CROSS REFERENCE

This application claims priority to U.S. provisional application number 63/413,670, filed October 6, 2022, which is incorporated by reference herein in its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under 2019-67021-29940 awarded by the United States Department of Agriculture, National Institute of Food and Agriculture (USDA- NIFA). The government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to a method of producing fine, long, and soft fibers with high retention of stretchability from coarse, short, and stiff animal fibers via controlled keratin rearrangement and stepwise stretching.

BACKGROUND

Fiber shortages, especially non-petroleum-based fibers, have become one of the most prominent concerns globally. The share of synthetic fibers from petroleum resources for textiles has increased dramatically from 59% in 2011 to 75% in 2020. Those synthetic fibers after disposal easily transformed into micro- and nano-particles, which are hardly degradable and reportedly pose health concerns to animals and humans. The rapid increase in the production of synthetic fibers inevitably cause more and more pressure on the environment and society. Therefore, it is urgent to find natural and degradable materials to replace the current petroleumbased products.

An effective way to replace petroleum-based fibers is to convert natural polymeric materials, which are largely available, cost-effective, and tough, into fibers. Affected by climate change, production costs, land and water limitations, production of commercially available non- petroleum-based fiber, such as cotton and wool, could not be further boosted. For example, wool is 100% natural and biodegradable with many unique features such as good resistance to wrinkle, odor, stain, fire and UV, and excellent breathability, softness, thermal insulation, and elasticity. However, restricted by factors such as land, environment, animal welfare, and cost of production, wool fibers account for less than 1% of textile fibers. Although wool exists in vast quantities, the amount used in apparel is minimal and decreasing. According to the International Wool Textile Organization, only 37% of wool produced are fine wool suitable for textiles, with diameter smaller than 40 micrometers, and the rest are coarse wool with low or zero values, mainly from meat goats. The hairs from meat goats had a diameter higher than 60 micrometers and became waste because they could not be used on a large scale, especially not suitable as fibers for textiles. Despite countless worldwide initiatives, short and coarse wools still lack demands, especially high-value demands due to limited property improvements. Slenderizing short and coarse hair fibers of meat goats into fine and long fibers with excellent properties remains a challenge to scientists and engineers to meet the market demand for fine protein fibers and add values to meat goats and food industry'.

Few technologies developed previously have been used on an industrial scale due to limited degrees of fiber extensions and poor retention of elasticity’ of stretched fibers. Limited stretchability and flexibility retention of hair fibers mainly resulted from the difficulty in recovery of crosslinkages in stretched fibers. After fiber extensions, thiols between protein molecules in fibers cannot effectively re-establish crosslinkages due to the increased distances among cysteine residuals. As a result, lengths of protein molecules without crosslinkages were greatly shortened and mobility between protein molecules were seriously compromised. Therefore, coarse hairs had limitations in extension and flexibility retention. Most technologies on extensions of wool fibers developed since 1985 heavily focused on sufficient swell, plasticization, disulfide reduction, and post-stretch stabilization of coarse hair fibers with no study on recovery of cysteine crosslinkages. These extension technologies can only reduce fiber diameters by 20%, therefore, are not suitable for turning waste hairs with diameters larger than 60 micrometer from meat goats into textile fibers with required diameters of smaller than 40 micrometers. Furthermore, these extension technologies noticeably decreased elasticity' of stretched fibers, which lacked competitiveness against the natural wool fibers with the same diameter. For example, the breaking elongation of hair fibers reduced to less than 10% with a 90% extension compared to the breaking elongation of 45% before extension.

SUMMARY

Provided herein is a method of fiber slenderization. The method includes (a) reducing disulfide bonds of animal hair fibers, (b) extending the length of the animal hair fibers, (c) oxidizing the animal hair fibers in the presence of a crosslinker, and (d) annealing the animal hair fibers. The crosslinker includes a dithiol. In some embodiments, the method further includes repeating steps (a) through (d). In some embodiments, steps (a) through (d) are repeated until the breaking elongation of the animal hair fibers is less than about 10%.

In some embodiments, the reducing disulfide bonds is accomplished by a process comprising reacting the animal hair fibers with a reducing agent. In some embodiments, the reducing agent is an inorganic salt. In some embodiments, the reducing agent is a sodium salt. In some embodiments, the reducing agent is sodium sulfite.

In some embodiments, the animal hair fibers are added to an aqueous solution comprising the reducing agent.

In some embodiments, the reducing disulfide bonds further comprises exposing the animal hair fibers to a swelling agent. In some embodiments, the swelling agent comprises urea.

In some embodiments, the reducing disulfide bonds further comprises exposing the animal hair fibers to a stretchability enhancement agent. In some embodiments, the stretchability enhancement agent comprises glycerol.

In some embodiments, the extending is performed to a degree that less than about 40% of the animal fibers are broken.

In some embodiments, the oxidizing is accomplished by a process comprising reacting the animal hair fibers and the crosslinker with an oxidizing agent. In some embodiments, the oxidizing agent is an inorganic salt. In some embodiments, the oxidizing agent is a sodium salt. In some embodiments, the oxidizing agent is sodium periodate.

In some embodiments, the dithiol has a formula R-(SH) _n , wherein:

R is linear or branched comprising one or more structures of aliphatic, alicyclic, aromatic, ether, ester, and amide;

R is saturated or non-saturated;

R has a number of sigma bonds from 1 to 20; and n is an integer greater than or equal to 2.

In some embodiments, n is 2. In some embodiments, the dithiol has a formula HS-R-SH.

In some embodiments, R includes one or more structures of aliphatic and ester. In some embodiments, R is linear. In some embodiments, R is saturated.

In some embodiments, the dithiol is 1,2-ethanediol, ethylene glycol bisthioglycolate, or a mixture thereof. In some embodiments, the dithiol is ethylene glycol bisthioglycolate. In some embodiments, the dithiol is 1,2-ethanedithiol. In some embodiments, the annealing is performed at a temperature of about 125°C for about 30 minutes.

In some embodiments, the animal hair fibers have a diameter prior to the fiber slenderization of greater than about 60 micrometers. In some embodiments, the animal hair fibers have a diameter after the fiber slenderization of less than about 40 micrometers.

In some embodiments, the average diameter of the animal hair fibers is reduced by more than about 50%.

In some embodiments, the average length of the animal hair fibers is increased by at least about 100%. In some embodiments, the average length of the animal hair fibers is increased by at least about 150%. In some embodiments, the average length of the animal hair fibers is increased by at least about 200%. In some embodiments, the average length of the animal hair fibers is increased by at least about 250%. In some embodiments, the average length of the animal hair fibers is increased by at least about 300%.

In some embodiments, the tenacity of the animal hair fibers after the fiber slenderization is at least about 150% of the tenacity’ of the animal hair fibers prior to the fiber slenderization.

In some embodiments, the breaking elongation of the animal hair fibers after the fiber slenderization is at least about 50% of the breaking elongation of the animal hair fibers prior to the fiber slenderization.

Also provided herein are slenderized animal hair fibers which are produced by a methods disclosed herein.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 depicts an example method disclosed herein for the slenderization of coarse wool by stepwise reduction, extension, and oxidation.

Figure 2A is a graph showing distances between thiol groups on different keratin chains in fibers having 0% extension.

Figure 2B is a graph showing distances between thiol groups on different keratin chains in fibers having 100% extension.

Figure 2C is a graph showing distances between thiol groups on different keratin chains in fibers having 200% extension.

Figure 3A is a graph showing % of total sulfur is from cysteine, cystine, and dithiol in wool fibers using different reducing agents at 100% of extension. Figure 3B is an image showing the SDS-PAGEs of initial fibers (Lane 1), 100% extended fibers treated with sulfite (Lane 2), 100% extended fibers treated with 1,2-ethanedithiol (Lane 3), and 100% extended fibers treated with ethylene glycol bisthioglycolate (Lane 4).

Figure 4 depicts example chemical reactions involved with dithiols in extended wool fibers.

Figure 5 A is a graph showing mechanical properties of wool fibers after different extensions with different crosslinkers. wherein BS is Breaking Stress, BE is Breaking Elongation, and M is Young’s modulus.

Figure 5B is a graph showing the relationship between ratios of wet tenacity to dry tenacity of fibers and retention of crosslinkages (X _n ).

Figure 5C is a graph showing a comparison between the models (solid line and dashed line) derived from Equations 1 and 2 described herein.

Figure 6A depicts a proposed mechanism for dithiol-aided extension of wool fibers via stepwise reduction and oxidations.

Figure 6B shows images of wool fibers before and after extension.

Figure 6C shows SEM images of initial and extended wool fibers having different diameters. The bar shows 50 pm.

Figure 7 is a graph showing a comparison of secondary ⁷ structures of wool fibers treated with different sulfur compounds.

DETAILED DESCRIPTION

Provided in the present disclosure is a method of converting coarse animal hairs to fine, high-performance wool fibers using manipulation of keratin alignment and crosslinkages. The method involves stepwise, multiple cycles of decrosslinking, drawing, crosslinking, and annealing, using long-chain dithiols as crosslinkers.

Provided herein is a method of using dithiols with long backbones to manipulate the crosslinkages in proteins during the stepwise extension of coarse meat goat hairs. The method provided herein can effectively restore the crosslinking degrees of keratins and establish length- controllable crosslinkages. which affect interaction and degrees of movement between protein molecules. As a result, fibers after extension have a length of nearly 300% of their initial and maintain the flexibility and wet stability ⁷ of initial fibers. In an embodiment, the diameter of the fibers is reduced by 54% with a 350% length while retaining the desired performance properties of the original animal hair, such as tenacity and elasticity. Tenacity is a customary' measure of strength of a fiber or yam. As used herein, “tenacity ’" refers to the ultimate (breaking) force of the fiber (in gram-force units) divided by the denier, wherein denier refers to grams per 9,000 meters of fiber. Denier is a direct measure of linear density ⁷ and is commonly used to describe the fineness of fibers.

Elasticity can be evaluated by a measure of the breaking elongation of the fibers. As used herein, “breaking elongation” refers to the maximum elongation of fibers and textiles by stretching. A higher elongation means a better elasticity.

The method disclosed herein can provide a long and fine fiber from multiple cycles of decrosslinking, elongating, crosslinking, and annealing using dithiols as a crosslinker. In one embodiment, the dithiol is a long-chain dithiol.

In embodiments disclosed herein, high-value fibers can be attained from wasted animal hairs, and in some embodiments, can increase the value of already high-valued fine wool fibers. The method can be utilized in a continuous process with batches which can be used to aid in expanding the production to an industrial scale. The method disclosed herein can be applicable to any animal hairs having disulfide, hemiacetal, acetal, or ester bonds in a short length (i.e. about 5mm). The method can have benefits in waste management and can reduce the use of petroleumbased synthetic fibers.

The method provided herein can have applications in the textile industry', biomedical engineering, and the cosmetics and beauty industries, such as hair products. Textile fibers suitable for use in the method disclosed herein include, but are not limited to, wool, camel fiber, alpaca fiber, rabbit fiber, goat fiber, and the like.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials are described herein for use in the present invention; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

As used herein, “alkylene” refers to a bivalent straight chain or branched alkyl linking group.

Provided herein is a method of fiber slenderization, including:

(a) reducing disulfide bonds of animal hair fibers; (b) extending the length of the animal hair fibers;

(c) oxidizing the animal hair fibers in the presence of a crosslinker, wherein the crosslinker comprises a dithiol; and

(d) annealing the animal hair fibers.

Figure 1 depicts a schematic of an example method 100. Coarse animal hair fibers are subjected to a reduction step 102a that reduces the disulfide bonds of hair fibers to remove crosslinkages. In some embodiments, the reducing disulfide bonds is accomplished by a process comprising reacting the animal hair fibers with a reducing agent. Any suitable reducing agent may be used in the present method. In some embodiments, the reducing agent is an inorganic salt. In some embodiments, the reducing agent is a sodium salt. In some embodiments, the reducing agent is a sulfite compound. In some embodiments, the reducing agent is sodium sulfite. In some embodiments, the animal hair fibers are added to an aqueous solution including the reducing agent.

In some embodiments, the reducing disulfide bonds further comprises exposing the animal hair fibers to a swelling agent. As used herein, a “swelling agent’" refers to a compound that enables the swelling of protein hairs to allow for chemical penetration to occur to aid the reduction. In some embodiments, the swelling agent is a nitrogen-containing compound. In some embodiments, the swelling agent is an amide. In some embodiments, the swelling agent is urea.

In some embodiments, the reducing disulfide bonds further comprises exposing the animal hair fibers to a stretchability enhancement agent. As used herein, a “stretchability enhancement agent,” refers to a compound that can break the intermolecular attractions between keratin molecules to facilitate the movement of protein molecules. In some embodiments, the stretchability enhancement agent is an organic compound having one or more hydroxy groups. In some embodiments, the stretchability enhancement agent is a polyol. In some embodiments, the stretchability enhancement agent is glycerol.

Extension step 104a extends the length of the animal fibers such that the fibers have reduced diameter (c|)). In some embodiments, the extending is performed to a degree that less than about 50%, less than about 40%, less than about 30%, less than about 20%, or less than about 10% of the animal fibers are broken. In some embodiments, the extending is performed to a degree that less than about 40% of the animal fibers are broken.

The hairs are then oxidized in step 106a in the presence of a crosslinker (e.g., a dithiol as depicted in Figure 1). In some embodiments, the oxidizing is accomplished by a process comprising reacting the animal hair fibers and the crosslinker with an oxidizing agent. In some embodiments, the oxidizing agent is an inorganic salt. In some embodiments, the oxidizing agent is a sodium salt. For example, the oxidizing agent is sodium periodate.

In some embodiments, the dithiol has a formula R-(SH) _n , wherein:

R is linear or branched comprising one or more structures of aliphatic, alicyclic, aromatic, ether, ester, and amide;

R is saturated or non-saturated;

R has a number of sigma bonds from 1 to 20; and n is an integer greater than or equal to 2.

In some embodiments, R comprises one or more structures of aliphatic and ester. In some embodiments, R is linear. In some embodiments, R is saturated. In some embodiments, R is alkylene having 2 to 20 carbons, wherein the alkylene is optionally substituted with -COO- (ester) linkages. In some embodiments, R is alkylene having 2 to 10 carbons, wherein the alkylene is optionally substituted with -COO- (ester) linkages. In some embodiments, R is alky lene having 2 to 4 carbons, wherein the alky lene is optionally substituted with one or two -

O

COO- (ester) linkages. In some embodiments. R is ethylene or °

In some embodiments, n is 2. In some embodiments, the dithiol has a formula HS-R-SH.

In some embodiments, the dithiol is 1 ,2-ethanediol (ETT), ethylene glycol bisthioglycolate (EGB), or a mixture thereof. In some embodiments, the dithiol is 1,2- ethenedithiol. In some embodiments, the dithiol is ethylene glycol bisthioglycolate.

The hairs are annealed in step 108a to yield hairs with longer crosslinkages. In some embodiments, the annealing is performed at a temperature in a range of about 100°C to 150°C. In some embodiments, the annealing is performed at a temperature of about 125°C. In some embodiments, the annealing is performed for at least about 30 minutes. In some embodiments, the annealing is performed at a temperature of about 125°C for about 30 minutes.

In some embodiments, the method further includes repeating steps (a) through (d) (corresponding to steps 102a-108a in Figure 1). As shown in Figure 1, step 102b is a repeated reduction step, step 104b is a repeated extension step, step 106b is a repeated oxidation step, and step 108b is a repeated annealing step. In some embodiments, steps (a) through (d) are repeated until the breaking elongation of the animal hair fibers is less than about 25%. less than about 20%, less than about 15%, less than about 1 %, or less than about 5%. In some embodiments, steps (a) through (d) are repeated until the breaking elongation of the animal hair fibers is less than about 10%. The method of fiber slenderization provided herein can reduce the diameter of animal hair fibers. In some embodiments, the animal hair fibers have a diameter prior to the fiber slenderization of greater than about 60 micrometers. In some embodiments, the animal hair fibers have a diameter prior to the fiber slenderization of in a range of about 50 micrometers to about 100 micrometers. In some embodiments, the animal hair fibers have a diameter after the fiber slenderization of less than about 40 micrometers. In some embodiments, the animal hair fibers have a diameter after the fiber slenderization in a range of about 10 micrometers to about 40 micrometers.

In some embodiments, the average diameter of the animal hair fibers is reduced by about 20% to about 60%. In some embodiments, the average diameter of the animal hair fibers is reduced by more than about 20%, more than about 30%, more than about 40%. more than about 50%, or more than about 60%. In some embodiments, the average diameter of the animal hair fibers is reduced by more than about 50%.

The method of fiber slenderization provided herein can lengthen animal hair fibers. In some embodiments, the average length of the animal hair fibers is increased by about 10% to about 300%. In some embodiments, the average length of the animal hair fibers is increased by about 50% to about 300%. In some embodiments, the average length of the animal hair fibers is increased by about 20% to about 200%. In some embodiments, the average length of the animal hair fibers is increased by at least about 10%, at least about 25%, at least about 30%, at least about 40%. at least about 50%, at least about 100%. at least about 150%. at least about 200%, at least about 250%, or at least about 300%. In some embodiments, the average length of the animal hair fibers is increased by at least about 100%. In some embodiments, the average length of the animal hair fibers is increased by at least about 150%. In some embodiments, the average length of the animal hair fibers is increased by at least about 200%. In some embodiments, the average length of the animal hair fibers is increased by at least about 250%. In some embodiments, the average length of the animal hair fibers is increased by at least about 300%.

Any suitable animal hair fibers may be used in the disclosed methods. In some embodiments, the animal hair fibers are wool fibers, camel hair fibers, alpaca hair fibers, rabbit hair fibers, goat hair fibers, or a mixture thereof. In some embodiments, the animal hair fibers are wool fibers. In some embodiments, the animal hair fibers are goat hair fibers.

The methods provided herein can allow for retention of tenacity. In some embodiments, the tenacity of the animal hair fibers after the fiber slenderization is about 100% to about 200% of the tenacity of the animal hair fibers prior to the fiber slenderization. In some embodiments. the tenacity of the animal hair fibers after the fiber slenderization is at least about 100%, at least about 150%, at least about 160%, at least about 165%, at least about 170%, at least about 175%, or at least about 180% of the tenacity of the animal hair fibers prior to the fiber slenderization. In some embodiments, the tenacity of the animal hair fibers after the fiber slenderization is at least about 100% of the tenacity of the animal hair fibers prior to the fiber slenderization. In some embodiments, the tenacity of the animal hair fibers after the fiber slenderization is at least about 150% of the tenacity of the animal hair fibers prior to the fiber slenderization.

The methods provided herein can allow for retention of elasticity. In some embodiments, the breaking elongation of the animal hair fibers after the fiber slenderization is about 30% to about 175% of the breaking elongation of the animal hair fibers prior to the fiber slenderization. In some embodiments, the breaking elongation of the animal hair fibers after the fiber slenderization is at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 100%, or at least about 150% of the breaking elongation of the animal hair fibers prior to the fiber slenderization. In some embodiments, the breaking elongation of the animal hair fibers after the fiber slenderization is at least about 50% of the breaking elongation of the animal hair fibers prior to the fiber slenderization.

Also provided herein are slenderized animal hair fibers which are produced by a method disclosed herein.

EXAMPLES

Preparation: Extension of wool fibers via stepwise reduction and oxidations

As shown in Figure 1, coarse wool fibers first immersed in aqueous solutions containing 2 mol/L of urea, 10 mmol/L of sodium sulfite, and 5% of glycerol at 70°C for 30 min. Sodium sulfite is a reducing agent. Urea is a swelling agent to swell protein hairs to allow chemical penetration and complete reduction. Glycerol is a stretchability enhancement agent that breaks the intermolecular attractions between keratin molecules and facilitates the movements of protein molecules.

Then wool fibers were taken out for extension at a speed of 10 mm/min to a degree that less than 40% of fibers were broken. To investigate the effect of dithiols on fiber stretchability, stretched wool fibers were introduced into solution of dithiols with concentrations of 10 mmol/L for 30 min before transferred into an oxidation bath containing 1 g/L sodium periodate at room temperature for 30 min to form new crosslinkages in fibers. The washed fibers were dried and annealed at 125 °C for 30 min. Then the stretched fibers were again reduced in aqueous solutions containing 2 mol/L of urea, 10 mmol/L of sulfite and 5% of glycerol at 70° C for 30 min, stretched at a speed of 10 mm/min to a degree that less than 40% of fibers were broken, oxidized in a bath containing 1 g/L sodium periodate at room temperature for 30 min, and annealed at 125 °C for 30 min. Above steps of reduction, extension, oxidation and annealing were repeated until the breaking elongation of stretched fibers were less than 10%. For the control, after the first time of extension, wool fibers were directly immersed into the oxidation bath to recover disulfide crosslinkages. Then fibers went through several cycles of reduction, extension, oxidation and annealing till the breaking elongation of stretched fibers were less than 10%.

Characterization

Mechanical properties of wool fibers

To measure mechanical properties at dry state, wool fibers were conditioned at 21 °C and 65% relative humidity for 24 h prior to tests. To measure mechanical properties at wet state, fibers were immersed in water for 10 min before the test. Properties of wool fibers were obtained according to ASTM standard D-3822 using an Instron tensile testing machine (Norwood, MA). Gauge length set for testing was 1 inch, and crosshead speed was 18 mm min '.

Determination of sulfur content in wool fibers

The sulfur content in goat hair fibers was determined based on the method developed by Campanella et al [28], This method was to individually measure the concentrations of amino acid. In detail, dried goat hairs w ere hydrolyzed using 6N HC1 under 110 °C for 24 h to obtain amino acids. Phenylisothiocyanate was used for pre-column quantitative derivatization of amino acids using a HPTC, UltiMate 3000 series, USA, equipped with a C8 column (Acclaim 120, 120 A, 4.6 x 250 mm, 5 pm) and a UV detector with wavelength set at 254 nm. The flow ⁷ rate w as 1 ml mm '. and a ternary gradient was employed using 0.7 M sodium acetate with pH 6.4 (phase A), water (phase B) and acetonitrile/water with a volume ratio of 8: 2 (phase C). Gradient elution w as used. The portion of phase A gradually decreased from 20% to 10%, phase B decreased from 75% to 10%, while phase C increased from 5% to 80%. Total retention time was 30 min with an additional 10 min for column re-equilibration. Sulfur content containing cysteine and cystine could be obtained directly from HPLC. Other sulfur content, contributed by dithiols, were calculated based on the total sulfur content and contents of cysteine and cystine.

Secondary structure of w ool fibers

X-ray diffraction was carried out for secondary ⁷ structure analysis of original and extended wool fibers. X-ray diffraction was conducted using a Rigaku D/Max-B X-ray diffractometer with Bragg-Brentano parafocusing geometry; a diffracted beam monochromator, and a conventional copper target X-ray tube set ( = 1.54 A) to 40 kV and 30 mA at 26 °C. Diffraction intensities were recorded with 20 ranging from 3° to 40° at a scan speed of 0.05° per second. The degree of crystallinity was calculated using Jade 6.0 software (Materials Data Incorporated: Livermore, CA, USA) with Gaussian peak fittings.

Molecular weight of wool fibers after extension

Initial wool fibers and extended fibers treated with different sulfur-containing compounds were hydrolyzed in 3 N HC1 at 90 °C for 4 h to moderate decrease in overall molecular weight. The decrease in molecular weight of proteins should help determination of overall degrees of crosslinking in fibers. About 2 mg of the hydrolyzed proteins were dissolved in 100 gL of NuPAGE LDS sample buffer (4x) with excess mercaptoethanol and heated at 70 °C for 5 h. The solution was centrifuged prior to loading. Each sample of 10 p.L was loaded into an individual slot of the gel. A molecular marker from Spectra Multicolor Low Range Protein Ladder was used. The molecular weights of the protein standard mixture ranged from 4.6 to 62 kDa.

Morphologies of fibers

Morphologies of wool fibers before and after extension were characterized using a fieldemission scanning electron microscope (Hitachi S4700 Field-Emission SEM).

Simulation of distance between thiols in extended wool fibers

The simulation was conducted via combination of BIOVI A Materials Studio 2018 (18.1.0.2017) and LAMMPS (large-scale atomic/molecular massively parallel simulator). The basic model was built in BIOVIA Materials Studio and simulation was run by LAMMPS. Four typical sequence of amino acids of wool, T3C 1 th5, T3Clth8, T3Clth9 and T3Clthl 0, were adopted from a previous work [29], 20 repeating units of sequences were incorporated into an amorphous cell in Materials Studio. The polymer consistent force field (PCFF) was used to establish the amorphous cell before the final file was exported as .car. In LAMMPS, the model was relaxed under NVE with temperature set at 298 K. Each simulated step was set at 0.5 with total 50000 steps. The deform was conducted along x-axis.

Statistics

All the data points were compared using the one-way analysis of variance by the Scheff'e test with a confidence interval of 95%. A p value smaller than 0.05 indicated a statistically significant difference. All statistical analysis was conducted using SAS (SAS Institute Inc., Cary, NC). Long distances between thiol groups in extended wool fibers

Figures 2A-2C shows distance distributions of thiols on different protein chains, obtained by computation simulation, under different extensions. The results show that the greater the degree of wool extension, the greater the distribution distance of thiol groups on different protein molecules. Thiol groups were unlikely to meet efficiently to reform disulfide bonds in stretched wool fibers. As a result, the number of crosslinkages in stretched wools was remarkably reduced. As shown in Figure 2, in unstretched wool fibers, the dominant distance between thiol groups on different protein molecules was 2 A. The fiber extension significantly increased the distances between thiols on different protein molecules, resulting in an almost disappearing of initial distance, 2 A, in unstretched fibers. When the fiber was stretched to 100%, the dominant distance between thiols changed to 3.6 A. Further extension to 200% led to a thiol distance evenly distributed from 3.6 to 15 A. Among them, 12.3 A was the newly emerging thiol distribution distance. When the space was greater than 2A, it was unlikely to form disulfide bonds between thiols on different molecules through oxidation reactions. Therefore, novel methods need to be developed to rebuild the crosslinkages in stretched wool fibers.

Four typical sequences of amino acids of wool, T3C lth5, T3Clth8, T3Clth9 and T3ClthlO, were adopted (see T.C. Elleman, The amino acid sequence of protein SCMK-B2A from the high-sulphur fraction of wool keratin, Biochemical Journal 130 (1972) 833-845). 20 repeating units of sequences were incorporated. The radial distribution function (RDF) denoted in equations by g(r) defines the probability of finding a functional group at a distance r from another tagged functional group.

High recovery of crosslinkages by dithiols in extended wool fibers

Figure 3A and 3B show the effect of dithiols on crosslinkages in hair fibers. Figure 3A shows the distribution of sulfur content in stretched wool fibers at 100% of extension treated with different crosslinkers. The control was measured before the extension of wool fibers. The results show that long-chain dithiols efficiently reacted with thiols on proteins to establish crosslinkages. As shown in Figure 3A, only 60% of thiols reformed disulfide bonds either intermolecularly or intramolecularly in wools when only treated with sulfite. When dithiols, the long crosslinker, were present, most of the thiols on proteins reacted with dithiols. An increase in backbone length of dithiols improved the portions of thiols on proteins reacting with dithiols. For example, EGB, ethylene glycol bisthioglycolate, with a longer backbone length, was involved in nearly 80% of the disulfide bond establishment. In contrast, ETT, ethanedithiol, with a slightly shorter backbone length, was involved in 60% of the disulfide bond establishment. Dithiols, especially those with lengthy backbones, substantially reduced the amount of unreacted cysteine in wool fibers. The results in Figure 3A are consistent with the simulation results in Figure 2, indicating that it was not feasible to re-establish disulfide bonds in stretched wool fibers solely by the free thiol groups on protein chains. The key to maintaining the mechanical properties of stretched protein fibers, especially the breaking elongation, is to restore the crosslinking in stretched fibers.

Figure 3B compares molecular weights of stretched wool fibers treated with different sulfur compounds to the original unstretched wool fibers. Lane 1 is initial fibers, Lane 2 is 100% extended fibers treated with sulfite, Lane 3 is 100% extended fibers treated with ETT, and Lane 4 is 100% extended fibers treated with EGB. The sulfite was sodium sulfite.

The results show that consistent with the results in Figure 3A. crosslinkers with long backbone lengths effectively established crosslinkages. As shown in Figure 3B, the denser crosslinkages in fibers, the narrower the distribution of molecular weights of fibers. Failure to recover a portion of crosslinkages led to the generation of molecules with small molecule weights and thus the large scale of molecular distributions on SDS-PAGE. The unstretched coarse wool had the highest degree of crosslinking and, therefore, the narrowest molecular weight distribution. Stretching sulfite-treated wool resulted in the most extensive molecular weight distribution because of the lowest recovery' of crosslinkages compared to dithiol-treated wool.

Without wishing to be bound by theory, Figure 4 proposes the protein reduction, fiber extension and protein oxidation to rebuild crosslinkages in stretched fibers based on the results of Figures 2A-C & 3A-B. For easy move between protein molecules and desirable stretchability of fibers, wool keratin was reduced to cleave disulfide bonds before extension, as shown in the reduction part. After fiber drawing, relative distances between thiol groups on different protein molecules would increase substantially. As a result, thiols on different protein molecules were less likely to re-form intermolecular crosslinkages under oxidations. The addition of dithiols, especially those with length of backbones equal to or greater than the distance of thiol groups on different protein molecules after fiber extension, would react with thiols on two protein molecules to rebuild crosslinkages via oxidations. Next, the effect of protein crosslinkages reconstructed by dithiols on fiber stretchability and mechanical properties of stretched fibers will be investigated.

Desirable retention of elasticity of meat goat hairs after high degrees of extensions Figure 5 A shows the effects of various sulfur-containing chemicals on the mechanical properties of wool fibers with different extensions. In Figure 5A, BS is Breaking Stress, BE is Breaking Elongation, and M is Young’s modulus. The sulfite was sodium sulfite. The results show that crosslinkers with lengthy backbones substantially improved retention of mechanical properties and wet stability of stretched fibers compared to unstretched meat goat hairs. Figure 5A shows that meat goat hairs could only be stretched up to 150% without dithiols. Dithiols helped stretch the same goat hairs to 320%, an increase of 115% compared to stretched hairs without dithiols. More importantly, even at the same extensions of hairs, breaking stress, breaking elongation, and Young's modulus of dithiol -treated hairs w ere remarkably higher than those without dithiol treatment. Figure 5A shows that increase in fiber extension resulted in the improvement in breaking stress of goat hairs. However, dithiol-treated goat hairs had a greater improvement than sulfite-treated hairs. The increase in breaking stress of stretched hairs resulted from improvement in molecular regularity' brought by drawing. Dithiols had a better capability' to balance the stress among protein molecules because of their lengthy backbones of dithiols. Therefore, dithiol-treated hairs had higher breaking stress after drawing. Figure 5A also shows that the breaking elongation of sulfite-treated fibers decreased from an initial 45% to about 10%, with increasing extensions. While the breaking elongation of dithiol-treated hair fibers increased first w hen the extension ratio of hair fibers increased to 100%. Further extension of the dithiol- treated hairs gradually lowered the breaking elongation because distances between thiols on different protein molecules were too lengthy after fiber extension to re-establish disulfide crosslinkages. As a result, degrees of crosslinking in stretched fibers decreased considerably. A low' degree of crosslinking reduced the length of protein molecules, w eakened the relative movement betw een protein molecules, and thus reduced breaking elongation. Because of their lengthy backbones, Dithiols reacted with those free thiols in stretched hair fibers. As a result, degrees of crosslinking in stretched fibers were recovered substantially. Those recovered crosslinkages substantially increased the lengths of keratin molecules and improved the move between proteins. Therefore, the breaking elongation of dithiol-treated fibers after the same degrees of extension was higher than that of sulfite-treated fibers. The increase in the length of dithiol backbones promoted the recovery of crosslinkages and prolonged the overall length of protein molecules. Besides, lengthy dithiol contributed to the flexibility' of crosslinkages. Therefore, fibers treated with EGB, with relatively long backbone, had a better breaking elongation than those treated with ETT, with relatively short backbone. As further extension fibers, the number of crosslinkages in protein gradually decreased, even for fibers treated with EGB, a lengthy dithiol. As a result, the breaking elongation of dithiol-treated fibers decreased gradually with increasing in draw ratios higher than 100%. Figure 5 A further shows that compared with dithiols. Young's modulus of the sulfite-treated fiber was much lower than that of dithiol-treated fibers. Young's modulus of fibers was directly determined by concentrations of crosslinkages in proteins. Since EGB and sulfite provided the highest and lowest retention of crosslinkages in stretched fibers, respectively, EGB-treated and sulfite-treated fibers had the highest and lowest retention of Young’s modulus, respectively.

Figure 5B shows the change in the ratio of fiber wet tenacity to dry tenacity under different retention of crosslinkages. The results show that the retention of crosslinkages in fibers was linearly related to the ratio. The larger the ratio, the better the wet properties of the fiber. Dithiols effectively established crosslinkages. thus enhancing the wet properties of fibers.

TR=0.312*D+0.009*O+0.963 Adj R ² =0.893. Eq. 1

ER=-0.644*D+0.107*O+0.904 Adj R ² =0.884. Eq. 2

XR=-0.471*D+0.056*O+0.947 Adj R ² =0.882. Eq. 3

Where TR and ER are the retention of tenacity and breaking elongation of fibers, respectively. The retention of tenacity and breaking elongation of fibers were calculated based on the ratio of properties of extended fibers to initial raw fibers. D is the drawing ratio of fibers, and o is the sigma bonds in crosslinkages. XR is the recovery of crosslinkages in fibers.

Eqs. 1-3 quantifies the effects of extensions of wool fiber and number of sigma bonds in crosslinkages on retentions of fiber tenacity, fiber elasticity, and crosslinkages in fibers. The results show that extensions of fibers linearly increased the retention tenacity and decreased the elongation retention at break. At the same time, the lengths of crosslinkages, that is, the number of sigma bonds in the crosslinkages, linearly increased both retentions of tenacity and breaking elongation of fibers. Furthermore, Eq. 3 shows that the extensions of fibers and length of crosslinkages changed mechanical properties of the fibers by controlling the retention of degree of crosslinkings in fibers. According to Eq. 1, fiber drawings improved retention of fiber tenacity while lengthy crosslinkages enhanced the improvement. Eq. 2 shows that the loss of fiber elasticity due to fiber drawings could be partially offset by extending the length of crosslinkages. As shown in Eq. 3, an increase in fiber drawings reduced the recovery of crosslinking degrees while the extension of the crosslinkages promoted the crosslinking recovery in fibers. Extensions decreased the amount of crosslinkages and thus promoted the regularly arranged protein molecules. As a result, external forces could be evenly distributed in fiber molecules. Fiber tenacity improved accordingly. Nevertheless, lowered crosslinkage by extension reduced overall lengths of protein molecules and weakened relative moves between molecule chains. Therefore, retentions of breaking elongation of fibers decreased. An increase in length of crosslinkages not only balanced external forces in fibers, but also enhanced relative moves between protein chains. Therefore, the retention of tenacity and elasticity of fibers were both improved by lengthy crosslinkages.

In addition, the model of Eqs. 1-3 not only quantitatively explained the reasons for the limited extension ratio of coarse wool and a sharp decrease in elasticity of stretched fibers but also introduced the approach to developing fine and elastic wool fibers with substantial degrees of extensions. The model developed in this disclosure also fits well with data in previous work on wool extension, as shown in Figures 5C. Figure 5C shows a comparison between the models (solid line and dashed line) derived from Eqs. 1 and 2 from the present disclosure and data (circles and squares) from the literature. For the model, o=l was used in Eqs. 1 and 2 assuming only disulfide bonds formed in wool fibers. See J. A. Rippon, J.R. Christoe, R.J. Denning, D.J. Evans, M.G. Huson, RR. Lamb, K.R. Millington, A.P Pierlot, Wool: Structure, properties, and processing. Encyclopedia of Polymer Science and Technology (2002) 1-46; H. Liu, W. Yu. Microstructural transformation of wool during stretching with tensile curves. Journal of applied polymer science 104 (2007) 816-822; J. Yao, Y. Liu, S. Yang, J. Liu, Characterization of secondary structure transformation of stretched and slenderized w ool fibers with FTIR spectra, Journal of Engineered Fibers and Fabrics 3 (2008); M. Hosseinkhani, M. Montazer, S. Eskandamejad, M. Rahimi, Simultaneous in situ synthesis of nano silver and w ool fiber fineness enhancement using sulphur based reducing agents. Colloids and Surfaces A: Physicochemical and Engineering Aspects 415 (2012) 431-438; J. Liu, Y. Weng, Effect of stretching slenderization treatment for microstructure of yak hair, Advanced Materials Research, Trans Tech Publ, 2012, pp. 1231-1234: M. Hosseinkhani, M. Montazer. S. Eskandamejad, T. Harifi, Optimization of wool slenderizing along with in-situ synthesis of silver nanoparticles using box-behnken design, Journal of Natural Fibers 14 (2017) 175-184.

Traditional methods of w ool extension counted on the recovery of disulfide crosslinkages in fibers via reactions between thiol groups on different protein molecules. Therefore, the sigma number in crosslinkages should be 1. According to Eq. 2, when the fiber was drawn by 155%, the retention of breaking elongation of fibers reduced to 0, indicating that the maximum extensions of fibers from traditional methods cannot exceed 150%. In fact, in previous work, the maximum extension ratio of wool was less than 150%. According to the model, the stretchability of fibers can be improved by increasing the number of sigma bonds in crosslinkages. Long crosslinkers such as dithiols can easily increase the sigma number of protein fiber crosslinkage through reduction & oxidation. Increasing the sigma bonds in crosslinkages significantly offset the decrease in elongation retention at break caused by drawing. Therefore, fibers still had desirable elasticity after high degrees of drawing.

Without wishing to be bound by theory', Figure 6A show s a proposed mechanism for the improvement in stretchability and high retention of mechanical properties of animal hair fibers via reduction, extension, and dithiol crosslinking. As shown in Figure 6A, to increase the drawability’ of protein molecules, the disulfide crosslinkages were cleaved via reduction prior to extension, as shown in Stage b. After extension, distances between thiols on different protein molecules increased. As a result, the crosslinkages cannot be re-formed through the oxidation reaction. Eventually, there was no overlap of protein molecules, resulting in "weak points", as shown in Stage c., and breakage of fibers. Crosslinkers with long backbones such as dithiols efficiently formed new- crosslinkages by reacting with thiols in fibers. Those "weak points" in fibers disappeared due to the formation of crosslinkages, as shown in Stage d. At the same time, the crosslinkages extended the length of protein molecules and improved the relative slippages between molecules. Therefore, after dithiol crosslinking, the fibers not only can be further stretched but also have an increase in breaking elongation, as shown in Stage f. Dithiols continuously cleave and rebuild crosslinkages so that "weak points" are eliminated, and slippages between molecules are reinforced. After multiple cycles of reduction & oxidation, the number of crosslinkages gradually decreased, resulting in weakened slippages between molecules. Finally, fibers cannot be further drawn since the breaking elongation is gradually reduced.

Figure 6B show s morphologies of initial and stretched meat goat hairs. The initial diameters were 74 pm and the final diameters were 38 pm. Compared to initial hair, stretched hairs were shinier. As a result, values of stretched meat goat hairs would increase greatly.

Figure 6C shows the SEM images of meat goat hairs before and after extension. The results show that the overlap scale layers on hair fibers were remarkably reduced and the area of individual scale exposed on the fiber surface increased after drawing.

Figure 7 shows the changes in secondary' structures of stretched meat goat hairs treated with different sulfur compounds. The results show that the fibers treated with dithiols had a higher crystallinity and a higher proportion of B-sheet cry stals due to the larger extensions. A higher drawability of dithiol-treated goat hairs contributed to the formation of more ordered structures of fibers. Therefore, the total degrees of crystallinity had a moderate improvement. At the same time, extension caused partial conversion from a-helix to B-sheet.

Comparison

Table 1 shows the changes in mechanical properties of wools after different degrees of extensions and compares the results to those from previous technologies. Results show that the method disclosed herein realized high ratios of fiber extension, high retention of elasticity of stretched fibers, high tenacity, and high degree of fiber fineness. In traditional technologies, fibers can only be stretched up to 110%, while fibers disclosed herein could be stretched almost 270%.

Table 1. Comparison of mechanical properties and final diameter changes of fibers among previously disclosed methods and the method disclosed herein.

[1] J. Yao, Y. Liu, S. Yang, J. Liu, Characterization of secondary structure transformation of stretched and slenderized wool fibers with FTIR spectra, Journal of Engineered Fibers and Fabrics 3 (2008).

[2] M. Hosseinkhani. M. Montazer, S. Eskandamejad. T. Harifi. Optimization of wool slenderizing along with in-situ synthesis of silver nanoparticles using box-behnken design, Journal of Natural Fibers 14 (2017) 175-184.

[3] J.A. Rippon, J.R. Christoe, R.J. Denning, D.J. Evans, M.G. Huson, P.R. Lamb, K.R. Millington. A. R Pierlot, Wool: Structure, properties, and processing, Encyclopedia of Polymer Science and Technology (2002) 1-46.

[4] H. Liu, W. Yu, Microstructural transformation of wool during stretching with tensile curves, Journal of applied polymer science 104 (2007) 816-822. [5] M. Hosseinkhani, M. Montazer, S. Eskandamejad, M. Rahimi, Simultaneous in situ synthesis of nano silver and wool fiber fineness enhancement using sulphur based reducing agents, Colloids and Surfaces A: Physicochemical and Engineering Aspects 415 (2012) 431-438.

- not conducted or not reported.

* All extension values were obtained from a one-time fiber stretching.

**Extension values were obtained by the method of fiber slenderization disclosed herein.

The coarse hairs from meat goats were successfully transferred into usable textile wool fibers with desirable fineness and elasticity via manipulating crosslinkages using dithiols. Dithiols effectively restored the crosslinking degrees in extended fibers and established the length-controllable crosslinking bonds, which affected interaction forces and degrees of movement between protein molecules. As a result, final fibers after extension had a length of 300% of their initial, a diameter of 50% of their initial, and maintained the flexibility' and wet stability of initial fibers. Produced fibers from meat goat hairs meet the requirements of textile applications.

OTHER EMBODIMENTS

It is to be understood that while the compounds and methods have been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.