Multifunctional double network composite hydrogel and preparation method and application thereof

Abstract

The present application belongs to the technical field of biosensing, and particularly relates to a multifunctional double-network composite hydrogel and a preparation method and application thereof. The present application uses polyvinyl alcohol and ammonium glycyrrhizinate as raw materials, utilizes polyvinyl alcohol to form hydrogen bonds through cyclic freezing and thawing, and ammonium glycyrrhizinate to construct a double-network hydrogel system through self-assembly, and introduces two-dimensional nanomaterial MXene and Hofmeister effect into the system to endow the system with conductivity and mechanical enhancement, and further prepares an ion-electron dual-conductive multifunctional double-network composite hydrogel for application in the field of flexible sensors. The preparation method of the multifunctional double-network composite hydrogel is green and simple, does not need to introduce other cross-linking agents, and has excellent mechanical properties, conductivity, sensing performance, photothermal performance and anti-swelling performance, and can be used for human signal monitoring. Meanwhile, the hydrogel also has excellent degradability and recyclability, providing a new idea for solving the increasingly serious problem of electronic waste pollution.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of biosensing, and particularly relates to a multifunctional double-network composite hydrogel as well as a preparation method and application thereof. BACKGROUND

[0002] Hydrogel is a kind of polymer material with excellent biocompatibility and unique network structure. Hydrogel is considered as the most ideal choice for preparing flexible sensors, but it still faces many challenges in practical application, including insufficient mechanical properties, poor conductivity, low sensing sensitivity, easy damage, non-degradability and poor adhesion, which seriously restricts the application of hydrogel in the field of high-performance flexible sensors. In particular, traditional hydrogel sensors are difficult to realize simultaneous high-sensitivity monitoring of full-scale human activities and tiny electro-physiological signals. And the hydrogel sensors reported at present usually only have sensing and diagnostic functions, and do not have degradability and reusability, and will produce a large amount of electronic waste after being damaged, which will cause certain impact on the ecological environment. Therefore, it is urgent to develop multifunctional hydrogel materials with good mechanical properties, conductivity, sensing performance, photothermal performance, degradability, recyclability and other excellent properties for the assembly of high-performance flexible skin sensors to realize high-sensitivity human health care sensing and open up a new way for the realization of wearable intelligent diagnosis and treatment. However, how to integrate good mechanical properties, recyclability and good sensing performance and other properties in a single hydrogel is still a challenge.

[0003] The polymer materials for constructing hydrogel-based flexible sensors can be roughly divided into natural polymers and synthetic polymers. Ammonium glycyrrhizinate (GA) is a derivative of a natural terpene compound extracted from the roots and roots of leguminous plants. Due to its good biocompatibility and self-assembly ability, it is widely used in the preparation of hydrogels. In addition, its unique reducing property can spontaneously reduce silver ions to silver nanoparticles, which can provide excellent antibacterial ability for hydrogels. Polyvinyl alcohol (PVA) has good biocompatibility, non-immunogenicity and various physical properties, and is one of the most important hydrophilic polymers in the field of hydrogel flexible sensors. PVA can form stable and elastic hydrogels through physical or covalent cross-linking. Polyvinyl alcohol has great application prospect in hydrogel flexible sensors.

[0004] Hofmeister effect, also known as Hofmeister series or salt effect, refers to the rearrangement of the polymer chain in the polymerization state under the salt effect or salt-induced effect, resulting in different structures and properties. Based on the direct interaction between salt ions and polymer macromolecules and their hydration shell, a hydrophobic interaction zone will be formed in the gel network structure. However, in the Hofmeister salt effect, although the introduction of ions endows the hydrogel with conductivity, this process also increases the crosslinking degree of the hydrogel, which further reduces the porous structure and adversely affects the ion conduction rate, thereby inhibiting the conductivity.

[0005] MXene, as a new two-dimensional nanometer conductive material, has attracted much attention due to its rich surface functional groups, excellent conductivity, good hydrophilicity and excellent photothermal performance. The chemical formula of MXene is M n+1 X n T x (n = 1-4), wherein M represents early transition metals such as Ti and Mo, X is a third or fourth main group element, and T x represents a series of different surface functional groups such as -OH, =O and F. Existing research shows that the combination of MXene and hydrogel can effectively improve the electronic conductivity of hydrogel and endow the hydrogel with excellent photothermal performance and biocompatibility. Based on the advantages and disadvantages of ion conduction and electronic conduction, it has good application value to prepare a hydrogel sensor with ion-electronic dual conductivity.

[0006] At present, there is no literature or patent information reported that ammonium glycyrrhizinate and polyvinyl alcohol are used as raw materials to introduce Hofmeister effect and MXene to prepare an ion-electronic dual conductive multifunctional hydrogel. SUMMARY

[0007] Therefore, in order to overcome the problems of poor mechanical properties, conductivity, degradability and anti-swelling performance of traditional hydrogels, the present application uses polyvinyl alcohol and ammonium glycyrrhizinate as raw materials, uses polyvinyl alcohol to form hydrogen bonds by cyclic freezing and thawing, and uses ammonium glycyrrhizinate to self-assemble to construct a double network hydrogel system, and introduces two-dimensional nanomaterial MXene and Hofmeister effect in the system, to prepare a multifunctional double network composite hydrogel with excellent mechanical properties, conductivity, sensing performance, photothermal performance, degradability and recyclability, which provides technical support for subsequent design of multifunctional hydrogel flexible sensors with high mechanical properties, conductivity and high sensitivity.

[0008] One of the purposes of the present application is to provide a multifunctional double network composite hydrogel.

[0009] To achieve the above object, the present application adopts the following technical solutions:

[0010] The multifunctional double-network composite hydrogel is composed of polyvinyl alcohol, ammonium glycyrrhizinate and MXene based on Hofmeister effect, wherein the concentration of MXene is 0.05wt%-0.15wt%, the concentration of polyvinyl alcohol is 5wt%-10wt%, and the concentration of ammonium glycyrrhizinate is 1wt%-5wt%.

[0011] Preferably, the concentration of MXene is 0.08wt%-0.12wt%, and most preferably 0.1wt%; the concentration of polyvinyl alcohol is 8wt%; and the concentration of ammonium glycyrrhizinate is 2wt%.

[0012] Further, the multifunctional double-network composite hydrogel is a double-network hydrogel system, and polyvinyl alcohol is self-assembled by hydrogen bond combination of ammonium glycyrrhizinate through cyclic freezing and thawing to form a double-network hydrogel system.

[0013] Further, the Hofmeister effect is to immerse the hydrogel in a salt solution to produce Hofmeister effect.

[0014] The introduction of Hofmeister effect makes the crystalline region structure of the hydrogel collapse and rearrange to give it higher crystallinity, thereby greatly enhancing the mechanical properties of the hydrogel. And with the increase of the concentration of sodium citrate solution, the elastic modulus and toughness of the hydrogel gradually increase. This is because with the gradual increase of the concentration of sodium citrate solution, the Hofmeister effect between the hydrogel and citrate ions gradually increases, resulting in more hydrophobic interaction domains, leading to dehydration of the hydrogel and enhancement of hydrogen bonding. At the same time, the introduction of Hofmeister effect salt solution endows the hydrogel with certain electrical conductivity.

[0015] The ion conductivity of the hydrogel prepared based on the above-mentioned Hofmeister effect has certain limitations, and preparing an ion-electron dual-conductive hydrogel is a better strategy. The hydrogel is endowed with excellent electronic conductivity by adding MXene. However, the more MXene is added, the better. The present application found that with the continuous increase of the amount of MXene, the conductivity of PVA / GA / MXene hydrogel showed a trend of first increasing and then decreasing, and the conductive performance was best when the content of MXene was 0.1wt%. This is because with the increase of MXene, the degree of overlap between MXene is higher, and more conductive paths are formed, and then the content of MXene exceeds a certain amount, which will affect the transmission efficiency of electrons between MXene layers. In addition, stacking may also lead to an increase in the gap and defects between MXene layers, which will also become an obstacle to electron transmission, further reducing the conductivity.

[0016] The present application also endows the PVA / GA / MXene hydrogel with excellent photothermal warming performance by adding MXene, and the PVA / GA / MXene hydrogel with 0.1wt% MXene has the strongest photothermal effect, which can quickly reach the equilibrium temperature in a relatively short time of about 400 seconds. And the PVA / GA / MXene hydrogel can be recycled stably in this process, showing excellent photothermal stability.

[0017] The multifunctional double-network composite hydrogel of the present application has excellent anti-swelling performance. Firstly, the introduction of MXene makes the hydrogel contain a large number of hydroxyl and aldehyde groups, thereby making it have higher hydrophilicity. When the hydrophilic colloid is put into water, water molecules will penetrate into the inside of the colloid, making the colloid volume swell. Secondly, due to the introduction of Hofmeister in the hydrogel system, a hydrophobic domain is generated on the hydrogel, the hydrogel dehydrates, the hydrogen bond is enhanced, the physical interaction is enhanced, the crosslinking density of the hydrogel is increased, and the anti-swelling property is improved.

[0018] The multifunctional double-network composite hydrogel of the present application has excellent degradability and recyclability. Because there are hydrophilic functional groups such as -OH on the surface of MXene in the gel network, the hydrogel is endowed with excellent hydrophilic performance. With the increase of temperature, the crosslinked network structure gradually collapses, releasing more free hydrophilic groups, and more water molecules penetrate into the gel network, causing the hydrogel to swell, and finally realizing degradation. In view of the non-toxic and harmless characteristics of PVA / GA / MXene hydrogel, the solution after degradation can be directly discharged to the environment after simple treatment, showing good environmental friendliness. The reversible hydrogen bond interaction between the components in the gel network endows the multifunctional double-network composite hydrogel with excellent recyclability.

[0019] Further, the MXene is a single-layer sheet structure, and the chemical formula of the MXene is Ti3C2.

[0020] The second object of the present application is to provide a preparation method of a multifunctional double-network composite hydrogel.

[0021] To achieve the above object, the present application adopts the following technical solutions:

[0022] The preparation method of the multifunctional double-network composite hydrogel comprises the following steps:

[0023] (1) mixing the polyvinyl alcohol, the ammonium glycyrrhizinate salt and the MXene to obtain a hydrogel precursor solution by reaction;

[0024] (2) introducing Hofmeister effect by freezing-thawing the hydrogel precursor solution obtained in step (1) and immersing it in a salt solution to obtain the multifunctional double-network composite hydrogel.

[0025] Further, the freezing-thawing cycle is 2-5 times, and each freezing is 6-10 hours and each thawing is 3-5 hours.

[0026] Preferably, the freezing-thawing cycle is 3 times, each freezing is 8 hours and each thawing is 4 hours.

[0027] Further, the salt solution is any one or more of sodium sulfate and sodium citrate; and the concentration of the salt solution is 10wt%-25wt%.

[0028] Preferably, the salt solution is sodium citrate; and the concentration of the salt solution is 22.5wt%.

[0029] Further, the soaking time is 10-24 hours, preferably 12 hours.

[0030] Further, the MXene is prepared by selectively removing the aluminum layer in the MAX phase precursor (Ti3AlC2) by using the HCl-LiF acid etching method and ultrasonic stripping.

[0031] Preferably, the MXene is prepared by the following method:

[0032] 1) adding LiF into a hydrochloric acid solution, stirring for 20 minutes to obtain a hydrofluoric acid mixed etching solution;

[0033] 2) slowly adding the raw material MAX phase powder into the hydrofluoric acid mixed etching solution obtained in step 1) in multiple times to react, stirring at 35℃ for 24 hours, and washing by multiple centrifugation operations until the pH of the supernatant is 6 to obtain a reaction solution;

[0034] 3) The reaction solution obtained in step 2) is added with deionized water and shaken uniformly, nitrogen bubbling is carried out, and ice bath ultrasonic treatment is carried out for 60 min; the MXene suspension is centrifuged at a speed of 3500 rpm for 60 min to obtain stably dispersed MXene.

[0035] The present application introduces Hofmeister effect by simple soaking method, and endows the hydrogel with excellent mechanical and conductive properties. The effect of soaking is reflected in two aspects. On the one hand, salt ions directly interact with the polymer chains of the hydrogel, thereby producing a crosslinked network to enhance the hydrogel. On the other hand, the salting-out effect, the concentrated salt solution will make the volume of the hydrogel shrink, and then some active sites or groups on the polymer chains will be closer to form specific interactions, such as microcrystals, hydrogen bonds and hydrophobic interactions. In this process, there may be different interactions between ions, polymer chains and water, which depends on the type of ions, which will lead to the difference of the microstructure and performance of the polymer. The present application preferably soaks the sample in 22.5wt% sodium citrate solution, which endows the hydrogel with excellent mechanical properties.

[0036] The third object of the present application is to provide a flexible sensor for human signal monitoring.

[0037] To achieve the above object, the present application adopts the following technical scheme:

[0038] The flexible sensor for human signal monitoring contains the aforementioned multifunctional double-network composite hydrogel.

[0039] The fourth object of the present application is to provide an application of the multifunctional composite hydrogel in preparing a flexible sensor for human signal monitoring.

[0040] To achieve the above object, the present application adopts the following technical scheme:

[0041] The application of the multifunctional composite hydrogel in preparing a flexible sensor for human physiological activity monitoring.

[0042] Further, the human physiological activity monitoring includes health monitoring, sports monitoring, underwater human signal monitoring.

[0043] The present application has the following beneficial effects:

[0044] 1. Ammonium glycyrrhizinate has excellent self-assembly behavior, antibacterial performance and biocompatibility, polyvinyl alcohol has excellent mechanical strength, the present application utilizes the cyclic freezing and thawing of polyvinyl alcohol to form hydrogen bonds and ammonium glycyrrhizinate self-assembly to construct a PVA / GA double-network hydrogel system, which does not introduce toxic crosslinking agents and has a simple and green preparation method.

[0045] 2. The present application introduces MXene and Hofmeister effect in the PVA / GA double network hydrogel system, which not only enhances the nanometer, but also endows the hydrogel with high-strength mechanical properties, conductive properties, sensing properties, photothermal properties, degradability and recyclability, etc., providing a basis for a new type of MXene-based nanocomposite hydrogel flexible sensor.

[0046] 3. The PVA / GA / MXene hydrogel of the present application has excellent mechanical tensile properties, with an elastic modulus of 380 KPa, a toughness of 14200 KJ / m 2 , a maximum elongation at break of 1200%, and a tensile strength of 2700 KPa. At the same time, it also has excellent compression performance, with an elastic modulus of 325 KPa and a maximum pressure of 250 KPa.

[0047] 4. The PVA / GA / MXene hydrogel of the present application has excellent conductive properties, with an electrical conductivity of 0.601 S / m.

[0048] 5. The PVA / GA / MXene hydrogel of the present application exhibits excellent photothermal warming performance and photothermal stability, and can quickly reach an equilibrium temperature of 54℃ in a short time of 400 seconds, with a temperature change of 28℃.

[0049] 6. The present application introduces Hofmeister effect in the hydrogel system, which produces hydrophobic interaction on the hydrogel, dehydrates the hydrogel, enhances the hydrogen bond and physical interaction, increases the crosslinking density of the hydrogel, and improves the anti-swelling property. Therefore, the PVA / GA / MXene hydrogel of the present application has excellent anti-swelling performance, with a swelling rate of about 3.5 g / g, and has great application potential in the field of underwater movement monitoring.

[0050] 7. The PVA / GA / MXene hydrogel of the present application has high sensitivity in a wide strain range of 0-1200%, with a GF of 0.431, and has reliable sensing stability and good actual movement monitoring effect, and can be used as a sensor to measure human activity.

[0051] 8. The PVA / GA / MXene hydrogel of the present application has good degradability and recyclability. The hydrogel has non-toxic and harmless properties, and the solution after degradation can be directly discharged to the environment after simple treatment, showing good environmental friendliness. It can also be melted and reshaped into a new hydrogel product. The present application provides a new idea to solve the increasingly serious problem of electronic waste pollution.

[0052] 9.The PVA / GA / MXene hydrogel is prepared by one-pot method, which overcomes the problems of poor mechanical properties, poor electrical conductivity, poor degradability and poor anti-swelling performance of traditional hydrogels, realizes wide range and stable health monitoring of hydrogel sensors, and integrates photothermal performance and degradability, which is expected to realize photothermal synergistic therapy in wound flexible sensor monitoring. The hydrogel sensor provides a new idea for the integration of wearable intelligent medical diagnosis and treatment. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 Preparation flow chart of MXene;

[0054] Figure 2 Preparation schematic diagram of PVA / GA / MXene hydrogel;

[0055] Figure 3 XRD test result chart;

[0056] Figure 4 Hydrogel gelation chart, wherein, Figure 4 -A is a PVA / GA aqueous solution chart; Figure 4 -B is a PVA / GA hydrogel gelation chart; Figure 4 -C is a PVA / GA / MXene aqueous solution chart; Figure 4 -D is a PVA / GA / MXene hydrogel gelation chart;

[0057] Figure 5 Hofmeister effect salt bubble stretching test chart, wherein, Figure 5 -A is a stress-strain curve chart; Figure 5 -B is an elastic modulus and toughness chart; Figure 5 -C is a maximum tensile stress chart; Figure 5 -D is a statistical chart of breaking elongation;

[0058] Figure 6 Stretching test chart of bubbles with different concentrations of sodium citrate, wherein, Figure 6 -A is a stress-strain curve chart; Figure 6 -B is an elastic modulus and toughness chart; Figure 6 -C is a maximum tensile stress chart; Figure 6 -D is a statistical chart of breaking elongation;

[0059] Figure 7 MXene content on hydrogel tensile property test chart, wherein, Figure 7 -A is a stress-strain curve chart; Figure 7 -B is an elastic modulus and toughness chart; Figure 7 -C is a maximum tensile stress and breaking elongation chart; Figure 7-D is the tensile experiment process diagram;

[0060] Figure 8 The compression performance test diagram of the hydrogel with different MXene contents, wherein, Figure 8 -A is the stress-strain curve diagram; Figure 8 -B is the elastic modulus diagram; Figure 8 -C is the maximum stress diagram and the maximum strain diagram; Figure 8 -D is the compression experiment process diagram;

[0061] Figure 9 -A is the tensile curve diagram of ten continuous cycles at 50% strain; Figure 9 -B is the loading-unloading test result diagram at different strains;

[0062] Figure 10 The conductivity test diagram, wherein, Figure 10 -A is the PVA / GA / MXene conductivity diagram with different MXene contents; Figure 10 -B is the luminance diagram of the light-emitting diode after the switch is closed; Figure 10 -C is the luminance diagram of the light-emitting diode after the hydrogel is stretched;

[0063] Figure 11 -A is the temperature change curve diagram of the hydrogel under continuous infrared irradiation; Figure 11 -B is the infrared thermal imaging diagram; Figure 11 -C is the temperature change curve diagram under different power infrared irradiation; Figure 11 -D is the cycle stability test diagram of the photothermal performance;

[0064] Figure 12 The PVA / GA / MXene hydrogel swelling experiment diagram;

[0065] Figure 13 The sensing performance test diagram, wherein, Figure 13 -A is the hydrogel sensitivity test diagram; Figure 13 -B is the ΔR / R0 diagram of multiple finger bending movements when the PVA / GA / MXene hydrogel sensor is fixed at the finger joint; Figure 13 -C is the ΔR / R0 diagram of multiple wrist bending movements when the PVA / GA / MXene hydrogel sensor is fixed at the wrist; Figure 14 -D is the ΔR / R0 diagram of different degrees of finger bending;

[0066] Figure 14 -A is the diagram before and after the degradation of the hydrogel; Reagent name -B is the plasticity and recyclability test result diagram of the PVA / GA / MXene hydrogel. DETAILED DESCRIPTION

[0067] The technical solutions of the present application will be described further in detail below in combination with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments in the present application without creative labor fall within the protection scope of the present application.

[0068] In the embodiments of the present application, the main experimental reagents are shown in Table 1.

[0069] Table 1. List of experimental reagents

[0070] Molecular formula Purity Ammonium glycyrrhizinate salt Polyvinyl alcohol C 42 H 65 NO 16 ]]> —— Sodium chloride [-CH2CHOH-] n ]]> —— NaCl Analytically pure Sodium sulfate Analytically pure Na2SO4 Zinc sulfate Analytically pure ZnSO4 Sodium citrate Analytically pure [C6H5Na3O7] Lithium fluoride LiF Analytically pure Hydrochloric acid HCl Analytically pure Titanium aluminum carbide Analytically pure (Ti3AlC2)MAX Figure 1

[0071] In the embodiments of the present application, the main experimental instruments are shown in Table 2.

[0072] Table 2. List of experimental instruments

[0073]

[0074]

[0075] Example 1. Preparation of PVA / GA / MXene hydrogel

[0076] (1) Preparation of MXene

[0077] The preparation process is shown in Figure 2 . The aluminum layer in the MAX phase precursor (Ti3AlC2) is selectively removed by using an HCl-LiF acid etching method to prepare ultra-thin two-dimensional MXene nanosheets (Ti3C2). The specific steps include: first, 1g of LiF is added to 20ml of hydrochloric acid solution (9M) and stirred for 20 minutes to obtain a hydrofluoric acid mixed etching solution. Second, the raw material MAX phase powder is slowly added to the hydrofluoric acid etching solution in multiple times, ensuring that it is fully reacted with the etching solution. Continue to stir and react at 35℃ for 24h, then wash the obtained material by multiple centrifugation operations until the pH of the upper clear liquid is about 6. Finally, add a proper amount of deionized water to the obtained material, shake well, and perform nitrogen bubbling, ice bath ultrasonic for 60min. Finally, centrifuge the MXene suspension at a speed of 3500rpm for 60 minutes to obtain a stable dispersion of single-layer MXene aqueous dispersion. The MXene dispersion is placed in a refrigerator (4℃) for low temperature preservation for subsequent use.

[0078] (2) Preparation of PVA / GA / MXene hydrogel

[0079] The preparation process of PVA / GA / MXene hydrogel is shown in Sample numberPVA / GA / MXene hydrogel was prepared by a simple one-pot method according to the experimental conditions of Tables 3-5. First, PVA, GA aqueous solution (ammonium glycyrrhizinate solution) and MXene were mixed to obtain a hydrogel precursor solution. After cooling at room temperature, the solution was subjected to 3 cycles of freezing and thawing, each cycle involving freezing for 8 h and thawing for 4 h. The Hofmeister effect was induced by soaking the solution in a salt solution to obtain the PVA / GA / MXene hydrogel. The PVA / GA / MXene hydrogel was prepared.

[0080] Table 3. Experimental grouping of salt-swollen hydrogels

[0081] PVA (wt%) GA (wt%) MXene (wt%) Salt (20 wt%) Soaking time (h) Sample number 1 8 2 0.1 Na2SO4 12 2 8 2 0.1 Na2SO4 24 3 8 2 0.1 ZnSO4 12 4 8 2 0.1 ZnSO4 24 5 8 2 0.1 [C6H5Na3O7] 12 6 8 2 0.1 [C6H5Na3O7] 24

[0082] Table 4. Experimental grouping of hydrogels swelled in different concentrations of sodium citrate

[0083] PVA (wt%) GA (wt%) MXene (wt%) Soaking time (h) Sodium citrate concentration (wt%) Sample number 1 8 2 0.1 12 10 2 8 2 0.1 12 15 3 8 2 0.1 12 20 4 8 2 0.1 12 22.5

[0084] Table 5. Experimental grouping of hydrogels with different MXene contents

[0085] PVA (wt%) GA (wt%) Sodium citrate concentration (wt%) Soaking time (h) MXene (wt%) Figure 3 1 8 2 22.5 12 0 2 8 2 22.5 12 0.05 3 8 2 22.5 12 0.1 4 8 2 22.5 12 0.12 5 8 2 22.5 12 0.15

[0086] Example 2. Characterization of MXene material

[0087] (1) Tyndall effect

[0088] A 10 mg / ml MXene solution was prepared in a 20 ml transparent glass bottle. A bright light path was observed when a laser pointer was used to focus a light beam through the glass bottle and the MXene solution, indicating that the material had surface charges and colloidal particles and exhibited a Tyndall effect, which could be effectively applied to the preparation of hydrogel materials.

[0089] (2) X-ray diffractometer (XRD)

[0090] A 3 ml MXene dispersion was filtered by vacuum suction to form a film, and XRD was used to further investigate the structural composition of the MAX phase, single-layer MXene and multi-layer MXene sample materials. The spectra of the single-layer MXene and multi-layer MXene materials and the MAX phase were observed, and the changes in the characteristic peaks after etching, intercalation and further delamination were compared. The scanning angle range was 5-90°.

[0091] The results are shown in Figure 4As shown, it can be seen that compared with MAX, the Al element in MXene is etched after etching, which corresponds to the obvious weakening of the (104) characteristic peak at 39°. At the same time, the test results show that the (002) characteristic peak is shifted to the left (9.5° to 6.5°). According to the Bragg diffraction equation 2dsinθ = λ, λ is a fixed value, θ decreases, d increases, indicating that the interlayer spacing of the MXene nanosheet increases, and also indicating that the Al element is etched. In addition, we observe that the multi-layer MXene still has the characteristic peak of Ti3AlC2, which is derived from the product obtained after etching and centrifugal washing of the MAX phase, and part of the Ti3AlC2 that is not completely etched is inevitably present. In contrast, single-layer MXene is obtained by ultrasonic peeling of multi-layer MXene and centrifugation. Due to the density difference of Ti3AlC2, it will be precipitated at the bottom of the centrifuge tube, so the single-layer MXene almost does not contain Ti3AlC2 component, and the (104) Al characteristic peak at 39° in the XRD pattern disappears.

[0092] Example 3. Hydrogel gelation test

[0093] A 2wt% GA aqueous solution was prepared and placed in a glass bottle for heating and cooling at high temperature (85°C). The self-assembly of the GA aqueous solution into a gel was observed by the tilting method. A PVA / GA / MXene hydrogel solution was prepared by one-pot method and transferred to a glass bottle. The PVA / GA / MXene hydrogel gelation was observed by the tilting method.

[0094] Results: As shown in Figure 4 -A and Figure 4 -B, the 2wt% GA aqueous solution was heated and self-assembled during the cooling process to form a GA hydrogel after about 10 min. This is because the hydrophobic group on the GA chain is connected to the hydrophilic group, which can self-assemble into a supramolecular hydrogel by non-covalent interaction. Subsequently, based on the PVA / GA hydrogel double network system, two-dimensional nanomaterial MXene was introduced to construct a PVA / GA / MXene hydrogel. The introduction of MXene provides a large number of hydroxyl and carboxyl groups, which can better react with PVA / GA to form a gel after 5 min of cooling. Details are shown in Figure 4 -C and Figure 5 -D.

[0095] Example 4. Mechanical property test

[0096] The PVA / GA / MXene double network hydrogel is formed by the supermolecular network of GA self-assembly and the large number of hydrogen bond networks formed by the cyclic freezing and thawing of PVA, and both are physical interaction gel with poor mechanical properties. Therefore, the present application introduces a simple and green mechanical performance enhancement strategy Hofmeister to enhance the mechanical performance and improve the gel conductivity at the same time. In order to detect the strength of the prepared hydrogel, the mechanical properties of the PVA / GA / MXene hydrogel are measured by a mechanical universal testing machine, and the effects of the type, time, concentration of salt and MXene content of Hofmeister effect on the mechanical properties of the hydrogel are explored. Specifically, according to the Hofmeister effect cation sequence K + >Na + >Cs + >Li + >NH + >Ca 2+ >Mg 2+ >Al 3+ , anion sequence Cit 3- >SO4 2- >CO3 2- S2O3 2- >Ac - >Cl - >NO 3- >I - , the effects of sodium citrate, sodium sulfate and zinc sulfate on the mechanical properties of PVA / GA / MXene hydrogel are explored, and the Hofmeister effect is verified. The effects of salt type and soaking time on the mechanical properties of the hydrogel are explored, and the grouping is shown in Table 3. After obtaining the best salt type and soaking time, the effects of salt solution concentration and MXene content on the mechanical properties of PVA / GA / MXene hydrogel are explored, as shown in Tables 4 and 5.

[0097] 1. Test method

[0098] (1) Compression test

[0099] The square sample (4.5mm in length) of the hydrogel prepared in Example 1 was subjected to compression test, and the measurement was carried out at ambient temperature with a loading rate of 5mm / min. The compression load (F) and displacement (x) were recorded.

[0100] The calculation formula of compression stress is as follows:

[0101]

[0102] wherein, Loadvalue is the negative weight value, unit N; a is the length of the hydrogel, unit mm.

[0103] The formula for calculating the compressive strain is as follows:

[0104]

[0105] where Position is the displacement in mm; h is the height of the hydrogel in mm.

[0106] Five parallel samples were tested for each group, and the average value of the test results was recorded.

[0107] The elastic modulus of the hydrogel was estimated from the slope of the straight line fitted to the stress-strain curve from 0% strain to 10% of the strain before the fracture strain.

[0108] (2) Tensile test

[0109] Tensile tests were performed on the rectangular samples (30 mm long, 10 mm wide, and 2 mm high) of the hydrogel prepared in Example 1, and the measurements were performed at ambient temperature with a loading rate of 50 mm / min, with a measurement length of 5 mm between the clamps. The tensile load (F) and displacement (x) were recorded.

[0110] The formula for calculating the tensile stress is as follows:

[0111]

[0112] where F is the tensile load of the hydrogel in N; S is the cross-sectional area in mm 2 .

[0113] The formula for calculating the tensile strain is as follows:

[0114]

[0115] where x is the sensor displacement, and l0 is the distance between the two clamps in mm.

[0116] The toughness (E) of the hydrogel is the area enclosed by the stress-strain curve from 0% strain to the fracture strain (ζβ), and the formula for calculating it is as follows:

[0117]

[0118] (3) Cyclic tensile test

[0119] Tensile test was performed on the rectangular sample (30 mm in length, 10 mm in width, and 2 mm in height) of the hydrogel prepared in Example 1, and the measurement length between the grips was 5 mm. The measurement was performed at an ambient temperature at a loading rate of 50 mm / min, and 10 continuous loading-unloading cycle tests were performed at a strain of 50%, and the sample returned to the initial position was one cycle, and the tensile load (F) and displacement (x) were recorded. At the same time, the cyclic tensile test was performed at a strain of 60%, 100%, 200%, and 300%. The calculation formula of the tensile stress and strain was the same as that in "(2) Tensile test".

[0120] 2. Test results

[0121] (1) Analysis of test results of the effect of salt type and time on the mechanical properties of the hydrogel

[0122] According to the Hofmeister effect sequence, the cation sequence is K + > Na + > Cs + > Li + > NH + > Ca 2+ > Mg 2+ > Al 3+ , and the anion sequence is Cit 3- > SO4 2- > CO3 2- S2O3 2- > Ac - > Cl - > NO 3- > I - Three kinds of salts, sodium citrate, sodium sulfate, and zinc sulfate, were selected to soak the PVA / GA / MXene hydrogel to verify the Hofmeister effect. The effect of salt type and soaking time on the stress-strain curve of the PVA / GA / MXene hydrogel was tested by a mechanical universal testing machine. As shown in Figure 5 , the mechanical properties of the hydrogel without salt soaking were poor and could not be tested. The mechanical properties of the hydrogel after salt soaking were significantly enhanced, which was because of the Hofmeister effect, the collapse of the crystal region structure inside the hydrogel, and the rearrangement to give it higher crystallinity, thereby greatly enhancing the mechanical properties of the hydrogel. As shown in Figure 5 -A, the mechanical properties of the hydrogel soaked in zinc sulfate salt solution for 12 h were the worst, with a breaking stress of only 173.9 KPa and a breaking strain of 257%. The second was sodium sulfate and sodium citrate, and the hydrogel soaked in sodium sulfate salt solution for 12 h reached 249 KPa and 431%, and the breaking stress and strain of the hydrogel soaked in sodium citrate salt solution for 12 h reached 1320 KPa and 480%. According to Figure 6The results of toughness and elastic modulus of the hydrogel sample represented by B show that the toughness and considerable elastic modulus of PVA / GA / MXene hydrogel soaked in sodium citrate solution for 12h are 3252KJ / m 2 , 284KPa. Since soaking in sodium citrate for 24h takes too long and does not greatly improve the mechanical properties of the hydrogel, the samples in the subsequent experiments on sodium citrate concentration are soaked in sodium citrate solution for 12h.

[0123] (2) Test results and analysis of the effect of salt concentration on the mechanical properties of hydrogel

[0124] The tensile stress-strain curves of PVA / GA / MXene hydrogel soaked in different concentrations of sodium citrate solution were tested by a mechanical universal testing machine. As shown in Figure 7 , the mechanical properties of the hydrogel are best when the concentration of sodium citrate solution is 22.5wt%, with a breaking stress of 2500KPa and a breaking strain of 1200%. As the concentration of sodium citrate solution increases, the elastic modulus and toughness of the hydrogel gradually increase, with the elastic modulus increasing from 55KPa to 373KPa and the toughness increasing from 55KJ / m 2 to 380KJ / m 2 . This is because as the concentration of sodium citrate solution increases, the Hofmeister effect between the hydrogel and citrate ions gradually increases, resulting in more hydrophobic interaction domains, causing the hydrogel to dehydrate, the hydrogen bond to strengthen, and the crosslinking density to increase. Therefore, the elastic modulus increases continuously, consistent with the trend of the tensile stress-strain curve. Therefore, the subsequent experiments use sodium citrate solution with a concentration of 22.5wt%.

[0125] (3) Test results and analysis of the effect of MXene content on the mechanical properties of hydrogel

[0126] The effect of MXene content on the tensile stress-strain curve of PVA / GA / MXene hydrogel was tested by a mechanical universal testing machine. The results, as shown in Figure 8 , show that as the MXene content increases, the elastic modulus and toughness of the hydrogel first increase and then decrease, and when the MXene content is 0.1wt%, the elastic modulus and toughness reach their maximum values of 380KPa and 14200KJ / m 2 , respectively. This may be because when the MXene content is too high, the hydrogel dehydrates severely during the salting-out process, and the overly dense network structure inside the hydrogel makes it hard and tends to plasticize, thus reducing the toughness.

[0127] The compression performance test results of hydrogels with different MXene contents are as follows Figure 8 -A Figure 8-C shows that the compression performance of PVA / GA / MXene hydrogel with different MXene contents is similar, and the elastic modulus is 300-350 KPa, and the maximum pressure is 250 KPa. Figure 9 -D shows that the hydrogel can withstand a large pressure and quickly recover to the initial state after the pressure is removed. As can be seen, the PVA / GA / MXene hydrogel with different MXene contents has good compression resistance.

[0128] (4) Results and analysis of cyclic tensile test

[0129] In order to verify the cyclic stability and energy dissipation characteristics of the PVA / GA / MXene hydrogel, the present application carries out cyclic tensile loading-unloading test. As shown in Figure 9 As shown in -A, the hydrogel still exhibits excellent resilience after 10 cycles. In the first loading-unloading cycle, the area surrounded by the stress-strain curve is the largest, which is due to the breaking of a large number of hydrogen bonds, which protects the gel network structure. In the subsequent 9 cycles, the shape and size of the stress-strain curve are almost the same, which indicates that the hydrogel has excellent fatigue resistance. As shown in Figure 10 -B shows that when the PVA / GA / MXene 0.1wt% hydrogel is tested under different strain conditions (60%-300%), the stress-strain curve exhibits different sizes of hysteresis loops, which strongly proves that the hydrogel has excellent energy dissipation capacity. With the increase of strain, the area of stress-strain hysteresis loop gradually increases, indicating that the greater the strain, the more energy needs to be dissipated. The energy dissipation mechanism of the hydrogel may be related to the breaking of hydrogen bonds inside the hydrogel. With the increase of strain, more hydrogen bonds are broken to dissipate energy to protect the hydrogel. Based on the experimental results of the tensile test, the PVA / GA / MXene hydrogel has excellent tensile strength and fatigue resistance, and has great application prospect in the field of hydrogel flexible sensor.

[0130] In summary, when the PVA / GA / MXene 0.1wt% hydrogel is soaked in 22.5wt% sodium citrate solution for 12h, the mechanical tensile properties are the best, the elastic modulus is 380KPa, the toughness is 14200KJ / m 2 , the maximum elongation at break is 1200%, and the tensile strength is 2700KPa. The compression test results show that the compression performance of PVA / GA / MXene hydrogel with different MXene contents is similar, and the elastic modulus is 325KPa, and the maximum pressure is 250KPa.

[0131] Example 5. Conductive performance test

[0132] In order to explore whether the PVA / GA / MXene hydrogel has the conductive ability as a flexible sensor, the conductivity of the PVA / GA / MXene hydrogel is tested. The resistance of the hydrogel sample is tested by DMM6500 source table. Four parallel samples are set for each group of samples, and the average value is obtained. The calculation formula of conductivity is as follows:

[0133]

[0134] wherein R represents the resistance value, unit is ohm (Ω); S represents the cross-sectional area of the hydrogel, unit is square meter (m 2 ); L represents the length of the hydrogel between two electrodes, unit is meter (m); σ is the conductivity, unit is siemens per meter (S / m).

[0135] The results are shown in Figure 10 -A, due to the introduction of MXene nanosheets, the PVA / GA / MXene hydrogel exhibits excellent conductivity. With the continuous increase of the amount of MXene, the conductivity of the PVA / GA / MXene hydrogel presents a trend of first increasing and then decreasing, and reaches the maximum value of 0.601 S / m when the content of MXene is 0.1wt%. Then with the increase of the amount of MXene, the conductivity presents a downward trend, which is due to the higher degree of overlap between MXene and the formation of more conductive paths with the increase of MXene. Then the content of MXene is too much to produce stacking. When the MXene layers are stacked together, the contact interface between them will form a certain resistance, which will affect the transmission efficiency of electrons between the MXene layers. In addition, stacking may also lead to an increase in voids and defects between MXene layers, which will also become an obstacle to electron transmission, further reducing its conductivity. The improvement of the conductivity of the hydrogel is conducive to the application of the hydrogel in flexible sensing to a certain extent, especially for the high-sensitivity monitoring of small electrical physiological signals. In addition, the hydrogel is connected in a closed loop, which more directly shows the conductive performance of the hydrogel, as shown in Figure 10 -B, after closing the switch, the light-emitting diode is lit, indicating that the hydrogel is conductive. As shown in Figure 11 -C, as the hydrogel is stretched, the brightness of the light-emitting diode decreases, indicating that the resistance of the hydrogel increases with the increase of the length of the gel, further proving the potential application of the hydrogel in sensors.

[0136] Example 6. Photo-thermal performance test

[0137] The photothermal properties of PVA / GA / MXene hydrogels with different MXene contents under infrared light irradiation were evaluated. The hydrogel samples with different MXene contents (5 mm in diameter and 2 mm in thickness) were placed in a culture dish and irradiated with 808 nm infrared light at a preset power density. A temperature sensor was used to test the sample temperature every 30 seconds, and the temperature change of the sample was recorded. Each group was set with 3 parallel samples. Finally, the temperature change curve of the sample under 808 nm infrared light irradiation was plotted according to the experimental data. At the same time, the photothermal stability of the hydrogel was tested by the method of 30s interval irradiation.

[0138] The results are shown in Figure 11 As shown in -A, the PVA / GA / MXene hydrogels with different MXene contents rapidly warmed up under continuous 808 nm infrared light irradiation, and the temperature change was stable at about 28℃, which proved that MXene had photothermal effect. And the PVA / GA / MXene hydrogel with 0.1wt% MXene content had the strongest photothermal effect, which could quickly reach the equilibrium temperature in a relatively short time of about 400 seconds, which was consistent with the results of the conductivity test. This is because too much MXene content will produce stacking, affecting the oscillation of electrons and hindering the formation of so-called surface plasmon waves. As a control, the hydrogel without MXene showed negligible temperature change under the same infrared light conditions, which indicated that the MXene hydrogel sensor had excellent photothermal warming performance. Figure 11 -B is the infrared imaging of PVA / GA / MXene hydrogel under 150s continuous infrared light irradiation, it can be seen that the photothermal imaging is consistent with Figure 11 -A data.

[0139] The present application further studied the relationship between different power infrared light and the temperature change of the hydrogel, and the results are shown in Figure 11 -C, the photothermal warming rate of the hydrogel showed an increasing trend with the increase of power. Since the photothermal stability is also a crucial indicator in photothermal therapy, the present application further evaluated the photothermal stability of PVA / GA / MXene hydrogel by switching the infrared switch. The results are shown in Figure 12 -D, when the infrared light switch is turned on, the hydrogel can quickly warm up; when the infrared light is turned off, the temperature of the hydrogel decreases rapidly. The PVA / GA / MXene hydrogel can be recycled stably in this process, which proves its excellent photothermal stability.

[0140] In summary, the PVA / GA / MXene hydrogel exhibits excellent photothermal warming performance, and the PVA / GA / MXene hydrogel with a MXene content of 0.1wt% has the strongest photothermal effect, which can quickly reach an equilibrium temperature of 54℃ in a relatively short time of about 400 seconds, with a temperature change of 28℃. The hydrogel without MXene shows negligible temperature change under the same infrared light conditions. Moreover, the hydrogel also exhibits excellent photothermal stability.

[0141] Example 7. Swelling performance test

[0142] The same size of hydrogel samples (5mm in diameter and 2mm in thickness) were prepared, and 5 parallel samples were set in each group. After freeze-drying, the mass of the hydrogel was weighed and recorded as W d Then it was soaked in ionized water, and after different time intervals, the hydrogel was taken out, the surface water of the hydrogel was absorbed with filter paper, and then weighed. At this time, the sample weight was recorded as W s After weighing, the hydrogel was continuously soaked, and the above operation was repeated until the hydrogel reached swelling equilibrium and was weighed. Finally, the swelling rate of the hydrogel was calculated.

[0143] The calculation formula of the swelling rate is as follows:

[0144]

[0145] Wherein, W s : the mass of the hydrogel after swelling, W d : the mass of the hydrogel after freeze-drying.

[0146] The results are shown in Figure 13 The swelling rate of the hydrogel rapidly increased in the first 3h, and reached swelling equilibrium around 24h. The swelling ratios of hydrogels with different MXene contents were all around 3.5g / g. This is because the introduction of MXene makes the hydrogel contain a large number of hydroxyl and aldehyde groups, thereby making it have high hydrophilicity. When the hydrophilic colloid is placed in water, water molecules will penetrate into the interior of the colloid, causing the colloid to swell in volume. In addition, due to the introduction of Hofmeister in the hydrogel system, a hydrophobic domain is generated on the hydrogel, the hydrogel dehydrates, the hydrogen bond is enhanced, the physical interaction is enhanced, the crosslinking density of the hydrogel is increased, and the swelling resistance is improved, which has great application potential in the field of underwater movement monitoring.

[0147] Example 8. Sensing test

[0148] 1. Strain sensing performance test of PVA / GA / MXene 0.1wt% hydrogel

[0149] In order to further study the strain sensing performance of the PVA / GA / MXene 0.1wt% hydrogel, the present application quantitatively evaluates by analyzing the relative resistance change signal in the tensile process. The PVA / GA / MXene 0.1wt% hydrogel is selected as the test sample (30mm long, 10mm wide, 2mm high), the universal testing machine and the DMM6500 source table are used to quantitatively evaluate the strain sensitivity measurement factor (GF) of the hydrogel, and the loading rate is 50mm / min, and the calculation formula is as follows:

[0150]

[0151] Wherein, ΔR (ΔR = R-R0) is the change of resistance, R0 is the initial resistance of the hydrogel, R is the resistance at a certain moment in the tensile process, and ε is the strain at the moment.

[0152] The results are shown in Figure 13 -A, and the strain factor of the hydrogel is 0.431 in a wide strain range of 0-1200%.

[0153] 2. Human signal monitoring

[0154] In order to further verify the potential application of PVA / GA / MXene hydrogel in the field of human motion detection, the hydrogel is connected with multiple parts of the human body for human signal monitoring. The hydrogel sensor is directly attached to different parts of the human body (such as fingers, wrists, etc.) and fixed by means of conductive copper tape, and at the same time, it is connected with DMM6500 source table, the finger and wrist joint bending process is repeated, and human physiological activity monitoring is carried out, the real-time resistance change of the sensor is recorded by the source table, and the stability of the hydrogel is verified.

[0155] The results are shown in Figure 13 -B, the PVA / GA / MXene hydrogel sensor is fixed at the finger joint by conductive copper tape to monitor the bending motion of the fingers at different angles. When the fingers are bent from straight to 0° and 45°, the value of ΔR / R0 continuously increases, and all the monitored electrical signals show good repeatability and stability. As shown in 14-C, for different degrees of wrist joint bending, the value of ΔR / R0 is different, so the bending angle can be distinguished according to the value of ΔR / R0. As shown in Figure 14 -D, the sensor fixed at the finger can also obtain stable and repeatable ΔR / R0 change value. From the above experimental results, it can be seen that the PVA / GA / MXene hydrogel sensor can monitor the strain of the human body in real time. And it has significant regularity and relative stability.

[0156] In summary, PVA / GA / MXene 0.1wt% hydrogel has high sensitivity in the range of 0-1200% strain, GF is 0.431; and has good repeatability and stability, which can be used as a sensor to measure human activity.

[0157] Example 9. Degradability and plasticity, recyclability test

[0158] To evaluate the degradation characteristics of PVA / GA / MXene hydrogel, the hydrogel was recycled, melted and re-injected into the mold, and the recyclability was observed by freeze-thaw cycling. The hydrogel was placed in hot water at 90°C to observe its degradation behavior.

[0159] The results are shown in Figure 14 -A, after 20 minutes, the hydrogel was completely degraded, indicating the degradability of PVA / GA / MXene hydrogel. This is because there are hydrophilic functional groups such as -OH on the surface of MXene in the gel network, which endows the hydrogel with excellent hydrophilic properties. With the increase of temperature, the cross-linked network structure gradually collapses, releasing more free hydrophilic groups, and more water molecules penetrate into the gel network, causing the hydrogel to swell, and finally achieve degradation. Given the non-toxic and harmless characteristics of PVA / GA / MXene hydrogel, the solution after degradation can be directly discharged to the environment after simple treatment, showing good environmental friendliness. In addition, the reversible hydrogen bond interaction between the components in the gel network endows PVA / GA / MXene hydrogel with excellent recyclability. As shown in ​ -B, during the reshaping process, the rectangular hydrogel was melted by heating. In this process, the crystalline regions and hydrogen bonds in the gel network were destroyed, leading to the collapse of the gel structure. Subsequently, the sol state of the gel was re-injected into the mold, and its fluidity was used to reshape it into any shape, forming a new hydrogel product. In today's growing electronic waste pollution, good degradability and recyclability provide a possible method to solve electronic waste pollution.

Claims

1. A multifunctional double network composite hydrogel, characterized by, The hydrogel is composed of polyvinyl alcohol, ammonium glycyrrhizinate salt and MXene based on Hofmeister effect; in the hydrogel, the concentration of MXene is 0.1 wt%, the concentration of polyvinyl alcohol is 8 wt%, and the concentration of ammonium glycyrrhizinate salt is 2 wt%. The preparation method of the multifunctional double-network composite hydrogel is as follows: (1) mixing the polyvinyl alcohol, the ammonium glycyrrhizinate salt and the MXene to obtain a hydrogel precursor solution; (2) after the hydrogel precursor solution obtained in step (1) is subjected to freeze-thaw cycles, it is immersed in a salt solution to introduce Hofmeister effect, thereby obtaining the multifunctional double-network composite hydrogel; the salt solution is a sodium citrate solution with a concentration of 22.5 wt%, and the immersion time is 12 hours; the MXene has a single-layer sheet structure, and the chemical formula of the MXene is Ti3C2.

2. The hydrogel of claim 1, wherein, The multifunctional double-network composite hydrogel is a double-network hydrogel system, in which the polyvinyl alcohol is self-assembled with ammonium glycyrrhizinate through hydrogen bonding after being subjected to freeze-thaw cycles, thereby forming a double-network hydrogel system.

3. The hydrogel of claim 1, wherein, The freeze-thaw cycle number is 2-5, and each freezing time is 6-10 h, and each thawing time is 3-5 h.

4. A flexible sensor for human signal monitoring, characterized by, The flexible sensor contains the multifunctional double-network composite hydrogel according to any one of claims 1-3.

5. Use of the multifunctional double-network composite hydrogel according to any one of claims 1-3 in the preparation of a flexible sensor for monitoring human physiological activities.

6. Use according to claim 5, characterized in that, The monitoring of human physiological activities includes health monitoring, sports monitoring and underwater human signal monitoring.

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