Flexible Strain Sensor Based on Conductive Hydrogel, Preparation Method Thereof and Application

Abstract

The present invention discloses a flexible strain sensor based on conductive hydrogel, its preparation method and application. One layer of conductive hydrogel of the flexible strain sensor is sandwiched between two layers of copper foil electrode sheets. The preparation method includes: preparing a prepolymer solution using graphene oxide, sodium alginate and methacrylamide, heating the prepolymer solution to polymerize into a polymeric hydrogel, soaking it in a calcium chloride solution, rinsing after the soaking is completed and then soaking it in deionized water, and achieving a swelling equilibrium state after sufficient water absorption to obtain the conductive hydrogel; encapsulating three layers and forming a flexible strain sensor based on the conductive hydrogel. The sensor of the present invention is applied to flexible wearable electronic devices to monitor the relative resistance change caused by the pressure generated by the deformation during the movement of different parts of the human body, and has the advantages of high sensitivity, low detection limit, wide detection range, good linearity and excellent stability, etc., and can effectively broaden the practical application of flexible wearable devices.

Description

Technical Field

[0001] The present invention relates to a flexible strain sensor, and specifically to a flexible strain sensor based on conductive hydrogel, its preparation method and application. Background Art

[0002] Sodium alginate, a polysaccharide extracted from algae, has excellent biocompatibility and has been widely used in the fields of food and medicine. At the same time, it has abundant carboxyl groups, which can interact with the amino groups of other materials to generate hydrogen bonds, making it highly concerned. Graphene oxide is an oxide of graphene, with a typical quasi-two-dimensional spatial structure. Its sheet contains abundant oxygen-containing functional groups, and has advantages such as ultra-high specific surface area, good hydrophilicity and mechanical properties. Conductive hydrogel is an intelligent hydrogel that combines a conductive material with a three-dimensional network structure of hydrogel. Therefore, it can have both the electrical properties of a conductor and the mechanical properties of a hydrogel, and can simulate the mechanical and sensory characteristics of human skin, having great development prospects in the fields of medical health, sports fitness and human-machine interface.

[0003] Flexible strain sensors based on hydrogels have advantages such as better biocompatibility, flexibility and adjustable Young's modulus on traditional flexible substrates. However, existing flexible strain sensors based on conductive nanomaterial hydrogels generally have disadvantages such as being easily damaged, unable to balance high strength and self-healing performance, and low reliability. Summary of the Invention

[0004] The present invention provides a flexible strain sensor based on conductive hydrogel, its preparation method and application. Aiming at problems such as poor flexibility, low sensitivity and small detection range of flexible strain sensors, a simple method for preparing GO / SA / PMAAm / Ca 2+ conductive hydrogel without chemical cross-linking agent is provided, and a flexible strain sensor is made based on this conductive hydrogel. This flexible strain sensor has advantages such as high sensitivity, low detection limit, wide detection range, good linearity and excellent stability, and can effectively broaden the practical application of flexible wearable devices.

[0005] The technical solution adopted by the present invention is as follows:

[0006] I. A flexible strain sensor prepared based on conductive hydrogel:

[0007] The flexible strain sensor includes two layers of copper foil electrode sheets and one layer of conductive hydrogel, and one layer of conductive hydrogel is sandwiched between two layers of copper foil electrode sheets; two wires are respectively led out from two layers of copper foil electrode sheets and connected to an external signal acquisition device.

[0008] II. A preparation method of a flexible strain sensor:

[0009] The method includes the following steps:

[0010] 1) Preparation of conductive hydrogel:

[0011] Preparation of prepolymer solution: Dissolve the graphene oxide dispersion in deionized water to obtain a diluted graphene oxide dispersion; dissolve sodium alginate powder in the diluted graphene oxide dispersion, stir at room temperature until completely swollen and then heat and stir to obtain a graphene oxide / sodium alginate solution; dissolve methylacrylamide particles in the graphene oxide / sodium alginate solution, stir at room temperature to obtain a graphene oxide / sodium alginate / methylacrylamide solution; dissolve the thermal initiator potassium persulfate in the graphene oxide / sodium alginate / methylacrylamide solution, stir and then evacuate to remove air bubbles to obtain a homogeneous prepolymer solution.

[0012] Preparation of conductive hydrogel: Use a syringe to draw 10 ml of the prepolymer solution and slowly inject it into a glass mold. When injecting, pay attention to avoiding the generation of small air bubbles that affect the performance of the prepared hydrogel. Seal the glass mold with a sealing film to prevent air from entering; place the glass mold in an electrothermal constant temperature blast drying oven for heating polymerization and then take it out. Then take out the polymerized hydrogel in the glass mold; soak the polymerized hydrogel in a calcium chloride solution. After soaking, rinse the polymerized hydrogel with deionized water. After rinsing, soak it in deionized water to fully absorb water and reach the swelling equilibrium state to obtain a conductive hydrogel block. Divide the conductive hydrogel block evenly to obtain several sheet-shaped conductive hydrogels.

[0013] 2) Preparation of flexible strain sensor:

[0014] In the order of copper foil electrode sheet, conductive hydrogel, and copper foil electrode sheet, use VHB tape to encapsulate the three layers and form a flexible strain sensor based on conductive hydrogel. Two copper foil electrode sheets respectively lead out two wires and connect to an external signal acquisition device.

[0015] In the preparation of the prepolymer solution described above, the concentration of the graphene oxide dispersion is 10 mg / g, and the concentration of the diluted graphene oxide dispersion is 0.16 - 1.46 mg / g.

[0016] In the preparation of the prepolymer solution described above, the mass of the graphene oxide dispersion is 0.1 wt% - 0.9 wt% of the total mass of sodium alginate powder and methylacrylamide particles; the mass of sodium alginate powder is 5 wt% - 25 wt% of the total mass of sodium alginate powder and methylacrylamide particles; the mass of methylacrylamide particles is 75 wt% - 95 wt% of the total mass of sodium alginate powder and methylacrylamide particles; the mass of the thermal initiator potassium persulfate is 0.5 wt% - 1 wt% of the mass of methylacrylamide particles.

[0017] In the preparation of the prepolymer solution, it is stirred at room temperature until completely swollen and then heated and stirred to obtain a graphene oxide / sodium alginate solution. Specifically, it is placed on a magnetic stirrer at room temperature and stirred for 4 to 5 h until completely swollen, and then heated to 50 °C and stirred for more than 30 min to obtain a graphene oxide / sodium alginate solution; after stirring at room temperature, a graphene oxide / sodium alginate / methacrylamide solution is obtained. Specifically, a graphene oxide / sodium alginate / methacrylamide solution is obtained after vigorously stirring at room temperature for 2 h; after stirring, the bubbles are removed by vacuum to obtain a homogeneous prepolymer solution. Specifically, it is sufficiently stirred for 15 min, and then the bubbles are removed by vacuum to obtain a homogeneous prepolymer solution.

[0018] In the preparation of the conductive hydrogel, the glass mold is placed in an electrothermal constant temperature blast drying oven for heating polymerization and then peeled off. Specifically, the glass mold is placed in an electrothermal constant temperature blast drying oven and heated to 50 °C for polymerization for 6 h and then peeled off.

[0019] In the preparation of the conductive hydrogel, the polymerized hydrogel is immersed in a calcium chloride solution. Specifically, the polymerized hydrogel is immersed in a calcium chloride solution with a content of 1 wt% to 10 wt% for 24 h to complete the immersion.

[0020] The application of the flexible strain sensor in flexible and wearable electronic devices.

[0021] The flexible strain sensor is installed on a flexible wearable electronic device, and the flexible wearable electronic device is worn on the human body. When the human body is in a moving state, the flexible strain sensor deforms due to the pressure of the human body on it, and then a relative resistance change occurs. The external signal acquisition device acquires the relative resistance change to monitor the motion state of the human body.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. The present invention prepares a flexible strain sensor based on a conductive hydrogel containing physical crosslinking. It is formed by physical crosslinking through non-covalent bonds (hydrogen bonds) and metal coordination bonds. Both the hydrogen bonds and metal coordination bonds in the gel network can be repaired again after being broken, making the flexible strain sensor have self-healing properties. By introducing Ca 2+ the mechanical properties of the hydrogel are enhanced.

[0024] 2. For hydrogels, one of their main disadvantages is poor water retention ability. Ca 2+ ions can absorb moisture in the air and enhance the water retention ability of the hydrogel. Encapsulation with VHB tape can delay the dehydration of the hydrogel.

[0025] 3. By adding the conductive substance graphene oxide, the conductivity and electrochemical properties of the hydrogel are improved, an intelligent conductive hydrogel is obtained, and the application range of the hydrogel is expanded.

[0026] In summary, the flexible strain sensor of the present invention has the advantages of high sensitivity, low detection limit, wide detection range, good linearity, and excellent stability, and can effectively broaden the practical applications of flexible wearable devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the tensile stress-strain curve after the hydrogel self-healing;

[0028] Figure 2 is the maximum strain detection limit diagram of the flexible strain sensor;

[0029] Figure 3 is the minimum strain detection limit diagram of the flexible strain sensor;

[0030] Figure 4 is the relative resistance change rate-time change curve of the conductive flexible strain sensor obtained in Example 2 at bending angles of 15°, 30°, 45°, 60°, 75°, and 90° of the finger;

[0031] Figure 5 (a) of is the relative resistance change rate-time change curve of the conductive flexible strain sensor obtained in Example 2 when bending at the wrist;

[0032] Figure 5 (b) of is the relative resistance change rate-time change curve of the conductive flexible strain sensor obtained in Example 2 when bending at the elbow;

[0033] Figure 5 (c) of is the relative resistance change rate-time change curve of the conductive flexible strain sensor obtained in Example 2 when bending at the knee;

[0034] Figure 5 (d) of is the relative resistance change rate-time change curve of the conductive flexible strain sensor obtained in Example 2 when bending at the ankle. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The specific embodiments of the present invention are as follows:

[0037] Example 1:

[0038] 1) Preparation of conductive hydrogel:

[0039] Preparation of precursor solution: 1.05 g of graphene oxide dispersion was dissolved in 12.6912 g of deionized water to obtain a diluted graphene oxide dispersion; 0.315 g of sodium alginate powder was dissolved in the diluted graphene oxide dispersion, and it was placed on a magnetic stirrer at room temperature and stirred for 4 h until completely swollen, then heated to 50 °C and stirred for 30 min to obtain a graphene oxide / sodium alginate solution; 1.785 g of methacrylamide particles were dissolved in the graphene oxide / sodium alginate solution, and vigorously stirred at room temperature for 2 h to obtain a graphene oxide / sodium alginate / methacrylamide solution; 0.02835 g of thermal initiator potassium persulfate was dissolved in the graphene oxide / sodium alginate / methacrylamide solution, stirred thoroughly for 15 min, and then degassed by vacuum to obtain a uniform precursor solution, which was stored in the refrigerator for later use.

[0040] Preparation of conductive hydrogel: Use a syringe to draw 10 ml of the precursor solution and slowly inject it into a glass mold. During injection, care should be taken to avoid generating small bubbles that may affect the performance of the prepared hydrogel. Seal the glass mold with a sealing film to prevent air from entering; Place the glass mold in an electrothermal constant temperature blast drying oven, heat it to 50 °C and polymerize for 6 h, then peel it off. Then take out the polymerized hydrogel in the glass mold; Immerse the polymerized hydrogel in a calcium chloride solution with a content of 8 wt% for 24 h. After the immersion is completed, rinse the polymerized hydrogel with deionized water. After rinsing, soak it in deionized water to fully absorb water and reach the swelling equilibrium state to obtain a conductive hydrogel block. The conductive hydrogel block was evenly divided to obtain several rectangular sheet-like conductive hydrogels with a length, width and thickness of 50 mm × 10 mm × 1 mm. Use nitrogen to dry the surface of the conductive hydrogel for 30 s to remove the moisture on the surface of the conductive hydrogel, so as to improve the adhesion between the conductive hydrogel and the VHB tape.

[0041] 2) Preparation of flexible strain sensor:

[0042] In the order of copper foil electrode sheet, conductive hydrogel, and copper foil electrode sheet, use VHB tape to encapsulate the three layers and form a flexible strain sensor based on conductive hydrogel. Two copper foil electrode sheets respectively lead out two wires and connect to an external signal acquisition device.

[0043] As Figure 1 shown, it is the tensile stress-strain curve of the hydrogel after self-healing. During the tensile process, when the tensile fracture stress of the hydrogel reaches 0.09008 MPa, the hydrogel spline will break again from the original cut. At this time, the tensile fracture elongation is 33%. The original tensile fracture elongation and tensile fracture stress of the hydrogel are 114% and 0.2645 MPa respectively. This indicates that although the fractured hydrogel can be reconstructed through dynamic physical cross-linking, the reconstruction rate between bonds is not 100%.

[0044] Example 2:

[0045] 1) Preparation of conductive hydrogel:

[0046] Preparation of prepolymer solution: 1.47 g of graphene oxide dispersion was dissolved in 12.6912 g of deionized water to obtain a diluted graphene oxide dispersion; 0.315 g of sodium alginate powder was dissolved in the diluted graphene oxide dispersion, and the mixture was placed on a magnetic stirrer and stirred for 4 h at room temperature until completely swollen, and then heated to 50° C. and stirred for 30 min to obtain a graphene oxide / sodium alginate solution; 1.785 g of methacrylamide particles was dissolved in the graphene oxide / sodium alginate solution, and the mixture was vigorously stirred at room temperature for 2 h to obtain a graphene oxide / sodium alginate / methacrylamide solution; 0.02835 g of thermal initiator potassium persulfate was dissolved in the graphene oxide / sodium alginate / methacrylamide solution, and the mixture was thoroughly stirred for 15 min. After vacuuming to remove bubbles, a uniform prepolymer solution was obtained, which was placed in a refrigerator for later use.

[0047] Preparation of conductive hydrogel: Use an injection syringe to extract an appropriate amount of prepolymer solution and slowly inject it into a glass mold. During injection, care should be taken to avoid the generation of small bubbles that affect the performance of the prepared hydrogel. Use sealing film to seal the glass mold to prevent air from entering; place the glass mold in an electric constant temperature blast drying oven, heat it to 50°C for polymerization for 6 hours, then peel it out, and then take out the polymerized hydrogel in the glass mold; soak the polymerized hydrogel in 8wt% calcium chloride solution for 24 hours to complete the soaking. After the soaking is completed, use deionized water to rinse the polymerized hydrogel. After rinsing, soak it in deionized water to fully absorb water and reach a swelling equilibrium state to obtain a conductive hydrogel block. After evenly dividing the conductive hydrogel block, obtain several rectangular sheets of conductive hydrogel with a length, width and thickness of 50mm×10mm×1mm. Use nitrogen to dry the surface of the conductive hydrogel for 30s to remove moisture on the surface of the conductive hydrogel to improve the adhesion between the conductive hydrogel and the VHB tape.

[0048] 2) Preparation of flexible strain sensor:

[0049] In the order of copper foil electrode sheet, conductive hydrogel, and copper foil electrode sheet, VHB tape is used to encapsulate the three layers and form a flexible strain sensor based on conductive hydrogel. Two wires are led out from the two layers of copper foil electrode sheets and connected to external signal acquisition equipment.

[0050] like Figure 2As shown, it is the maximum detection limit of the flexible strain sensor. R0 is the initial resistance of the sensor when it is not stretched, with the unit of ohm (Ω). ΔR is the resistance change of the sensor during the stretching process, with the unit of ohm (Ω). GF is the sensitivity factor of the sensor, which refers to the ratio of the relative change in resistance to the strain change under the stable working state. The hydrogel substrate has high stretchability and flexibility, and the encapsulated VHB tape also has good flexibility, which matches the mechanical properties of the hydrogel, improving the detection range of the sensor. GO / SA / PMAAm / Ca 2+ The maximum detection strain of the flexible strain sensor is 178.37%, and it still has a high sensitivity of GF = 4.46 under a high tensile strain close to 180%, which demonstrates the excellent sensing performance of the sensor.

[0051] As Figure 3 shown, it is the minimum detection limit of the conductive flexible strain sensor. Under extremely small tensile strains, the flexible strain sensor still has a relatively stable relative resistance change, and the minimum strain detection limit reaches 0.4%.

[0052] As Figure 4 shown, it describes the relative resistance change rate - time change curves of the flexible strain sensor at finger bending angles of 15°, 30°, 45°, 60°, 75°, and 90°. It can be seen from the figure that as the finger bending angle gradually increases, the relative resistance change of the flexible strain sensor also continuously increases. In the figure, when the finger bending angle reaches 90°, the relative resistance change reaches the highest at this time, about 76%. This precisely shows that GO / SA / PMAAm / Ca 2+ The flexible strain sensor can accurately monitor different finger bending angles. The reason is that during the finger bending process, the GO sheets are continuously broken, resulting in a corresponding gradual increase in resistance and a gradual decrease in the current in the electrical conduction path. In addition, as the finger bending angle increases, the relative resistance change of the sensor shows an upward trend, and the increment of the relative resistance change is almost constant. When the finger is in the bent state and the restored state, the relative resistance change can also remain relatively stable and return to the initial state, which shows that GO / SA / PMAAm / Ca 2+ The flexible strain sensor has good electrical stability.

[0053] As Figure 5As shown, the relative resistance change rate-time change curves of the flexible strain sensor when the human body parts such as the wrist, elbow, knee, and ankle are bent are shown. From the figure, we can see that as the bending degree of each human body part increases, the relative resistance change of the flexible strain sensor gradually increases accordingly. Among them, the maximum of the wrist part is 49%, the maximum of the elbow part is 84%, the maximum of the knee part is 114%, and the maximum of the ankle part is 61%, showing the difference in relative resistance change. The reason for this is that due to the difference in the amplitude during the bending process of different human body parts, the degree of GO sheet layer fracture is different, resulting in different amplitudes of resistance increase. In addition, after the flexible strain sensor returns to the initial state, its relative resistance change can almost return to the initial value again, which also indicates that the flexible strain sensor has good electrical stability.

[0054] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the technical principle of the present invention, several improvements and deformations can still be made, and these improvements and deformations should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a flexible strain sensor based on a conductive hydrogel, the flexible strain sensor comprising two layers of copper foil electrode sheets and one layer of conductive hydrogel, with one layer of conductive hydrogel sandwiched between the two layers of copper foil electrode sheets; two wires are respectively led out from the two layers of copper foil electrode sheets and connected to an external signal acquisition device, and it is characterized in that: The method includes the following steps: 1) Preparation of conductive hydrogel: Preparation of prepolymer solution: Dissolve the graphene oxide dispersion in deionized water to obtain a diluted graphene oxide dispersion; dissolve sodium alginate powder in the diluted graphene oxide dispersion, stir at room temperature until completely swollen and then heat and stir to obtain a graphene oxide / sodium alginate solution; dissolve methacrylamide particles in the graphene oxide / sodium alginate solution, stir at room temperature to obtain a graphene oxide / sodium alginate / methacrylamide solution; dissolve the thermal initiator potassium persulfate in the graphene oxide / sodium alginate / methacrylamide solution, stir and then evacuate to remove air bubbles to obtain a prepolymer solution; Preparation of conductive hydrogel: Use a syringe to extract the prepolymer solution and inject it into a glass mold, seal the glass mold with a sealing film to prevent air from entering; place the glass mold in an electrothermal constant temperature blast drying oven for heating and polymerization and then take it out, and then take out the polymerized hydrogel in the glass mold; soak the polymerized hydrogel in a calcium chloride solution, after soaking, rinse the polymerized hydrogel with deionized water, and soak it in deionized water to fully absorb water until it reaches a swelling equilibrium state to obtain a conductive hydrogel block, and evenly divide the conductive hydrogel block to obtain several sheet-shaped conductive hydrogels; 2) Preparation of flexible strain sensor: In the order of copper foil electrode sheet, conductive hydrogel, and copper foil electrode sheet, use VHB tape to encapsulate the three layers and form a flexible strain sensor based on conductive hydrogel, and lead out two wires from the two copper foil electrode sheets respectively and connect them to an external signal acquisition device.

2. The preparation method of the flexible strain sensor based on conductive hydrogel according to claim 1, characterized in that: In the preparation of the prepolymer solution, the concentration of the graphene oxide dispersion is 10 mg / g, and the concentration of the diluted graphene oxide dispersion is 0.16 - 1.46 mg / g.

3. The preparation method of the flexible strain sensor based on conductive hydrogel according to claim 1, characterized in that: In the preparation of the prepolymer solution, the mass of the graphene oxide dispersion is 0.1 wt% - 0.9 wt% of the total mass of the sodium alginate powder and methacrylamide particles; the mass of the sodium alginate powder is 5 wt% - 25 wt% of the total mass of the sodium alginate powder and methacrylamide particles; the mass of the methacrylamide particles is 75 wt% - 95 wt% of the total mass of the sodium alginate powder and methacrylamide particles; the mass of the thermal initiator potassium persulfate is 0.5 wt% - 1 wt% of the methacrylamide particles.

4. The preparation method of the flexible strain sensor based on conductive hydrogel according to claim 1, characterized in that: In the preparation of the prepolymer solution, it is stirred at room temperature until completely swollen and then heated and stirred to obtain a graphene oxide / sodium alginate solution. Specifically, it is placed on a magnetic stirrer at room temperature and stirred for 4 to 5 h until completely swollen, and then heated to 50 °C and stirred for 30 min to obtain a graphene oxide / sodium alginate solution; a graphene oxide / sodium alginate / methacrylamide solution is obtained after stirring at room temperature. Specifically, a graphene oxide / sodium alginate / methacrylamide solution is obtained after stirring at room temperature for 2 h; a prepolymer solution is obtained after removing bubbles by vacuum pumping after stirring. Specifically, it is stirred for 15 min, and then a prepolymer solution is obtained after removing bubbles by vacuum pumping.

5. The preparation method of the flexible strain sensor based on conductive hydrogel according to claim 1, wherein: In the preparation of the conductive hydrogel, the glass mold is placed in an electrothermal constant temperature forced air drying oven for heating and polymerization and then peeled off. Specifically, the glass mold is placed in an electrothermal constant temperature forced air drying oven and heated to 50 °C for polymerization for 6 h and then peeled off.

6. The preparation method of the flexible strain sensor based on conductive hydrogel according to claim 1, wherein: In the preparation of the conductive hydrogel, the polymerized hydrogel is immersed in a calcium chloride solution. Specifically, the polymerized hydrogel is immersed in a calcium chloride solution with a content of 1 wt% to 10 wt% for 24 h to complete the immersion.

7. The application of a flexible strain sensor prepared by the preparation method according to any one of claims 1-6, wherein: The application of the flexible strain sensor in flexible wearable electronic devices.

8. The application method of a flexible strain sensor prepared by the preparation method according to any one of claims 1-6, wherein: The flexible strain sensor is installed on a flexible wearable electronic device, and the flexible wearable electronic device is worn on the human body. The flexible strain sensor deforms due to the pressure of the human body on the flexible strain sensor during the human body movement state, thereby generating a relative resistance change, and monitoring the movement state of the human body.

Images