sol of S5 is poured onto the surface of the sensing layer gel in the "semi-freeze-thaw state" of S4. After standing and leveling, the composite system is subjected to freeze-thaw cycle treatment to finally obtain an asymmetric structure hydrogel with an adhesive layer on the upper layer and a sensing layer on the lower layer. S7. Install electrodes to prepare strain sensor: Take the asymmetric structure hydrogel from S6 out of the mold, cut it, attach metal sheets to both ends, and use insulating tape to fix the metal sheets as electrodes to obtain the asymmetric structure hydrogel flexible strain sensor.

2. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The aqueous dispersant includes one or more of sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, polyvinylpyrrolidone, TNWDIS, and carboxymethyl cellulose; the mass ratio of carbon nanotubes to aqueous dispersant is 1:1 to 10.

3. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The concentration of the carboxylic acid modified polyvinyl alcohol solution is 5-15 wt%, and the mass ratio of the carboxylic acid modified polyvinyl alcohol, glycerol and water is 1:1-5:4-10; the mass ratio of the carbon nanotube dispersion to the carboxylic acid modified polyvinyl alcohol solution is 1-4:10-30.

4. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The ferric salts include one or more of ferric chloride, ferric sulfate, and ferric nitrate.

5. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The mass ratio of the trivalent iron salt to water is 1:6 to 15; the mass ratio of the ion-conducting additive to the polyvinyl alcohol / carbon nanotube dispersion is 1:8 to 15.

6. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The freeze-thaw cycle process described in S4 involves first freezing at -50°C for 6 to 18 hours, then thawing at room temperature for 6 to 18 hours, then freezing at -50°C for 10 to 15 hours, and finally thawing at room temperature for 10 to 60 minutes.

7. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The tannic acid solution has a tannic acid concentration of 5% to 15% by mass; the mass ratio of the tannic acid solution to the carboxylic acid-modified polyvinyl alcohol solution is 1:5 to 15; and the mass ratio of the ion-conducting additive to the polyvinyl alcohol-tannic acid mixed solution is 1:20 to 40.

8. The method for fabricating an asymmetric hydrogel flexible strain sensor according to claim 1, characterized in that, The freeze-thaw cycle process described in S6 involves first freezing at -50°C for 6 to 18 hours, then thawing at room temperature for 6 to 18 hours, and repeating this freeze-thaw process 5 to 8 times.

9. An asymmetric structure hydrogel flexible strain sensor prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the asymmetric structure hydrogel flexible strain sensor according to claim 9 in the fabrication of wearable electronic devices.

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