CN114112123B - Application of folic acid-metal ion hydrogel, double-network gel and preparation method and application thereof - Google Patents
Application of folic acid-metal ion hydrogel, double-network gel and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses application of a folic acid-metal ion hydrogel, a double-network gel, a preparation method and application thereof. The double-network gel is formed by folic acid-metal ion hydrogel and polymer gel, and the folic acid-metal ion hydrogel is used as a heat-resistant factor to endow the double-network gel with high-temperature stability and excellent mechanical property and conductivity, so that the double-network gel can be applied to flexible electronic devices, for example, can be used for preparing pressure sensors, and provides possibility for real-time monitoring of material deformation at high temperature.
Description
Technical Field
The invention relates to the technical field of hydrogel, in particular to application of folic acid-metal ion hydrogel, double-network gel and a preparation method and application thereof.
Background
The hydrogel can be used in the design and manufacture of wearable equipment due to the stretchability, flexibility and electrical conductivity, but most of hydrogels have a narrow working range, and generally, a polymer gel freezes and becomes brittle at 0 ℃ to lose mechanical strength, and melts to a liquid state at about 40-50 ℃ to lose support property, so that the need for researching a low-temperature-resistant and high-temperature-resistant gel material is urgent and has important significance.
In recent years, in order to improve the low temperature resistance of gel materials, researchers have developed several low temperature resistant gels based on glycol-water mixed systems or inorganic salt systems, which can maintain the gel state at a low temperature of-30 ℃.
However, at high temperatures, gel melting is still an unavoidable problem.
Disclosure of Invention
In order to overcome the problems, the inventors of the present invention have conducted intensive studies to study the use of a folic acid-metal ion hydrogel, a double-network gel, a preparation method and an application thereof, the folic acid-metal ion hydrogel has excellent temperature stability, can form the double-network gel with a polymer gel, is used as a heat-resistant factor to endow the double-network gel with high temperature stability, excellent mechanical properties and electrical conductivity, overcomes the problem that the polymer gel is easily melted at high temperature, can be applied to flexible electronic devices, can develop flexible devices with operating temperature light, such as pressure sensors, and provide possibility for real-time monitoring of material deformation at high temperature, and the method provided by the present invention is simple and easy to implement, thereby completing the present invention.
It is an object of the present invention to provide the use of a folate-metal ion hydrogel in flexible electronic devices, preferably for the preparation of sensors.
The second aspect of the invention provides a double-network gel, which is formed by a folic acid-metal ion hydrogel and a polymer gel, preferably, the folic acid-metal ion hydrogel has a mass fraction of 5-30%.
A third aspect of the present invention provides a method of preparing a double-network gel, preferably a method of preparing a double-network gel according to the second aspect of the present invention, the method comprising the steps of:
and 3, obtaining the double-network gel.
A fourth aspect of the invention provides the use of a double-network gel in a flexible electronic device, preferably for the manufacture of a sensor, for example for the manufacture of a pressure sensor.
The invention has the following beneficial effects:
(1) The invention provides an application of a folic acid-metal ion hydrogel in a flexible electronic device, which can be used for preparing a sensor, wherein the folic acid-metal ion hydrogel has excellent high-temperature stability and is in a gel state at 25-90 ℃, the storage modulus and the loss modulus are not obviously reduced, and the pterin quadruplet chiral accumulation of folic acid is still maintained;
(2) The invention provides a double-network gel, which is formed by folic acid-metal ion hydrogel and high polymer gel, wherein the folic acid-metal ion hydrogel is used as a heat-resistant gene to endow the double-network gel with high-temperature stability and excellent high-temperature stability and mechanical property, and the folic acid-metal ion hydrogel can be introduced into various types of high polymer hydrogels by different preparation methods to obtain the double-network gel, so that the defect that the traditional high polymer gel is easy to melt at high temperature is overcome;
(3) The double-network gel provided by the invention can conduct electricity within the temperature range of-20-80 ℃, has different stretching degrees and different resistance change degrees, so that the double-network gel has a huge application prospect in the aspect of flexible electronic devices, and provides possibility for real-time monitoring of material deformation at high temperature;
(4) The double-network gel can be developed into a flexible device with wide working temperature, and the preparation method of the double-network gel is simple, easy to realize and suitable for large-scale popularization.
Drawings
FIG. 1 shows a phase diagram of folate-zinc ions, including solutions, gels and suspensions, of a preferred embodiment of the invention;
FIG. 2 shows the change of modulus with temperature of the folic acid-zinc ion hydrogel prepared by the invention in example 1;
FIG. 3 is a photograph showing a folic acid-zinc ion hydrogel obtained in example 1 of the present invention;
FIG. 4 shows stress sweep results at different temperatures for folic acid-zinc ion hydrogels prepared according to examples of the present invention;
FIG. 5 shows the frequency scanning results of the folic acid-zinc ion hydrogel prepared in example 1 of the present invention at different temperatures;
FIG. 6 shows TEM images of the folate-zinc ion hydrogel obtained in example 1 of the present invention at 25 ℃ and 80 ℃;
FIG. 7 shows the results of circular dichroism spectroscopy of the folic acid-zinc ion hydrogel obtained in example 1 of the present invention at different temperatures;
FIG. 8 shows the curve of the circular dichroism peak at 318nm of the folic acid-zinc ion hydrogel obtained in example 1 of the present invention as a function of temperature;
FIG. 9 shows photographs of the double-network gels obtained in examples 3 to 5 of the present invention and the single-network gels obtained in comparative examples 1 to 3 at different temperatures;
FIG. 10 shows the change of modulus with temperature of the folic acid-zinc ion hydrogel obtained in example 1 of the present invention, the double network gel obtained in example 3, and the polyvinyl alcohol single network gel obtained in comparative example 1;
FIG. 11 shows the results of mechanical property tests of the double-network gel obtained in example 3 of the present invention;
FIG. 12 shows the conductivity test results for the dual network gel obtained in example 3 of the present invention;
FIG. 13 shows the results of the conductivity test of the double-network gel in Experimental example 6 of the present invention;
FIG. 14 is a photograph showing a pressure sensor based on the double network gel obtained in example 3 in Experimental example 7 of the present invention, and the pressure sensor fixed to a finger;
FIG. 15 shows the relative resistance change of the pressure sensor when the finger is repeatedly fully and relaxed in Experimental example 7;
fig. 16 is a schematic view showing the structure of a metal bending sensor in experimental example 8;
fig. 17 shows the resistance change of the metal bending sensor at 25 c and 80 c in experimental example 9.
The reference numbers illustrate:
1-double network gel film;
2-a metal sheet;
3-conductive silver adhesive;
4-silver wire.
Detailed Description
The invention is explained in more detail below with reference to the drawings and preferred embodiments. The features and advantages of the present invention will become more apparent from the description.
According to the present invention there is provided the use of a folate-metal ion hydrogel in flexible electronic devices, preferably in the manufacture of sensors, such as for the manufacture of pressure sensors.
According to the present invention, the folate-metal ion hydrogel comprises a coordination linkage of metal ions and folate molecules.
The folic acid-metal ion hydrogel is a complex of folic acid molecules and metal ions which are fully crosslinked and coordinated to form hydrogel molecules with a net structure.
Pterin head group of folic acid forms tetrad through hydrogen bond, pi-pi stacking between tetrads forms fiber, metal ion coordinates with carboxylic acid of folic acid, and the fiber is crosslinked and intertwined to form a net structure.
According to the present invention, a folic acid-metal ion hydrogel is prepared by a method comprising the steps of:
step (1), preparing a folic acid aqueous solution;
step (2), preparing a metal ion aqueous solution;
and (3) uniformly mixing the folic acid aqueous solution and the metal ion aqueous solution at room temperature, and standing to obtain the folic acid-metal ion hydrogel.
According to the present invention, in the step (1), folic acid is dissolved in an aqueous solution comprising water or an aqueous buffer solution to prepare an aqueous folic acid solution.
Since water may contain undesirable cations and anions, it is not intended that folic acid and the set metal ion form a complex in the present invention.
According to a preferred embodiment of the present invention, the water used in the present invention is purified water containing no ions, and more preferably ultrapure water.
In the present invention, when a folic acid aqueous solution is prepared using pure water, an alkaline agent is added dropwise to make the pH of the aqueous solution alkaline, preferably 7.0 to 9.5, in order to increase the solubility and dissolution rate of folic acid.
The alkaline reagent is selected from one or more of ammonia water, sodium carbonate, sodium bicarbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide and cesium hydroxide; preferably one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide and cesium hydroxide; more preferably sodium hydroxide or potassium hydroxide.
When the hydroxide is used for reaction with carboxyl in folic acid, the types and the contents of inorganic anions in the aqueous solution of folic acid are not increased, and the interference of coordination of folic acid and metal ions is reduced as much as possible.
The inventors have discovered that sodium, potassium, lithium, rubidium, and/or cesium metal ions can be added to aqueous folic acid solutions by adjusting the pH of the solutions using one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide. Sodium, potassium, lithium, rubidium and/or cesium metal ions may play a stabilizing role to a certain extent on the tetranection formed by folic acid and metal ions during the formation of the hydrogel.
The aqueous buffer solution is selected from one or more of Acetic acid-sodium acetate, boric acid-borax, HEPES (N- (2-hydroxyethyl) piperazine-N' -2-ethanesulfonic acid), MOPS (3-morpholinopropanesulfonic acid), MES (2- (N-morpholino) ethanesulfonic acid), trimethylolmethylamine-hydrochloric acid (Tris-HCl), trimethylolmethylamine-Boric acid (Tris-Boric acid), and trimethylolmethylamine-Acetic acid (Tris-Acetic acid).
When aqueous folic acid solution is prepared using an aqueous buffer solution, preferably, the aqueous buffer solution is selected from one or more of trimethylol methylamine-hydrochloric acid, trimethylol methylamine-boric acid, and trimethylol methylamine-acetic acid.
In order to improve the solubility of folic acid in an aqueous buffer solution, the pH value of the aqueous buffer solution is 7.0-9.5.
In the aqueous solution of folic acid obtained in step (1), the concentration of folic acid is 0.5mM-200mM, preferably 5mM-100mM.
According to the present invention, in step (2), the same aqueous solution as in step 1 is used to prepare an aqueous solution of metal ions.
According to the invention, the metal ion is selected from the group consisting of chemically stable metal ions in the monovalent or divalent state, preferably from the group consisting of sodium, potassium, lithium, rubidium, cesium, zinc (Zn) 2+ ) Copper (Cu) 2+ ) Nickel (Ni) 2+ ) Cobalt (Co) 2+ ) Manganese (Mn) 2+ ) Chromium (Cr) 3+ ) Molybdenum (Mo) 2+ ) Silver (Ag) + ) Cadmium (Cd) 2+ ) Magnesium (Mg) 2+ ) Calcium (Ca) 2+ ) Lead (Pb) 2+ ) And one or more of metal ions such as barium ions.
Preferably, the metal ions include one or more of zinc, copper, nickel, cobalt, manganese, chromium, molybdenum, silver, cadmium, magnesium, calcium, and lead ions,
more preferably, the metal ions include one or more of zinc, silver, magnesium and calcium ions.
According to the present invention, when the aqueous solution of metal ions is prepared, it is preferable to use an inorganic salt containing the metal ions, for example, a hydrochloride, a nitrate, a sulfate, or the like, and it is preferable to use a nitrate or a sulfate, because most nitrates or sulfates have relatively good solubility and the aqueous solution of metal ions can be prepared more easily.
According to the present invention, in the aqueous solution of metal ions prepared in step (2), the concentration of the metal ions is 0.5mM to 500mM, preferably 5mM to 400mM.
According to the present invention, in the step (3), the folic acid aqueous solution and the metal ion aqueous solution are uniformly mixed to obtain a mixed system, and the mixing method is not particularly limited as long as the mixing can be uniformly performed, and examples thereof include vortexing, stirring, and ultrasound.
After mixing, the final concentration of the folic acid in the mixed system is 0.5mM-200mM, preferably 5mM-100mM, more preferably 5mM-50mM, e.g. 15mM.
After mixing, the final concentration of the metal ion in the mixed system is 0.5mM-500mM, preferably 5mM-400mM, more preferably 10mM-200mM, for example 27mM.
Research shows that the viscosity of the prepared solution or the mechanical strength (modulus) of the hydrogel can be obviously improved by increasing the concentration of folic acid; when the concentration of folic acid is too low, a hydrogel cannot be obtained.
In the step (3), after the folic acid aqueous solution and the metal ion aqueous solution are mixed, the molar concentration ratio of folic acid to metal ions is 1: (0.1-10).
In order to further optimize the mechanical strength of the hydrogel and make the hydrogel have good mechanical strength and suitable for skin application, the molar ratio of folic acid to metal ions in the mixed system after mixing is preferably 1: (0.4-10), preferably 1 (1-5).
After the folic acid aqueous solution and the metal ion aqueous solution are mixed, the dosage proportion of folic acid and metal ions in the mixed system can influence the fluid property of the folic acid-metal ion hydrogel. In the present invention, the folic acid-metal ion hydrogel has a fluid state such as a solution, a gel, or a suspension.
Specifically, in a preferred embodiment, the phase diagram of the folate-zinc ion hydrogel is shown in fig. 1, wherein in fig. 1: with the increase of the concentration of folic acid, the lower limit of the folic acid/zinc ion molar ratio of the folic acid-zinc ion hydrogel forming a gel state is gradually reduced, the upper limit is gradually increased, and the gel area is enlarged; correspondingly, with the reduction of the folic acid concentration, the lower limit of the folic acid/zinc ion molar ratio of the folic acid-zinc ion hydrogel forming a gel state is gradually increased, the upper limit is gradually decreased, and the gel area is reduced. As can also be seen from fig. 1, when the molar ratio of folic acid/zinc ion is gradually increased under the condition of a constant folic acid concentration, the folic acid-zinc ion hydrogel changes from a solution state to a gel state and then to a suspension state.
Preferably, when the folic acid concentration is 15mM, the folic acid aqueous solution and the zinc ion aqueous solution are mixed, and when the folic acid/zinc ion molar ratio is 1 (0-0.9), the obtained hydrogel is in a solution state; when the molar concentration ratio of folic acid to zinc ions is 1 (0.9-1.9), the obtained hydrogel is in a gel state; when the molar concentration ratio of folic acid to zinc ions is 1 (2.0-10), preferably 1 (2.0-2.2), the hydrogel obtained is in the form of a suspension,
preferably, when the selected metal ions are zinc ions and the concentration of folic acid is 10mM, the molar concentration ratio of folic acid to zinc ions is 1 (0-1.0), the obtained hydrogel is in a solution state; when the molar concentration ratio of folic acid to zinc ions is 1 (1.0-1.9), the obtained hydrogel is in a gel state; when the molar concentration ratio of folic acid to zinc ions is 1 (2.0-10), preferably 1 (2.0-2.2), the hydrogel obtained is in the form of a suspension.
Preferably, when the selected metal ions are zinc ions and the concentration of folic acid is 2mM, the molar concentration ratio of folic acid to zinc ions is 1 (0-1.5), the obtained hydrogel is in a solution state; when the molar concentration ratio of folic acid to zinc ions is 1 (1.5-1.7), the obtained hydrogel is in a gel state; when the molar concentration ratio of folic acid to zinc ions is 1 (1.9-10), preferably 1 (2.0-2.2), the hydrogel obtained is in the form of a suspension.
Further research finds that the pH value of the mixed system may influence the forming and properties of the hydrogel, and the mechanical strength of the hydrogel prepared when the mixed system is too basic is obviously weakened; when the alkalinity is too low or even when the acidity is too low, precipitation occurs in the mixed system and hydrogel formation is not possible.
Optionally, the pH of the mixed system solution is adjusted to be neutral or alkaline, preferably 7.0-9.5.
And standing the mixed system to ensure that the metal ions and the folic acid are fully crosslinked and coordinated to form hydrogel molecules.
It was found that the formation of metal ion hydrogel based on folic acid (folate-metal ion gel) is a multi-step process: firstly, pterin head group of folic acid forms tetrad through hydrogen bond, and the tetrad is further pi-pi stacked to form fiber; finally, the metal ions and the carboxylic acid of the folic acid are coordinated, the fibers are crosslinked, and the fibers are intertwined to form the hydrogel molecules with a net structure.
Forming a solution of hydrogel when hydrogel molecules of a network structure formed by metal ions and folic acid molecules are uniformly dispersed in water; when the hydrogel molecules with the network structure have good dispersibility and can wrap water in the hydrogel molecules to cause the hydrogel molecules to lose fluidity, a gel state is formed; when the hydrogel molecules with the network structure can not be dispersed in water, a suspension of the folic acid-metal ion hydrogel is formed.
The mixed system is preferably allowed to stand at room temperature, which means 10 to 35 ℃, preferably 15 to 30 ℃, more preferably 20 to 30 ℃, for example 25 ℃.
In the step (3), the time for forming the hydrogel by standing is related to the type of the selected metal ions; preferably, the hydrogel is allowed to stand for 1 to 2 days, so that the crosslinking molecules in the hydrogel can be more stable.
In the preparation method provided by the invention, the metal ions are prepared into the aqueous solution in advance, so that the non-uniformity of gelling of the metal ion solid and the folic acid aqueous solution can be avoided, and more uniform and reliable folic acid-metal ion hydrogel can be prepared.
According to the invention, the folic acid-metal ion hydrogel has high temperature resistance, and can keep the solid-like form of the gel without flowing, namely, the folic acid-metal ion hydrogel can not be melted and converted into a liquid form at the temperature of 25-90 ℃.
According to a preferred embodiment of the present invention, the folate-metal ion hydrogel substantially maintains a storage modulus G 'and a loss modulus G' at 30-75 ℃, wherein the storage modulus is in the storage modulus range of 10 1 -10 6 Pa, preferably 10 2 -10 4 Pa, loss modulus in the range of 10 0 -10 5 Pa, preferably 10 1 -10 3 Pa。
According to the invention, the folic acid-metal ion hydrogel has typical gel rheological characteristics, and the storage modulus of the folic acid-metal ion hydrogel is higher than the loss modulus by one order of magnitude in the temperature range of 25-55 ℃, namely the ratio of G 'to G' is 10 1 The gel structure is always maintained, and the gel is not converted into the solution.
In the invention, the high temperature resistance of the folic acid-metal ion hydrogel is because the folic acid-metal ion hydrogel has strong coordination effect in the structure and is slightly influenced by temperature, and the folic acid-metal ion hydrogel can still maintain the action strength at high temperature.
According to the folic acid-metal ion hydrogel system, the fiber network in the system still exists at a high temperature of 80 ℃ and is more compact compared with the fiber network at a low temperature of 25 ℃, so that after the temperature is increased, the coordination effect is enhanced, the crosslinking between the fiber bundle and the fiber network is promoted, and the gel structure is maintained.
Due to the chiral accumulation of the folate tetrad in the folate-metal ion hydrogel, the folate-metal ion hydrogel has strong circular dichroism signals, and the circular dichroism signals are almost unchanged at the temperature range of 25-70 ℃, which indicates that the chiral accumulation of the pterin tetrad of the folate is still maintained in the temperature rise process, so that the hydrogel is in a gel structure form. Chiral stacking peaks for the folate pterin quadruplets are present in the circular dichroism near 280nm and 318 nm.
Due to the high temperature resistance of the folic acid-metal ion hydrogel, the folic acid-metal ion hydrogel can be taken as a heat-resistant factor or a heat-resistant gene to be introduced into the traditional polymer gel to prepare high-temperature stable gel, is applied to flexible electronic devices, and is preferably used for preparing sensors, such as pressure sensors and metal bending sensors.
According to the present invention, there is provided a double-network gel formed of a polymer gel and a folic acid-metal ion hydrogel.
According to a preferred embodiment of the present invention, the weight fraction of the folic acid-metal ion hydrogel in the double-network gel is 5 to 30%, preferably 10 to 25%.
According to the invention, the double-network gel is in a solid-like state, namely a gel state, within the temperature range of 25-90 ℃.
According to the present invention, a polymer gel is formed from a polymer gel precursor.
According to a preferred embodiment of the present invention, the polymer gel precursor is a precursor capable of forming polymer gel in the art, and is preferably selected from one or more of polyvinyl alcohol, agarose and acrylamide, and the formed double-network gel is polyvinyl alcohol/folic acid-metal ion double-network gel, agarose/folic acid-metal ion double-network gel, or polyacrylamide/folic acid-metal ion double-network gel.
According to the present invention, the method for preparing the double-network gel is different from that used for the precursors of different polymer gels, and preferably, the double-network gel is prepared by one of a freezing-melting method, a heating-cooling method and an in-situ polymerization method, for example, the polyvinyl alcohol/folic acid-metal ion double-network gel can be prepared by the freezing-melting method, the agarose/folic acid-metal ion double-network gel can be prepared by the heating-cooling method, and the polyacrylamide/folic acid-metal ion double-network gel can be prepared by the in-situ polymerization method.
According to the invention, a preparation method of the double-network gel is provided, which comprises the following steps:
and 3, obtaining the double-network gel.
According to a preferred embodiment of the present invention, in step 1, the raw material is polyvinyl alcohol, agarose or acrylamide,
preferably, the polyvinyl alcohol has a degree of hydrolysis of 98 to 99% and a molecular weight of 88000 to 97000.
According to the invention, in step 1, the solvent is a glycol-water binary solvent system, wherein the volume ratio of glycol to water is 1.
According to the invention, in the step 1, the raw materials are added into the solvent and mixed to obtain the mixed solution, wherein the mass fraction of the raw materials in the mixed solution is 0.5-40 wt%, preferably 1-30 wt%.
According to a preferred embodiment of the present invention, when the raw material is polyvinyl alcohol, the mass fraction of polyvinyl alcohol in the mixed solution is 1 to 10wt%, preferably 1 to 5wt%, for example 3wt%.
According to another preferred embodiment of the invention, when the starting material is agarose, the mass fraction of agarose in the mixed solution is 1-15 wt.%, preferably 1-10 wt.%, for example 5 wt.%.
According to a further preferred embodiment of the invention, when the starting material is acrylamide, the mass fraction of acrylamide in the mixed solution is between 20 and 40 wt.%, preferably between 25 and 35 wt.%, for example 30 wt.%.
According to a preferred embodiment of the present invention, in step 1, when the raw material is polyvinyl alcohol or agarose, after obtaining the mixed solution, heating the mixed solution to obtain a homogeneous solution system;
the heating temperature is 70-90 ℃, the heating time is 10-40 min, preferably, the heating time is 20-30 min,
for example, when the raw material is polyvinyl alcohol, the heating temperature is 90 ℃, and the heating time is 20min;
when the raw material is agarose, the heating temperature is 70 deg.C, and the heating time is 20min.
According to another preferred embodiment of the present invention, in step 1, when the starting material is acrylamide, an initiator, preferably Ammonium Persulfate (APS),
further, the amount of the initiator to be added is 5 to 15. Mu.L, preferably 8 to 12. Mu.L, based on 1mL of the mixed solution.
According to the present invention, in step 1, the mixing method is not particularly limited, and stirring is preferable.
According to the invention, in step 2, mother liquor of folic acid and mother liquor of metal inorganic salt are added into the homogeneous mixed solution obtained in step 1, and are mixed to obtain homogeneous viscous liquid.
According to the invention, in step 2, mother liquor of folic acid is obtained by dissolving folic acid in a solvent, mother liquor of metal inorganic salt is obtained by dissolving metal inorganic salt in a solvent,
the solvent is preferably glycol-water binary solvent, the metal inorganic salt is inorganic salt containing metal ions, the metal inorganic salt is one or more selected from hydrochloride, nitrate or sulfate containing metal ions, preferably nitrate or sulfate, such as zinc nitrate.
According to the invention, the molar concentration of folic acid in the mother liquor of folic acid is 50 to 150mM, preferably 80 to 120mM, more preferably 100mM;
the molar concentration of the metal ion in the mother liquor of the metal inorganic salt is 50 to 150mM, preferably 80 to 120mM, and more preferably 100mM.
According to the invention, in step 2, mother liquor of folic acid and mother liquor of metal inorganic salt are added into the homogeneous solution system obtained in step 1, and are uniformly mixed, preferably by stirring, to obtain homogeneous viscous liquid.
According to a preferred embodiment of the invention, the stirring is carried out by means of magnetic stirring, at a speed of 200-800rpm, for a time of 10-30 minutes.
According to a preferred embodiment of the present invention, in step 2, the homogeneous solution system obtained in step 1 is first cooled, preferably to 50 to 60 ℃, and then the mother liquor of folic acid and the mother liquor of metal inorganic salt are added, for example, when the raw material is polyvinyl alcohol, the temperature is reduced to 60 ℃; when the raw material is agarose, the temperature is reduced to 50 ℃.
According to the invention, in the homogeneous viscous liquid obtained in step 2, the ratio of the molar concentration of folic acid to the molar concentration of metal ions is 1: (0.1 to 10), preferably 1 (1 to 5), more preferably 1: (1.0 to 1.9), for example, 1.4, 1.
According to a preferred embodiment of the invention, in step 2, a coagulant is also added and mixed homogeneously to obtain a homogeneous viscous liquid,
preferably, the coagulant is N, N, N ', N' -Tetramethylethylenediamine (TEMED), which catalyzes the generation of free radicals by APS to accelerate the polymerization of acrylamide gels.
According to the invention, the mass concentration of the starting materials in the resulting homogeneous viscous liquid is from 1 to 30% by weight, preferably from 2 to 25% by weight.
According to a preferred embodiment of the invention, the resulting homogeneous viscous liquid has a mass concentration of starting materials (e.g. in the case of polyvinyl alcohol and agarose) of 3 to 10 wt.%, preferably 4 to 8 wt.%, for example 5 wt.%;
according to another preferred embodiment of the invention, the resulting homogeneous viscous liquid has a mass concentration of starting material (e.g. acrylamide) of 5 to 20 wt.%, preferably 10 to 20 wt.%, for example 15 wt.%.
According to a preferred embodiment of the invention, the double-network gel is obtained by a freeze-thaw method, for example, when polyvinyl alcohol is used as the starting material,
specifically, in the step 3, the homogeneous phase viscous liquid obtained in the step 2 is injected into a mold, and is circularly frozen and melted after being cooled to room temperature, so as to obtain the double-network gel, preferably, the double-network gel is frozen for 30min at the temperature of-20 ℃, is melted at the room temperature (25 ℃) after being taken out, and the process is repeated for 3 times, so as to obtain the double-network gel.
In the present invention, the mold can be a self-made mold, and different molds can be used to control the shape of the double network gel, for example, a silica gel pad of 1mm or 2mm is used to separate between two glass plates of 10cm × 10cm, such a mold can prepare a very thin double network gel film. Or a cylindrical mold was used to obtain a double network gel that could be tested for compression properties.
According to another preferred embodiment of the present invention, the double-network gel is obtained by a heating-cooling method, for example, when agarose is used as the raw material,
specifically, in step 3, the homogeneous viscous liquid obtained in step 2 is poured into a mold, cooled, preferably to room temperature (25 ℃) to obtain a double-network gel.
According to a further preferred embodiment of the present invention, the double-network gel is obtained by an in-situ polymerization process, for example, when acrylamide is used as the starting material,
specifically, in step 3, the homogeneous viscous liquid obtained in step 2 is injected into a mold, and is allowed to stand for 10 to 60min, preferably 20 to 40min, for example, 30min, to complete in-situ polymerization, so as to obtain the double-network gel.
In the present invention, three typical double-network gels are obtained in the above embodiment, and the other polymer gels can also be formed into double-network gels by introducing folic acid-metal ion gels in a similar manner as described above.
According to the invention, the folic acid-metal ion hydrogel with the mass fraction of 5-30% (preferably 10-25%) is introduced into the polymer gel to form the double-network gel, and the folic acid-metal ion hydrogel is introduced into the polymer gel as a heat-resistant factor or a heat-resistant factor, namely a high-temperature stabilizer, so that the polymer gel is endowed with excellent high-temperature stability.
The double-network gel has high-temperature stability, for example, the double-network gel can still keep the solid-like property without melting at the temperature of 25-90 ℃, namely, the gel form is kept, and the single-network gel without introducing the folic acid-metal ion hydrogel melts into liquid at the temperature of about 60 ℃.
According to the invention, the storage modulus and loss modulus of the double-network gel at-30-80 ℃ are respectively in the storage modulus range of 10 1 -10 6 Pa, preferably 10 2 -10 4 Pa, loss modulus in the range of 10 0 -10 5 Pa, preferably 10 1 -10 3 Pa。
In the invention, the storage modulus and loss modulus of the double-network gel and the folic acid-metal ion hydrogel are basically consistent, which shows that the obtained double-network gel has high temperature resistance.
According to the invention, the tensile stress and the tensile deformation of the double-network gel are both higher than those of the traditional polymer gel without the folic acid-metal ion hydrogel, the tensile deformation is improved by 15 percent, even by more than 30 percent, and the tensile stress is improved by more than 35 percent, even by more than 110 percent.
In the invention, under the strong promotion of coordination, the formation of folic acid tetrad and the accumulation of folic acid tetrad are still maintained at high temperature, and the strength of the double-network gel cannot be reduced along with the increase of temperature because the coordination provides enough cross-linking points.
According to the invention, the tensile deformation of the double-network gel is 900-1000%, and the tensile stress is 1500-1900 kPa.
According to the invention, due to the good modulus and tensile properties of the double-network gel, the double-network gel can bear the cutting of a blade, i.e. can not be cut by the blade, and the mechanical properties of the double-network gel are kept unchanged at high temperature, such as the double-network gel can be subjected to compression-recovery cycle at 80 ℃ and then can be recovered.
According to the invention, the double-network gel has conductivity in the range of-20-80 ℃, and the conductivity of the double-network gel can reach 1.2S/m through the precise regulation and control of inorganic salt such as potassium nitrate, thereby meeting the requirement of the conductivity of a flexible device.
The double-network gel of the present invention has excellent light transmittance and stretchability.
In the invention, the double-network gel has excellent high-temperature stability, mechanical property and conductivity, so that the double-network gel has great potential in the aspect of flexible electronic devices.
According to the present invention, there is provided a double-network gel prepared according to the above method.
According to the present invention there is provided the use of a double network gel in flexible electronic devices, preferably for the manufacture of sensors, for example pressure sensors.
According to the present invention, the electrical resistance of the double-network gel is changed by stretching the double-network gel to different degrees, and the degree of stretching is increased to increase the change in electrical resistance.
The double-network gel has excellent temperature stability, can normally work at high temperature when used as a pressure sensor, has the same resistance change amplitude within the range of 20-80 ℃, and can provide possibility for real-time monitoring of material deformation at high temperature.
According to the invention, the obtained double-network gel is used as a metal bending sensor (or pressure sensor) and has a relative resistance change rate (delta R/R) within 20-80 DEG C 0 ) 0% to 110%, the resistance of the double-network gel becomes large when stretched, and the resistance can be more than twice as high as the original resistance, so that the range of the relative resistance change rate is large.
The invention utilizes the excellent high-temperature stability, mechanical property and conductivity of the double-network gel, is applied to flexible electronic devices, overcomes the problem of high-temperature melting of the traditional polymer gel, and provides a direction for developing flexible devices with wide working temperature.
Examples
Example 1
Weighing 10.74mg of folic acid in a 2mL test tube, adding 0.225mL of ultrapure water, adding potassium hydroxide to adjust the pH of the solution to about 9 to obtain folic acid aqueous solution;
taking another test tube, adding 10.04mg of zinc nitrate and 1.275ml of ultrapure water to obtain a metal ion aqueous solution;
the aqueous solution of metal ions was added to the aqueous folic acid solution so that the concentration of folic acid in the mixed solution was 15mM and the concentration of zinc ions was 22.5mM. And (3) stirring uniformly by vortex, standing, wherein the mixed solution is turbid initially and becomes clear and transparent after 10 minutes to obtain the folic acid-zinc ion hydrogel.
Example 2
The procedure of example 1 was repeated except that the metal ions were exchanged for Cu, respectively 2+ 、Ni 2+ 、Mg 2+ 、Pb 2+ 、Ag + Respectively preparing folic acid-copper ion hydrogel, folic acid-nickel ion hydrogel, folic acid-magnesium ion hydrogel, folic acid-lead ion hydrogel and folic acid-silver ion hydrogel.
Example 3
Adding polyvinyl alcohol into a glycol-water binary solvent to obtain a mixed solution, wherein the mass fraction of the polyvinyl alcohol is 3wt%, and heating the mixed solution at 90 ℃ for 20min to obtain a colorless and transparent homogeneous solution system;
preparing mother liquor of folic acid: adding folic acid into a glycol-water binary solvent, and uniformly mixing to obtain a folic acid mother solution, wherein the molar concentration of folic acid in the folic acid mother solution is 100mM;
preparing a zinc nitrate mother solution: adding zinc nitrate into a glycol-water binary solvent, uniformly mixing to obtain a zinc nitrate mother solution, wherein the molar concentration of zinc nitrate in the zinc nitrate mother solution is 100mM,
then slowly cooling the homogeneous solution system to 60 ℃, taking 6.4mL of the homogeneous solution system cooled to 60 ℃, adding 1.5mL of folic acid mother liquor and 2.1mL of zinc nitrate mother liquor in sequence, and stirring to be homogeneous to obtain viscous liquid;
injecting the viscous liquid into a mold, cooling to room temperature, freezing in a refrigerator at-20 deg.C, taking out after 30min, thawing at room temperature, and repeating the process for three times to obtain the double-network gel of polyvinyl alcohol/folic acid-zinc ions.
Example 4
Adding agarose with certain mass into a glycol-water binary solvent to ensure that the mass fraction of the agarose is 5wt%, and heating at 70 ℃ for 20min to obtain a colorless and transparent homogeneous solution system;
preparing mother liquor of folic acid: adding folic acid into a glycol-water binary solvent, and uniformly mixing to obtain a folic acid mother solution, wherein the molar concentration of folic acid in the folic acid mother solution is 100mM;
preparing a zinc nitrate mother solution: adding zinc nitrate into a glycol-water binary solvent, uniformly mixing to obtain a zinc nitrate mother solution, wherein the molar concentration of zinc nitrate in the zinc nitrate mother solution is 100mM,
slowly cooling the obtained homogeneous solution system to 50 ℃, taking 6.4mL of homogeneous solution system cooled to 60 ℃, sequentially adding 1.5mL of mother solution of folic acid and 2.1mL of mother solution of zinc nitrate, stirring to obtain homogeneous viscous liquid,
and injecting the obtained homogeneous phase viscous liquid into a mold, and cooling to room temperature to obtain the agarose/folic acid-zinc ion double-network gel.
Example 5
Preparing a folic acid solution: adding folic acid into a glycol-water binary solvent, and uniformly mixing to obtain a folic acid mother solution, wherein the molar concentration of folic acid in the folic acid mother solution is 100mM;
preparing a zinc nitrate solution: adding zinc nitrate into a glycol-water binary solvent, and uniformly mixing to obtain a mother solution of zinc nitrate, wherein the molar concentration of zinc nitrate in the mother solution of zinc nitrate is 100mM;
adding 29 μ L of 10wt% APS to 2.7mL of 30wt% acrylamide in ethylene glycol-water solution to obtain a homogeneous solution system;
then 600. Mu.L of folic acid solution (100 mM) and 840. Mu.L of zinc nitrate solution (100 mM) were added to the system, followed by sufficient stirring, and finally 3. Mu.L of TEMED was added to obtain a homogeneous viscous liquid;
and injecting the obtained homogeneous phase viscous liquid into a mold, and standing for 30min to obtain the polyacrylamide/folic acid-zinc ion double-network gel.
Comparative example
Comparative example 1
Adding polyvinyl alcohol into a glycol-water binary solvent to ensure that the mass fraction of the polyvinyl alcohol is 3wt%, and heating at 90 ℃ for 20min to obtain a colorless and transparent homogeneous solution system;
then slowly cooling the homogeneous solution system to 60 ℃, and stirring to obtain viscous liquid;
and (3) injecting the viscous liquid into a mold, cooling to room temperature, putting into a refrigerator at the temperature of-20 ℃, taking out after 30min, melting at room temperature, and repeating the process for three times to obtain the polyvinyl alcohol single-network gel.
Comparative example 2
Adding agarose with a certain mass into a glycol-water binary solvent to ensure that the mass fraction of the agarose is 5wt%, and heating at 70 ℃ for 20min to obtain a colorless and transparent homogeneous solution system;
slowly cooling to 50 ℃, then adding mother liquor of folic acid and mother liquor of zinc nitrate in sequence, stirring to obtain homogeneous viscous liquid,
and filling the obtained homogeneous phase viscous liquid into a self-made mould, and cooling to room temperature to obtain the agarose/folic acid-zinc ion double-network gel.
Comparative example 3
Adding 29 μ L of 10wt% APS to 2.7mL of 30wt% acrylamide in ethylene glycol-water solution, stirring well, adding 3 μ L of TEMED to obtain homogeneous viscous liquid;
and injecting the obtained homogeneous phase viscous liquid into a mold, and standing for 30min to obtain the polyacrylamide single-network gel.
Examples of the experiments
Experimental example 1
The temperature-changing rheological properties of the folic acid-zinc ion hydrogel obtained in example 1, namely the change rule of the storage modulus G 'and the loss modulus G' with temperature, were tested by using a ThermoHaake RS300 rheometer, and the obtained test results are shown in FIG. 2.
As can be seen from FIG. 2, the storage modulus G 'and the loss modulus G' of the folic acid-zinc ion hydrogel did not decrease significantly during the temperature increase from 30 ℃ to 70 ℃, both G 'and G' increased during the temperature increase from 30 ℃ to 50 ℃, and both G 'and G' remained substantially stable when the temperature reached above 50 ℃.
The folic acid-zinc ion hydrogel was placed in a vial and heated to 90 c, and the photograph thereof is shown in fig. 3, and it can be seen that the folic acid-zinc ion hydrogel can maintain the solid form of the gel without flowing even at an elevated temperature of 90 c.
The results of the stress scanning and the frequency scanning of the folic acid-zinc ion hydrogel obtained in example 1 at different temperatures (25 ℃,37 ℃,45 ℃ and 55 ℃) are respectively shown in fig. 4 and 5 by adopting a ThermoHaake RS300 rheometer, and it can be seen from fig. 4 and 5 that the storage modulus G 'of the folic acid-zinc ion hydrogel is always higher than the loss modulus G' by one order of magnitude in the range of 25-25 ℃, which is a typical gel rheology characteristic, which shows that the folic acid-zinc ion hydrogel always keeps a gel structure in the test temperature range and does not generate the conversion from gel to solution.
Experimental example 2
The transmission electron microscope test was performed on the folate-zinc ion hydrogel obtained in example 1, and the test results are shown in FIG. 6, and TEM images of the folate-zinc ion hydrogel at 25 ℃ and 80 ℃ are shown in FIGS. 6 (a) and 6 (b), respectively.
As can be seen from FIG. 6, the fiber network in the folic acid-metal ion hydrogel still exists at 80 ℃, compared with that at 25 ℃, the nanofiber network in the folic acid-metal ion hydrogel is more compact and the aggregation of the fibers is more remarkable at 80 ℃, which shows that the coordination function is enhanced after the temperature is increased, and the crosslinking between the fiber bundle and the fiber network is promoted.
Example 3
As the tetrad of folic acid is subjected to chiral accumulation, the folic acid-zinc ion hydrogel has strong circular dichroism signals, circular dichroism spectra of the folic acid-zinc ion hydrogel obtained in the example 1 at different temperatures (25 ℃,30, 35 ℃, 40 ℃,45 ℃,50 ℃, 55 ℃, 60 ℃, 65 ℃ and 70 ℃) are tested, the test result is shown in figure 7, and the change curve of the circular dichroism peak intensity at 318nm along with the temperature is shown in figure 8.
As can be seen from FIG. 7, the signals of the circular dichroism spectra of the folic acid-zinc ion hydrogel hardly changed in the temperature range of 25 to 70 ℃. The results are further confirmed in fig. 8. As the peaks near 280nm and 318nm in the circular dichroism are derived from the chiral accumulation of pterin group tetrad in the folic acid, the pterin tetrad chiral accumulation of the folic acid is still maintained in the temperature rising process, so that the folic acid-zinc ion hydrogel has the characteristic of high temperature resistance.
Experimental example 4
The photographs of the double-network gels obtained in examples 3 to 5 and the single-network gels obtained in comparative examples 1 to 3 at room temperature and high temperature, respectively, are shown in FIG. 9, (a), (b), (c) are photographs of the double-network gels obtained in examples 3 to 5 at room temperature, (d), (e), (f) are photographs of the double-network gels obtained in examples 3 to 5 at 90 ℃, respectively, and (g), (h) and (i) are photographs of the single-network gels obtained in comparative examples 1 to 3 at 60 ℃.
As can be seen from FIG. 9, when 10-25% by mass of folic acid-zinc ion hydrogel was introduced into a single polymer hydrogel, the three types of double-network gels could maintain the solid-like properties of the gel at 90 ℃ without melting, while the single-network gels obtained in comparative examples 1-3 melted into liquid at about 60 ℃.
Experimental example 5
The folate-zinc ion hydrogel (DN) obtained in example 1 and the double-network gel (F-Zn) obtained in example 3 were tested using a ThermoHaake RS300 rheometer 2+ ) And the temperature-variable rheological property of the single network gel (PVA) obtained in the comparative example 1, the test temperature is-30 ℃ to 80 ℃, and the test result is shown in figure 10.
As can be seen from FIG. 10, the storage modulus G 'and the loss modulus G' of the folic acid-zinc ion hydrogel and the double-network gel can be maintained at-30 ℃ to 80 ℃, but the storage modulus G 'and the loss modulus G' of the single-network gel are reduced from 40 ℃ and are melted to 50 ℃, so that the rheological property cannot be continuously tested.
It can be seen that the folic acid-zinc ion hydrogel only accounts for 10-25% of the weight of the double-network gel system, which is enough to endow the double-network hydrogel with high-temperature stability, and the formation of the double-network gel also obviously improves the mechanical strength of the polyvinyl alcohol gel.
The storage modulus G 'of the polyvinyl alcohol/folic acid-zinc ion double-network gel reaches 4016Pa, and the storage modulus G' of the polyvinyl alcohol single-network gel with the same concentration is only 2723Pa.
Mechanical property tests were performed on the dual-network gel of example 3 and the single-network gel of comparative example 1 using a ThermoHaake RS300 rheometer, and the test results are shown in fig. 11, where curve a is the single-network gel of comparative example 1 and curve B is the dual-network gel of example 3.
As can be seen from FIG. 11, the stretchable deformation of the PVA single-network gel is 750 + -53%, which can reach a tensile stress of 1007 + -166 kPa, while the stretchable deformation of the PVA/folate-zinc ion double-network gel can reach 976 + -23%, which is 1706 + -112 kPa, which shows that the double-network gel has a modulus and a tensile property, so that the double-network gel can bear the cutting of a blade without breaking.
The polyvinyl alcohol/folic acid-zinc ion double-network gel is subjected to compression-recovery cycle test at 80 ℃, and the obtained double-network gel can still maintain the mechanical property at 80 ℃, which indicates that the double-network gel has high-temperature stability.
The results show that the folic acid-zinc ion hydrogel is used as a heat-resistant factor to endow the traditional polymer gel with high-temperature stability, so that the folic acid-zinc ion hydrogel can be used as a heat-resistant gene to improve the thermal stability of the common hydrogel.
Experimental example 6
The double-network gel has conductivity because inorganic salt ions dissolved in a water-glycol solvent still exist in the double-network gel system. The polyvinyl alcohol/folic acid-zinc ion double-network gel, the power supply and the LED bulb are connected into a circuit, and the conductivity can be macroscopically known through the brightness of the LED bulb. The folic acid-zinc ion hydrogel of example 1, the double-network gel of example 3 and the single-network gel of comparative example 1 were subjected to conductivity tests, wherein (a), (d), (g) are the conductivity of the folic acid-zinc ion hydrogel of example at-20 ℃, 25 ℃ and 80 ℃, respectively, (b), (e), (h) are the conductivity of the polyvinyl alcohol single-network gel of comparative example 1 at-20 ℃, 25 ℃ and 80 ℃, respectively, and (c), (f) and (i) are the conductivity of the double-network gel of example 3 at-20 ℃, 25 ℃ and 80 ℃.
As shown in fig. 12, both the folic acid-zinc ion hydrogel and the double-network gel can conduct electricity at a temperature ranging from-20 ℃ to 80 ℃ to light the LED lamp, but the polyvinyl alcohol single-network gel has poor conductivity and gradually melts at high temperature.
The conductivity of the double-network gel can be accurately controlled by adding inorganic salt, fig. 13 shows the conductivity of the double-network gel obtained by adding different amounts of potassium nitrate at different temperatures, and as can be seen from fig. 9, the conductivity of the double-network gel can reach 1.2S/m at room temperature by 500mM of potassium nitrate, which meets the conductivity requirement of the flexible electronic device.
Experimental example 7
A double-network hydrogel film having a thickness of 1mm was prepared as described in example 3, and the film was subjected to light transmittance measurement and stretch measurement, and found to have excellent light transmittance and stretchability.
Cutting the film into strips with the length of 3cm and the width of 0.5cm as pressure sensors, connecting a circuit through conductive silver glue and silver wires, fixing the pressure sensors on fingers as shown in fig. 14 (a), measuring the influence of double-network gel film stretching caused by bending (stretch) and relaxing (relax) of the fingers on resistance as shown in fig. 14 (b), wherein the test result is shown in fig. 15, large and small respectively represent the bending degree of the fingers and the stretching degree of the gel, when the fingers are repeatedly and completely relaxed, the resistance of the sensors can be periodically changed, when the bending angle of the fingers is increased, the resistance change is increased, and the relative resistance change rate (delta R/R) is increased 0 ) The range is 0% -110%.
In order to measure the sensing behavior of the double-network gel film as a pressure sensor at high temperature, the pressure sensor is fixed at two ends of the metal sheet and connected into the circuit through the silver lead, the structural schematic diagram is shown in fig. 16, the double-network gel film is stretched through the bending of the metal sheet, the resistance change of the metal bending sensor is tested, and the test result is shown in fig. 17.
As can be seen from FIG. 17, the magnitude and speed of the change in resistance of the pressure sensor are identical at 25 deg.C and 80 deg.C, the relative rate of change in resistance (Δ R/R) 0 ) The change within 0-80% indicates that the pressure sensor has excellent temperature stability, and the polyvinyl alcohol single-network gel is melted at 60 ℃ and cannot meet the requirement of 80 DEG CThe sensing operation of (2). The dual-network gel taking the folic acid-zinc ion hydrogel as the heat-resistant gene can be used for preparing flexible electronic devices with wide working temperature range.
The invention has been described in detail with reference to the preferred embodiments and illustrative examples. It should be noted, however, that these specific embodiments are only illustrative of the present invention and do not limit the scope of the present invention in any way. Various modifications, equivalent substitutions and alterations can be made to the technical content and embodiments of the present invention without departing from the spirit and scope of the present invention, and these are within the scope of the present invention. The scope of the invention is defined by the appended claims.
Claims (8)
1. The double-network gel is characterized by being formed by folic acid-metal ion hydrogel and polymer gel, wherein the folic acid-metal ion hydrogel accounts for 5-30% of the mass fraction;
the double-network gel is prepared by a method comprising the following steps:
step 1, adding raw materials into a solvent, mixing to obtain a mixed solution, wherein the mass fraction of the raw materials in the mixed solution is 1-30 wt%, heating the mixed solution to 70-90 ℃ to obtain a homogeneous solution system, and the raw materials are polyvinyl alcohol or agarose;
step 2, adding mother liquor of folic acid and mother liquor of metal inorganic salt into the homogeneous phase solution system to obtain homogeneous phase viscous liquid, wherein the ratio of the molar concentration of folic acid to the molar concentration of metal ions in the obtained homogeneous phase viscous liquid is 1 (1-5), the mass concentration of raw materials is 3-10 wt%, the metal inorganic salt is inorganic salt containing metal ions, and the metal ions are zinc ions;
and 3, injecting the homogeneous viscous liquid obtained in the step 2 into a mold, and circularly freezing and melting after cooling to room temperature to obtain the double-network gel.
2. The double-network gel according to claim 1, characterized in that it is in a solid-like state at a temperature in the range of 25 to 90 ℃ and/or
The tensile deformation of the double-network gel is 900-1000%, and the tensile stress is 1500-1900 kPa.
3. A method for preparing a double-network gel according to claim 1 or 2, characterized in that it comprises the following steps:
step 1, adding raw materials into a solvent to obtain a homogeneous solution system;
step 2, adding mother liquor of folic acid and mother liquor of metal inorganic salt to obtain homogeneous viscous liquid;
step 3, obtaining double-network gel;
in the step 2, mother liquor of folic acid is obtained by dissolving folic acid in a solvent, mother liquor of metal inorganic salt is obtained by dissolving metal inorganic salt in the solvent,
in the obtained homogeneous phase viscous liquid, the liquid is dissolved in the solvent,
the molar concentration ratio of folic acid to metal ions is 1: (0.1-10) of,
the mass concentration of the raw materials is 2-25 wt%.
4. A process according to claim 3, wherein the resulting homogeneous viscous liquid,
the molar concentration ratio of folic acid to metal ions is 1 (1-5),
the mass concentration of the raw materials is 3-10 wt%.
5. The method of claim 3,
step 1, adding the raw materials into a solvent, mixing to obtain a mixed solution, and heating the mixed solution to obtain a homogeneous solution system, wherein the heating temperature is 70-90 ℃, and the heating time is 10-40 min;
in the step 2, cooling the homogeneous solution system to 50-60 ℃, and adding a mother solution of folic acid and a mother solution of metal inorganic salt to obtain a homogeneous viscous liquid;
and in the step 3, cooling or repeatedly freezing and melting the obtained homogeneous viscous liquid to obtain the double-network gel.
6. The method of claim 3,
step 1, adding raw materials into a solvent, mixing to obtain a mixed solution, and adding an initiator into the mixed solution to obtain a homogeneous solution system;
in the step 2, mother liquor of folic acid and mother liquor of metal inorganic salt are added, and coagulant is added to obtain homogeneous viscous liquid;
and 3, standing the homogeneous phase viscous liquid obtained in the step 2 for 10-60 min to obtain the double-network gel.
7. Use of the double network gel according to claim 1 or 2 in a flexible electronic device for the preparation of a sensor.
8. Use according to claim 7 for the preparation of a pressure sensor.
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