Preparation method of high-performance strain sensor based on nano-silver nano-cellulose seepage network

Abstract

The invention belongs to the field of flexible electronic devices, and discloses a preparation method of a high-performance strain sensor based on a nano-silver nano-cellulose seepage network. According to the hydrogel, polyvinyl alcohol is used as a matrix, a seepage network formed by polydopamine anchored nano-silver / nano-cellulose PCNF is used as a conductive phase, glycerin is used as an anti-freezing humectant, and through the synergistic effect of a three-dimensional seepage conductive network and a dynamic hydrogen bond network, the defects that traditional hydrogel is poor in sensing stability, poor in environmental stability and low in mechanical strength are overcome. The preparation process is simple, the conductive hydrogel is high in tensile strength, high in sensitivity and good in cycling stability and has an antibacterial function, and an assembled strain sensor can monitor a human motion signal in real time and is suitable for the fields of wearable equipment, electronic skin and medical monitoring in an extreme environment. Through material system and process innovation, integration of high performance and multiple functions is achieved, and wide application prospects are achieved.

Description

Technical Field

[0001] The present invention belongs to the field of flexible electronic devices, and particularly relates to a high-performance strain sensor based on a silver nanocellulose percolation network, and more particularly to a highly sensitive strain sensor based on a polydopamine-anchored silver nanocellulose percolation network and its applications, which are applicable to wearable devices, human motion monitoring, human-computer interaction, intelligent healthcare, and other flexible electronic sensing systems. Background Art

[0002] In recent years, with the rapid development of flexible electronic technology, the demand for high-performance strain sensors in the fields of wearable devices, intelligent healthcare, human-computer interaction, and health monitoring has increased significantly. Strain sensors can be used to detect human motion, physiological signals (such as breathing, heart rate, muscle contraction), and external mechanical stimuli, and have broad application prospects in health monitoring, motion analysis, and intelligent robots. However, the current hydrogel flexible strain sensors on the market still face major challenges.

[0003] Traditional conductive fillers (such as carbon nanotubes, graphene, metal nanoparticles) require a large amount of use, are prone to aggregation or separation in the flexible matrix, the conductive network is unstable, and the sensing sensitivity is limited. The sensitivity factor (GF) of existing strain sensors is usually high in the low-strain range, but the conductive network is prone to breakage under large-strain conditions, resulting in attenuation of the detection signal and making it difficult to achieve high-precision measurement. After multiple cyclic deformations, signal drift is likely to occur due to the destruction of the conductive network, affecting the stability and service life of the sensor. The mechanical strength of hydrogel flexible strain sensors is relatively low, and they are prone to breakage or permanent deformation during long-term use or under high strain. Due to the high water content of hydrogel flexible sensors, they are prone to freezing in low-temperature environments, resulting in the embrittlement of the flexible material and loss of stretchability. In high-temperature or dry environments, hydrogels are prone to rapid water loss, resulting in performance degradation or even complete failure, seriously affecting the durability and reliability of the sensor and limiting its application in wearable devices.

[0004] In recent years, nanocellulose (CNF) has been used as a reinforcing material for flexible sensors. A single nanocellulose material itself does not have conductivity and needs to be combined with conductive fillers to construct a stable conductive network. However, simple compounding or template loading cannot solve the irregular migration of conductive particles such as shedding, aggregation, and separation. Therefore, there is an urgent need to develop nanocellulose-based strain sensors with high sensitivity, excellent mechanical properties, environmental stability, and reusability. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that when the existing flexible strain sensor materials are used for a long time in a harsh environment, the conductive fillers are prone to phenomena such as shedding, aggregation, separation, and other irregular migrations, resulting in attenuation of the sensing performance and inability to achieve long-term accurate sensing.

[0006] The present invention provides a strain sensor based on a polydopamine-anchored silver nanocellulose percolation network. By constructing polydopamine-anchored silver nanocellulose particles and introducing a polyvinyl alcohol / glycerol / water system, a stable conductive percolation network is formed, achieving high sensitivity, excellent mechanical properties, and long-term environmental stability, and being able to maintain stable sensing performance in extreme environments.

[0007] A preparation method of a high-performance strain sensor based on a silver nanocellulose percolation network includes the following steps:

[0008] (1) Preparation of a polydopamine-anchored silver nanocellulose (PCNF) composite by a one-step reduction method:

[0009] Add dopamine to an aqueous solution of nanocellulose, adjust the pH to 8.5 under Tris buffer conditions, and react at room temperature for 12 hours to form a coating layer of PDA on the surface of CNF; subsequently, slowly mix the AgNO3 solution and ammonia water, control the reduction conditions, and uniformly deposit and anchor silver nanoparticles on the surface of PDA-modified CNF to obtain PDA@CNF-Ag, that is, the PCNF composite.

[0010] (2) Preparation of a PVA-PCNF-GL hydrogel

[0011] Disperse the PCNF composite obtained in step (1) in an aqueous solution of polyvinyl alcohol PVA / glycerol GL at a predetermined ratio and stir evenly, and then perform centrifugal degassing; pour the mixed solution into a mold, place it in an environment of -20°C for 12 hours, and then naturally melt at room temperature for 1 hour to form a conductive hydrogel with a polydopamine-anchored silver nanocellulose percolation network structure.

[0012] (3) Preparation of a low-temperature flexible strain sensor

[0013] Adhere conductive electrodes on both sides of the conductive hydrogel obtained in step (2) to prepare a low-temperature flexible strain sensor.

[0014] Connect the prepared low-temperature flexible strain sensor to a digital multimeter. This device can realize real-time monitoring of the movement state of human joints (such as fingers, wrists, knees, etc.) by detecting the resistance change of the hydrogel under different strain states.

[0015] In step (1), the dosage ratio of the aqueous solution of nanocellulose, dopamine, and AgNO3 solution is 100 mL: 0.2 g: 50 mL. Among them, the concentration of the AgNO3 solution is 0.1 mol / L, and the aqueous solution of nanocellulose is diluted to 0.5 wt.%.

[0016] In step (2), PCNF, polyvinyl alcohol (PVA), and glycerol are mixed in the following three mass ratios: 0.2:4:6 to 18.

[0017] In step (2), in the conductive hydrogel, the mass percentage of glycerol in the total hydrogel system is 15 to 45 wt%; the loading amount of silver nanoparticles is controlled within 0.20 wt%.

[0018] In step (2), the glycerol / water binary solvent system can effectively improve the anti-freezing ability of the sensor, enabling the sensor not to freeze for 120 minutes under the condition of low temperature -20°C and still maintain flexibility for 120 minutes in a high-temperature environment of 70°C.

[0019] In the present invention: The percolation network formed by the PCNF composite material serves as a high-performance adjustable conductive channel, ensuring that mechanical properties and adjustable electron transport are maintained even during large deformations; the polyvinyl alcohol matrix: serves as a flexible support material, endowing the hydrogel with excellent flexibility; the glycerol-water binary solvent system: utilizes the synergistic hydrogen bonds formed between glycerol molecules, water, and polyvinyl alcohol to effectively inhibit freezing at low temperatures and water evaporation at high temperatures, ensuring that the sensor maintains stable performance in the range of -20°C to 70°C.

[0020] The strain sensor prepared by the present invention is used in flexible electronic devices and is applicable to fields such as wearable health monitoring, human motion tracking, human-computer interaction, and rehabilitation medicine.

[0021] The beneficial effects of the present invention are as follows:

[0022] The strain sensor prepared by the present invention has the following remarkable advantages, making it have great potential in the field of wearable electronic devices, especially in applications under low-temperature environments:

[0023] (1) High sensitivity, wide strain detection range, and stable signal output: The sensor has stable high sensitivity, and the sensitivity factor (GF) reaches 2.21, enabling a linear response in a wide strain range from 1% to 100%. This is attributed to the uniform nanofibrillated cellulose-silver nanoparticle percolation network, which constitutes a stable and adjustable conductive channel. The conduction path of the sensor under strain can be dynamically regulated, enabling it to still maintain a stable signal output after high-strain deformation and being suitable for long-term repeated use.

[0024] (2) Excellent mechanical strength and durability: The interfacial bonding force between the PCNF composite material and the PVA matrix is enhanced through hydrogen bonding, significantly improving the overall mechanical properties of the hydrogel, and its maximum tensile stress reaches 154.6 kPa. In addition, the multiple hydrogen bond networks formed between the PVA matrix and glycerol also greatly improve the cyclic stability of the hydrogel, and it can still maintain a stable electrical response even after 300 tensile cycles, ensuring the excellent durability and reusability of the sensor.

[0025] (3) Excellent environmental stability: Due to the introduction of the glycerol-water binary solvent system, the sensor of the present invention will not freeze under low-temperature conditions (down to -20°C), thus ensuring that the sensor still has stable conductivity in extreme environments. The sensor of the present invention can still maintain more than 70% of its initial mass after being exposed to air for 144 hours, significantly improving its moisture retention and environmental stability.

[0026] (4) Antibacterial properties and biocompatibility: The hydrogel has antibacterial properties and can inhibit the growth of Gram-positive bacteria (S. aureus) and Gram-negative bacteria (E. coli), thereby improving the biocompatibility and durability of the sensor.

[0027] (5) Self-adhesion and wearability: The sensor has good self-adhesive characteristics and can quickly adhere to the human skin in a natural state. It can still maintain stable adhesion even under low-temperature conditions, meeting the requirements of an adhesive wearable sensor.

[0028] In summary, the strain sensor material of the present invention, with its high-sensitivity strain detection ability, excellent mechanical strength and durability, excellent low-temperature adaptability, stable signal output, good moisture retention and environmental stability, antibacterial properties, and self-adhesion and wearability, is particularly suitable for real-time monitoring and evaluation of human movements in low-temperature environments and has significant industrial promotion and market application value. Description of the Drawings

[0029] Figure 1 It is a schematic diagram of the synthesis of PVA-PCNF-GL hydrogel.

[0030] Figure 2 It is the FTIR diagram (a) and SEM structure (b) of the freeze-dried sample of PVA-PCNF-GL hydrogel.

[0031] Figure 3 It is the mechanical properties of PVA-PCNF-GL hydrogel.

[0032] Figure 4 It is the test results of the antifreeze, moisture retention and heat resistance properties of PVA-PCNF-GL hydrogel.

[0033] Figure 5 It is the thermogravimetric analysis and differential scanning calorimetry analysis of PVA-PCNF-GL hydrogel.

[0034] Figure 6 It is the analysis of the conductive and sensing properties of the sensor assembled from PVA-PCNF-3.

[0035] Figure 7Electromyogram of human motion monitored by the strain sensor assembled with PVA-PCNF-3.

[0036] Figure 8 Resistance change rate of the strain sensor assembled with PVA-PCNF-3 under different strain conditions.

[0037] Figure 9 Antibacterial performance analysis of PVA-PCNF-GL hydrogel. Detailed implementation mode

[0038] The technical solution of this application will be described in detail in combination with the embodiments shown in the drawings. It should be emphasized that the listed embodiments are only presented exemplarily and are not intended to limit the protection scope of this application. Based on the present disclosure, any equivalent technical solutions obtained by those skilled in the art through conventional technical means without departing from the core concept of this application fall within the protection scope of the claims of this application.

[0039] Example 1

[0040] A preparation method of a strain sensor based on PVA-PCNF-GL conductive hydrogel, using the one-pot method. The schematic diagram is shown in Figure 1 , and the steps are as follows:

[0041] (1) Preparation of PCNF composite

[0042] Weigh 100 mL of 0.5 wt% aqueous nanocellulose solution and add 1.21 g of Tris buffer. After adjusting the pH of the solution to 8.5, add 0.2 g of dopamine (DA) and react at room temperature for 12 hours under magnetic stirring to polymerize DA on the surface of CNF to form PDA@CNF. Filter and wash the reacted solution with a PVDF membrane to obtain PDA@CNF.

[0043] Slowly add the prepared silver ammonia solution to the PDA@CNF solution, stir for 30 minutes, remove the suspended impurities by high-speed homogenization and centrifugation (4000 rpm, 5 minutes), and wash thoroughly to obtain a uniformly dispersed PCNF composite.

[0044] (2) Preparation of PVA-PCNF-GL hydrogel solution

[0045] Add 4 g of polyvinyl alcohol (PVA) and the pre-prepared PCNF (containing 0.2 g of effective PCNF, and the specific amount can be adjusted according to requirements) into a container. Add 6 g of glycerol so that the mass ratio of glycerol in the whole solution is 15 wt%, 30 wt% or 45 wt% respectively. At the same time, add an appropriate amount of deionized water to adjust the total mass. Place the mixed solution in a 95 °C water bath and stir at a low speed for 1 hour until the PVA is completely dissolved and fully mixed with PCNF and glycerol. Centrifuge and degas the solution to obtain a uniform hydrogel solution. Then pour it into a dumbbell-shaped or other required-shaped mold. Place the mold in a -20 °C refrigerator and freeze for 12 hours, and then transfer it to room temperature to thaw naturally for 1 hour to obtain a PVA-PCNF-GL conductive hydrogel. The products corresponding to different glycerol dosages are named PVA-PCNF-1, PVA-PCNF-2, and PVA-PCNF-3 respectively.

[0046] (3) Prepare a low-temperature flexible strain sensor by adding a conductive electrode

[0047] The sensing performance was measured by directly clamping the hydrogel with an alligator clip and fixing it with tape winding.

[0048] Note: In this example, the performance of the hydrogel can be further regulated by adjusting parameters such as the addition amounts of PVA, glycerol, and PCNF.

[0049] Comparative Example 1

[0050] Compared with Example 1, glycerol was not added, and the product was a PVA-PCNF hydrogel.

[0051] I. Structural Characterization

[0052] The prepared PVA-PCNF-GL conductive hydrogel was analyzed for its structure and chemical composition to verify the structure and performance of the PCNF composite material and the hydrogel material.

[0053] (1) Fourier transform infrared spectroscopy (FT-IR) test

[0054] As Figure 2 shown in a of, use a Bruker ALPHA type Fourier transform infrared spectrometer to test the infrared spectrum of the freeze-dried hydrogel sample in the range of 500 - 4000 cm-1. It was observed that the -OH stretching vibration peak (about 3287 cm-1) and C–O stretching vibration peak (about 1128 cm-1) in the PVA matrix shifted to lower wavenumbers after the addition of glycerol, indicating an enhanced hydrogen bond interaction; the presence of the characteristic peaks of metal–oxygen and other functional groups in PCNF proved that silver nanoparticles had been successfully loaded on the nanocellulose.

[0055] (2) Scanning electron microscope (SEM) analysis

[0056] Using MIRA, the freeze-dried hydrogel samples were observed with a TESCAN scanning electron microscope. As Figure 2 shown in the SEM image of b in, a uniform cross-linked three-dimensional porous structure was formed inside the sample, and dispersed nanoparticles could be observed on the pore walls, demonstrating the uniform distribution of the PCNF composite material in the hydrogel network, which helps to form a continuous conductive path.

[0057] II. Mechanical and environmental performance tests

[0058] The mechanical properties of the hydrogel were tested using an MTS-E43 electronic universal material testing machine, and its environmental stability was evaluated under different temperature and humidity environments.

[0059] (1) Mechanical property tests, as Figure 3 shown;

[0060] The stress-strain curves of the hydrogel under tension and compression were tested. The results showed that:

[0061] The PVA-PCNF hydrogel without glycerol had a higher tensile strain but a lower tensile strength;

[0062] After adding 15wt%, 30wt%, and 45wt% glycerol, the maximum tensile strength of the hydrogel increased respectively, up to 154.6 kPa at most, and the elongation at break showed a moderate downward trend;

[0063] After 300 strain cycle tests, the hydrogel still maintained high electrical response and mechanical stability. The cyclic performance test is as Figure 8 shown;

[0064] (2) Environmental stability tests, as Figure 4 shown;

[0065] The prepared hydrogels were respectively placed under low temperature of -20 °C, high temperature of 70 °C, and conditions of 25 °C and 60% relative humidity for moisture retention tests. After 144 hours of testing, the hydrogel samples containing glycerol still maintained more than 70% of the initial mass, showing excellent anti-freezing and moisture retention properties; under low temperature environments, the hydrogel did not freeze, and there was no obvious water loss or hardening under high temperature conditions.

[0066] (3) The thermal stability of the samples was tested using a TGA 4000 thermogravimetric analyzer and a DSC 4000 differential scanning calorimeter. As Figure 5 shown in a and b of, which represent the curves of the sample mass loss rate changing with temperature, and the three stages of sample degradation, 30 °C to 200 °C, 200 °C to 400 °C, and 400 °C to 800 °C. Figure 5The c indicates that as the glycerol content in the sample increases, the anti-freezing ability of the sample enhances. For PVA-PCNF and PVA-PCNF-1, obvious crystallization peaks start to appear at 0 °C, which corresponds to the formation of ice crystals from the water in the hydrogel system at this point. As the glycerol content increases, the position of the crystallization peak of the PVA-PCNF-2 hydrogel drops to -10 °C, while no obvious crystallization peak appears for PVA-PCNF-3, indicating that PVA-PCNF-3 has the best anti-freezing ability.

[0067] III. Conductive and Strain Sensing Performance Tests

[0068] (1) Conductivity test, as shown in Figure 6 a and b of;

[0069] Construct a simple LED circuit with the hydrogel as the conductive medium. The test shows that:

[0070] At room temperature, the hydrogel can conduct electricity stably, and the brightness of the LED lamp is uniform;

[0071] Under low-temperature (-20 °C) conditions, the hydrogel regulated by appropriate glycerol can still maintain high conductivity.

[0072] (2) Strain sensitivity test, as shown in Figure 6 c, d and e of;

[0073] Make the hydrogel sample into a sensor patch, connect the resistance measurement circuit, and record the resistance change during the loading–unloading cycle. When applying strain (0% < Δε < 100%), the measured sensitivity coefficient GF is about 2.21;

[0074] (3) Demonstration of wearable applications as Figure 7 shown;

[0075] Directly attach the hydrogel flexible sensor to human joints, such as the wrist d, elbow c, neck f, and fingers a and b), and the leg e to real-time monitor the joint bending and movement states.

[0076] The test results show that the resistance change signals caused by different joint movements have their own characteristics and can accurately distinguish large-amplitude and subtle movements; at the same time, the sensor can also effectively identify the stroke changes during writing, proving its potential application value in the field of intelligent character recognition.

[0077] IV. Antibacterial Performance Test,

[0078] such as Figure 9As shown, the PVA-PCNF-GL hydrogel has antibacterial properties. A transparent antibacterial zone appears around the hydrogel, and the diameters of the antibacterial zones against Escherichia coli and Staphylococcus aureus are 3.2 mm and 4.2 mm, respectively. This antibacterial property is mainly attributed to the slow release of AgNPs, and the continuously generated Ag + can inhibit bacterial growth.

[0079] The above embodiments have described in detail the preparation, structural characterization, mechanical properties, environmental stability, and electrical and strain sensing properties of the present invention. All test results show that the highly adjustable conductive percolation network constructed by PCNF loaded with anchored silver nanoparticles of the present invention, in synergistic action with the PVA-PCNF-GL hydrogel matrix, not only greatly improves the conductivity and sensitivity of the sensor, but also significantly improves its mechanical properties and environmental adaptability, and has broad application prospects.

[0080] It should be noted that the details not described in this embodiment are well-known to those of ordinary skill in the art. The above is only a specific embodiment of the present invention and should not be regarded as a limitation on the protection scope of the present invention. Any modifications, improvements, and equivalent replacements within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a high-performance strain sensor based on a nanosilver nanocellulose percolation network, characterized in that: The following steps are involved: (1) Preparation of polydopamine-anchored nanosilver nanocellulose PCNF composites by one-step reduction method: Dopamine was added to the nanocellulose aqueous solution, the pH was adjusted under Tris buffer conditions, and the reaction was carried out at room temperature to form a PDA coating layer on the CNF surface; then the AgNO3 solution was slowly mixed with ammonia water, and the reduction conditions were adjusted to make the silver nanoparticles uniformly deposited and anchored on the PDA-modified CNF surface to obtain PDA@CNF-Ag, i.e., PCNF composite material; (2) Preparation of PVA-PCNF-GL hydrogel: The PCNF composite material obtained in step (1) is dispersed in a polyvinyl alcohol (PVA) / glycerol (GL) aqueous solution of a predetermined ratio and stirred evenly, followed by centrifugal degassing; the mixed solution is poured into a mold, frozen, and then naturally thawed at room temperature to form a conductive hydrogel having a polydopamine-anchored nanosilver nanocellulose percolation network structure; (3) Preparation of low temperature flexible strain sensor: Conductive electrodes are adhered to both sides of the conductive hydrogel obtained in step (2) to produce a low-temperature flexible strain sensor.

2. The preparation method according to claim 1, characterized in that In step (1), the dosage ratio of the nanocellulose aqueous solution, dopamine and AgNO3 solution is 100 mL: 0.2 g: 50 mL, wherein the concentration of the AgNO3 solution is 0.1 mol / L, and the nanocellulose aqueous solution is diluted to 0.5 wt.%.

3. The preparation method according to claim 1, characterized in that: In step (1), the pH is adjusted to 8.5; the reaction time at room temperature is 12 hours.

4. The preparation method according to claim 1, characterized in that: In step (2), PCNF, polyvinyl alcohol (PVA) and glycerol are mixed according to the following three mass ratios: 0.2:4:6-18.

5. The preparation method according to claim 1, characterized in that: In step (2), in the conductive hydrogel, the mass percentage of glycerol in the total hydrogel system is 15-45wt%; and the loading amount of silver nanoparticles is controlled within 0.20wt%.

6. The preparation method according to claim 1, characterized in that: In step (2), the freezing is: freezing at -20°C for 12 hours.

7. The preparation method according to claim 1, characterized in that: In step (2), the mixture is naturally thawed at room temperature for 1 hour.

8. The high-performance strain sensor based on the nanosilver nanocellulose percolation network prepared by the preparation method according to any one of claims 1 to 7 is used in flexible electronic devices, which are suitable for wearable health monitoring, human motion tracking or human-computer interaction and rehabilitation medicine.

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