A silk fibroin fiber and its preparation method

Abstract

This invention belongs to the field of fiber processing technology and relates to a silk fibroin fiber and its preparation method. The preparation method involves dissolving pure silk fibroin in formic acid to obtain a formic acid solution of silk fibroin, and then wet spinning the formic acid solution of silk fibroin to obtain silk fibroin fiber. The concentration of the formic acid solution of silk fibroin is 18-25 wt%. The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding; the coagulation bath is water, and the temperature of the coagulation bath is 30-35℃. In the wet spinning process, this invention improves the orderliness of the silk fibroin molecular chains along the fiber axis by adjusting the temperature of the coagulation bath and the concentration of the formic acid solution of silk fibroin, omitting complex post-processing steps, reducing production costs and process complexity. The prepared silk fibroin fiber has excellent breaking strength and elongation at break, and its mechanical properties are close to those of natural silk, showing broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of fiber processing technology and relates to a silk fibroin fiber and its preparation method. Background Technology

[0002] Silk fibroin fiber is a regenerated silk fibroin fiber obtained by first preparing a silk fibroin solution from degummed natural silk and then spinning the silk fibroin solution. It has a unique molecular structure, excellent mechanical properties and good biocompatibility, and has been widely used in new materials fields such as medical care and biomedicine. Therefore, it is of great significance to study the preparation method of silk fibroin fiber.

[0003] However, silk fibroin has weak mechanical properties in its nascent state and is easily soluble in water, limiting its practical applications. Currently, researchers generally improve the mechanical properties of fibers by adjusting their condensed-state structure (β-sheet and crystallinity). Increasing the β-sheet can enhance the rigidity and water resistance of the fibers.

[0004] To promote the formation and orientation of the β-sheet structure of silk fibroin, there are currently two main methods:

[0005] The first method involves introducing external additives. For example, patent application CN115947945A discloses a modified silk fibroin material, its preparation method, and its application. This material utilizes the interaction between flexible segments of flexible polymers (polyethylene glycol, polyvinyl alcohol, polylactic acid, etc.) and silk fibroin through specific hydrogen bonding synergistic effects, forming a β-sheet structure. However, the maximum tensile strength of this supramolecularly cross-linked silk fibroin is only 19.8 MPa, and its mechanical properties are still significantly inferior to those of natural silk.

[0006] The second method involves post-treatment of the spun fibers, including stretching, steam treatment, and soaking in organic solvents. For example, in the literature (Dry and Wet Spinning of Regenerated Silk Fibers and Their Properties [D]. Suzhou: Suzhou University, 2014:31-34.), a regenerated silk fibroin solution was prepared by dissolving degummed natural silk in a CaCl2 / HCOOH binary solvent system. Then, a wet spinning process was used to prepare nascent regenerated silk fibroin fibers. Finally, the obtained nascent regenerated silk fibroin fibers were subjected to stretching and other post-treatments in a 75% (v / v) ethanol aqueous solution to obtain regenerated silk fibroin filaments. In the wet spinning process, the concentration of the regenerated silk fibroin solution was 12-18 wt%, deionized water was used as the coagulation bath, and the temperature of the coagulation bath was 25℃. The dry breaking strength of the obtained regenerated silk fibroin fibers was 428.42±43.07 MPa, and the dry breaking elongation was 26.50±2.43%. Although its mechanical properties are similar to those of natural silk, the post-processing involves high energy consumption, high cost, and low efficiency in stretching. Furthermore, the immersion in organic solvents can lead to ethanol volatilization, posing potential environmental safety issues. Experiments show that if the post-processing step is omitted, the dry breaking strength of the resulting regenerated silk fibroin fiber is only 260-280 MPa, and the dry breaking elongation is only 30-40%, resulting in a significant reduction in mechanical properties.

[0007] Therefore, it is necessary to develop a method for preparing silk fibroin fibers with excellent mechanical properties that requires no post-treatment. Summary of the Invention

[0008] The purpose of this invention is to solve the above-mentioned problems existing in the prior art and to provide a method for preparing silk fibroin fibers.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing silk fibroin fiber involves dissolving pure silk fibroin in formic acid to obtain a formic acid solution of silk fibroin, and then performing wet spinning on the formic acid solution of silk fibroin to obtain silk fibroin fiber.

[0011] The concentration of formic acid solution for silk fibroin is 18-25 wt%.

[0012] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0013] The coagulation bath is water, and the temperature of the coagulation bath is 30-35℃.

[0014] Compared with the literature "Dry and wet spinning of regenerated silk fibroin filaments and its performance study [D]. Suzhou: Suzhou University, 2014:31-34.", this invention can produce silk fibroin fibers with excellent mechanical properties without omitting the post-treatment process. This is mainly because the present invention increases the concentration of formic acid solution of silk fibroin and the temperature of coagulation bath.

[0015] When a formic acid solution of silk fibroin is introduced into water, the silk fibroin molecular chains migrate and diffuse at the formic acid-water interface. This migration and diffusion exhibits asymmetry, meaning there is a difference in the migration and diffusion behavior between the formic acid side and the water side, i.e., the diffusion rates are inconsistent. This asymmetry is more pronounced when the concentration of the formic acid solution and the temperature of the coagulation bath are high, because the higher concentration of silk fibroin causes the silk fibroin molecules to remain on the formic acid side for a longer time, further exacerbating the difference in migration and diffusion between the formic acid and water sides. At higher coagulation bath temperatures, the water molecules move more vigorously, exerting a stronger impact on the silk fibroin molecules, resulting in faster migration and diffusion on the water side, further enhancing this asymmetry. The more pronounced this asymmetry, the more conducive it is to driving the silk fibroin molecular chains to align orderly along the fiber axis, because the asymmetric migration and diffusion behavior makes the silk fibroin molecules on the water side more easily guided and aligned by water molecules. The polarity of water molecules promotes the alignment of silk fibroin molecules along the fiber axis, forming a more ordered structure. Due to the different diffusion rates of formic acid and water, silk fibroin molecules are subjected to a "stretching" or "guiding" effect at the interface, making them easier to migrate along the fiber axis, thereby enhancing the orderliness of the molecular chains. The higher the orderliness of the silk fibroin molecular chains along the fiber axis, the better it is for improving the fiber orientation, reducing defects such as porosity and cracks, and improving the crystallinity of the fiber, thus enhancing the stability of the microstructure. Therefore, the overall mechanical properties of the fiber are superior.

[0016] As a preferred technical solution:

[0017] The preparation method of silk fibroin fiber as described above involves the following steps: dissolving degummed silk fibers in a formic acid solution of calcium chloride to form a membrane, and then removing the calcium chloride from the membrane to obtain pure silk fibroin.

[0018] In the above-described method for preparing silk fibroin fibers, the extrusion speed is 1-1.5 mL / min, the ambient temperature is 20±5℃, and the relative humidity is 30±5%.

[0019] In the method for preparing silk fibroin fibers as described above, the length of the coagulation bath is 25-30 cm.

[0020] The method for preparing silk fibroin fibers as described above involves a winding speed of 3-3.5 m / min.

[0021] The present invention also provides a silk fibroin fiber, which is prepared by a method for preparing silk fibroin fiber as described in any of the preceding claims.

[0022] As a preferred technical solution:

[0023] The silk fibroin fiber described above has a dry breaking strength of 400-420 MPa and a dry breaking elongation of 1.5-1.7%, and a wet breaking strength of 410-425 MPa and a wet breaking elongation of 23-26%.

[0024] Beneficial effects:

[0025] (1) In the wet spinning process, the present invention improves the orderliness of the silk fibroin molecular chains along the fiber axis by adjusting the temperature of the coagulation bath and the concentration of the formic acid solution of silk fibroin. Without any post-treatment, the content of β-sheet in the fiber is directly increased during the spinning process, eliminating the complex post-treatment process and reducing production costs and process complexity.

[0026] (2) The present invention does not require volatile organic solvents such as ethanol, and the formic acid in the wet spinning process has a low degree of volatility, thus avoiding potential environmental safety issues.

[0027] (3) The silk fibroin fiber prepared by the present invention has excellent breaking strength and breaking elongation, and its mechanical properties are similar to those of natural silk, and it has broad application prospects. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0029] The following are the relevant performance testing methods for each embodiment and comparative example:

[0030] (1) Dry fracture strength: The test was conducted using a universal testing machine. The test parameters were: clamp distance 5cm, tensile rate 1.5mm / min, ambient temperature 23±5℃, and ambient relative humidity 40±5%.

[0031] (2) Elongation at break in dry state: The test was conducted using a universal testing machine. The test parameters were: clamp distance of 5 cm, tensile rate of 1.5 mm / min, ambient temperature of 23±5℃, and ambient relative humidity of 40±5%.

[0032] (3) Wet fracture strength: The sample to be tested was soaked in water for 48 hours, then taken out and tested using a universal testing machine. The test parameters were: clamp distance of 5 cm, tensile rate of 1.5 mm / min, ambient temperature of 23±5℃, and ambient relative humidity of 40±5%.

[0033] (4) Elongation at break in wet conditions: The sample to be tested was soaked in water for 48 hours, then taken out and tested using a universal testing machine. The test parameters were: clamp distance of 5 cm, tensile rate of 1.5 mm / min, ambient temperature of 23±5℃, and ambient relative humidity of 40±5%.

[0034] Example 1

[0035] A method for preparing silk fibroin fibers, the specific steps of which are as follows:

[0036] (1) Prepare raw materials;

[0037] Degummed silk: Degummed silkworm silk;

[0038] Formic acid;

[0039] Formic acid solution of calcium chloride: calcium chloride concentration is 4 wt%;

[0040] PDMS film: 200µm thick;

[0041] Water: Deionized water;

[0042] (2) Preparation of pure silk fibroin;

[0043] The degummed filaments were placed in a calcium chloride formic acid solution at a weight ratio of 1:25. The solution was stirred continuously with a magnetic stirrer at 400 rpm at 25°C until the degummed filaments were completely dissolved.

[0044] The stirring time was 15 min to obtain a silk fibroin-calcium chloride-formic acid solution; the silk fibroin-calcium chloride-formic acid solution was evenly spread on a PDMS membrane and dried on a hot table at 60℃ for 4 h to obtain a silk fibroin membrane; the silk fibroin membrane was peeled off from the PDMS membrane and soaked in water for 24 h to remove calcium chloride; the soaked silk fibroin membrane was taken out and dried at room temperature to obtain pure silk fibroin.

[0045] (3) Preparation of silk fibroin fibers;

[0046] Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber.

[0047] The concentration of the formic acid solution in silk fibroin was 23 wt%.

[0048] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0049] During extrusion by the spinneret, the extrusion speed is 1 mL / min, the ambient temperature is 20℃, and the ambient relative humidity is 35%.

[0050] The coagulation bath is water, the temperature of the coagulation bath is 30℃, and the length of the coagulation bath is 25cm.

[0051] The winding speed is 3m / min.

[0052] The final silk fibroin fiber had a dry breaking strength of 420 MPa and a dry breaking elongation of 1.6%, and a wet breaking strength of 425 MPa and a wet breaking elongation of 25%.

[0053] Comparative Example 1

[0054] A method for preparing silk fibroin fiber is basically the same as in Example 1, except that in step (3), the concentration of the formic acid solution of silk fibroin is 15wt%.

[0055] The final silk fibroin fiber had a dry breaking strength of 300 MPa and a dry breaking elongation of 1%, and a wet breaking strength of 310 MPa and a wet breaking elongation of 10%.

[0056] Compared with Example 1, the change in dry breaking elongation of Comparative Example 1 was not significant, the dry breaking strength decreased by 28.6%, the wet breaking strength decreased by 27.1%, and the wet breaking elongation decreased by 60%. This is because when the concentration of the formic acid solution of silk fibroin is 15wt%, the concentration of silk fibroin is too low, which will lead to a lower viscosity of the spinning solution, making it easy for defects or loose structures to occur during fiber forming, thus reducing mechanical properties.

[0057] Comparative Example 2

[0058] A method for preparing silk fibroin fiber is basically the same as in Example 1, except that in step (3), the concentration of the formic acid solution of silk fibroin is 28wt%.

[0059] The final silk fibroin fiber had a dry breaking strength of 350 MPa and a dry breaking elongation of 1.2%, and a wet breaking strength of 365 MPa and a wet breaking elongation of 20%.

[0060] Compared with Example 1, the changes in dry and wet breaking elongation of Comparative Example 2 were not significant. The dry breaking strength decreased by 16.7% and the wet breaking strength decreased by 14.1%. This is because when the concentration of the formic acid solution of silk fibroin was 28wt%, the concentration of silk fibroin was too high. During the spinning process, it is easy to generate crystal blocks or agglomerates inside the fiber, resulting in uneven fiber structure and reduced overall strength.

[0061] Comparative Example 3

[0062] A method for preparing silk fibroin fiber is basically the same as in Example 1, except that the temperature of the coagulation bath in step (3) is 25°C.

[0063] The final silk fibroin fiber had a dry breaking strength of 303 MPa and a dry breaking elongation of 1%, and a wet breaking strength of 314 MPa and a wet breaking elongation of 18%.

[0064] Compared with Example 1, the changes in dry and wet breaking elongation of Comparative Example 3 were not significant. The dry breaking strength decreased by 27.9% and the wet breaking strength decreased by 26.1%. This is because when the temperature of the coagulation bath is 25°C, the excessively low coagulation bath temperature will slow down the solidification speed of the fiber in the coagulation bath, resulting in insufficient crystallization or poor molecular orientation of the fiber, which affects the mechanical properties.

[0065] Comparative Example 4

[0066] A method for preparing silk fibroin fiber is basically the same as in Example 1, except that the temperature of the coagulation bath in step (3) is 40°C.

[0067] The final silk fibroin fiber had a dry breaking strength of 320 MPa and a dry breaking elongation of 1.5%, and a wet breaking strength of 335 MPa and a wet breaking elongation of 20%.

[0068] Compared with Example 1, the changes in dry and wet breaking elongation of Comparative Example 4 were not significant. The dry breaking strength decreased by 23.8% and the wet breaking strength decreased by 21.2%. This is because when the temperature of the coagulation bath is 40°C, the excessively high coagulation bath temperature may cause the fiber part to soften or deform excessively, forming microcracks or defects, which reduces the breaking strength.

[0069] Example 2

[0070] A method for preparing silk fibroin fibers, the specific steps of which are as follows:

[0071] (1) Prepare raw materials;

[0072] Degummed silk: Degummed silkworm silk;

[0073] Formic acid;

[0074] Formic acid solution of calcium chloride: The concentration of calcium chloride is 3.5 wt%;

[0075] PDMS film: 200µm thick;

[0076] Water: Deionized water;

[0077] (2) Preparation of pure silk fibroin;

[0078] Degummed fibrous fibers were placed in a calcium chloride-formic acid solution at a weight ratio of 1:20. The solution was stirred continuously with a magnetic stirrer at 100 rpm for 40 minutes at 20°C until completely dissolved, yielding a silk fibroin-calcium chloride-formic acid solution. This solution was then evenly spread onto a PDMS membrane and dried on a 40°C hot plate for 8 hours to obtain a silk fibroin membrane. The silk fibroin membrane was then peeled off the PDMS membrane and soaked in water for 12 hours to remove calcium chloride. Finally, the soaked silk fibroin membrane was removed and dried at room temperature to obtain pure silk fibroin.

[0079] (3) Preparation of silk fibroin fibers;

[0080] Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber.

[0081] The concentration of the formic acid solution for silk fibroin was 25 wt%.

[0082] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0083] During extrusion by the spinneret, the extrusion speed was 1.5 mL / min, the ambient temperature was 25℃, and the relative humidity was 32%.

[0084] The coagulation bath is water, the temperature of the coagulation bath is 35℃, and the length of the coagulation bath is 28cm.

[0085] The winding speed is 3.2 m / min.

[0086] The final silk fibroin fiber had a dry breaking strength of 400 MPa and a dry breaking elongation of 1.5%, and a wet breaking strength of 410 MPa and a wet breaking elongation of 23%.

[0087] Example 3

[0088] A method for preparing silk fibroin fibers, the specific steps of which are as follows:

[0089] (1) Prepare raw materials;

[0090] Degummed silk: Degummed silkworm silk;

[0091] Formic acid;

[0092] Formic acid solution of calcium chloride: calcium chloride concentration is 5 wt%;

[0093] PDMS film: 200µm thick;

[0094] Water: Deionized water;

[0095] (2) Preparation of pure silk fibroin;

[0096] Degummed fibrous fibers were placed in a calcium chloride-formic acid solution at a weight ratio of 1:22. The solution was stirred continuously with a magnetic stirrer at 200 rpm for 20 minutes at 40°C until completely dissolved, yielding a silk fibroin-calcium chloride-formic acid solution. This solution was then evenly spread onto a PDMS membrane and dried on a 70°C hot plate for 3 hours to obtain a silk fibroin membrane. The silk fibroin membrane was then peeled off the PDMS membrane and soaked in water for 18 hours to remove calcium chloride. Finally, the soaked silk fibroin membrane was removed and dried at room temperature to obtain pure silk fibroin.

[0097] (3) Preparation of silk fibroin fibers;

[0098] Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber.

[0099] The concentration of the formic acid solution in silk fibroin is 20 wt%.

[0100] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0101] During extrusion, the spinneret speed was 1.4 mL / min, the ambient temperature was 23℃, and the relative humidity was 30%.

[0102] The coagulation bath is water, the temperature of the coagulation bath is 31℃, and the length of the coagulation bath is 30cm.

[0103] The winding speed is 3.5 m / min.

[0104] The final silk fibroin fiber had a dry breaking strength of 416 MPa and a dry breaking elongation of 1.7%, and a wet breaking strength of 415 MPa and a wet breaking elongation of 24%.

[0105] Example 4

[0106] A method for preparing silk fibroin fibers, the specific steps of which are as follows:

[0107] (1) Prepare raw materials;

[0108] Degummed silk: Degummed silkworm silk;

[0109] Formic acid;

[0110] Formic acid solution of calcium chloride: The concentration of calcium chloride is 2.5 wt%;

[0111] PDMS film: 200µm thick;

[0112] Water: Deionized water;

[0113] (2) Preparation of pure silk fibroin;

[0114] Degummed fibrous fibers were placed in a calcium chloride-formic acid solution at a weight ratio of 1:30. The solution was stirred continuously with a magnetic stirrer at 300 rpm for 18 minutes at 30°C until completely dissolved, yielding a silk fibroin-calcium chloride-formic acid solution. This solution was then evenly spread onto a PDMS membrane and dried on a 50°C hot plate for 6 hours to obtain a silk fibroin membrane. The silk fibroin membrane was then peeled off the PDMS membrane and soaked in water for 20 hours to remove calcium chloride. Finally, the soaked silk fibroin membrane was removed and dried at room temperature to obtain pure silk fibroin.

[0115] (3) Preparation of silk fibroin fibers;

[0116] Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber.

[0117] The concentration of the formic acid solution in silk fibroin was 18 wt%.

[0118] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0119] During extrusion, the spinneret speed was 1.3 mL / min, the ambient temperature was 15℃, and the relative humidity was 25%.

[0120] The coagulation bath is water, the temperature of the coagulation bath is 33℃, and the length of the coagulation bath is 27cm.

[0121] The winding speed is 3.1 m / min.

[0122] The final silk fibroin fiber had a dry breaking strength of 415 MPa and a dry breaking elongation of 1.5%, and a wet breaking strength of 420 MPa and a wet breaking elongation of 26%.

[0123] Example 5

[0124] A method for preparing silk fibroin fibers, the specific steps of which are as follows:

[0125] (1) Prepare raw materials;

[0126] Degummed silk: Degummed silkworm silk;

[0127] Formic acid;

[0128] Formic acid solution of calcium chloride: calcium chloride concentration is 2 wt%;

[0129] PDMS film: 200µm thick;

[0130] Water: Deionized water;

[0131] (2) Preparation of pure silk fibroin;

[0132] Degummed fibrous fibers were placed in a calcium chloride-formic acid solution at a weight ratio of 1:27. The solution was stirred continuously with a magnetic stirrer at 350 rpm for 25 minutes at 35°C until completely dissolved, yielding a silk fibroin-calcium chloride-formic acid solution. This solution was then evenly spread onto a PDMS membrane and dried on a hot plate at 80°C for 2 hours to obtain a silk fibroin membrane. The silk fibroin membrane was then peeled off the PDMS membrane and soaked in water for 6 hours to remove calcium chloride. Finally, the soaked silk fibroin membrane was removed and dried at room temperature to obtain pure silk fibroin.

[0133] (3) Preparation of silk fibroin fibers;

[0134] Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber.

[0135] The concentration of the formic acid solution in silk fibroin was 22 wt%.

[0136] The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding.

[0137] During extrusion, the spinneret speed was 1.2 mL / min, the ambient temperature was 18℃, and the relative humidity was 28%.

[0138] The coagulation bath is water, the temperature of the coagulation bath is 32℃, and the length of the coagulation bath is 29cm.

[0139] The winding speed is 3.3 m / min.

[0140] The final silk fibroin fiber had a dry breaking strength of 409 MPa and a dry breaking elongation of 1.6%, and a wet breaking strength of 418 MPa and a wet breaking elongation of 23%.

[0141] In all the above embodiments and comparative examples, the preparation process of degummed silk is as follows: clean silkworm cocoons are manually peeled into thin layers, boiled in sodium carbonate aqueous solution to remove sericin, then taken out and washed clean with water; the above degumming-washing steps are repeated twice to finally obtain degummed silk.

Claims

1. A method for preparing silk fibroin fibers, characterized in that, Pure silk fibroin is dissolved in formic acid to obtain a formic acid solution of silk fibroin. The formic acid solution of silk fibroin is then wet-spun to obtain silk fibroin fiber. The concentration of formic acid solution for silk fibroin is 18-25 wt%. The wet spinning process is as follows: extrusion through a spinneret → coagulation in a coagulation bath → winding, without any post-processing required; The coagulation bath is water, and the temperature of the coagulation bath is 30-35℃; The dry breaking strength of silk fibroin fiber is 400-420 MPa, and the dry breaking elongation is 1.5-1.7%. The wet breaking strength is 410-425 MPa, and the wet breaking elongation is 23-26%.

2. The method for preparing silk fibroin fiber according to claim 1, characterized in that, The preparation process of pure silk fibroin is as follows: after dissolving degummed silk in a formic acid solution of calcium chloride, a membrane is formed, and then the calcium chloride in the membrane is removed to obtain pure silk fibroin.

3. The method for preparing silk fibroin fiber according to claim 1, characterized in that, When the spinneret is extruded, the extrusion speed is 1-1.5 mL / min, the ambient temperature is 20±5℃, and the ambient relative humidity is 30±5%.

4. The method for preparing silk fibroin fiber according to claim 1, characterized in that, The length of the coagulation bath is 25-30cm.

5. The method for preparing silk fibroin fiber according to claim 1, characterized in that, The winding speed is 3-3.5 m / min.

6. A silk fibroin fiber, characterized in that, The silk fibroin fiber is prepared by any one of the preparation methods described in claims 1 to 5.