Preparation method of chitosan composite fiber with high silk fibroin content

Abstract

The invention relates to a preparation method of a chitosan composite fiber with high silk fibroin content. The preparation method comprises the following steps: mixing oxalic acid dihydrate and choline chloride, and stirring to prepare a clear and transparent eutectic solvent; the preparation method comprises the following steps: adding silk into a eutectic solvent, forming a silk / eutectic solvent mixed pasty material under heating and stirring conditions, and enabling silk fibers to fall into micron-sized fibers; removing the eutectic solvent from the mixed paste through cleaning and suction filtration to obtain micron-sized silk fibers, ultrasonically stripping silk nanofibers, and finally centrifuging to prepare a uniformly dispersed silk nanofiber solution; adjusting the pH value of the obtained silk nanofiber solution with acetic acid, concentrating or diluting according to the required amount, adding chitosan, dissolving and uniformly mixing to prepare a mixed spinning solution; carrying out wet spinning on the mixed spinning solution, and carrying out coagulating bath coagulating regeneration, washing, drafting and drying to prepare chitosan composite fibers; and the content of high silk fibroin is up to 66.5% or above.

Description

Technical Field

[0001] The present invention relates to a preparation method of a chitosan composite fiber with a high fibroin content, belonging to the technical fields of textile engineering and functional textiles, biomedical engineering, preparation of polymer composites, etc. Background Art

[0002] Silk is a natural protein fiber with good biocompatibility. During the production, processing, and weaving of silk products, a large number of various wastes are generated, including cocoon shells, silkworm pupae, silk waste, etc. Among them, the texture of cocoon shells is very rough and uneven, and the dyeing performance is low; the fibroin content in silk reeling waste, silk weaving waste, etc. is relatively low and is usually regarded as waste. For such wastes, most of the current treatment methods still stay at the stages of incineration, burial, etc., resulting in a great waste of resources. Therefore, it is of great significance to explore and improve the recycling process of waste silk, improve the recycling efficiency of waste silk, and realize the recycling and reuse of waste silk. The comprehensive utilization industry of silk waste has broad development prospects. Some enterprises have successfully developed comprehensive utilization technologies for silk waste and produced products such as silk powder, silk peptides, and silk protein films, but it is still in its infancy.

[0003] Protein nanofibrils have good biocompatibility and broad application prospects in the biomedical field. Among them, silk nanofibrils are a kind of natural protein nanofibers that can be extracted from silk fibers on a large scale by a top-down method. Mechanically exfoliated silk nanofibers have attracted much attention in the fields of materials, biomedicine, etc. in recent years due to their wide sources and excellent mechanical properties. At present, there have been reports on the preparation of natural silk nanofiber biomimetic microspheres with the potential of biomedical materials (Yan S, Wang L, ACS Nano, 2022, 16(9): 15115-23). This method prepares silk nanofibers by mechanical exfoliation in an aqueous solution. The fiber preparation process is completely based on an aqueous solution system, which can maintain its excellent biocompatibility. Silk nanofiber microspheres can be further functionalized and applied to trypsin detection and blood purification, showing the potential for application in the biomedical field. In addition, it has been reported that silk waste can be recycled by a physical method. This method better retains the excellent natural structure and properties of silk fibroin (Zhang C, Xia L, ACS Applied Materials & Interfaces, 2020, 12(22): 25409-18), and does not produce secondary pollutants. By physically forming methods to regulate the assembly mode of polymer chains and the interfacial force between filler particles and the polymer matrix, a toughened polyurethane / unmodified silk fibroin powder composite film has been prepared. However, the multi-level structure formed by the unmodified silk fibroin powder recovered by physical means and the polymer material has relatively low mechanical properties of the composite material, which limits the industrial application scenarios of the recovered silk fibroin powder. Silk proteins usually also use top-down preparation methods, and the process is relatively mature. Silk protein aqueous solutions are prepared through steps such as degumming, dissolution, and dialysis. Patent CN104532365 A discloses a method for preparing silk nanofibers. By transferring degummed silk to a solution composed of small molecule compounds, deionized water, and alcohols, after soaking, the binding force between nanofibrils is weakened, and then the solution is transferred to a crusher to prepare silk nanofibers by mechanical crushing. This method has a short preparation time, simple equipment, and is easy to control. However, the silk needs to be degummed before treatment, and the solution composed of small molecule compounds, deionized water, and alcohols after preparation cannot be reused, resulting in wastewater discharge and resource waste.

[0004] Chitosan fibers have defects such as poor continuity in the textile process and relatively poor mechanical properties, which affect their usability. Patent CN119465444A discloses a preparation method of a low-viscosity chitosan solution and high-strength chitosan fibers. Through the combined action of a weak acid and a lower alcohol, chitosan molecules are dissolved and the hydration shell layer of chitosan molecules is reduced. At the same time, the lower alcohol plays a role in lubrication and dispersion, greatly reducing the viscosity of the chitosan solution. The low-viscosity chitosan solution is extruded into an organic solvent containing a metal salt to solidify into filaments, and then, after drawing and drying, high-strength chitosan fibers are obtained. It is suitable for high-speed spinning and meets the requirements of efficient and clean production of chitosan fibers. However, the dissolution step of chitosan is relatively complex and requires the compounding of weak acids such as boric acid and lactic acid with lower alcohols, resulting in a relatively high cost.

[0005] Co-preparing chitosan composite fibers by mixing other materials with chitosan to make up for the above-mentioned defects is an effective method. Since the solubility of both silk fibroin and chitosan is affected by pH, when the two are mixed, the pH value will deviate from their respective stable ranges, resulting in aggregation and precipitation. Moreover, the change of ionic strength in the solution will affect the charge distribution and hydration layer of protein and polysaccharide molecules. When mixing, the hydration layer of silk fibroin and chitosan molecules will be destroyed, changing their electrostatic interaction, increasing the intermolecular attraction, and causing the two to aggregate and precipitate. Therefore, the method of preparing chitosan composite fibers with a high silk fibroin content by mixing aqueous silk fibroin solution and chitosan solution is not an effective method. Further, Patent CN113005532 A discloses a method for continuously preparing composite fibers of silk nanofibers and chitin or chitosan. The degummed silk is rapidly stirred in a solvent composition containing potassium hydroxide, urea, water, etc. to uniformly disperse the silk in the solution, and then directly ultrasonically dispersed into uniform silk nanofibers without separation. Then chitin or chitosan is added and stirred to rapidly dissolve chitin or chitosan to prepare a mixed solution, avoiding the problem of easy aggregation and precipitation when the aqueous silk fibroin solution and chitosan solution are mixed. By changing the proportion of silk nanofibers in the chitosan composite fibers to increase the content of silk fibroin, and then through steps such as regeneration, drawing, washing, and drying, composite fibers are prepared, which has good ability to prepare composite fibers. However, the silk still needs to be degummed before treatment, and the components of the spinning solution are relatively complex, resulting in high costs. Moreover, the highest content of silk fibroin in the composite fibers is only 50%. In addition, it has been reported that wet-spun chitosan composite fibers with enhanced mechanical strength and hierarchical structure have been successfully prepared using silk nanofibers as reinforcing materials. The separated silk nanofibers are uniformly distributed in the chitosan fibers and arranged in the composite fibers after extrusion and drawing. Silk fibroin exists in the form of nanofibers. When the content of silk fibroin in the composite fibers is up to 50%, its tensile strength and modulus can be increased by 2.9 times and 3.5 times respectively. This work provides a feasible strategy for optimizing the macroscopic structure of chitosan fibers and promoting their physical properties (J. Xiao, L. Li, H. You, S. Zhou, Y. Feng, R. You, Polym. Eng. Sci. 2023, 63(2), 379), but the prepared composite fibers still have disadvantages such as a diameter greater than 100 μm, uneven and rough fiber surface, and low silk fibroin content that need to be improved.

[0006] Since it is extremely easy to produce aggregation and precipitation when mixing aqueous silk fibroin solution and chitosan solution, it is very difficult to prepare chitosan composite fibers with a high silk fibroin content in this way, and it is not easy for industrial production. Summary of the Invention

[0007] In view of the problems existing in the prior art, the present invention uses a deep eutectic solvent composed of choline chloride and oxalic acid dihydrate to prepare a stable aqueous solution of silk nanofibers from degummed silk. After adjusting the pH, it is directly compounded with chitosan, so that silk fibroin is compounded with chitosan in the form of silk nanofibers. After heating and stirring, a uniform spinning dope is formed. The solution has good stability and moderate viscosity, which is convenient for storage and transportation. Subsequently, wet spinning is carried out, and the spinning and post-treatment conditions are optimized to successfully prepare a composite fiber of silk nanofibers and chitosan. As the proportion of silk nanofibers increases, its tensile strength gradually increases. Among them, the ratio of silk nanofibers to chitosan reaches a maximum of 2:1, and the silk fibroin content is at least 55%. During the spinning process, the filaments are smooth, not easily blocked, and the filament forming effect is good. The surface of the silk fiber is smooth, and the average diameter is less than 30 μm, with excellent spinnability. It can be used in industrial production to efficiently produce composite fibers, providing a practical solution for the reprocessing and efficient utilization of waste silk and the preparation of high-performance chitosan composite fibers.

[0008] The present invention provides a method for preparing a chitosan composite fiber with a high silk fibroin content. It can conveniently and quickly mix silk fibroin with chitosan in the form of nanofibers to make a mixed spinning dope, and successfully prepare a chitosan composite fiber through wet spinning, realizing a high ratio of silk fibroin in the chitosan composite fiber while controlling the fiber diameter.

[0009] The technical solution of the present invention is as follows:

[0010] A method for preparing a chitosan composite fiber with a high silk fibroin content, comprising the following steps:

[0011] (1) Mix oxalic acid dihydrate / choline chloride and prepare a clear and transparent deep eutectic solvent through heating and stirring;

[0012] (2) Add the cleaned and dried silk to the deep eutectic solvent in step (1), and under the condition of heating and stirring, form a paste-like material of silk / deep eutectic solvent mixture, and the silk fibers fall off into micron-sized fibers;

[0013] (3) Wash and filter the mixed paste obtained in step (2) to remove the deep eutectic solvent, obtain micron-sized silk fibers, then ultrasonically exfoliate to obtain silk nanofibers, and finally centrifuge to prepare a uniformly dispersed silk nanofiber solution;

[0014] (4) Adjust the pH of the silk nanofiber solution obtained in step (3) with acetic acid, concentrate or dilute it according to the required amount, add chitosan, dissolve and mix it evenly to prepare a mixed spinning dope;

[0015] (5) The mixed spinning dope obtained in step (4) is subjected to coagulation regeneration in a coagulation bath by wet spinning, followed by washing with water, drawing, and drying to prepare chitosan composite fibers.

[0016] Preferably, in step (1), the molar ratio of choline chloride to oxalic acid dihydrate is 1:(1 - 2), the heating temperature is 80 - 90 °C, the heating time is 1 - 3 h, and the stirring speed is 300 - 1000 rpm.

[0017] Preferably, in step (2), the mass ratio of silk to the deep eutectic solvent is 1:(50 - 100), the heating temperature is 80 - 100 °C, the heating time is 3 - 6 h, and the stirring speed is 300 - 500 rpm.

[0018] Preferably, in step (3), the ultrasonic power is 300 - 400 W, and the centrifuge speed is 2000 - 3428×g.

[0019] Preferably, in step (4), the degree of deacetylation of chitosan is greater than 95%, the pH of the obtained mixed spinning dope is 1.0 - 3.0, the concentration of chitosan in the obtained mixed spinning dope is 2.0 - 3.0% (w / v), and the mass ratio of silk fibroin to chitosan in the obtained mixed spinning dope is 1:(0.5 - 0.8).

[0020] Preferably, in step (5), the flow rate of the wet spinning injection pump is 5 - 10 μL / min, the pore diameter of the spinning orifice is 130 - 250 μm, the coagulation bath is an aqueous sodium hydroxide solution and ethanol with different volume ratios, the stretching baths are an aqueous sodium chloride solution and an aqueous ethanol solution respectively, and the drying temperature is 40 - 55 °C.

[0021] Preferably, the concentration of the aqueous sodium hydroxide solution in the coagulation bath is 1% - 5% (w / v), and the volume ratio of the aqueous sodium hydroxide solution to ethanol is (1 - 2):3; the concentration of sodium chloride in the stretching bath is 1 - 5% (w / v), and the ethanol concentration is 20% - 50% (v / v).

[0022] The prepared chitosan composite fibers have a high silk fibroin content of up to more than 66.5%.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. The present invention utilizes a deep eutectic solvent system composed of choline chloride and oxalic acid dihydrate, which has the advantages of simple preparation, low cost, good thermal stability, reusability, low toxicity, etc. By directly heating and stirring the cleaned and dried silk without degumming treatment, thus saving the cumbersome steps such as degumming and reducing costs. Due to the acidity and swelling ability of the deep eutectic solvent system, non-covalent or covalent interactions and hydrogen bonds will be weakened or destroyed, and the strength of hydrophobic interactions in proteins will be reduced, directly stripping silk nanofiber with certain mechanical strength from natural silk fibroin. Then, the deep eutectic solvent is removed by washing and suction filtration, and the deep eutectic solvent can be reused after evaporating water. The silk fiber is then peeled into silk nanofibers by an ultrasonic cell disruptor, and finally, uniformly dispersed and uniformly sized silk nanofibers are prepared by centrifugation. After adjusting the pH, chitosan is added in different proportions, and the ratio of silk nanofibers to chitosan reaches up to 2:1 at most. By heating and stirring, chitosan is dissolved to prepare a uniformly mixed solution. By changing the proportion of silk nanofibers in the chitosan composite fiber, the content of silk fibroin is increased, avoiding the problem of easy aggregation and precipitation when a high-content aqueous solution of silk protein is mixed with a chitosan solution. The solution has good stability and moderate viscosity, which is convenient for storage and transportation. Finally, composite fibers are prepared by wet spinning. During the spinning process, the filaments are discharged smoothly, not easily blocked, and the filament forming effect is good. The average diameter is less than 30 μm, with excellent spinnability, and can be applied to industrial production for efficient production of composite fibers.

[0025] 2. In the present invention, the components such as the deep eutectic solvent system composed of choline chloride and oxalic acid dihydrate, the sodium hydroxide aqueous solution and ethanol mixed coagulation bath, and the sodium chloride and low-concentration ethanol stretching bath are safe, low-toxic, recyclable and reusable. The technological process is simple, the investment in safety and environmental protection is low, the cost is saved, and at the same time, the resource waste caused by silk waste during the recycling process is solved, the pollution is reduced, and the development of circular economy is promoted. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to illustrate the technical solutions in different embodiments or techniques of the present invention, the drawings required in the embodiments are introduced.

[0027] Figure 1 Scanning electron microscope photos of the silk nanofibers prepared in Examples 1-4

[0028] Figure 2 Scanning electron microscope photo of the surface of the chitosan composite fiber prepared in Example 1

[0029] Figure 3 Scanning electron microscope photo of the surface of the chitosan composite fiber prepared in Example 2

[0030] Figure 4 Scanning electron microscope photo of the surface of the chitosan composite fiber prepared in Example 3

[0031] Figure 5 Scanning electron microscope photograph of the surface of the chitosan composite fiber prepared in Example 4

[0032] Figure 6 Bar chart of the diameters of the chitosan composite fibers with different fibroin contents prepared in Examples 1-4

[0033] Figure 7 Bar chart of the tensile strengths of the chitosan composite fibers with different fibroin contents prepared in Examples 1-4

[0034] Figure 8 Bar chart of the elongation at break of the chitosan composite fibers with different fibroin contents prepared in Examples 1-4

[0035] The following will further illustrate the specific preparation process and beneficial technical effects of the present invention in conjunction with the accompanying drawings to fully understand the purpose, features and effects of the present invention. Detailed implementation manners

[0036] To better understand the present invention, the following examples further illustrate the present invention, but the content of the present invention is not limited to the following examples only.

[0037] Example 1

[0038] Step (1): Mix oxalic acid dihydrate / choline chloride in a molar ratio of 1:1, and prepare a clear and transparent deep eutectic solvent by heating at 80°C and stirring at 1000 rpm for 3 h;

[0039] Step (2): Add the cleaned and dried silk to the deep eutectic solvent prepared in step (1) in a mass ratio of 1:100, heat at 100°C, and stir at a rotation speed of 500 rpm for 6 h to form a paste-like material of silk / deep eutectic solvent mixture, and the silk fibers fall off into micron-sized fibers;

[0040] Step (3): Wash and filter the mixed paste obtained in step (2) to remove the deep eutectic solvent to obtain micron-sized silk fibers, then ultrasonically exfoliate the silk nanofibers at 400 W, and finally centrifuge at 3428×g to prepare a uniformly dispersed silk nanofiber solution;

[0041] Step (4): Adjust the pH of the aqueous solution of silk nanofibers obtained in step (3) to 3.0 with acetic acid and dilute it to the required amount, and mix it with chitosan with a deacetylation degree greater than 95% in a mass ratio of 4:5. Dissolve and mix evenly to obtain a mixed spinning dope with a final chitosan concentration of 3.0% (w / v);

[0042] Step (5): The mixed spinning dope obtained in step (4) is extruded through a spinneret with a pore diameter of 130 μm at a flow rate of 5 μL / min by an injection pump into a sodium hydroxide aqueous solution - ethanol mixed coagulation bath (the concentration of the sodium hydroxide aqueous solution is 5% (w / v), and the volume ratio of the sodium hydroxide aqueous solution to ethanol is 2:3), and is coagulated and drawn into filaments. The obtained filament fibers are thoroughly washed with deionized water, drawn in a 5% (w / v) sodium chloride solution and a 20% (v / v) ethanol solution respectively, and dried thoroughly in an oven at 55 °C to obtain a chitosan composite fiber with a mass ratio of silk fibroin to chitosan of 5:4, where the silk fibroin content is approximately 55.6%.

[0043] The silk nanofibers prepared above were observed by scanning electron microscopy, as shown in Figure 1 Appendix Figure 1 A. Figure A is a scanning electron micrograph of the obtained silk nanofibers magnified 30,000 times. The diameter of the silk nanofibers is 200 - 300 nm, and the length is 500 - 700 nm.

[0044] The chitosan composite fibers prepared above were observed under a scanning electron microscope, as shown in Figure 2 Appendix Figure 2 Figure is a scanning electron micrograph of the obtained chitosan composite fibers magnified 5,000 times. The chitosan composite fibers are uniform in thickness, smooth on the surface, and have no obvious defects; Figure 6 Figure is a column chart of the diameter of the obtained chitosan composite fibers. The average diameter of the chitosan composite fibers is approximately 25.8 μm, which is less than 30 μm; The mechanical properties of the chitosan composite fibers prepared above were tested, as shown in Figure 7 Figure is a column chart of the tensile strength of the chitosan composite fibers. The average tensile strength is approximately 126 MPa, Figure 8 Figure is a column chart of the elongation at break of the chitosan composite fibers. The average elongation at break is approximately 5%.

[0045] Example 2

[0046] Step (1): Oxalic acid dihydrate / choline chloride is mixed at a molar ratio of 1:1.5, and a clear and transparent deep eutectic solvent is prepared by heating at 85 °C and stirring at 1000 rpm for 2 h;

[0047] Step (2): The cleaned and dried silk is added to the deep eutectic solvent prepared in step (1) at a mass ratio of 1:75, heated at 95 °C, and stirred at a speed of 500 rpm for 5 h to form a paste-like material of silk / deep eutectic solvent mixture, and the silk fibers are exfoliated into micron-sized fibers;

[0048] Step (3): The eutectic solvent in the mixed paste obtained in step (2) was removed by washing and suction filtration to obtain micron-scale silk fibers. Then, silk nanofibers were exfoliated by 400W ultrasonic treatment. Finally, a uniformly dispersed silk nanofiber solution was prepared by centrifugation at 3000×g.

[0049] Step (4): The pH of the aqueous silk nanofiber solution obtained in step (3) was adjusted to 2.5 with acetic acid and concentrated to the required amount. Chitosan with a degree of deacetylation greater than 95% was mixed with it at a mass ratio of 3:4. After dissolving and mixing evenly, a mixed spinning dope with a final chitosan concentration of 2.5% (w / v) was obtained.

[0050] Step (5): The mixed spinning dope obtained in step (4) was extruded through a spinneret with a pore diameter of 130 μm at a flow rate of 8 μL / min by an injection pump into a coagulation bath of sodium hydroxide aqueous solution - ethanol (the concentration of sodium hydroxide aqueous solution was 3% (w / v), and the volume ratio of sodium hydroxide aqueous solution to ethanol was 1:3), and formed into filaments through coagulation and stretching. The obtained filament fibers were washed thoroughly with deionized water, stretched in 4% (w / v) sodium chloride solution and 30% (v / v) ethanol solution respectively, and then dried thoroughly in an oven at 50 °C to obtain a chitosan composite fiber with a mass ratio of silk fibroin to chitosan of 4:3, and the silk fibroin content was about 57.1%.

[0051] The prepared silk nanofibers were observed by scanning electron microscopy, as shown in Figure 1 Appendix Figure 1 B. Appendix B is a scanning electron micrograph of the obtained silk nanofibers magnified 30,000 times. The diameter of the silk nanofibers is 200 - 300 nm, and the length is 500 - 700 nm.

[0052] The prepared chitosan composite fibers were observed under a scanning electron microscope, as shown in Figure 3 Appendix Figure 3 Appendix is a scanning electron micrograph of the obtained chitosan composite fibers magnified 5000 times. The chitosan composite fibers are uniform in thickness, smooth on the surface, without obvious defects, and dot-like textures gradually appear on the surface with the increase of silk protein content; Figure 6 Appendix is a column chart of the diameter of the obtained chitosan composite fibers. The average diameter of the chitosan composite fibers is about 27.6 μm, less than 30 μm, indicating that the increase of silk fibroin content has no obvious effect on its diameter; The mechanical properties of the prepared chitosan composite fibers were tested, Figure 7 Appendix is a column chart of the tensile strength of the chitosan composite fibers. Its average tensile strength is about 142 MPa, indicating that with the increase of silk fibroin content, the tensile strength of the chitosan composite fibers gradually increases, Figure 8It is a bar chart of the elongation at break of chitosan composite fibers. The average elongation at break is about 7%, indicating that as the content of silk fibroin increases, the elongation at break of chitosan composite fibers also increases, and its ductility is enhanced.

[0053] Example 3

[0054] Step (1): Mix oxalic acid dihydrate / choline chloride in a molar ratio of 1:1.5, and prepare a clear and transparent eutectic solvent by heating at 85°C and stirring at 600 rpm for 2 h.

[0055] Step (2): Add the cleaned and dried silk to the eutectic solvent prepared in step (1) at a mass ratio of 1:75, heat at 90°C, and stir at a speed of 400 rpm for 4 h to form a paste-like material of silk / eutectic solvent mixture, and the silk fibers fall off into micron-sized fibers.

[0056] Step (3): Wash and filter the mixed paste obtained in step (2) to remove the eutectic solvent, obtain micron-sized silk fibers, then ultrasonically exfoliate the silk nanofibers at 350 W, and finally centrifuge at 2000×g for 10 min to prepare a uniformly dispersed silk nanofiber solution.

[0057] Step (4): Adjust the pH of the aqueous solution of silk nanofibers obtained in step (3) to 1.5 with acetic acid and concentrate it according to the required amount. Mix chitosan with a degree of deacetylation greater than 95% at a mass ratio of 3:5. Dissolve and mix evenly to obtain a mixed spinning dope with a final chitosan concentration of 2.5% (w / v).

[0058] Step (5): Pass the mixed spinning dope obtained in step (4) through an injection pump at a flow rate of 10 μL / min and spray it through a spinneret with a pore diameter of 180 μm into a sodium hydroxide aqueous solution-ethanol mixed coagulation bath (the concentration of the sodium hydroxide aqueous solution is 2% (w / v), and the volume ratio of the sodium hydroxide aqueous solution to ethanol is 1:2), and form filaments through coagulation and stretching. Wash the obtained filament fibers thoroughly with deionized water, stretch them in a 2% (w / v) sodium chloride solution and a 40% (v / v) ethanol solution respectively, and dry them thoroughly in an oven at 45°C to obtain chitosan composite fibers with a mass ratio of silk fibroin to chitosan of 5:3, and the silk fibroin content is about 62.5%.

[0059] The prepared silk nanofibers were observed by scanning electron microscopy, as shown in Figure 1 Figure C. Figure 1 Figure C is a scanning electron micrograph of the obtained silk nanofibers magnified 30,000 times. The diameter of the silk nanofibers is 200 - 300 nm, and the length is 500 - 700 nm.

[0060] The chitosan composite fibers prepared above were observed under a scanning electron microscope, as shown in the appendix Figure 4 below. Figure 4 Figure Figure 4 is a scanning electron microscope image of the obtained chitosan composite fibers magnified 5000 times. The chitosan composite fibers are uniform in thickness, smooth on the surface, without obvious defects, and the surface dot-like textures increase with the increase of the silk fibroin content; Figure 6 Figure Figure 6 is a column chart of the diameters of the obtained chitosan composite fibers. The average diameter of the chitosan composite fibers is about 26.5 μm, which is less than 30 μm, indicating that the increase in the silk fibroin content has no obvious effect on its diameter; The mechanical properties of the above-prepared chitosan composite fibers were tested. Figure 7 Figure Figure 7 is a column chart of the tensile strength of the chitosan composite fibers. The average tensile strength is about 166 MPa, and the tensile strength increases with the increase of the silk fibroin content in the chitosan composite fibers, and the ability to resist tensile failure is improved. Figure 8 Figure Figure 8 is a column chart of the elongation at break of the chitosan composite fibers. The average elongation at break is about 15%, indicating that the elongation at break of the chitosan composite fibers is continuously increasing with the increase of the silk fibroin, and the ductility of the fibers is enhanced.

[0061] Example 4

[0062] Step (1): Oxalic acid dihydrate / choline chloride was mixed at a molar ratio of 1:2, and a clear and transparent deep eutectic solvent was prepared by heating at 90 °C and stirring at 300 rpm for 1 h.

[0063] Step (2): The cleaned and dried silk was added to the deep eutectic solvent prepared in step (1) at a mass ratio of 1:50, heated at 80 °C, and stirred at a speed of 300 rpm for 3 h to form a paste-like material of silk / deep eutectic solvent mixture, and the silk fibers were exfoliated into micron-sized fibers.

[0064] Step (3): The mixed paste obtained in step (2) was filtered by washing to remove the deep eutectic solvent to obtain micron-sized silk fibers, and then the silk nanofibers were exfoliated by 300 W ultrasound, and finally centrifuged at 2000×g for 10 min to prepare a uniformly dispersed silk nanofiber solution.

[0065] Step (4): The aqueous solution of silk nanofibers obtained in step (3) was adjusted to pH 1.0 with acetic acid and concentrated according to the required amount, and chitosan with a deacetylation degree greater than 95% was mixed with it at a mass ratio of 1:2. It was dissolved and mixed evenly to obtain a mixed spinning dope with a final chitosan concentration of 2.0% (w / v).

[0066] Step (5): The mixed spinning dope obtained in step (4) is extruded through a spinneret with a pore diameter of 250 μm at a flow rate of 8 μL / min by an injection pump into a mixed coagulation bath of sodium hydroxide aqueous solution - ethanol (the concentration of the sodium hydroxide aqueous solution is 1% (w / v), and the volume ratio of the sodium hydroxide aqueous solution to ethanol is 1:3), and is coagulated and drawn into filaments. The obtained filament fibers are thoroughly washed with deionized water, drawn in a 1% (w / v) sodium chloride solution and a 50% (v / v) ethanol solution respectively, and dried thoroughly in an oven at 40 °C to obtain a chitosan composite fiber with a mass ratio of silk fibroin to chitosan of 2:1, where the silk fibroin content is approximately 66.7%.

[0067] The prepared silk nanofibers were observed by scanning electron microscopy, as shown in Figure 1 Appendix Figure 1 D. Appendix

[0068] The prepared chitosan composite fibers were observed under a scanning electron microscope, as shown in Figure 5 Appendix Figure 5 is a scanning electron micrograph of the obtained chitosan composite fiber magnified 5000 times. The chitosan composite fiber is uniform in thickness, smooth on the surface, without obvious defects, and the surface dot-like texture increases with the increase of the silk protein content; Appendix Figure 6 is a column chart of the diameter of the obtained chitosan composite fiber. The average diameter of the chitosan composite fiber is about 28.4 μm, which is less than 30 μm, indicating that the increase in the silk fibroin content has no obvious effect on the diameter of the chitosan composite fiber. Even when the silk fibroin content increases to 66.7%, it can ensure that the chitosan composite fiber has a filament diameter less than 30 μm; the mechanical properties of the prepared chitosan composite fiber were tested, Appendix Figure 7 is a column chart of the tensile strength of the chitosan composite fiber. Its average tensile strength is about 247 MPa, indicating that when the silk fibroin content increases to 66.7%, it can greatly improve its tensile strength, and the quality of the fiber has been improved. Appendix Figure 8 is a column chart of the elongation at break of the chitosan composite fiber. The average elongation at break is about 5%, indicating that too high a silk fibroin content will instead reduce its elongation at break and decrease its ductility.

[0069] The preparation method described in the present invention is to directly place the cleaned and dried silk in a deep eutectic solvent composed of choline chloride and oxalic acid dihydrate without degumming treatment, heat and stir to break the hydrogen bonds in fibroin and reduce the strength of hydrophobic interactions in the protein, then remove the deep eutectic solvent by washing and suction filtration, and then use an ultrasonic cell disruptor to strip the silk fibers into silk nanofibers. Finally, uniformly dispersed silk nanofibers are prepared by centrifugation. After adjusting the pH, different proportions of chitosan are added to make fibroin compound with chitosan in the form of silk nanofibers, and a uniformly mixed solution is prepared by heating and stirring, and then a composite fiber is prepared by wet spinning. The content of fibroin in the composite fiber can reach more than 66.5% at most. The technical problem that it is difficult to form filaments with a high content of fibroin in the chitosan composite fiber is solved. At the same time, the problem of recycling waste silk is solved, and the recycling of resources is realized. The chitosan composite fiber with a high fibroin content prepared by the present invention has broad application prospects in frontier fields such as the textile industry, industry, and biomedicine.

[0070] The specific embodiments of the present invention have been described in detail above. Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the specification of the present invention. Those skilled in the art can make different modifications and changes to the present invention without creative labor. Therefore, all technical solutions that can be obtained through logical analysis, reasoning, or limited experiments based on the existing invention should be within the protection scope determined by the claims.

Claims

1. A preparation method of a chitosan composite fiber with a high fibroin content, characterized in that It includes the following steps: (1) Mix oxalic acid dihydrate with choline chloride, and prepare a clear and transparent eutectic solvent by heating and stirring; (2) Add the cleaned and dried silk to the eutectic solvent in step (1), and under the conditions of heating and stirring, form a paste-like material of silk / eutectic solvent mixture, and the silk fibers fall off into micron-sized fibers; (3) Wash and filter the mixed paste obtained in step (2) to remove the eutectic solvent, obtain micron-sized silk fibers, then ultrasonically exfoliate silk nanofibers, and finally centrifuge to prepare a uniformly dispersed silk nanofiber solution; (4) Adjust the pH of the silk nanofiber solution obtained in step (3) with acetic acid, concentrate or dilute it according to the required amount, add chitosan, dissolve and mix it evenly to obtain a mixed spinning dope; (5) Pass the mixed spinning dope obtained in step (4) through wet spinning, coagulate and regenerate it through a coagulation bath, wash it with water, and draw and dry it to prepare chitosan composite fibers.

2. The preparation method according to claim 1, wherein: In step (1), the molar ratio of choline chloride / oxalic acid dihydrate is 1:(1 - 2), the heating temperature is 80 - 90 °C, the heating time is 1 - 3 h, and the stirring speed is 300 - 1000 rpm.

3. The preparation method according to claim 1, wherein: In step (2), the mass ratio of silk to the eutectic solvent is 1:(50 - 100), the heating temperature is 80 - 100 °C, the heating time is 3 - 6 h, and the stirring speed is 300 - 500 rpm.

4. The preparation method according to claim 1, characterized in that: In step (3), the ultrasonic power is 300 - 400 W, and the centrifuge speed is 2000 - 3428×g.

5. The preparation method according to claim 1, characterized in that: In step (4), the deacetylation degree of chitosan is greater than 95%, the pH of the obtained mixed spinning dope is 1.0 - 3.0, the concentration of chitosan in the obtained mixed spinning dope is 2.0 - 3.0% (w / v), and the mass ratio of fibroin to chitosan in the obtained mixed spinning dope is 1:(0.5 - 0.8).

6. According to the preparation method described in claim 1, characterized in that: In step (5), the flow rate of the wet spinning injection pump is 5 - 10 μL / min, the pore diameter of the spinning orifice is 130 - 250 μm, the coagulation bath is an aqueous sodium hydroxide solution and ethanol with different volume ratios, the drawing baths are an aqueous sodium chloride solution and an aqueous ethanol solution respectively, and the drying temperature is 40 - 55 °C.

7. According to the preparation method described in claim 6, characterized in that: The concentration of the aqueous sodium hydroxide solution in the coagulation bath is 1% - 5% (w / v), and the volume ratio of the aqueous sodium hydroxide solution to ethanol is (1 - 2):3; the concentration of sodium chloride in the drawing bath is 1 - 5% (w / v), and the ethanol concentration is 20% - 50% (v / v).

Images