Full-bio-based physical crosslinking hydrogel and preparation method thereof

Abstract

The invention discloses full-bio-based physical crosslinking hydrogel and a preparation method thereof, and relates to the field of preparation of hydrogel functional materials. In the application, the preparation method of the full-bio-based physical crosslinking hydrogel comprises the following specific steps: S1, mixing gelatin and starch, heating, and standing at low temperature to obtain full-bio-based original gel; s2, carrying out primary salting-out on the whole bio-based original gel; s3, performing directional stretching on the hydrogel subjected to the first salting-out treatment; and S4, salting out the stretched hydrogel again. The tensile strength of the full-bio-based physical crosslinking hydrogel prepared in the invention can reach 7.81 to 10.37 MPa, and the Young modulus range of the full-bio-based physical crosslinking hydrogel can reach 12.32 to 47.32 MPa.

Description

Technical Field

[0001] This application relates to the field of preparation of hydrogel functional materials, specifically to a fully bio-based physically cross-linked hydrogel and its preparation method. Background Technology

[0002] Hydrogels have shown broad application prospects in food engineering, tissue engineering, wearable sensors, and other fields. Currently, fully bio-based hydrogels are attracting significant attention from researchers due to their biodegradability. However, most fully bio-based hydrogels prepared using existing technologies suffer from poor tensile strength and Young's modulus, which greatly limits their development and practical application. Therefore, this application specifically proposes a fully bio-based physically crosslinked hydrogel with superior tensile strength and Young's modulus, along with its preparation method. Summary of the Invention

[0003] To address the above problems, this invention provides a fully bio-based physically cross-linked hydrogel and its preparation method.

[0004] To achieve the above objectives, the technical solution adopted in this experiment is as follows: A fully bio-based physically crosslinked hydrogel, wherein the tensile strength ranges from 7.81 to 10.37 MPa and the Young's modulus ranges from 12.32 to 47.32 MPa.

[0005] A method for preparing a fully bio-based physically crosslinked hydrogel includes the following steps: S1. Mix gelatin and starch, heat and then let stand at low temperature to obtain a fully bio-based progel. S2. Perform initial salting out of the all-bio-based pro-gel; S3. The hydrogel that has undergone initial salting-out treatment is stretched in a specific direction. S4. The stretched hydrogel is subjected to salting out again.

[0006] Preferably, step S1 specifically includes the following steps: placing gelatin and starch in ultrapure water, heating and stirring to obtain a uniform mixed solution; subjecting the mixed solution to ultrasonic treatment to effectively remove air bubbles, obtaining an ultrasonically treated liquid; then, pouring the ultrasonically treated liquid into a glass mold, allowing it to stand at low temperature to obtain a fully bio-based protogel.

[0007] Preferably, in step S1, the mass percentage of gelatin is 5% to 7% of the total mass of gelatin, starch, and ultrapure water; the mass percentage of starch is 3% to 5% of the total mass of gelatin, starch, and ultrapure water; and the mass percentage of ultrapure water is 88% to 92% of the total mass of gelatin, starch, and ultrapure water.

[0008] Preferably, in step S1, the heating and stirring conditions are: stirring temperature of 75-90 °C and stirring time of 1.5-3.5 h.

[0009] Preferably, in step S1, the ultrasonic power is 90-120 W and the ultrasonic time is 8-15 min during ultrasonic treatment.

[0010] Preferably, in step S1, the conditions for low-temperature standing are: a low temperature range of 2 to 6 °C and a standing time of 1 to 3 h.

[0011] Preferably, step S2 specifically includes the following steps: cutting the all-bio-based progenitor gel into rectangular hydrogels, soaking the rectangular hydrogels in an organic salt solution to obtain a preliminary hydrogel product, and absorbing the organic salt solution off the surface of the preliminary hydrogel product with absorbent paper to obtain a hydrogel with the first salting-out treatment.

[0012] Preferably, in step S2, the molar concentration of the organic salt solution is 1–2.5 mol / L, and the soaking time is 1–1.5 h.

[0013] Preferably, in step S2, the organic salt solution is an organic salt solution with citrate as the anion.

[0014] Preferably, in step S2, one of sodium citrate aqueous solution, ammonium citrate aqueous solution, and potassium citrate aqueous solution is used.

[0015] Preferably, step S3 specifically includes the following steps: stretching the hydrogel treated by the first salting out, so that the hydrogel treated by the first salting out is stretched to 150% to 250% of its original length along the length direction; and then fixing the stretched state of the hydrogel treated by the first salting out to obtain the stretched hydrogel.

[0016] Preferably, step S4 specifically includes the following steps: soaking the stretched hydrogel in an organic salt solution to obtain a re-salting hydrogel; absorbing the organic salt solution on the surface of the re-salting hydrogel with absorbent paper to obtain a fully bio-based physically cross-linked hydrogel.

[0017] Preferably, in step S4, the molar concentration of the organic salt solution is 1–2.5 mol / L, and the soaking time is 12–20 h.

[0018] Preferably, in step S4, the organic salt solution is one of sodium citrate aqueous solution, ammonium citrate aqueous solution, and potassium citrate aqueous solution.

[0019] Compared with the prior art, the beneficial technical effects of this application are as follows: Since gelatin can solidify after heating and then being allowed to stand at a low temperature, in step S1 of this application, mixing gelatin and starch, followed by heating and then allowing them to stand at a low temperature, allows for physical cross-linking of the gelatin and starch, resulting in a fully bio-based protogel. Specifically, this allows the amino and carboxyl groups on the gelatin to form multiple intermolecular hydrogen bonds with the hydroxyl groups on the starch. However, this physical cross-linking method is difficult to disrupt the inherent interactions within the individual molecules of gelatin and starch, preventing the gelatin and starch molecules from fully interacting and fusing. Therefore, the fully bio-based protogel obtained in this step is very fragile. For example, the fully bio-based protogel may break during further processing. When subjected to mechanical stretching, it can only withstand stretching to about 25% of its original length, and cannot withstand further mechanical stretching, which greatly limits the improvement of the mechanical properties of the all-bio-based original gel. In the existing technology, the mechanical properties of hydrogels are usually improved by directional stretching, and the stretching length is usually at least 40% to 50% of the original length of the hydrogel. However, due to the strong intramolecular interactions of different biopolymers in gels prepared by physical cross-linking of two or more natural polymers, it is difficult for different biopolymer molecules in the gel to interact and fuse. Therefore, it is difficult to directly improve the mechanical properties of hydrogels by directional stretching. Therefore, this application first performs initial salting out on the all-bio-based protogel, then directional stretching, and then performs a second salting out. Specifically, this application first performs initial salting out on the all-bio-based protogel, utilizing the salting out effect in the Hofmeister effect. Citrate ions in the organic salt solution significantly reduce the solubility of gelatin and starch in the water inside the all-bio-based protogel by competing for water molecules, stripping the hydration layer, and strengthening hydrophobic interactions. This not only induces the aggregation and arrangement of gelatin molecules but also the aggregation and arrangement of starch molecules, forming starch crystal structures and gelatin crystal structures. Furthermore, it enhances the hydrogen bonding interactions between starch and gelatin molecules, thereby effectively strengthening the interactions between starch and gelatin molecules. Interactive fusion; then, this application performs directional stretching treatment on the first salting-out fully bio-based physically cross-linked hydrogel. This application uses directional stretching to gradually straighten the bent gelatin and starch molecules in the first salting-out fully bio-based physically cross-linked hydrogel, so that the gelatin and starch molecules in the first salting-out fully bio-based physically cross-linked hydrogel can be oriented and arranged, further forming starch crystal structure and gelatin crystal structure, to obtain directional stretched fully bio-based physically cross-linked hydrogel; then, the directional stretched fully bio-based physically cross-linked hydrogel forms oriented gelatin crystals and starch crystals again under the salting-out effect, while reducing the distance between gelatin molecules and starch molecules, and enhancing the compactness between molecules inside the fully bio-based physically cross-linked hydrogel, thereby obtaining a fully bio-based physically cross-linked hydrogel with high density.

[0020] Furthermore, tests show that the fully bio-based physically cross-linked hydrogel prepared in this application has a maximum tensile strength of 10.37 MPa, a maximum Young's modulus of 47.32 MPa, and a maximum crystallinity of 5.63%. Moreover, the fully bio-based physically cross-linked hydrogel prepared in this application is also completely degradable. Attached Figure Description

[0021] Figure 1 Figures a and b are scanning electron microscope images of the fully bio-based progel and the fully bio-based physically cross-linked hydrogel prepared in Example 1, respectively; the scale of both figures a and b is 10 μm. Figure 2 X-ray diffraction patterns of the fully bio-based progel (a) and the fully bio-based physically crosslinked hydrogel (b) prepared in Example 1; Figure 3 Tensile curves of the fully bio-based protogel (a) and the fully bio-based physically crosslinked hydrogel (b) prepared in Example 1; Figure 4 The puncture curve of the fully bio-based physically cross-linked hydrogel prepared in Example 1; Figure 5 Photographs showing the dissolution process of the fully bio-based physically cross-linked hydrogel prepared in Example 1 in water at 40°C; Figure 6 The image shows a scanning electron microscope image of the fully bio-based physically cross-linked hydrogel prepared in Example 2; the scale in the image is 10 μm. Figure 7 X-ray diffraction pattern of the fully bio-based physically cross-linked hydrogel prepared in Example 2; Figure 8 The image shows the stretching curve of the fully bio-based physically cross-linked hydrogel prepared in Example 2. Figure 9 The puncture curve is shown in Example 2 for the fully bio-based physically cross-linked hydrogel. Figure 10 Photographs showing the dissolution process of the fully bio-based physically cross-linked hydrogel prepared in Example 2 in water at 60°C; Figure 11 This is a scanning electron microscope image of the fully bio-based physically cross-linked hydrogel prepared in Example 3; the scale in the image is 10 μm. Figure 12 X-ray diffraction pattern of the fully bio-based physically cross-linked hydrogel prepared in Example 3; Figure 13 The image shows the stretching curve of the fully bio-based physically cross-linked hydrogel prepared in Example 3; Figure 14 The puncture curve is shown for the fully bio-based physically cross-linked hydrogel prepared in Example 3. Figure 15Photographs showing the dissolution process of the fully bio-based physically cross-linked hydrogel prepared in Example 3 in α-amylase solution; Figure 16 This is a schematic diagram of the fixture's structure; Figure 17 This is a side view schematic diagram of the connection relationship between a set of clamping components and screws; Figure 16 and Figure 17 In the middle, 1. rectangular frame, 2. clamping component, 21. fixing plate, 22. clamping plate, 23. vertical connecting plate, 3. screw. Detailed Implementation

[0022] The pigskin gelatin, starch, sodium citrate, and ammonium citrate used in this application were all purchased from the market and applied directly.

[0023] Example 1:

[0024] S1. Mix gelatin and starch, heat, and then let stand at a low temperature to obtain a fully bio-based progel; specifically including the following steps: 5.0 g of gelatin and 3.0 g of starch were placed in 92.0 g of ultrapure water and stirred at 75 °C for 1.5 h to obtain a homogeneous mixed solution. The mixed solution was then subjected to ultrasonic treatment at a power of 90 W for 8 min to effectively remove air bubbles, resulting in an ultrasonically treated liquid. The ultrasonically treated liquid was then poured into a glass mold and allowed to stand at 2 °C for 1 h to obtain a fully bio-based protogel. In this Example 1, the prepared fully bio-based protogel had a length of 12 cm, a width of 8 cm, and a thickness of 0.3 cm. The gelatin used in Example 1 was pigskin gelatin. S2. Perform initial salting-out treatment on the all-bio-based protogel, specifically including the following steps: The bio-based progenitor gel obtained in step S1 was cut into rectangular hydrogels with a length of 12 cm, a width of 1 cm, and a thickness of 0.3 cm. The rectangular hydrogels were then soaked in a sodium citrate aqueous solution with a molar concentration of 1 mol / L for 1 h to obtain the initial hydrogel product. The sodium citrate aqueous solution on the surface of the initial hydrogel product was then absorbed with absorbent paper to obtain the hydrogel after the first salting-out treatment. S3. Perform directional stretching on the hydrogel after the initial salting-out treatment, specifically including the following steps: Place one end of the hydrogel from the first salting-out treatment between the two clamping plates of one set of clamping members in the fixture, and place the other end of the hydrogel from the first salting-out treatment between the two clamping plates of another set of clamping members in the fixture. Then use screws to tighten the two clamping plates of each set of clamping members. Then, the fixing plates of the two sets of clamping parts are pulled in opposite directions, causing the hydrogel treated by the first salting out to be stretched along its length to 150% of its original length. Then, screws are used to tighten the corresponding fixing plates in each set of clamps to fix the stretched state of the hydrogel after the first salting-out treatment, and the stretched hydrogel is obtained. At this time, the stretched hydrogel is fixed by clamps. The structure of the clamp in this application is as follows: Figure 16 and Figure 17 As shown, the clamp includes a rectangular frame 1 cut from a polytetrafluoroethylene sheet and four clamping members 2; the outer frame of the rectangular frame 1 has a length of 25 cm, a width of 15 cm, and a thickness of 1 cm, and the width between the inner frame and the outer frame of the rectangular frame 1 is 1 cm; the four clamping members 2 have the same structure and are divided into two groups, with one group of clamping members 2 symmetrically arranged on the upper and lower sides of the rectangular frame 1 along the width direction; in this application, the clamping member 2 includes two horizontal fixing plates 21, a horizontal clamping plate 22, and two vertical connecting plates 23, with the two fixing plates 21 respectively fixedly connected to the two ends of the clamping plate 22 through the two vertical connecting plates 23; in each group of clamping members 2, the clamping plates 22 of the two clamping members 2 are detachably connected by screws 3; in each group of clamping members 2, the two fixing plates 21 of the two clamping members 2 located on the same side are also detachably connected by screws 3; after the clamp is assembled, the vertical connecting plate 23 contacts the rectangular frame 1; S4. The stretched hydrogel is subjected to a second salting-out treatment, specifically including the following steps: The stretched hydrogel, fixed by a clamp, was immersed in a sodium citrate aqueous solution for 12 h. The molar concentration of the sodium citrate aqueous solution was 1 mol / L. After immersion, the clamp and the stretched hydrogel it fixed were removed from the sodium citrate aqueous solution. Then, the screws on the clamp were removed, the clamping parts were taken off, and the hydrogel was taken out. This hydrogel is the re-salting hydrogel. The sodium citrate aqueous solution on the surface of the re-salting hydrogel was absorbed with absorbent paper to obtain a fully bio-based physically cross-linked hydrogel.

[0025] Test 1: The fully bio-based progel prepared in step S1 of Example 1 and the fully bio-based physically crosslinked hydrogel prepared in step S4 were characterized and tested. Specifically: (1) Scanning electron microscopy tests of fully bio-based progels and fully bio-based physically cross-linked hydrogels: The fully bio-based protogel and the fully bio-based physically cross-linked hydrogel were subjected to brittle fracture in liquid nitrogen, sputter-coated with gold, and their microstructures were observed using scanning electron microscopy. The results are as follows: Figure 1 As shown in Figures a and b.

[0026] from Figure 1 As shown in Figure a, the fully bio-based protogel has loose pores; when the fully bio-based protogel is stretched, the loose pores cause the fully bio-based protogel to break under relatively small tensile force. from Figure 1 As shown in Figure b, the fully bio-based physically cross-linked hydrogel exhibits no loose pores and has a relatively dense structure with only a few gaps on its surface, as indicated by the yellow dashed box in Figure b. When the fully bio-based physically cross-linked hydrogel is stretched, its dense structure requires a relatively large tensile force to break. This implies that the fully bio-based physically cross-linked hydrogel has a higher tensile strength and Young's modulus than the original fully bio-based hydrogel.

[0027] (2) Crystal structure characterization of fully bio-based progels and fully bio-based physically cross-linked hydrogels: The fully bio-based progel and the fully bio-based physically crosslinked hydrogel were placed on the X-ray diffraction stage, respectively. The diffraction angle range was set to 5°–70°, and the scanning rate was 5° / min. The XRD curves were obtained as follows: Figure 2 As shown; Figure 2 In the figure, curve a represents the XRD curve of the fully bio-based protogel, and curve b represents the XRD curve of the fully bio-based physically cross-linked hydrogel. from Figure 2 As shown in curve a, the XRD pattern of the all-bio-based progenitor gel exhibits diffraction peaks at both 27° and 39°. The diffraction peak at 27° is related to the random coiling and dispersion of individual polypeptide chains within the all-bio-based progenitor gel, indicating that a specific spatial sequence has not been formed. The diffraction peak at 39° is associated with the amorphous regions of gelatin. The presence of these diffraction peaks at both 27° and 39° suggests that an ordered crystalline structure has not formed within the all-bio-based progenitor gel. Furthermore, calculations using Jade software show that the crystallinity of the all-bio-based progenitor gel is only 0.26%. from Figure 2As shown in curve b, the XRD curve of the fully bio-based physically cross-linked hydrogel exhibits diffraction peaks at 8.2° and 17.1°. The diffraction peak at 8.2° is related to the formation of a triple helix in gelatin, which (approximately 6–9°) indicates the formation of a crystal structure. The diffraction peak at 17.1° is related to the crystallization of starch. The presence of diffraction peaks at both 8.2° and 17.1° in the fully bio-based physically cross-linked hydrogel indicates the formation of a crystalline structure. Furthermore, calculations using Jade software show that the crystallinity of the fully bio-based physically cross-linked hydrogel is 5.42%, meaning that the fully bio-based physically cross-linked hydrogel... The crystallinity of the gel increased by 1984.6% compared to the original bio-based gel, indicating the formation of more crystalline structures in the bio-based physically crosslinked hydrogel. Because of the numerous crystalline structures formed within the bio-based physically crosslinked hydrogel, these structures, acting as physical crosslinking points, enhance the structural stability of the gel network. When the bio-based physically crosslinked hydrogel is stretched, the enhanced structural stability of the gel network effectively hinders the relative slippage of the polymer chains, thereby effectively improving the deformation resistance of the bio-based physically crosslinked hydrogel and ultimately increasing its tensile strength and Young's modulus.

[0028] (3) Tensile property tests of fully bio-based protogels and fully bio-based physically crosslinked hydrogels: The fully bio-based protogel and the fully bio-based physically crosslinked hydrogel were fixed on a smart tensile testing machine, and the tensile rate was set to 50 mm / min to obtain the tensile curves, as shown below. Figure 3 As shown; Figure 3 In the figure, curve a represents the stretching curve of the fully bio-based protogel, and curve b represents the stretching curve of the fully bio-based physically cross-linked hydrogel.

[0029] from Figure 3 It can be seen that the tensile strength of the fully bio-based protogel prepared in Example 1 is 0.01 MPa, and the Young's modulus is 0.004 MPa; the tensile strength of the fully bio-based physically cross-linked hydrogel prepared in Example 1 is 8.19 MPa, and the Young's modulus is 12.32 MPa. Clearly, after the fully bio-based protogel underwent initial salting-out treatment, directional stretching treatment, and subsequent salting-out treatment, the tensile strength and Young's modulus increased by 81800% and 307900%, respectively. This indicates that the sequential initial salting-out treatment, directional stretching treatment, and subsequent salting-out treatment of the fully bio-based protogel can effectively improve the tensile strength and Young's modulus of the fully bio-based hydrogel.

[0030] (4) Puncture test of fully bio-based physically cross-linked hydrogels: A fully bio-based physically cross-linked hydrogel was fixed between two steel rings on a texture analyzer for puncture experiments. A 2 mm diameter needle was used, and the needle tip moved downwards at a speed of 1 mm / s. The puncture test results are as follows: Figure 4 As shown. From Figure 4 It can be seen that the puncture resistance strength when the needle tip penetrates is 3.21 MPa and the puncture displacement is 9.64 mm. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 1 has a strong puncture resistance, which corresponds to its high tensile strength and Young's modulus.

[0031] (5) Degradation test of fully bio-based physically cross-linked hydrogels: The fully bio-based physically crosslinked hydrogel prepared in Example 1 was cut into hydrogel blocks measuring 3.43cm × 2.04cm × 0.26cm. The hydrogel blocks were then placed in water at 40°C, and photographs were taken at 0s, 18s, and 63s. The photographs are shown below. Figure 5 As shown. From Figure 5 As can be seen, the fully bio-based physically cross-linked hydrogel rapidly changes from a gel state to a sol state within 0s to 18s, and by 63s, the fully bio-based physically cross-linked hydrogel is completely miscible with water. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 1 can be completely degraded in water at 40℃, and the degradation rate is relatively fast.

[0032] Example 2:

[0033] S1. Mix gelatin and starch, heat, and then let stand at a low temperature to obtain a fully bio-based progel; specifically including the following steps: 6.0 g of gelatin and 4.0 g of starch were placed in 90.0 g of ultrapure water and stirred at 80 °C for 2 h to obtain a homogeneous mixed solution. The mixed solution was then subjected to ultrasonic treatment at a power of 110 W for 12 min to effectively remove air bubbles, resulting in an ultrasonically treated liquid. The ultrasonically treated liquid was then poured into a glass mold and allowed to stand at 4 °C for 1 h to obtain a fully bio-based protogel. The fully bio-based protogel prepared in Example 2 had a length of 12 cm, a width of 8 cm, and a thickness of 0.3 cm. The gelatin used in Example 2 was pigskin gelatin. S2. The initial salting-out treatment of the all-bio-based protogel includes the following steps: The bio-based progenitor gel obtained in step S1 was cut into rectangular hydrogels with a length of 12 cm, a width of 1.2 cm, and a thickness of 0.3 cm. The rectangular hydrogels were then soaked in an ammonium citrate aqueous solution with a molar concentration of 1.5 mol / L for 1.2 h to obtain a preliminary hydrogel product. The ammonium citrate aqueous solution on the surface of the preliminary hydrogel product was then removed with absorbent paper to obtain the hydrogel after the first salting-out treatment. S3. The hydrogel undergoes initial salting-out treatment followed by directional stretching, specifically including the following steps: Place one end of the hydrogel from the first salting-out treatment between the two clamping plates of one set of clamping members in the fixture, and place the other end of the hydrogel from the first salting-out treatment between the two clamping plates of another set of clamping members in the fixture. Then use screws to tighten the two clamping plates of each set of clamping members. Then, the fixing plates of the two sets of clamping parts are pulled in opposite directions, causing the hydrogel treated by the first salting out to be stretched along its length to 200% of its original length; Then, screws are used to tighten the corresponding fixing plates in each set of clamps to fix the stretched state of the hydrogel after the first salting-out treatment, and the stretched hydrogel is obtained. At this time, the stretched hydrogel is fixed by clamps. S4. The hydrogel is stretched and then subjected to a second salting-out treatment, which includes the following steps: The stretched hydrogel, fixed by a clamp, was immersed in an ammonium citrate aqueous solution for 16 h. The molar concentration of the ammonium citrate aqueous solution was 1.5 mol / L. After immersion, the clamp and the stretched hydrogel fixed by it were removed from the ammonium citrate aqueous solution. Then, the screws on the clamp were removed, the clamping parts were removed, and the hydrogel was taken out. This hydrogel is the re-salting hydrogel. The ammonium citrate aqueous solution on the surface of the re-salting hydrogel was absorbed with absorbent paper to obtain a fully bio-based physically cross-linked hydrogel.

[0034] Test 2: The fully bio-based physically crosslinked hydrogel prepared in Example 2 was characterized and tested: (1) Microstructure characterization of fully bio-based physically cross-linked hydrogels: The fully bio-based physically cross-linked hydrogel was subjected to brittle fracture in liquid nitrogen, sputter-coated with gold, and its microstructure was observed using a scanning electron microscope. The results are as follows: Figure 6 As shown.

[0035] from Figure 6 It can be seen that: no loose pores are observed in the fully bio-based physically cross-linked hydrogel, and the structure of the fully bio-based physically cross-linked hydrogel is relatively dense, with only a few gaps appearing on its surface. Among them, the gaps are as follows: Figure 6 The area highlighted by the yellow dashed box is shown in the image. When fully bio-based physically cross-linked hydrogels are stretched, their relatively dense structure means that they require a large tensile force to break. This also means that fully bio-based physically cross-linked hydrogels have a large tensile strength and Young's modulus.

[0036] (2) Crystal structure characterization of the fully bio-based physically cross-linked hydrogel: The fully bio-based physically crosslinked hydrogel was placed on the X-ray diffractometer stage. The diffraction angle range was set to 5°–70°, and the scanning rate was 5° / min. The XRD curves were obtained as follows: Figure 7 As shown.

[0037] from Figure 7 It can be seen that the XRD curves of the fully bio-based physically cross-linked hydrogel show diffraction peaks at 8.2° and 17.1°, respectively. The diffraction peak at 8.2° is related to the formation of a triple helix in gelatin, and the formation of a triple helix in gelatin (approximately 6–9°) indicates the formation of a crystal structure. The diffraction peak at 17.1° is related to the crystallization of starch. The presence of diffraction peaks at both 8.2° and 17.1° in the fully bio-based physically cross-linked hydrogel indicates that it has formed a crystalline structure. Furthermore, calculations using Jade software show that the crystallinity of the fully bio-based physically cross-linked hydrogel is [insert value here]. The 5.24% indicates the formation of numerous crystalline structures within the fully bio-based physically crosslinked hydrogel. These crystalline structures, acting as physical crosslinking points, enhance the structural stability of the gel network. When stretched, the strengthened gel network effectively hinders the relative slippage of the polymer chains, thereby significantly improving the hydrogel's resistance to deformation and ultimately increasing its tensile strength and Young's modulus.

[0038] (3) Tensile property test of fully bio-based physically cross-linked hydrogels: A fully bio-based physically cross-linked hydrogel was fixed on a smart tensile testing machine, and a tensile rate of 50 mm / min was set to obtain a tensile curve, as shown below. Figure 8 As shown.

[0039] from Figure 8 It can be seen that the tensile strength of the fully bio-based physically crosslinked hydrogel prepared in Example 2 is 7.81 MPa and the Young's modulus is 14.32 MPa. This indicates that the initial salting-out treatment, directional stretching treatment, and subsequent salting-out treatment can effectively improve the tensile strength and Young's modulus of the fully bio-based hydrogel.

[0040] (4) Puncture test of fully bio-based physically cross-linked hydrogels: A fully bio-based physically cross-linked hydrogel was fixed between two steel rings on a texture analyzer for puncture experiments. A 2 mm diameter needle was used, and the needle tip moved downwards at a speed of 1 mm / s. The puncture test results are as follows: Figure 9 As shown.

[0041] from Figure 9It can be seen that the puncture resistance strength when the needle tip penetrates is 7.62 MPa and the puncture displacement is 3.22 mm. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 2 has a strong puncture resistance, which corresponds to its high tensile strength and Young's modulus.

[0042] (5) Degradation test of fully bio-based physically cross-linked hydrogels: The fully bio-based physically crosslinked hydrogel prepared in Example 2 was cut into hydrogel blocks measuring 2.78cm × 2.91cm × 0.27cm. The hydrogel blocks were then placed in water at 60°C, and photographs were taken at 0s, 5s, and 30s. The photographs are shown below. Figure 10 As shown.

[0043] from Figure 10 As can be seen, the fully bio-based physically cross-linked hydrogel prepared in Example 2 rapidly changed from a gel state to a sol state within 0s to 5s, and by 30s, the fully bio-based physically cross-linked hydrogel was completely miscible with water. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 2 can be completely degraded in water at 60℃, and the degradation rate is relatively fast.

[0044] Example 3:

[0045] S1. Mix gelatin and starch, heat, and then let stand at a low temperature to obtain a fully bio-based progel; specifically including the following steps: 7.0 g of gelatin and 5.0 g of starch were weighed and placed in 88.0 g of ultrapure water, and stirred at 90 °C for 3.5 h to obtain a homogeneous mixed solution. The mixed solution was then subjected to ultrasonic treatment at a power of 120 W for 15 min to effectively remove air bubbles and obtain an ultrasonically treated liquid. The ultrasonically treated liquid was then poured into a glass mold and allowed to stand at 6 °C for 3 h to obtain a fully bio-based protogel. The fully bio-based protogel prepared in this Example 3 has a length of 12 cm, a width of 8 cm, and a thickness of 0.3 cm. S2. Perform initial salting-out treatment on the all-bio-based protogel, specifically including the following steps: The bio-based progenitor gel obtained in step S1 was cut into rectangular hydrogels with a length of 12 cm, a width of 1.5 cm, and a thickness of 0.3 cm. Then, the rectangular hydrogels were soaked in a sodium citrate aqueous solution at 20°C for 1.5 h to obtain a preliminary hydrogel product. The sodium citrate aqueous solution was a saturated sodium citrate aqueous solution at 20°C. The sodium citrate aqueous solution on the surface of the preliminary hydrogel product was then absorbed using absorbent paper to obtain a hydrogel with the first salting-out treatment. The gelatin used in Example 3 was pigskin gelatin. S3. Perform directional stretching on the hydrogel after the initial salting-out treatment, specifically including the following steps: Place one end of the hydrogel from the first salting-out treatment between the two clamping plates of one set of clamping members in the fixture, and place the other end of the hydrogel from the first salting-out treatment between the two clamping plates of another set of clamping members in the fixture. Then use screws to tighten the two clamping plates of each set of clamping members. Then, the fixing plates of the two sets of clamping parts are pulled in opposite directions, causing the hydrogel treated in the first salting-out process to be stretched along its length to 250% of its original length. Then, screws are used to tighten the corresponding fixing plates in each set of clamps to fix the stretched state of the hydrogel after the first salting-out treatment, and the stretched hydrogel is obtained. At this time, the stretched hydrogel is fixed by clamps. S4. The hydrogel is stretched and then subjected to a second salting-out treatment, which includes the following steps: At 20°C, the stretched hydrogel, fixed by a clamp, was immersed in a sodium citrate aqueous solution for 20 h, wherein the sodium citrate aqueous solution was a saturated sodium citrate aqueous solution at 20°C. After immersion, the clamp and the stretched hydrogel fixed therein were removed from the sodium citrate aqueous solution. Then, the screws on the clamp were removed, the clamping parts were taken off, and the hydrogel was taken out to obtain a re-salting hydrogel. The sodium citrate aqueous solution on the surface of the re-salting hydrogel was absorbed with absorbent paper to obtain a fully bio-based physically crosslinked hydrogel.

[0046] Test 3: The fully bio-based physically crosslinked hydrogel prepared in Example 3 was characterized and tested, specifically: (1) Scanning electron microscopy test of the fully bio-based physically cross-linked hydrogel: The fully bio-based physically cross-linked hydrogel was subjected to brittle fracture in liquid nitrogen, sputter-coated with gold, and its microstructure was observed using a scanning electron microscope. The results are as follows: Figure 11 As shown.

[0047] from Figure 11 It can be seen that: no loose pores are observed in the fully bio-based physically cross-linked hydrogel, and the structure of the fully bio-based physically cross-linked hydrogel is relatively dense, with fewer gaps on its surface. Among them, the gaps are as follows: Figure 11 The area highlighted in the yellow dashed box is shown in the image. When fully bio-based physically cross-linked hydrogels are stretched, their denser structure requires a greater tensile force to break them. This means that fully bio-based physically cross-linked hydrogels have greater tensile strength and Young's modulus.

[0048] (2) Crystal structure characterization of the fully bio-based physically cross-linked hydrogel: The fully bio-based physically crosslinked hydrogels were placed on the X-ray diffractometer stage, with the diffraction angle range set to 5–70° and the scanning rate to 5° / min. The XRD curves were obtained as shown below. Figure 12 As shown; from Figure 12 It can be seen that the XRD curves of the fully bio-based physically cross-linked hydrogel show diffraction peaks at 8.2° and 17.1°, respectively. The diffraction peak at 8.2° is related to the formation of a triple helix in gelatin, and the formation of a triple helix in gelatin (approximately 6–9°) indicates the formation of a crystal structure. The diffraction peak at 17.1° is related to the crystallization of starch. The presence of diffraction peaks at both 8.2° and 17.1° in the fully bio-based physically cross-linked hydrogel indicates that it has formed a crystalline structure. Furthermore, calculations using Jade software show that the crystallinity of the fully bio-based physically cross-linked hydrogel is [insert value here]. The 5.63% indicates the formation of numerous crystalline structures within the fully bio-based physically cross-linked hydrogel. These crystalline structures, acting as physical cross-linking points, enhance the structural stability of the gel network. When stretched, the strengthened gel network effectively hinders the relative slippage of the polymer chains, thereby significantly improving the hydrogel's resistance to deformation and ultimately increasing its tensile strength and Young's modulus.

[0049] (3) Tensile property test of fully bio-based physically cross-linked hydrogels: A fully bio-based physically cross-linked hydrogel was fixed on a smart tensile testing machine, and a tensile rate of 50 mm / min was set to obtain a tensile curve, as shown below. Figure 13 As shown; from Figure 13 It can be seen that the tensile strength of the fully bio-based physically crosslinked hydrogel prepared in Example 3 is 10.37 MPa, and the Young's modulus is 47.32 MPa. This indicates that the tensile strength and Young's modulus of the fully bio-based hydrogel can be effectively improved by sequentially performing primary salting-out treatment, directional stretching treatment, and secondary salting-out treatment.

[0050] (4) Puncture test of fully bio-based physically cross-linked hydrogels: A fully bio-based physically cross-linked hydrogel was fixed between two steel rings on a texture analyzer for puncture experiments. A 2 mm diameter needle was used, and the needle tip moved downwards at a speed of 1 mm / s. The puncture test results are as follows: Figure 14 As shown.

[0051] from Figure 14It can be seen that the puncture resistance strength when the needle tip penetrates is 8.28 MPa and the puncture displacement is 4.95 mm. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 3 has a strong puncture resistance, which corresponds to its high tensile strength and Young's modulus.

[0052] (5) Degradation test of fully bio-based physically cross-linked hydrogels: The fully bio-based physically cross-linked hydrogel prepared in Example 3 was cut into hydrogel blocks measuring 3.14cm × 3.25cm × 0.28cm. The hydrogel blocks were then placed in an α-amylase solution at room temperature. The α-amylase solution was obtained by dissolving α-amylase in deionized water, with an α-amylase concentration of 1.5 g / L. The hydrogel blocks were then subjected to further treatment on day 0 (i.e., Figure 15 0d), 1 day (i.e.) Figure 15 1 day), 2 days (i.e.) Figure 15 2 days), 5 days (i.e.) Figure 15 5 days) and 7 days (i.e. Figure 15 Take photos at 7d (mid-day), the photos are as follows Figure 15 As shown.

[0053] from Figure 15 As can be seen, the fully bio-based physically cross-linked hydrogel rapidly transitioned from a gel state to a sol state from day 0 to day 5 (i.e., the first five days), and by day 7, the fully bio-based physically cross-linked hydrogel was completely miscible with water. This indicates that the fully bio-based physically cross-linked hydrogel prepared in Example 3 can be completely degraded in an α-amylase solution with a concentration of 1.5 g / L at room temperature.

Claims

1. A fully bio-based physically cross-linked hydrogel, characterized in that: The tensile strength of the fully bio-based physically crosslinked hydrogel ranges from 7.81 to 10.37 MPa, and the Young's modulus ranges from 12.32 to 47.32 MPa.

2. A method for preparing a fully bio-based physically cross-linked hydrogel, characterized in that: The fully bio-based physically crosslinked hydrogel is the fully bio-based physically crosslinked hydrogel according to claim 1; the preparation method of the fully bio-based physically crosslinked hydrogel includes the following steps: S1. Mix gelatin and starch, heat and then let stand at low temperature to obtain a fully bio-based protogel; S2. Perform initial salting out of the all-bio-based protogel; S3. The hydrogel that has undergone initial salting-out treatment is stretched in a specific direction. S4. The stretched hydrogel is subjected to salting out again.

3. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 2, characterized in that: Step S1 specifically includes the following steps: placing gelatin and starch in ultrapure water, heating and stirring to obtain a uniform mixed solution; subjecting the mixed solution to ultrasonic treatment to effectively remove air bubbles, obtaining an ultrasonically treated liquid; then, pouring the ultrasonically treated liquid into a glass mold, allowing it to stand at low temperature to obtain a fully bio-based protogel.

4. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 3, characterized in that: In step S1, the mass percentage of gelatin is 5% to 7% of the total mass of gelatin, starch, and ultrapure water; the mass percentage of starch is 3% to 5% of the total mass of gelatin, starch, and ultrapure water; and the mass percentage of ultrapure water is 88% to 92% of the total mass of gelatin, starch, and ultrapure water.

5. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 3, characterized in that: In step S1, the heating and stirring conditions are: stirring temperature of 75-90 ℃ and stirring time of 1.5-3.5 h.

6. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 3, characterized in that: In step S1, during ultrasonic treatment, the ultrasonic power is 90–120 W and the ultrasonic time is 8–15 min.

7. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 3, characterized in that: In step S1, the conditions for low-temperature settling are: a low temperature range of 2–6 °C and a settling time of 1–3 h.

8. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 2, characterized in that: Step S2 specifically includes the following steps: cutting the all-bio-based progenitor gel into rectangular hydrogels, soaking the rectangular hydrogels in an organic salt solution to obtain a preliminary hydrogel product, and absorbing the organic salt solution off the surface of the preliminary hydrogel product with absorbent paper to obtain a hydrogel with the first salting-out treatment.

9. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 2, characterized in that: Step S3 specifically includes the following steps: stretching the hydrogel after the first salting-out treatment, so that the hydrogel after the first salting-out treatment is stretched along the length direction to 150% to 250% of its original length; and then fixing the stretched state of the hydrogel after the first salting-out treatment to obtain the stretched hydrogel.

10. The method for preparing a fully bio-based physically cross-linked hydrogel according to claim 2, characterized in that: Step S4 specifically includes the following steps: soaking the stretched hydrogel in an organic salt solution to obtain a re-salting hydrogel; absorbing the organic salt solution from the surface of the re-salting hydrogel with absorbent paper to obtain a fully bio-based physically cross-linked hydrogel.

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