Method for producing silk protein powder, and electrospinning thereof

Abstract

The present invention relates to a method for producing silk protein powder, comprising the successive steps of: a) providing silk; b) dissolving the silk in an aqueous solution of zinc chloride ZnCl2 at a temperature above 70°C, to obtain an aqueous solution of silk proteins; c) precipitating the silk proteins by introducing the aqueous solution of silk proteins into an excess of alcohol in order to obtain an aqueous solution of zinc chloride and alcohol and a coprecipitate of silk proteins and zinc; d) filtering the aqueous solution of zinc chloride and alcohol comprising the co-precipitate of silk proteins and zinc in order to collect said co-precipitate; e) optionally, washing said co-precipitate with alcohol; f) drying the optionally washed co-precipitate so as to obtain a powder of silk proteins and zinc; and wherein the powder of silk proteins and zinc obtained at the end of step f) is such that it comprises, as a percentage by weight relative to the total weight of the powder, from 8% to 12% of zinc, the zinc being derived from the aqueous solution of zinc chloride ZnCl2 used in step b). The invention also relates to a powder of silk proteins and zinc, to a method for electrospinning said powder and to an associated nonwoven fabric.

Description

[0001] TITLE: Process for the production of silk protein powder and its electrospinning

[0002] The present invention relates to a process for producing silk protein powder from any type of silk, including recycled silk, as well as a silk protein powder suitable for electrospinning. More particularly, the invention relates to a complete chemical recycling process for silk waste, including silk waste from textile industries or silk cocoons of insufficient quality for conventional spinning processes, to produce high-quality regenerated silk yarns directly usable by the silk industry. In addition to recycling lower-quality silk waste and cocoons, the process is optimized to minimize its environmental impact by using non-toxic solvents that are recyclable within the process itself and by requiring minimal water.

[0003] Silk is the filamentous product of the silk glands of spiders and the larvae of various insects. The material most commonly used to make so-called "natural silk" textiles is the secretion of silkworm caterpillars (Bombyx mori) when they build their cocoons to house their pupae. Wild silk, or tussah, is obtained from cocoons other than those of Bombyx mori; it is generally from the larvae of moths of the genus Antheraea that live on oak trees in India or China.

[0004] Raw silk (or "egg silk") is composed of approximately 65% ​​fibroin by weight. The nature (amino acid composition) and quantity of this protein vary depending on the origin of the raw silk. Furthermore, in raw silk, which is extracted directly from the cocoon, the fibroin is surrounded by another protein called sericin, which is removed (degumming), at least partially, when the silk is to be used in the textile industry. Sericin is a protein that generally constitutes between 20 and 25% by weight of the raw silk filament. Finally, in addition to fibroin and sericin, raw silk contains water (approximately 12%) and traces of fats, waxes, dyes, and minerals. One cocoon yields one strand of raw silk, ranging in length from 800m to 1500m.

[0005] In traditional spinning methods using cocoons, to prevent the raw silk thread from being cut when the moth emerges from the cocoon, the chrysalis is killed inside the cocoon, for example, by exposing it to hot air or placing it in a drying oven. The cocoon is then immersed in boiling water and brushed to remove the small, discontinuous threads on the outside of the cocoon (blazes) and expose the free end of the thread, which is then reeled. The silk thread can then undergo various treatments such as:

[0006] - The unwinding process, which consists of assembling the previously obtained yarn; it is finally arranged in skeins on a reel called a winder;

[0007] - The throwing process, also called "twisting," which consists of twisting the raw silk thread to increase its strength; this operation also allows for the removal of certain impurities that still adhere to the thread;

[0008] - Degumming, which consists of removing the sericin (sericin) sheath from the silk thread to give it suppleness and shine. If the degumming is complete, the silk is supple; if it is only partial, the silk is said to be cooked, or semi-cooked if it is even more imperfect.

[0009] The waste resulting from all the operations described above can possibly be recovered and processed; it is made into a thread called "schappe" which is used in certain weaving operations.

[0010] Furthermore, silk can be dyed. However, the sandstone (sericin sheath) is not compatible with dyeing, so it is necessary to carry out at least a partial degumming operation.

[0011] It is clear from the above that there are many different types and qualities of silk. The silk cocoons themselves can vary depending on their origin. Furthermore, only cocoons of excellent quality are suitable for spinning. Each stage of silk processing also generates silk waste. Finally, woven silk also varies in quality depending on whether it has been twisted, spun, degummed, dyed, etc., using various processes.

[0012] Numerous processes for dissolving silk, particularly for dissolving fibroin from silk cocoons, have already been proposed. However, such processes are generally unsuitable for use with silk waste from various sources, such as waste from the textile industry. This waste may include pure silk fabrics, blended fabrics, highly twisted silk yarns, or undegummed silk. Furthermore, many of these processes involve complex solvent mixtures, large quantities of water, or solvents with low recyclability. For example, Antonios Keirouz et al. (Keirouz, A., Zakharova, M., Kwon, J., Robert, C., Koutsos, V., Callanan, A., Radacsi, N.; “High-throughput production of silk fibroin-based electrospun fibers as biomaterial for skin tissue engineering applications”; Mater. Sc / . Eng.(2020, C112, 110939) proposed a process involving dissolving silk cocoons in lithium bromide (LiBr), followed by precipitation in ethanol, repeated washing of the precipitate in water to remove LiBr, and drying of the resulting gel. The regenerated silk thus produced was dissolved in formic acid (FA) and calcium chloride to produce electrospunable fibroin fibers with poly(glycerol sebacate) and / or poly(caprolactone) in hexafluoroisopropanol (HFIP). However, LiBr solutions have a significant environmental impact and are difficult to recycle. The repeated washing with water requires large volumes and results in regenerated silk fibroin in gel form. The electrospun fibers from this fibroin gel consist of a mixture of silk fibroin solution with poly(glycerol sebacate) and / or poly(caaprolactone).The solvent system used is complex and includes HFIP, FA, and CaCh. HFIP has adverse effects on health and the environment. Furthermore, the described process is only suitable for silk cocoons.

[0013] A similar process was reported by Yifeng Huang et al. (Huang, Y., Bailey, K., Wang, S., Feng, X. “Silk fibroin films for potential applications in controlled release”; React. Fund. Polym. 2017, 116, pp. 57-68). This process uses LiBr (9.5M) as a solvent and ethanol to precipitate the silk solutions. The silk precipitate was washed several times in water to remove residual LiBr and then dried in an oven to yield a silk fibroin with a gelatinous texture. The regenerated silk was dissolved in formic acid to produce fibroin films that can be used for controlled drug release. The resulting regenerated silk fibroin has a gelatinous texture, unsuitable for electrospinning, and the solvent used is not recyclable.

[0014] In document EP 0 011 161 A1, a fine silk fibroin powder was produced by dissolving silk fibroin in an aqueous ZnCh solution at 70°C. This was followed by precipitation using a coagulating salt such as ammonium sulfate or the addition of sulfuric acid to produce a silk gel. The gel was washed several times with water to remove the coagulating salt. It was then treated at a high temperature (90-100°C) to yield silk powder. The solvent used in this case is not recyclable, making the process polluting. The process is preceded by degumming steps, and the final powder obtained contains only fibroin as the silk protein. The process yields a high-purity fibroin powder suitable for use in cosmetic and pharmaceutical preparations but not suitable for electrospinning.

[0015] A publication by Michael Wöltje et al. (Wöltje, M., Kölbel, A., Aibibu, D., Cherif, C. “A fast and reliable process to fabricate regenerated silk fibroin solution from degummed silk in 4 hours”; Int. J. Mol. Sci. 2021, 22(19), 10565) reports the rapid dissolution of silk fibroin, particularly from cocoons (10%), in a zinc chloride solution (9.6 M) at 45 °C within one hour, to produce a silk suspension. Prior to the dissolution step, the silk is completely degummed. The solutions are then purified by gel filtration through crosslinked dextran (Zetadex-25) by passing them several times through a desalting column, resulting in pure fibroin containing no residual zinc. The silk solutions are then poured into films. Furthermore, the solvent used in this process cannot be recycled.

[0016] None of the aforementioned processes are suitable for the chemical recycling of any type of silk, including silk in cocoon form, raw silk, silk that is at least partially degummed, silk that is at least partially spun, and / or woven silk, dyed or undyed, and especially silk derived from silk waste from textile industries or silk cocoons whose quality is insufficient for conventional spinning processes. Furthermore, none of these processes is suitable for producing a silk protein powder that can be directly electrospun to create high-quality nonwovens from regenerated silk yarns. Finally, the processes mentioned above have a significant environmental impact because they involve the use of toxic and / or non-recyclable solvents within the process itself and / or require a large quantity of water.

[0017] The aim of the invention is therefore to propose a process for producing silk protein powder that overcomes all the disadvantages of prior art processes.

[0018] The invention thus relates to a process for producing silk protein powder, comprising the successive steps of: a) supplying silk, in particular in the form of cocoons, raw silk, silk at least partially degummed, silk at least partially milled, woven silk dyed or undyed, in particular recycled silk; b) dissolving the silk in an aqueous solution of zinc chloride ZnCh at a temperature above 70°C, preferably from 75°C to 100°C, preferably from 80°C to 90°C, preferably for a period less than or equal to 3h, preferably less than or equal to 2h, more preferably from 5 min to 2h, to obtain an aqueous solution of silk proteins;c) precipitation of silk proteins by introducing the aqueous solution of silk proteins, preferably drop by drop, into an excess of alcohol to obtain an aqueous solution of zinc chloride and alcohol and a co-precipitate of silk proteins and zinc; d) filtration of the aqueous solution of zinc chloride and alcohol comprising the co-precipitate of silk proteins and zinc to collect said co-precipitate; e) optionally, washing said co-precipitate with alcohol; f) drying of the co-precipitate, optionally washed, preferably at a temperature of 40°C to 80°C, in order to obtain a silk and zinc protein powder; and wherein the silk and zinc protein powder obtained at the end of steps f) is such that it comprises, as a percentage by weight relative to the total weight of the powder, 8 to 12% zinc, preferably 9 to 11% zinc, the zinc being derived from the aqueous solution of zinc chloride ZnCh implemented in step b).;

[0019] The invention also relates to a powder of silk and zinc proteins obtained according to the above process, this powder being suitable for direct electrospun production.

[0020] The invention also relates to a process for electrospinning a silk protein powder comprising the steps of: i) dissolving a silk and zinc protein powder obtained according to the above-described process or a powder as previously described in a solution comprising at least one organic acid and at least one polymer to obtain an electrospinning solution; and ii) electrospinning the solution obtained in step i) to produce a silk thread.

[0021] The invention finally relates to a non-woven fabric of silk thread(s) obtained according to the electrospinning process described above.

[0022] Process for producing silk protein powder

[0023] The invention relates first of all to a process for producing a silk protein powder, comprising the successive steps of: a) supplying silk, in particular in the form of cocoons, raw silk, silk at least partially degummed, silk at least partially milled, woven silk dyed or undyed, in particular recycled silk; b) dissolving the silk in an aqueous solution of zinc chloride ZnCh at a temperature above 70°C, preferably from 75°C to 100°C, preferably from 80°C to 90°C, preferably for a period less than or equal to 3h, preferably less than or equal to 2h, more preferably from 5 min to 2h, to obtain an aqueous solution of silk proteins;c) precipitation of silk proteins by introducing the aqueous solution of silk proteins, preferably dropwise, into an excess of alcohol to obtain an aqueous solution of zinc chloride and alcohol and a co-precipitate of silk proteins and zinc; d) filtration of the aqueous solution of zinc chloride and alcohol comprising the co-precipitate of silk proteins and zinc to collect said co-precipitate; e) optionally, washing said co-precipitate with alcohol; f) drying of the optionally washed co-precipitate, preferably at a temperature of 40°C to 80°C, in order to obtain a powder of silk proteins and zinc; and in which the silk and zinc protein powder obtained at the end of steps f) is such that it comprises, as a percentage by weight relative to the total weight of the powder, 8 to 12% of zinc, preferably 9 to 11%, the zinc being derived from the aqueous solution of zinc chloride ZnCh used in step b).;

[0024] The term “silk proteins” refers to the proteins that make up silk, namely fibroin and, possibly, depending on the origin of the silk supplied in step a), sericin.

[0025] A first step of the process according to the invention consists of supplying silk, in particular in the form of cocoons, raw silk, silk at least partially degummed, silk at least partially spun, woven silk dyed or undyed, in particular recycled silk.

[0026] The term "raw silk" or "unrefined silk" refers to silk directly from cocoons, therefore silk consisting mainly of fibroin surrounded by sericin.

[0027] The term "at least partially degummed silk" refers to silk composed of fibroin from which the sericin has been at least partially, or even completely, removed. The degumming process is well known to those skilled in the art. It can be carried out, in particular, by removing the sericin from the silk through washing it in hot, soapy water.

[0028] The process according to the invention does not require, preferably does not include, an additional degumming step when using raw and / or partially degummed silk.

[0029] The terms "at least partially spun silk" or "at least partially twisted silk" refer to silk whose yarn has undergone a twist, allowing bundles of fibers, yarns, or filaments to be joined together. Twist is preferably expressed in turns per meter (rpm). This involves counting the number of turns a yarn makes around its axis, relative to its length before untwisting. Twist is preferably evaluated according to ISO 2061:2015.

[0030] The term "woven silk" refers to silk made up of different silk threads woven together using any weaving technique. The weaving and knitting processes of silk threads are well known to those skilled in the art.

[0031] The term "recycled silk" refers to any silk derived from silk waste from textile industries or from silk cocoons whose quality is not sufficient for conventional spinning processes.

[0032] According to one embodiment, the silk supplied in step a) is shredded. "Frayed silk" means a silk fabric whose textile fibers are obtained after mechanical action (succession of cylinders by breaking down the threads) on a fabric, to obtain fiber fluff.

[0033] A second step in the process involves dissolving the silk in an aqueous solution of zinc chloride (ZnCh) at a temperature above 70°C to obtain an aqueous solution of silk proteins.

[0034] The use of zinc chloride is an essential step in the process. In particular, it allows for excellent dissolution of all types of silk and also makes it possible to obtain, at the end of the process according to the invention, a very particular silk powder in that it contains residual zinc from the dissolution solution.

[0035] According to a preferred embodiment, this step is carried out at a temperature of 75°C to 100°C, preferably 80°C to 90°C.

[0036] According to one embodiment, this step is carried out for a duration less than or equal to 3 hours, preferably less than or equal to 2 hours, more preferably from 5 minutes to 2 hours.

[0037] The temperature and duration conditions are adapted to the nature of the silk used in step a). They are optimized so as not to degrade the silk proteins and to allow homogeneous dissolution of the silk in a minimum time, advantageously less than or equal to 3h.

[0038] In a preferred embodiment, the aqueous zinc chloride (ZnCh) solution comprises, preferably, zinc chloride (ZnCh) at a concentration of 5 to 11 mol / L, preferably 7 to 10 mol / L, and more preferably 8 to 9 mol / L, dissolved in water, preferably demineralized water. In one embodiment, step b) of dissolution is carried out using 10 to 22 g, preferably 12 to 18 g, and more preferably 14 to 16 g by weight of silk per 100 mL of aqueous zinc chloride solution.

[0039] The process then includes a step of c) precipitating silk proteins by introducing the aqueous solution of silk proteins, preferably drop by drop, into an excess of alcohol to obtain an aqueous solution of zinc chloride and alcohol and a co-precipitate of silk and zinc proteins.

[0040] According to a preferred embodiment, the alcohol used in step c) is chosen from aliphatic mono-alcohols such as ethanol, methanol, propanol, isopropanol, butanol; aliphatic diols such as ethylene glycol, diethylene glycol, triethylene glycol and aliphatic triols such as glycerol; or a mixture thereof; preferably from ethanol, methanol, isopropanol or a mixture thereof; and even more preferably from ethanol.

[0041] Ethanol is particularly preferred for environmental reasons. Furthermore, it can be recycled and reused within the process itself.

[0042] According to an advantageous embodiment, the volume ratio between the aqueous solution of silk proteins and the alcohol during step c) of precipitation is 6 to 14, preferably 8 to 12, more preferably 9 to 11. Such a ratio ensures optimal precipitation of silk proteins.

[0043] The precipitation step is followed by a filtration step d). Filtration of the aqueous solution of zinc chloride and alcohol including the co-precipitate of silk proteins and zinc thus allows the collection of said co-precipitate.

[0044] The precipitation and filtration steps are essential components of the process. Unlike other techniques such as dialysis, these steps can be completed quickly and do not require large quantities of water. Furthermore, the precipitation method is scalable for industrial production and compatible with various solvents, including LiBr / EtOH / water, LiBr, CaCh / water, and ZnCh. In addition, this method produces silk powder that can be stored at room temperature for several months, unlike dialyzed silk, which gels within days, even at low temperatures.

[0045] According to a preferred embodiment, the filtrate from step d) of filtration, thus free of the co-precipitate, is subjected to step h) of evaporation so as to obtain two distinct compositions: one of alcohol and the other of an aqueous solution of zinc chloride (ZnCh). This embodiment is particularly advantageous in that it allows the alcohol and the aqueous zinc chloride solution to be recycled, thus enabling their reuse in the process according to the invention. The environmental impact of the process according to the invention is thereby significantly reduced.

[0046] Thus, according to one embodiment, the aqueous solution of zinc chloride ZnCh implemented in step b) of dissolution is obtained at a rate of at least 75%, preferably at least 80%, more preferably at least 85% of the recycling of the aqueous solution of zinc chloride obtained at the end of step h) of evaporation.

[0047] Advantageously, the alcohol used in step c) of precipitation is obtained from at least 75%, preferably at least 80%, more preferably at least 85% of the recycled alcohol obtained at the end of step h) of evaporation.

[0048] The process according to the invention finally includes a drying step of the optionally washed co-precipitate, preferably at a temperature of 40°C to 80°C, in order to obtain a powder of silk and zinc proteins.

[0049] Prior to drying, the co-precipitate can be washed with alcohol, preferably ethanol. This washing advantageously removes some of the zinc chloride, ultimately resulting in an optimized content in the silk powder, for example, preferably 8 to 12% by weight relative to the total weight of the powder. Furthermore, it facilitates the drying process by removing residual water.

[0050] At the end of the process according to the invention, a powder of silk proteins and zinc is obtained. Such a powder therefore comprises silk proteins but also residual zinc from the aqueous solution of zinc chloride ZnCh used in step b).

[0051] It has been demonstrated that such a powder of silk and zinc proteins can be electrospun. Electrospunting is particularly possible when zinc is present in the powder at a percentage by weight of 8 to 12%, preferably 9 to 11%.

[0052] According to the conditions of the previous steps of the process according to the invention, the silk and zinc protein powder can be washed with water (step g) at the end of step f) to lower the zinc content of said silk and zinc protein powder.

[0053] To obtain a silk and zinc protein powder that can be directly electrospun after dissolution, the silk and zinc protein powder obtained at the end of steps f) or g) is such that it comprises, as a percentage by weight relative to the total weight of the powder, 8 to 12% zinc, preferably 9 to 11% zinc, the zinc being derived from the aqueous zinc chloride (ZnCh) solution used in step b). Silk and zinc protein powder

[0054] The invention also relates to a silk and zinc powder obtained according to the process described above.

[0055] Such a silk and zinc protein powder comprises, as a percentage by weight relative to the total weight of the powder, 8 to 12% zinc, preferably 9 to 11% zinc, the zinc being derived from the aqueous solution of zinc chloride ZnCh implemented in step b) of the process according to the invention.

[0056] As will be shown in the examples, such a powder is quite unique. In particular, it is directly electrospuntable, unlike a silk protein powder to which zinc is added afterward, even at a rate of 9 to 11% by weight relative to the total weight of the powder. Furthermore, the powder according to the invention exhibits a specific decomposition profile during thermogravimetric analysis or thermal gravimetric analysis (TGA) and a unique NMR (nuclear magnetic resonance) spectrum in D2O / FA.

[0057] It is clear from the examples and the foregoing that within the silk and zinc protein powder according to the invention, the residual zinc from the process according to the invention has a very particular interaction with the silk proteins, giving said powder according to the invention very specific properties, making it in particular suitable for electrospinning.

[0058] Electrofilament process

[0059] The invention therefore also relates to a process for electrospinning a silk protein powder comprising the following steps: i) dissolving a silk and zinc protein powder obtained according to the process of the invention previously described or a powder according to the invention in a solution comprising at least one organic acid and a polymer in order to obtain an electrospinning solution; ii) electrospinning the solution obtained in step i) to produce a silk thread.

[0060] In one embodiment, the organic acid used in step i) of dissolution is selected from formic acid, trifluoroacetic acid, acetic acid, and mixtures thereof, with formic acid being preferred. In another embodiment, the polymer used in step i) of dissolution is selected from polyethylene glycol, poly(caprolactone), polyurethane, Nylon 6, and mixtures thereof, with polyethylene glycol being preferred.

[0061] Preferably, polyethylene glycol, when used in the process, is employed at a concentration of 0.4% to 1.5%, preferably 0.6% to 0.8% by volume relative to the total volume of the electrospinning solution. When poly(caprolactone), polyurethane, and / or Nylon 6 are used in the process, they are employed at a concentration of more than 5%, preferably more than 10% by volume relative to the total volume of the electrospinning solution.

[0062] Typically, the conductivity of the electrospinning solution obtained in step i) is 0.5 mS / cm to 3 mS / cm, preferably 1 mS / cm to 2 mS / cm.

[0063] Conductivity is measured using a Mettler-Toledo SevenGo™ Duo pH / conductivity SG23 laboratory conductivity meter. The conductivity electrode is immersed in the solution to be tested. This electrode emits an electric current through the solution, and the solution's ability to conduct this current is measured. The instrument calculates the conductivity based on the electrical response and displays the result in pS / cm (microsiemens per centimeter).

[0064] According to an embodiment particularly adapted to a silk and zinc protein powder obtained according to the process of the invention, in particular from undegummed silk, the viscosity of the electrospinning solution obtained in step i) is less than or equal to 400 MPa.s, preferably from 200 to 400 MPa.s. In particular when the process uses undegummed silk, such viscosity is obtained in particular by adjusting the quantity of silk and zinc protein powder in the electrospinning solution.

[0065] Viscosity is measured based on the flow ramp of the solution using the RM 200 laboratory rheometer. This device measures the viscosity of the solution by applying a stress (from 0.05 to 30 mNm) and measuring the resulting deformation.

[0066] Electrospinning is a process well known to those skilled in the art. Electrospinning is advantageously implemented using a powder solution according to the invention under the action of an intense electric field (on the order of 1 kV / cm) to create a fiber deposit. The fiber deposit constitutes a "nonwoven fabric," a term well-established and familiar to those skilled in the art.

[0067] The invention therefore also relates to a non-woven fabric made of silk thread(s) obtained according to the electrospinning process described above. The invention will become clearer upon reading the following examples, given solely by way of non-limiting illustration, and made with reference to the drawings in which:

[0068] [Fig. 1] Figure 1 illustrates the thermal gravimetric analysis profile of the "Invention 1" and "Comparative 1" samples from Example 3.

[0069] [Fig. 2] Figure 2 illustrates the first derivative of the thermal gravimetric analysis profile of the "Invention 1" and "Comparative 1" samples from Example 3.

[0070] [Fig. 3] Figure 3 illustrates the thermal gravimetric analysis profile of the "Invention 1" and "Comparative 2" samples from Example 3.

[0071] [Fig. 4] Figure 4 illustrates the first derivative of the thermal gravimetric analysis profile of the "Invention 1" and "Comparative 2" samples from Example 3.

[0072] [Fig. 5] Figure 5 illustrates the first derivative of the thermal gravimetric analysis profile of the samples "Invention 1", "Invention 2" and "Invention 3" of Example 3.

[0073] [Fig. 6] Figure 6 illustrates the N- MR spectra 1 H of the samples "Invention 1", "Invention 2" and "Invention 3" from example 3 dissolved in D2O / Formic Acid (300MHz).

[0074] [Fig. 7] Figure 7 illustrates the N- MR spectra 1 H of the samples "Invention 1", "Comparative 1" and "Comparative 2" of example 3 dissolved in D2O / Formic Acid (300MHz).

[0075] [Fig. 8] Figure 8 illustrates the ATR-IR (Attenuated Total Infrared Reflectance) spectra of the "Invention 1" and "de-coated silk" samples.

[0076] [Fig. 9] Figure 9 shows the ATR-IR spectra of electrospun degummed silk and electrospun non-degummed silk from the samples "invention 1" and "invention 2".

[0077] Examples

[0078] Example 1 of a method according to the invention

[0079] 1. Dissolving the silk and purifying the solution (laboratory scale)

[0080] 1.1 Dissolving silk

[0081] Solvent preparation: 24 g of zinc chloride are dissolved in 20 mL of demineralized water to prepare a solution with a concentration of 8.8 M. The resulting solution is heated to 85°C.

[0082] Sample preparation: The silk samples are cut into small pieces to facilitate solvent access to the fibers. The samples are introduced into a 16% w / v zinc chloride solution (8.8 M) at 85°C and then stirred until completely dissolved. The nature of the samples is detailed in Table 1. The dissolution conditions for the different silk samples are summarized in Table 1. The stirring time corresponds to the time required for complete dissolution of the sample (obtaining a homogeneous solution without any suspended and / or gelled residue). [Table 1] 1.2. Purification of silk solutions

[0083] The purification of silk solutions obtained after dissolution in aqueous zinc chloride solution, ZnCh, is carried out by precipitation in ethanol, EtOH, (technical or absolute).

[0084] Silk solutions (40 mL) are introduced dropwise into an excess of EtOH (400 mL), which causes the silk to precipitate as regenerated silk proteins (fibroin and, depending on the nature of the samples, sericin) (RSF) and zinc, also referred to hereafter as "Zn-RSF precipitate".

[0085] The ZnCh / EtOH / water fraction is collected as a filtrate while the Zn-RSF precipitate is collected and washed twice with EtOH, on a sintered glass.

[0086] 1.3. Drying of the Zn-RSF precipitate

[0087] The Zn-RSF precipitate is dried at 60°C in an oven for one hour, ground, and then stored at room temperature. The powder production yield is between 80 and 100%, indicating good silk recovery.

[0088] The process comprising steps 1.1 to 1.3 detailed above takes a maximum of three hours.

[0089] Precipitation of the silk protein solution using EtOH works for all types of silk, whether degummed, ungummed, yarn-dyed, or even in the form of woven or knitted and dyed fabrics. For dyed silk, the type of dyes used affects the final color of the powder. In some cases, the dyes were soluble in EtOH, and the resulting powder was therefore partially dyed. In other cases, the dye was insoluble in EtOH, resulting in a dyed powder.

[0090] After drying, the silk and zinc protein powder Zn-RSF contains approximately 9-11% residual zinc.

[0091] Some samples of dried Zn-RSF powder were subjected to several washes with deionized water to reduce the residual zinc content to 1.2%. The IR spectra of the "Invention 1" Zn-RSF powder in Example 3, obtained using this process from recycled degummed silk, revealed bands similar to those of native degummed silk fibers. For example, the characteristic NH vibration (bending) band of degummed silk yarns was around 3276 cm⁻¹. -1 overlaps exactly with the bands of the Zn-RSF powder, as illustrated in figure 8.

[0092] The C=O stretch bands of silk located around 1617 and 1513 cm -1 perfectly overlap the bands of the Zn-RSF powder produced by the process, as illustrated in Figure 8.

[0093] This demonstrates that there is no degradation of silk proteins during the implementation of the process according to the invention.

[0094] 2. Dissolving silk and purifying the solution (industrial scale)

[0095] Silk dissolution is scaled up using a double-jacketed reactor, a mechanical agitator, and an oil-based heating system.

[0096] This installation allows for efficient agitation and adequate heat transfer. Using this equipment, dissolution becomes much faster for fully degummed silk threads with very high twist (2500 rpm), requiring only one hour compared to two hours in the smaller-scale reactor.

[0097] Other types of silk have been dissolved on a large scale and have shown a similar decrease in dissolution time. Large-scale solutions are purified in the same way, by precipitation in EtOH.

[0098] Conditions for dissolution:

[0099] Quantity of sample to dissolve: 15% by weight / volume of solvent

[0100] Solvent: 300 mL of 8.8M ZnCh

[0101] Solvent temperature: 85°C

[0102] Table 2 summarizes the dissolution data and conditions of the different large-scale silk samples. The stirring time corresponds to the time required for complete dissolution of the sample. [Table 2] solvents:

[0103] To achieve an ecological and economical approach to silk recycling, solvent recovery is necessary.

[0104] During silk purification by precipitation, the collected filtrate, containing water, ZnCh, and ethanol, is recycled by evaporating the ethanol from the mixture. This step restores the aqueous phase containing ZnCh.

[0105] The ethanol is reused for the precipitation of the silk solution, while the aqueous solvent allows for the dissolution of another batch of silk.

[0106] This process allows the solution to be recycled three times. From the fourth cycle onwards, approximately 10% zinc chloride must be added to restore the dissolution efficiency.

[0107] 3. Electrofilament of silk solutions from Zn-RSF powder

[0108] After the regeneration of silk waste, new value-added fibers are produced by the electrospinning process. Electrospinning is a process well known to those skilled in the art.

[0109] Electrospinning is carried out using a Zn-RSF powder solution according to the invention under the action of an intense electric field (on the order of 1 kV / cm), in order to produce a deposit of fibers (or "non-woven").

[0110] Preparation of silk powder solutions suitable for electrospinning:

[0111] Polyethylene glycol (PEG, 900KD) is dissolved in formic acid (FA) and stirred to obtain a homogeneous viscous solution. Zn-RSF silk powder is then gradually added while stirring to obtain a homogeneous solution.

[0112] The solution is electrospun to form silk nonwovens. Table 3 summarizes the preparation conditions for Zn-RSF powder solutions suitable for electrospunting. Percentages are given as weight percentages relative to the total weight of the solution.

[0113] [Table 3]

[0114] Table 4 summarizes the electrospinning conditions from different types of Zn-RSF silk powder according to the invention obtained after the recycling steps detailed above. [Table 4]

[0115] FA = formic acid; cond. = conductivity in mS / cm; Zn = weight percentage in the powder relative to the total weight of the powder / weight percentage relative to the total weight of the solution. The homogeneity of electrospun nonwovens and the continuity of electrospinning depend on the homogeneity of the electrospinning solution and the parameters of the electrospinning apparatus. However, all the Zn-RSF powders according to the invention (examples EI1 to EI5), containing between 9 and 11.2% zinc, according to the invention, made it possible to obtain nonwovens by electrospinning, regardless of the origin of the silk (degummed or ungummed silk, ground silk waste, colored silk yarn, or colored jersey fabric). The presence of zinc in the electrospinning solution, derived from the Zn-RSF powders according to the invention (residual zinc), facilitates the dissolution of the silk, resulting in more homogeneous solutions that can be directly electrospinned. Effect of residual zinc on electrospinning

[0116] The effect of zinc cation on electrospinning was studied. Table 5 summarizes the tests carried out for this study.

[0117] [Table 5]

[0118] PEG = polyethylene glycol 900 kDa; FA = formic acid; Zn = weight percentage in the powder relative to the total weight of the powder / weight percentage relative to the total weight of the solution

[0119] Conclusions

[0120] A Zn-RSF powder obtained according to the invention and then washed so as to contain at most 1.2% of ZnCh (comparative tests EC2 and EC3) does not allow for obtaining satisfactory electrospun fibers.

[0121] Adding zinc chloride (at a rate of 2 to 8.5% by weight relative to the total weight of the solution) to a Zn-RSF powder solution obtained according to the invention and then washed so as to contain at most 1.2% of ZnCh (comparative tests EC4 to EC5) does not allow obtaining satisfactory electrospun fibers.

[0122] It is clear from the preceding examples that within the silk and zinc protein powder according to the invention, the residual zinc from the process according to the invention has a very particular interaction with the silk proteins, giving said powder according to the invention very specific properties, making it in particular suitable for electrospinning.

[0123] Furthermore, when zinc chloride is added to an electrospinning solution comprising a zinc powder according to the invention, including in particular a silk powder and 9.7% residual zinc (comparative test EC1), more fibrous fibers are obtained, but droplet ejection and fiber browning are also observed.

[0124] The method of preparing the silk powder provides an optimal range of ZnCh. This quantity of zinc, initially incorporated into the silk powder, allows for its good dissolution in formic acid and maintains the fibrous appearance during electrospinning without browning.

[0125] Effect of sericin on electrospinning:

[0126] PEG = polyethylene glycol 900 kDa; FA = formic acid; Zn = weight percentage in the powder relative to the total weight of the powder / weight percentage relative to the total weight of the solution

[0127] Compared to Zn-RSF powder derived from degummed silk, solutions obtained from undegummed silk exhibit the following properties:

[0128] For the same quantity of dissolved silk, the Zn-RSF powder derived from undegummed silk according to the invention yields solutions that are much more viscous than those obtained with degummed silk. Therefore, smaller quantities of Zn-RSF powder derived from undegummed silk should be used to prepare these formulations in order to obtain the appropriate viscosity range for electrospinning.

[0129] The conductivity of the solutions from degutted and undegutted silk is comparable, between 1 and 1.5 mS / cm.

[0130] The ATR-IR spectrum of electrospun fibers from undegummed silk revealed signals similar to those of electrospun fibers from degummed silk. The peaks corresponding to sericin (1394 cm⁻¹) -1 and 1453 cm -1) do not appear because sericin and fibroin are completely integrated into electrospun fibers, unlike natural fibers where sericin completely covers both fibroin chains, as illustrated in Figure 9.

[0131] In conclusion, the silk powder according to the invention, derived from undegummed silk, is directly electrospun and produces homogeneous nonwoven fabrics, provided that the viscosity of the electrospinning solution is less than or equal to 400 MPa·s. These fabrics are more fibrous than those obtained from degummed silk powder. Electrospinning undegummed silk eliminates the degumming step, which is water- and time-consuming, and also preserves the quality of the original silk.

[0132] Example 2 of comparative procedures

[0133] 1. Comparative dissolutions of silk

[0134] Table 6 presents the conditions and results of comparative dissolutions using, in particular, conventional solvents such as CaCh and LiBr-based solvents.

[0135] The silk samples correspond to degummed silk threads with a high twist (2500), this type of silk being in fact the most difficult to dissolve among those tested previously.

[0136] Unless otherwise stated in Table 6, the dissolution conditions were the same as those detailed in Example 1 (Laboratory-scale silk dissolution).

[0137] [Table 6] Dissolution of highly twisted degummed silk (2500) in a series of conventional solvents

[0138] S: complete dissolution of the sample after the indicated stirring time; IS: no or poor dissolution of the sample after the indicated stirring time; weight ratio; ** molar ratio

[0139] High-twist (2500) degummed silk fibers require a long time to dissolve in well-known solvents such as CaCh (20 hours) and LiBr (3 hours), even when using a higher salt concentration (9.3 M). In an aqueous LiCl solution, the silk sample remained intact even after 20 hours.

[0140] Although LiBr has shown a dissolution profile similar to that of ZnCh but with a longer stirring time, only a specific grade of LiBr can be used for dissolving silk. For example, any grade of ZnCh can be used to dissolve silk, whereas only anhydrous LiBr can dissolve silk fibroin.

[0141] 2. Comparative purification of recycled silk

[0142] 2.1 Purification by dialysis

[0143] The most commonly used technique is dialysis. Two types of dialysis bags were used, cellulose esters with a porosity threshold of 6000D (CE) and regenerated cellulose dialysis cassettes with a porosity of 3500D (RC).

[0144] After dissolving the degummed silk in aqueous solutions of LiBr or ZnCh, the solutions (10 mL) were cooled and dialyzed in distilled water (100 mL). The silk solutions were dialyzed for 24 or 48 hours, and the water was changed after 1, 2, 12, 6, and 12 hours. Table 7 summarizes the dissolution conditions in LiBr or ZnCh.

[0145] [Table 7]

[0146] 2.1.1. Dialysis of Li Br / silk solutions

[0147] Cellulose ester membranes

[0148] Dialysis of silk solutions obtained by dissolving in LiBr using cellulose ester membranes for 24 hours resulted in a decrease in solution viscosity and a color change from yellow to colorless. The solution's conductivity decreased significantly from 13–16 mS / cm to 0.3 mS / cm, indicating effective desalting. The resulting solution remained homogeneous for up to one week at 16°C, after which it began to gel. The final concentration of the solution after dialysis was 4%. Increasing the dialysis time to 48 hours produced a less stable solution that gelled within the dialysis membrane.

[0149] Conclusion: Although dialysis allows for the production of ultrapure silk protein solutions by completely removing the salt used, this step also results in excessively dilute silk solutions (4%) that are not electrospinnable. These solutions are unstable and must be stored at 4°C. Gel formation is observed after one week. The dialysis process itself is time-consuming (48-72 hours) and requires a large amount of water, up to 7 L of water per 10 mL of silk protein solution. Regenerated cellulose cassette

[0150] In the case of the cellulose cassette, similar results were obtained by increasing the dialysis time. Furthermore, diluted silk solutions were obtained due to cassette swelling, thus reducing the initial silk concentration by a factor of four.

[0151] 2.1.2. Dialysis of ZnCh / silk solutions

[0152] Dialysis of silk solutions obtained from dissolution with ZnCh using regenerated cellulose cassettes or cellulose ester membranes gave similar results to dialysis of silk solutions obtained from dissolution with Li Br: the solutions began to gel in the dialysis bags and showed little stability.

[0153] 2.1.3. Dialysis of LiBr / EtOH / water or CaCh / EtOH / water solutions

[0154] When LiBr / EtOH / water or CaCh / EtOH / water mixtures were used for dissolving silk and then dialyzed through a cellulose ester membrane, the membrane immediately broke down and a large portion of the solution was lost.

[0155] However, this problem was not encountered when using regenerated cellulose (RC) cassettes, which are more resistant. The stability of EtOH-containing solutions with respect to gelation is higher than that of aqueous solutions, in which they remained homogeneous for up to two weeks.

[0156] Conclusion

[0157] The dialysis procedure is lengthy (48 hours) and requires large quantities of water. Furthermore, it is not suitable for all solvent systems such as LiBr / EtOH / water and produces unstable solutions at low concentrations that are unsuitable for electrospinning.

[0158] Furthermore, the solvent is not recyclable during dialysis, making the process polluting. Indeed, during dialysis, some of the silk fibroin (short chains) also diffuses into the dialysis solution, thus contaminating it. Finally, this procedure is difficult to scale up industrially.

[0159] 2.2 Purification by precipitation in ethanol followed by washing with water (comparative process of any process between the two methods). The silk protein solution is added dropwise to an excess of ethanol. Complete desalting of the precipitate is achieved by successive washes with deionized water. However, this procedure led to the production of a silk protein powder with a gelatinous texture that was difficult to dry, even after several washes with ethanol. 3. Solvent recycling - comparative examples

[0160] The solvent recycling step is mandatory to provide an economically viable silk recycling process.

[0161] 3.1 The use of the conventional dialysis step makes solvent recycling impossible.

[0162] 3.2 During purification by precipitation, it is possible to recover the saline solution by simple distillation with ethanol.

[0163] Highly twisted, degummed silk yarns were dissolved in recycled LiBr or ZnCh solutions. The dissolution conditions and results are detailed in Table 8.

[0164] [Table 8] Dissolution of degummed silk in recycled solutions of ZnCh or LiBr.

[0165] S: complete dissolution of the sample after the indicated stirring time; IS: no or poor dissolution of the sample after the indicated stirring time

[0166] In both cases, the ethanol was removed, and then the aqueous saline solution was concentrated. The results revealed that the recyclability of solutions containing LiBr is limited to one cycle, while that of solutions containing ZnCh exceeds three cycles. The ZnCh can be regenerated by adding back the 10% initially lost.

[0167] These results make the aqueous solution containing ZnCh a superior choice to other solvents and make the recycling process environmentally friendly.

[0168] 3. Comparative ages of electrofil

[0169] Several electrospinning experiments were conducted using dialyzed silk protein solutions (“degrained silk protein solution from LiBr”), silk protein powder obtained by dissolution in LiBr (“Li-RSF powder”), or Zn-RSF powder to which a water-soluble polymer was not added to prepare the electrospinning solution. The solutions produced varied in terms of viscosity, silk protein concentration, and conductivity. Table 9 summarizes these experiments, indicating the properties of the solutions tested and the corresponding electrospinning results. [Table 9]

[0170] FA = formic acid; Cond = conductivity Electrospinning of the dialyzed solutions was not possible due to the low concentration of silk proteins and, consequently, the low viscosity.

[0171] Electrospinning of silk protein solutions or powders from LiBr-containing solutions was not possible due to the high viscosity of the solution. Even decreasing the silk protein concentration did not allow the production of electrospun fibers. The addition of PEG did not improve the electrospinning results for these solutions but increased their viscosity.

[0172] The electrospinning of Zn-RSF powder according to the invention is only possible in the presence of a water-soluble polymer such as PEG.

[0173] Example 3: Chemical analysis of silk and zinc protein powders

[0174] Various silk protein powders detailed in Table 10 below were subjected to thermogravimetric analysis or thermal gravimetric analysis (TGA).

[0175] This is a thermal analysis method in which the mass of a sample is measured over time as a function of temperature changes. Thermal gravimetric analysis was performed here over a temperature range of 25 to 700°C.

[0176] [Table 10] The profile of the thermal gravimetric analysis of the "Invention 1" samples and

[0177] "Comparative 1" are presented in Figure 1. Figure 2 represents the first derivative of this profile.

[0178] In relation to Figure 1, it appears that the residual mass of the "Invention 1" sample (9.2% zinc) is higher than the residual mass of the "Comparative 1" sample (1.2%). In particular, at 700°C the residual weight (weight relative to the initial weight) of the "Invention 1" sample is higher than that of the "Comparative 1" sample (61% and 53% respectively), due to a higher zinc content in the first sample (8% difference in residual mass, which corresponds to the percentage of zinc).

[0179] The decomposition profile of the "Invention 1" sample is therefore different from that of the "Comparative 1" sample.

[0180] The thermal gravimetric analysis profile of the "Invention 1" and "Comparative 2" samples is shown in Figure 3. Figure 4 represents the first derivative of this profile.

[0181] With regard to Figure 3, the residual mass of the "Invention 1" sample (9.2% zinc) is similar to the residual mass of the "Comparative 2" sample (9.2% including 1.2% residual zinc from the dissolution process and 8% additional zinc) obtained at 700°C since they have the same zinc content.

[0182] However, the decomposition profile of the "Invention 1" sample is different from that of the "Comparative 2" sample.

[0183] In view of figure 4, it appears that the decomposition takes place in four stages in the case of the "Invention 1" sample, while the decomposition of the "Comparative 2" sample is a classic two-stage decomposition.

[0184] Figure 5 illustrates the first derivative of the profiles of the thermal gravimetric analysis of the samples "Invention 1", "Invention 2" and "Invention 3".

[0185] The decomposition profiles of silk and zinc protein powders are similar, with decomposition occurring in four stages and their residual masses at 700°C being comparable.

[0186] The silk powders in Table 10 were also analyzed by nuclear magnetic resonance (NMR) using D2O and formic acid as solvents. NMR is a spectroscopic technique that applies nuclear magnetic resonance to the hydrogen nuclei present in the molecules of a substance in order to determine their structure.

[0187] N- MR spectra 1 Spectra of samples "Invention 1", "Invention 2" and "Invention 3" are shown in Figure 6. These spectra reveal similarities since they contain similar amounts of incorporated zinc cation, (9-11% of zinc).

[0188] Figure 7 illustrates the N- MR spectra 1H of the samples "Invention 1", "Comparative 1" and "Comparative 2". These spectra reveal a difference between the spectrum of "Invention 1"

[0189] Sample I contains 9.7% of the zinc cation incorporated, while "Comparative 1" contains a small amount of zinc cation (1.2%). By adding zinc chloride to sample "Comparative 1" to obtain sample "Comparative 2" (9.2%), the N-MR 1 H remained the same, which indicates that the silk powder in which the zinc cation is incorporated (residual zinc from the process according to the invention) has an NMR- 1 In particular, it should be noted that the sample of "Invention 1" in which the zinc cation is directly incorporated (residual zinc from the process according to the invention) is more soluble in D2O and formic acid, while the samples "Comparative 1" and "Comparative 2" tend to gel.

[0190] Conclusions

[0191] It appears that the silk and zinc protein powder according to the invention, comprising between 8 and 12% by weight of residual zinc, differs from silk protein powders obtained according to prior art protocols. The study of the profiles of thermal gravimetric analysis and N-MR 1 H demonstrates this difference and it turns out, moreover, as previously demonstrated, that only the powder obtained according to the invention is directly electrospuntable after being dissolved in the presence of at least one organic acid and at least one water-soluble polymer.

[0192] Example 4: Precipitation of the silk solution with solvents other than ethanol

[0193] The following solvents were tested: Ethanol is preferred due to its low toxicity, affordability, and accessibility (including from bioresources). Experimental section:

[0194] 9 g of highly twisted (2500 rpm) degummed silk were cut into small pieces (~ 4 cm 2 The silk pieces (9 g) were added to a 50 mL, 8.8 M ZnCl2 solution, heated to 85 °C. The solution was stirred for two hours at the same temperature (85 °C) to ensure complete dissolution. The solution was then precipitated in 200 mL of solvent (EtOH, technical grade EtOH, MeOH, or iProH). The resulting precipitate was filtered and then dried in an oven at 65 °C to obtain 10 g of silk powder.

[0195] The silk protein powders obtained contained between 8 and 12% by weight of residual zinc, making these powders directly electrospuntable after being dissolved in the presence of at least one organic acid and at least one water-soluble polymer.

Claims

33 DEMANDS 1. A process for producing silk protein powder, comprising the successive steps of: a) supplying silk, in particular in the form of cocoons, raw silk, silk at least partially degummed, silk at least partially milled, woven silk dyed or undyed, in particular recycled silk; b) dissolving the silk in an aqueous solution of zinc chloride ZnCh at a temperature above 70°C, preferably from 75°C to 100°C, preferably from 80°C to 90°C, preferably for a period of less than or equal to 3 hours, preferably less than or equal to 2 hours, more preferably from 5 minutes to 2 hours, to obtain an aqueous solution of silk proteins; c) precipitating the silk proteins by introducing the aqueous solution of silk proteins, preferably drop by drop, into an excess of alcohol to obtain an aqueous solution of zinc chloride and alcohol and a co-precipitate of silk proteins and zinc;d) filtration of the aqueous solution of zinc chloride and alcohol comprising the co-precipitate of silk proteins and zinc to collect said co-precipitate; e) optionally, washing of said co-precipitate with alcohol; f) drying of the co-precipitate, optionally washed, preferably at a temperature of 40°C to 80°C, in order to obtain a powder of silk proteins and zinc; and wherein the powder of silk proteins and zinc obtained at the end of steps f) is such that it comprises, as a percentage by weight relative to the total weight of the powder, 8 to 12% of zinc, preferably 9 to 11% of zinc, the zinc being derived from the aqueous solution of zinc chloride ZnCh carried out in step b).

2. A process according to claim 1, wherein the aqueous zinc chloride solution ZnCh of step b) comprises, preferably, zinc chloride ZnCh in a proportion of 5 to 11 mol / L, preferably 7 to 10 mol / L and, more preferably 8 to 9 mol / L, dissolved in water, preferably demineralized water.

3. A process according to any one of the preceding claims, wherein step b) of dissolution is carried out by using 10 to 22 g, preferably 12 to 18 g, more preferably 14 to 16 g by weight of silk per 100 mL of aqueous zinc chloride solution.

4. A method according to any one of the preceding claims, wherein the volume ratio between the aqueous solution of silk proteins and the alcohol during 34 of step c) of precipitation is from 6 to 14, preferably from 8 to 12, more preferably from 9 to 11.

5. A process according to any one of the preceding claims, wherein the alcohol used in step c) is selected from aliphatic monoalcohols such as ethanol, methanol, propanol, isopropanol, butanol; aliphatic diols such as ethylene glycol, diethylene glycol, triethylene glycol and aliphatic triols such as glycerol; or mixtures thereof; preferably from ethanol, methanol, isopropanol or mixtures thereof; and even more preferably from ethanol.

6. A process according to any one of the preceding claims, wherein the filtrate from step d) of filtration is subjected to a step h) of evaporation so as to obtain two distinct compositions of alcohol on the one hand and of an aqueous solution of zinc chloride on the other.

7. A process according to claim 6, wherein the aqueous zinc chloride solution used in step b) of dissolution is obtained from at least 75%, preferably at least 80%, more preferably at least 85% of the recycling of the aqueous zinc chloride solution obtained at the end of step h) of evaporation.

8. A process according to claim 6, wherein the alcohol used in step c) of precipitation is obtained from at least 75%, preferably at least 80%, more preferably at least 85% of the recycling of the alcohol obtained at the end of step h) of evaporation.

9. Silk and zinc protein powder obtained according to any one of claims 1 to 8.

10. A process for electrospinning a silk protein powder comprising the steps of: i) dissolving a silk and zinc protein powder obtained according to the process of any one of claims 1 to 8 or a powder according to claim 9 in a solution comprising at least one organic acid and at least one polymer to obtain an electrospinning solution; and ii) electrospinning the solution obtained in step i) to produce a silk thread.

11. Electrospinning process according to claim 10, wherein the organic acid used in step i) of dissolution is selected from formic acid, trifluoroacetic acid, acetic acid and mixtures thereof, preferably formic acid.

12. Electrospinning process according to claim 10 or 11, wherein the polymer used in step i) of dissolution is selected from polyethylene glycol, the polycaprolactone, polyurethane, nylon 6 and their mixtures, preferably polyethylene glycol.

13. Electrospinning process according to any one of claims 10 to 12, wherein the viscosity of the electrospinning solution obtained in step i) is less than or equal to 400 MPa.s, preferably from 200 to 400 MPa.s.

14. Non-woven fabric of silk yarn(s) obtained according to the process of any one of claims 10 to 13.

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