Production and Utilization of Keratin and Sericin-Based Electro-Spun Nanofibers: A Comprehensive Review

ABSTRACT This article review is devoted to throw the light on the unique characteristics of keratin- and sericin-based electro-spun nanofibers which make them suitable for various applications in different fields. The principles of electro-spinning together with the various devices usually used to fabricate nanofibers are also highlighted. The chemistry of keratin and sericin bio-polymers and the methods of extraction from their respective natural resources, such as wool and natural silk fibers, were criticized. Blending of keratin or sericin with various natural and synthetic polymeric materials to improve their rheological properties to obtain electro-spinnable composite suitable for production of functional nano-fibrous mat was discussed. Incorporation of nanosized metals and metal oxides as well as bioactive materials into keratin and sericin-based electro-spun nanofibers imparts new functions to the produced nanofibres. The utilization of these functional nano-fibrous mats in biomedical, filtration and smart textile applications was illustrated. The current status and future prospects of the electro-spun nanofibers were highlighted.


Introduction
Nanotechnology is one of the greatest prime foundations in science and technology within last century. The nanotechnology has altered the human-made equipment and lifestyle, and it is supposed to have a high economic impact all over the world. The materials subjected to the nanotechnology rules are prospective to exhibit more than one nano-scaled dimension, which exhibits new and distinct physical and chemical characteristics that attracted much more attention than that extrapolated from the same material properties at the larger scale. Nanoparticles, nano-rode, nanowire, nanofibers, carbon nanotube, etc., are among these nanomaterials. Nanofibers are categorized as a class of nanomaterials that are usually obtained by electro-spinning technique. The electro-spinning has been ascribed by many workers as the simplest method for production of micro/nanofibers. Various review articles have been devoted to assign the state-of-the-art in electro-spinning (Xue et al. 2017). The global nanofibers market analysis by product by applications indicated the nanofibers that produced from polymeric materials emerged as the main product segment.
Keratin and sericin are among proteinic biopolymers which are discharged into the environment as waste materials or by-products. Various studies have been achieved to extract and retrieve these natural materials from their respective resources for proper utilization in definite areas such as textile, food and water purification (El-Newashy et al. 2019;Maya et al. 2021). Electro-spinning of keratin and sericin-based composites into nano-fibers would afford appropriate candidate for various unconventional applications. In this article review, the different methods which can be adopted for electrospinning of keratin and sericin-containing composites were criticized and their current applications were outlined and their potential for new applications were proposed.

Production and application of electro-spun nanofibers
Nanotechnology is one of the greatest prime foundations in science and technology within last century. The nanotechnology has altered the human-made equipment and lifestyle, and it is supposed to have a high economic impact all over the world. The materials subjected to the nanotechnology rules are prospective to exhibit more than one nano-scaled dimension, which exhibit new and distinct physical and chemical characteristics that attracted much more attention than that extrapolated from the same material properties at the larger scale. Nanofibers are categorized as a class of nanomaterials that are usually obtained by electro-spinning technique. Nanofibers are produced by electro-spinning technique in which strong electric field is applied and the solution of polymeric material are converted into continuous nano-sized fibers (Agrahari et al. 2017). Whereas, the high electrostatic forces overcome the cohesive forces with simultaneous evaporation of the solvent resulting in formation of the jet and formation of the nano-sized fibers (Shahriar et al. 2019). Nanofibers were defined as fibers with diameters less than 1 μm (1000 nm) having relatively low density, large surface area to volume, large pore volume, small pore size resulting in their potential applications in diverse areas including air/ liquid filtration (Li et al. 2019), biomedical and tissue engineering , drug delivery energy generation and storage, sensors (Ziai et al. 2022), catalysts (Barhoum et al. 2019), and smart textiles (Mirjalili and Zohoori 2016;Zakrzewska et al. 2022). Figure 1 summarizes some of the applications the electro-spun nanofibers in different fields.
The word electro-spinning is derived from "electrostatic spinning" (Baniasadi et al. 2015) where the first concept of electro-spinning was introduced in the seventeenth century by physicist William Gilbert (Amna et al. 2020). He observed the deformation of water droplet when a piece of electrically charged amber was approaching near the droplet, where under the electrostatic force the spherical water droplets converted into a cone shape which then known as Taylor cone. Cooley and Morton were pioneers in production of fibers through electro-spinning (Wu et al. 2020) and the first effort on aspects of the electro-spinning process was made by Anton Formhals who filed series of 22 patents .
Since 1995, about two hundred universities and research centers were investigating the electrospinning operation and annual number of scientific articles concerned with electro-spinning is inclining (Nascimento et al. 2015). The electro-spinning process depends on three main factors including solution parameters, environmental conditions, and process setup parameters (Reddy et al. 2021). Concentration, surface tension, molecular weight, dielectric constant, viscosity, and conductivity are the affecting parameters of the solution toward formation of nanofiber. Additionally, environmental parameters including humidity and temperature also affect the electrospun nanofiber. The formation of nano-fibers is highly influenced by the adopted electric potential, rate of flow, and the distance between the solution output needle and the collector.

Types of electro-spinning
The main components of electro-spinning are a power supply with a high electric potential, a syringe pump, a spinneret (for example, a hypodermic needle with blunt tip), and a conductive collector. Basically, there are two conventional approaches in electro-spinning namely which comprise the utilization of needlelike nozzle, and needle-less electro-spinning (Odularu 2022). Recently, near-field electro-spinning (NFES) technique was introduced as a tool for manufacturing of micro-and nanofibrous materials by using direct current (DC). Their appropriate flexibility made NFES suitable for printing of sensors, cell scaffolds, and electronic components (Martinez-Prieto, Ehmann, and Cao 2020).

Needle -(Nozzle) -based electro-spinning
Principally, there are two standard electro-spinning setups horizontal and vertical (upward and the downward electro-spinning) as illustrated in Figure 2.The upward and downward electro-spinning setups are different in the fibers' orientation and beads' number. The formation of beads in the downward process setup will be more than with the upward process (Alghoraibi et al. 2018). For laboratory scale use, the downward electro-spinning setup is the most favorable, owing to the simplicity of monitoring and optimizing the entire operation, whereas the upward setup is suitable for the large-scale production (Abdel-Hady, Alzahrany, and Hamed 2011).
In the multi-needle spinnerets, different polymer solutions used to obtain mixed nanofibers owing to the interference taken place during electro-spinning operation (Subrahmanya et al. 2021). The coaxial spinnerets (bi-axial and triaxial) were applied to produce hollow, core -sheath and composite nanofibers by concurrent electro-spinning of two or more different polymers by utilizing co-axial capillaries for feeding the spinneret. The bi-component side by side electro-spinning (Two Nozzle Electro-spinning) is another type of needle electro-spinning setup in which each syringe contains a polymer solution existing in a side-by-side design and a shared syringe pump controlled the flow rate of the two polymer solutions (Gupta and Wilkes 2006). Furthermore, there are wide ranges of needlebased electro-spinning such as electro-blowing (gas-jet electro-spinning), magnetic field-assisted electro-spinning, conjugate electro-spinning, centrifugal electro-spinning, emulsion electro-spinning (Alghoraibi et al. 2018), etc.

Needle-less electro-spinning
In order to avoid using of capillaries and needles and their related limits and allow industrial-scale production of nanofibers, many authors have devised novel spinneret systems that are usually called needle-less electro-spinning . The distinguished characteristic feature of needle-less electro-spinning is the easiness of the formation of multiple nanofiber jets in the proper positions at the spinneret's surface. These spinnerets exclude the problem of blockage that occurred during needle electro-spinningand also enable faster production withgreater area deposition of nanofibers (Khan et al. 2013). During the needle-less electro-spinning, the waves of an electrically conductive liquid organize themselves on a mesoscopic scale. This is followed by jet formation when the intensity of the applied electric potential exceeds a critical value. The needleless electro-spinning setup may be categorized into two general categories: (i) needle-less electro-spinning in which the feeding system is confined, and (ii) that system with in which the feeding system is unconfined. In both groups, a high electric potential is used to create the nanofibers from the polymer jets. The solution of the used polymer afforded as a control sample of free surfacesolution on an appropriate support that might be in a cylindrical shape or a wire (in the 1 st group), or a liquid surface (in the 2 nd group) (Nazir et al. 2015).
Since the dawn of the new millennium, different shapes of needleless electro-spinning setup have been developed such as bubble electro-spinning, two-layer fluid electro-spinning, rotary cone electro- spinning, and edge electro-spinning (Begum and Khan 2017). Nano-spider electro-spinning is a commercial needle-less electro-spinning system which necessitates the use of an electrostatic field with high electric potential to form an electrically charged polymer stream and producing nonwoven webs in diameters of 50-300 nm (El-Newehy et al. 2012).

State of the art
Various review articles have been devoted to assign the state-of-the-art in electro-spinning (Xue et al. 2017). In early 20 th and 21 st centuries, electro-spinning has been getting more and more attention not only in industrially, but also in the scientific community. Nowadays, some companies have been designed and implemented industrial production lines for mass production of electro-spun nanofibers to enable regular flow of certain commercial products (Xue et al. 2019). Now, nanofibers-based commercial products have great share in the markets of sensors, ultra-filtration, and medical products.
The global nanofibers market analysis by product by applications indicated the nanofibers that produced from polymeric materials emerged as the main product segment, as shown in Figure 3. The easy to obtain and cost-effectiveness of various natural biopolymers and man-made ones are the main reasons that led to high demand of polymer nanofibers; in which, natural polymers (proteinic or cellulosic) have better biocompatibility and man-made polymers enhance the flexibility of the final product (Stojanov and Berlec 2020). Large-scale application of polymer nanofibers in high-efficiency air/water filters coupled with increasing usage in health care, energy storage, and protective clothing in the mature economies can be attributed to high penetration of polymer products in the industry.
It seems that nanofibres electro-spun from cellulosic substrates are more commonly used than those based on proteinic sources (Ali et al. 2022;Ara et al. 2021;Ganesh Kumar and Palani Rajan 2022). Herein, we highlight the previous and current trials for production of protein-containing nanofibers based on keratin and sericin and their current and future applications.

Production of keratin and sericin-based electro-spun nanofibers
Wet processing of textiles results in production of considerable amounts of by-products (El-Sayed, Abou Taleb, and Mowafi 2021 a) for example, during scouring of wool, large amounts of wool wax are discharged into the effluent. Furthermore, the short fibers produced from carding and combing of wool fleece could be a source of keratin (El-Sayed 2021). On the other hand, large quantities of sericin are produced during degumming of raw natural silk . Various studies have been achieved to extract and retrieve these natural materials from their respective resources for proper utilization in definite areas such as textile, food and water purification (El-Newashy et al. 2019;Mowafi, AbouTaleb, andEl-Sayed 2018 Maya et al. Maya et al. 2021).

Chemistry of keratin and sericin
Within the international market of proteinic biopolymers, keratin is always in the first place. Owing to its low toxicity and versatile properties, keratin has great share in various industrial and health-care sectors (Mowafi, Abou Taleb, and El-Sayed 2018). Generally, keratin presents in the chemical composition of the animal kingdom, constituting the highest proteinic parts therein. Keratin has 90% of proteins, with the major elements S (2-5%), N (15-18%), and other elements (3.2%), together with 1.27% fat. These elements are the nuclei of the amino acids forming the polypeptide chains of keratin; viz. cystine, arginine, glycine, aspartic acid, lysine, and serine (El-Sayed 2021).
Based on their structure, keratins are classified into α and β keratins. α-keratins (soft keratins) are low in sulfur content and usually exist in sheep wool, skin, and hair. β-keratins (hard keratins) are sulfur-rich and usually present in the hard tissues such as bird feather, horns, claws, nails, and hooves (Reddy, Zhou, and Ma 2020).  polypeptide chains are highly cross-linked by disulfide bonds, hydrophobic interactions and hydrogen bonding. There are many side groups in the amino acid residues along keratin macromolecules; Viz. acidic, basic, hydroxy, etc. (Rubing et al. 2022). These side groups acquire keratinous materials, like wool, certain desired properties such as hydrophilicity, substantivity toward dyes and metal ions, high capacity to bind with other polymers, . . . etc. (El-Fiky et al. 2021).
Sericin (amorphous and globular protein) constitutes a minor part of natural silk amounting up to 25% of the mass of raw natural silk (Mowafi, Abou Taleb, and El-Sayed 2018). The molecular mass of sericin derived from most of silkworm's species (e.g., Bombyx mori, Bombyx mandarins, and other species) ranges between 20 and 400 kDa, depending on the extraction method used.
Sericin is a hydrophilic macromolecular proteineous material containing eighteen amino acids; some of them exist with a relatively high ratio such as serine (ca.40%) and glycine (ca.16%) (Veiga et al. 2020). Hence, sericin contains polar side chain of about 42.3% (-OH, -COOH, and -NH 2 groups) as shown in Figure 4. These polar groups are responsible for moisturizing and oxidizing properties of sericin to enhance its efficiency to form cross-links, copolymers or blends with other polymers to produce sericin-based biomaterials of tailored desired properties.

Extraction of keratin
Globally, the amount of keratinous waste exceeds 5 million tons per annum (Du et al. 2022). These resources include wool, feathers, hair, horns, claws, and nails  and their emission without utilization resulting in secondary ecological pollution to the environment. Lately, a wide range of researches in utilization of keratin in several fields have been developed, such as medical, biological, and packing applications (Kooshamoghadam et al. 2021;Wu et al. 2018). Various methods, chemical or biological using an enzymatic method, have been explored for the dissolution and extraction of keratins. Chemical extraction of keratin includes alkali extraction , reduction (Aluigi et al. 2014), sulphitolysis (Sinkiewicz et al. 2017), oxidation , and ionic liquids (Li, Huang, and Yang 2017).

Extraction of sericin
Sericin is a sticky layer that acts as a gum binder that envelops the fibroin fiber to maintain the structural integrity of the cocoon. Moreover, sericin acts as a shield which minimizes the harmful effect of ultraviolet radiation as well as other environmental concerns on cocoons (Cao and Zhang 2016). Removal of sericin from raw natural silk is known as degumming, and is usually undergone to produce softer natural silk filaments. Annually, about fifty tons of sericin is discharged into the effluent as a result of degumming of more than 400,000 tons cocoons. Away from the serious environmental and ecological problems resulted from discharge of sericin into the degumming effluent, this also causes a loss of profits which can be reimbursed if sericin is properly extracted and utilized ). Sericin has been regarded to have various characteristics, such as antioxidant and anti-bacterial activity, UV resistance and moisture-absorbing properties, which make it a valuable component in some cosmetics and food products, as well as in biomedical applications (Hong et al. 2019). Several degumming methods have been reported utilizing different reagents such as boiled water (Nultsch et al. 2018), soap, alkalis (Vyas and Shukla 2016), acids (Hu et al. 2019) and enzymes (Zhu et al. 2022) ( Figure 5).

Electro-spinning of keratin and sericin
Electro-spinning of keratinous material faces some challenges such as the relatively high molar mass and low viscosity (Yildiz, Kara, and Acarturk 2020). To overcome the undesirable mechanical properties and brittleness of pure keratin nano-fibrous mats, keratin was blended with other polymeric materials, such as PVA, PEO, PAN, chitosan, gelatin, etc., in order to have been electro-spun into nanofibers .
In contrast to keratin, sericin was successfully electro-spun into nanofibers solution by using trifluoroacetic acid as a solvent (Zhang et al. 2012). Nevertheless, mechanical properties of the obtained sericin nanofibers were very weak, which is a drawback if they are to be used in various industrial fields (Khan and Tsukada 2014). Consequently, sericin has been blended with other natural and synthetic polymers like chitosan, silk fibroin, PVA, and poly(L-lactic acid), (PLA), to improve mechanical properties of the nanofiber mats to be applied in several industrial fields (Carissimi et al. 2019).

Keratin-based electro-spun nanofibers and their utilization
Keratin has a unique outstanding efficiency for capturing metal ions, dyes and volatile organic compounds, which makes it an appropriate candidate for water and air purification. Its intrinsic bioactivity allows its use in a wide range of biomedical applications (Monavari, Zohoori, and Davodiroknabadi 2022). Keratin/polymer blends can be dissolved only in definite solvents such as trifluoroethanol, acetic acid, formic acid, trichloro methane/N,N-dimethylformamide, and hexafluoro isopropanol (HFIP) to facilitate electro-spinning (Adeli, Khorasani, and Parvazinia 2019).

Keratin/Poly vinyl alcohol nanofibers and their utilization
Polyvinyl alcohol (PVA) is a bio-degradable, bio-compatible and nontoxic synthetic polymer with proper properties which make it suitable ingredient in electro-spinning of keratins to overcome their spinning limitations (Esparza et al. 2017). Various researches have been explored in the blending of keratin (from feather or wool) and PVA to be fabricated into nanofibers by electro-spinning (He et al. 2017;. Where, different ratios of keratin and PVA were dissolved in their appropriate solvent such as formic acid, acetic acid or in water at high temperature to obtain electrospuncomposite solution which then subjected to electro-spinning process under their suitable electrospinning conditions (electric potential difference, rate of flow and distance from the spinneret to collector).
It has been reported that keratin/PVA nanofibers are important candidates for dye and heavy metal adsorbents because of the large number of keratin functional groups and high specific surface of nanofibers . The electrostatic interactions between negatively and positively amino acid residues along keratin and cationic or anionic dyes resulting in high efficiency of keratin/PVA nanofibers to adsorb dye (Mowafi, Abou Taleb, and El-Sayed 2022). Figure (6) shows our study during a recent project, for absorption capacity of different ratios of feather keratin/PVA nanofibers to cationic and anionic dyes.
Furthermore, keratin/PVA nanofibers were applied for adsorption of toxic metal ion (copper, chromium, lead, mercury, etc.) from polluted water. This is due to the presence of high ratio of amino acids containing polar groups in their side chains which are capable of binding with charged particles such as metal cations. The free carboxylic groups of aspartic and glutamic acids are the most appropriate binding niches at different pH values ).
On the other hand, the amino acid sequences Arg-Gly-Asp and Leu-Asp-Val along the keratin macromolecules are able to form a bond with cell surface ligands acting as the extracellular matrix (ECM) that makes the cell-cell and cell-matrix interactions easier which enhances the cell adhesion and proliferation (Akhmetova and Heinz 2021). Several studies have been developed on biomedical purposes of keratin/PVA nanofibers . The thermally citric acid crosslinked FK/PVA nanofiber has been reported to be utilized as promising scaffolds for tissue engineering applications (Esparza et al. 2017).
Sanchez Ramirez et al. utilized PVA-NFs and keratin/PVA-NFs for designing and implementation a double-layered mat for wound healing . Whereas the upper layer contains cross-linked PVA nanofibers, the lower one is made from PVA/wool-keratin composite. The resulted keratin/PVA-NFs enhanced ability for cell bonding in in-vitro trials compared with pure PVA-NFs.
The incorporation of silver nanoparticles (AgNPs) into FK-based nanofibers for better microbiocidal activity has been also reported ). Amajuoyi et al. developed a novel approach to electro-spin a keratin/coenzyme/PVA composite into nanofibers followed by utilization in curing of infected wounds (Amajuoyi et al. 2020). The bactericidal agent mupirocin was added the said composite to enhance the antimicrobial activity of the resulted nanofibers. Keratin is capable of activation of keratinocytes within the wound bed, and their stimulation to proliferate and migrate leading to eventual wound closure (Kelly 2017). On the other side, mupirocin is widely used in promotion of pathogen-free wound bed, and the coenzyme (as an antioxidant) improves mitochondrial functions in diabetic wounds (Amajuoyi et al. 2020).
In another study, a nanofiber made from keratin/PVA-PLA hybridized with nano-fibrillated chitosan/Zinc oxide nanoparticles was recommended as an appropriate bactericidal scaffold for wound healing (Mohammadi, Shakoori, and Arab-Bafrani 2021). A new application for FK/PAV in the field of regeneration of nerves has been reported recently (Figure 7) (Khumalo et al. 2022). FK/ PVA nanofibers provide a frame that resembles the ECM of the natural fibrous structure of neural tissue, and thus can be used in nerve regeneration.

Keratin/Polyethylene oxide nanofibers and their utilization
Polyethylene oxide (PEO), a biocompatible water-soluble polymer, has crystalline, thermoplastic, nontoxic properties. By virtue of its excellent rheological properties, it can be often mixed with the low viscous materials to prepare composite nanofibers, to improve their spinnability (Li, Huang, and Yang 2017;Ruzgar, Kurtoglu, and Bhullar 2020). The electro-spinning of keratin/PEO composite as well as their potential applications in filtration and biomedical fields has been studied by some researchers (Ma et al. 2017). Li et al. reported that the extracted wool keratin by sulphitolysis was mixed with PEO to prepare the keratin/PEO aqueous solution used for electro-spinning. Where, the filtration efficiency of the keratin/PEO nanofibers reached up to 88% due to the effect of keratin in reducing the diameter of the produced nanofiber resulting in capturing of particles by virtue of adsorption function (Li, Huang, and Yang 2017).
The high-content human hair keratin/PEO nanofiber was developed by electro-spinning coupled with cross-linking within two successive processes. The said solution was thoroughly admixed with the crosslinking agent ethylene glycol diglycidyl ether (EGDE), followed by electro-spinning into nanofibers. The obtained nanofibers were cross-linked again with vapor of EGDE in order to lessen their water solubility, to be suitable for tissue engineering and cell-seeded scaffold .
In a similar work, Fan and coworkers employed protein-rich crosslinked keratin/PEO nanofibers in cell culture. The cell culture results emphasized that the obtained keratin/PEO blend nanofiber mat is suitable for cell adhesion, a property which is essential for tissue engineering and regenerative medicine (Ma et al. 2017). Recently, hydroxyapatite (HA) reinforced human hair keratin/PEO nanofiber membrane with improved mechanical properties has been developed for wound dressing ). Keratin/PEO composite was blended with two different collagens from two different sources before electro-spinning into new protein-based nanofibers. The in-vitro tests indicated the cytocompatibility of the obtained collagen-containing nanofibers. Consequently, the collagen-containing keratin/PEO nanofibers were introduced in the market for the bone tissue engineering or in biomimetic ECM for tissue regeneration (Râpă et al. 2020).
Lately, the cell viability of wool keratin-based nano-fibers was evaluated. Where three solvents, namely formic acid, HFIP, and water, were applied to obtain keratin/PEO electro-spun nano-fibers. In-vitro cell interactions validated a better response to cell proliferation within a period of 14 day in case of nanofibers electro-spun from HFIP solution. In case of the HFIP, the hydrogen bonds between the polypeptide chains of keratin macromolecules maintain the α-helix structures and enhance the capability of keratin-containing nanofibers during interaction with human mesenchymal stem cells (hMSC) .

Keratin/Gelatin nanofibers and their utilization
Gelatin, denatured collagen, is a natural protein with special bio-activity, bio-compatibility and biodegradability, non-antigenicity. Gelatin, a nontoxic, non-carcinogenic biopolymer, has no immunogenicity which makes it the proper choice for many biomedical applications (Ghosh et al. 2017). Different keratin/gelatin nanofibrous mats have been fabricated for wound dressing applications has been stated. Aiming to produce a double-layered membrane for wound dressing, Yao et al. blended human hair-keratin with gelatin for electro-spinning over polyurethane. The result cell viability assay revealed that the majority of cells migration was toward the gelatin/keratin regions by virtue of their compatibility for cellular interactions. Furthermore, the hydrophilicity of the said biopolymers as well as the high porosity of the electro-spun keratin/gelatin mats enhanced the absorption power for the exudates from the wound bed easier .
Similarly, Singaravelu et al. also generated durable horn keratin-based bi-layered electro-spun mats as smart biomaterial for wound healing. A blend of gelatin with poly (3-hydroxybutyric acid) was electro-spun over a keratin-containing sheet as a bi-layered scaffold. Results of this work showed the said scaffold can accelerate wound healing during in-vivo trials (Singaravelu et al. 2016). Similar findings have been obtained upon using keratin nanofibers with gelatin/methacrylate hydrogel (Kim et al., 2017).

Keratin/Polyacrylonitrile nanofibers and their utilization
Polyacrylonitrile, PAN, is a heat-and chemicals-resistant synthetic polymer having good mechanical properties. The active nitrile (CN) groups present in PAN make it efficient at metal ion adsorption through coordination. The modified PAN nanofiber membrane containing human hair keratin was synthesized or adsorption of gold (Au) and platinum (Pt) ions. The keratin-based PAN electro-spun membrane showed efficient adsorption for Au and Pt ions (Gökoğlua et al., 2021). Currently, keratin/ PAN was successfully electro-spun into nanofibers which may be used in future for biomedical applications. The extracted wool keratin was added to 15% PAN in DMSO solutions for electrospinning by a wire-based electro-spinning machine (Goyal et al., 2022).

Keratin/polyamide-6 nanofibers and their utilization
Polyamide 6 (PA6), also widely known as Nylon 6, is an important polyamide which is used in various applications including fibers, films, plastics etc. PA6 is characterized by its excellent thermomechanical properties, highly hygroscopic and water absorption (Donato and Mija 2019).
Blending keratin with PA6 was adopted for preparation of nanofibers and casting into films. Aluigi et al., electro-spun randomly oriented nanofibers with diameters between 230 and 130 nm by electrospinning of merino wool keratin/PA6 composite in formic acid (Aluigi et al., 2011). The keratin/ PA6nano-fibers showed higher adsorption capacities for Cu 2+ ion than pure PA6 nanofibers and this is due to complexes formation between Cu 2+ ions and free carboxyl groups of the amino acid residues of keratin including aspartic acid, glutamic acid, arginine, etc. Additionally, the cysteic acid residues in the extracted keratin are considered as the chelating agents for metal cations. The maximum adsorption capacities for keratin/PA6 nanofibers mats exceeds 100 (g/kg), compared to (~50 g/kg) in case of commercial activated carbon (Aluigi et al., 2011). The efficiency of keratin/PA 6 nanofibers for removal of Cr +6 ions (55.9 g/kg) from wastewater was also investigated. The hydrolyzed keratin/ PA6 nanofiber mats showed a higher adsorption capacity than pure PA6 nanofiber mats due to presence of binding sites of keratin amino acid residues (Aluigi et al., 2012). Recently, goat hair from tannery waste was used as a source keratin through sulphitolysis. The obtained keratin was then blended with PA 6 and electro-spun into a nanofibrous membrane for removal of anionic dye (David et al., 2020). The scanning electron micrographs of the produced nanofibers emphasized that the pore size of the electro-spun fibers decreased from 0.33, in case of PA6 nanofibers, to 0.18 μm, in case of keratin/PA 6 nanofibers; and thus, the adsorption of dye molecules was enhanced. The keratin/PA6 membrane showed better removal efficiency compared with the PA membrane due to the strong electrostatic interaction between the positively amino groups of keratin and dye anions.
Coarse wool grades are usually used for low-price products with low profits upon selling. In their attempts to attain proper utilization of coarse wool grades, Egyptian and Italian teams cooperated together to electro-spin coarse wool keratin/PA6 nanofibers mat, followed by utilization in removal of not only certain metal ions, but also acid and basic dyes from industrial drainage water . Recently, nanofibrous mats with antimicrobial properties were obtained by electro-spinning of Ag-doped keratin/PA6, and successfully used for air filtration (Shen et al. 2019). The Ag-keratin/PA6 composite solution was prepared by adding the keratin powder, extracted from the coarse wool by the reduction method, and AgNO 3 to a formic acid solution of PA6 to obtain electro-spun keratin/PA6 composite solution, as shown in Figure 8.

Keratin/Polycaprolactone nanofibers and their utilization
Polycaprolactone (PCL), aliphatic linear polyester, has recently applied as an important polymer due to its unique physico-mechanical characteristics, miscibility with a wide scope of polymers, and biodegradability (Guarino et al., 2017).
Keratin/PCL nanofibers were explored by many researchers as a scaffold for cell proliferation with tailored mechanical properties. An aqueous soluble keratin from hair was blended with different rations of PCL before being electro-spun into nanofibrous membrane appropriate to support 3T3 cell growth and proliferation (Edwards et al., 2015). Zhu and coworkers utilized L-cysteine-based redox system for extraction of keratin, which in turn, blended with PCL to prepare nanonets keratin of diameter of 299-624 nm via one-step electro-spinning from formic acid solution (Zhu et al., 2017).
An approach method to obtain a uniform coating of calcium phosphate onto electro-spun human hair keratin/PCL nanofibrous membrane for potential bone tissue regeneration has been developed . Keratin promoted the chelation of Ca 2+ and a uniform calcium phosphate coating onto PCL scaffolds resulting in increased the mechanical strength of the resultant scaffolds. Additionally, they observed the formed scaffolds with 2.66 nm had a high proliferation of hMSCs. Similarly, magnesium oxide included in the human hair keratin-PCL composite has been successfully fabricated into nanofibrous web and utilized in biomedical applications and musculoskeletal tissue engineering (Boakye et al., 2015). Subsequently, the in-vitro trials for cell proliferation performance of the nanofibrous mat obtained from electro-spinning of keratin/PCL from formic acid solution were studied (Wu et al., 2018). The addition of keratin to the PCL improved the hydrophilicity of the obtained nanofibrous mat increased and accelerated the rate of biodegradation (the loss in weight exceeds 25% within 7 weeks) and stimulated a more significant level of in-vitro tests.
Wan and coauthors electro-spun PCL/keratin/AuNPs mats with the ability to catalytically generate nitric oxide. The released nitrogenous gas could promote human umbilical vein endothelial cell growth (Wan et al., 2018). Gold nanoparticles enriched mats, release of NO, combined the biocompatibility of keratins and NO generating property. Accordingly, these said mats would be potent in vascular tissue engineering.

Keratin/Poly (butylene succinate) nanofibers and their utilization
Poly (butylenes succinate), PBS, is bio-based aliphatic polyester with good heat resistance, melting temperature and well-balanced mechanical properties provide a wide processing range. Owing to its bio-compatibility, bio-degradability and adequate ability to bind with natural fibers, PBS has received increasing attention, in the last years, for its utilization in drug delivery and in tissue engineering (Fabbri et al., 2018).
Guidotti et al. dissolved a mixture of keratin and PBS (50:50) in HFIP as common solvent (Guidotti et al., 2020). The blend solution, despite their immiscibility, was successfully electrospun into nanofibrous mats with improved mechanical properties owing to the existence of PBS. Additionally, the extents of swelling biodegradation of the obtained mats were promoted due to the hydrophilic nature of keratin. The latter feature would enhance the efficiency of the prepared mats for drug release and fibroblast proliferation. In another work, the same authors studied the effect of blending of rhodamine B (RhB) with keratin/PBS nanofibers for drug delivery applications (Guidotti et al., 2021).

Keratin/Poly (lactic-co-glycolic acid) nanofibers and their utilization
Poly(lactic-co-glycolic acid), which is usually designated as PLGA or PLG, is a linear aliphatic polyester (Structure 1). Because of its bio-degradability and bio-compatibility, PLGA has been usually used in fabrication of devices used in drug delivery and tissue engineering (Makadia and Siegel 2011). Zhang et al. prepared PLGA/wool keratin composite membranes by the solvent casting and electrospinning methods . Wool keratin could enhance the efficiency of PLGA toward cell affinity and bioactivity and lessen the occurrence of PLGA-induced aseptic inflammation. On the other side, PLGA can effectively improve the wool keratin mechanical properties. The results of this study indicated that the thermal stability and mechanical properties of the prepared PLGA/1.0% wool keratin composite nano-fibrous mats are satisfactory. Furthermore, the biological activity and cell compatibility of these mats are appropriate for promotion of periodontal tissues regeneration (after 12 weeks). Recently, the same group prepared antibacterial PLGA/wool keratin composite nanofibers decorated with ornidazole as antibacterial agent for oral periodontal guided tissue regeneration. The ornidazole-loaded PLGA/wool keratin had a high capacity for water absorption together with improved mechanical properties which make it suitable for in-vitro trials .

Other keratin-based nanofibers and their utilization
Being a biodegradable synthetic polymer, the FDA approved poly(L-lactide) (PLLA) as a candidate for tissue engineering. The large pore size of the fibrous structure of PLLA affords possible element for drug delivery applications . According to Zhang et al. PLLA was blended with keratin and the resulted composite was electro-spun into nanofibrous scaffold suitable for local tumor chemotherapy (Zhang et al., 2020b). The abundance of active groups along keratin macromolecules recommended its utilization in bonding with chemotherapeutic drugs release. The electro-spun suspension was prepared by adding the drug model 5-fluorouracil (5-FU)/keratin composite into the PLLA solution (chloroform/DMF). The kinetic study of the in-vitro drug release proved that the electrostatic force of attraction between keratin and 5-FU molecules makes the 5-FU-K-PLLA composite nanofibers able to expand the duration of 5-FU release at low pH. This would guarantee a prolonged effective inhibition in local tumor chemotherapy.
In another study, electro-spun wool keratin/silk fibroin composite nanofibers have been successfully fabricated and used to chelate and absorb Cu(II) ions from water (Ki et al. 2007). Due to the superior affinity of the hydrophilic amino acids of wool keratin toward metal ions, the blend nanofibers can capture about 2.8 mg/g Cu +2 ions from water, and the captured ions can be eluted from the nanofibers easily using dilute mineral acid. It has been found that, two strategies are usually adopted to promote the adsorption capacity of certain nanofibrous polymeric substrates toward metal ions: i) anchoring new active functional groups on the fiber surface, and ii) increasing surface area to improve adsorption capability (Fang, Wang, and Lin 2011).
Wool keratin/polyethylene terephthalate (PET) composite nanofiber membrane was successfully manufactured and utilized in removal of Cr +6 in acidic aqueous solutions (Jin et al. 2020).The maximum adsorption ability of wool keratin/PET composite nanofibrous membranes to Cr (VI) exceeds 75 g/kg, compared to about 27 g/kg in case of pure PET nanofiber membrane. This may be attributed to the electrostatic adsorption of the protonated amino groups, a pH value of 3, and the oxidation/reduction reaction of disulfide bond in cystine oxide in keratin. The possible adsorption mechanism for Cr (VI) removal is based on three approaches related to the protonation of the amide group, the electrostatic adsorption and the redox reaction.

Sericin-based electro-spun nanofibers and their utilization
To date, there are limited researches on electro-spinning of silk sericin (SS) alone into nanofibers by using trifluoroacetic acid, TFA, as solvent (Adeli, Khorasani, and Parvazinia 2019). Yang et al., developed microparticles and nanofibers sericin from low molecular weight sericin (LS) and high molecular weight sericin (HS), according to two different extraction conditions, by dissolving them in TFA followed by electro-spinning, respectively . Due to the weak mechanical properties of sericin nanofibers, SS has been blended with other natural and synthetic polymers like gelatin, chitosan, silk fibroin, PVA, PLLA, PCL, etc.in order to utilize in different areas including filtration, biomedical.

Sericin/Silk fibroin nanofibers and their utilization
In order to overcome the hydrophobicity drawback of silk fibroin resulting from the presence of a number of hydrophobic groups, silk sericin with excellent hydrophilicity was demonstrated to blend with silk fibroin (SF) by many researchers. An electro-spinnable blend solution of sericin and silk fibroin was prepared by dissolution in formic acid. The obtained SS/SF electro-spun nanofiber film dressings are biocompatible, and the presence of sericin enhanced the rate of cell growth (Lin and Zuo 2021).
Liu et al. studied the influence of SS content on the properties of the electro-spun silk nanofibrous membrane . The SS can impart both existence of silk I and II, which resulted in improvement in the mechanical characteristics of the obtained nanofiber. In another approach, the influence of SF/SS films on macrophage polarization and vascularization through in-vivo and in-vitro experiments has been studied .

Sericin/PVA nanofibers and their utilization
Many researchers have been prepared sericin/PVA nanofibers by electro-spinning. β-cyclodextrin (BC) together with sericin were merged with PVA nanofibers through electro-spinning and eventually crosslinking at high temperature in presence of citric acid. The obtained nanofibers were utilized in capturing of methylene blue (MB) from dye house . As indicated in Figure 9, at pH 8 the ability of PVA/SS/CD nanofibers for adsorption of MB can be explained in terms of two approaches: i) an electrostatic force of attraction between positively charged methylene blue and Figure 9. Adsorption mechanism of crosslinked sericin/β-cyclodextrin/pva composite nanofibers toward MB .
negatively charged carboxyl groups of sericin or citric acid adsorbed, ii) host -guest complex interaction of β-cyclodextrin holds the molecules of MB through host -guest inclusion complexes.
Recently, Mowafi et al. extracted and precipitated sericin from degumming effluent and blended it with PVA in formic acid for electro-spinning using wire electro-spinneret unit. The obtained SS/PVA nanofibers showed a high efficiency to capture acidic and basic colorants, together with metal ions due to presence of polar groups in sericin; viz. -OH, -COOH, and -NH 2 groups (Mowafi, Abou Taleb, and El-Sayed 2022).
In an attempt to make antimicrobial air filtration mask, sericin/PVA/Clay composite was electrospun into nanofibrous mats (Purwar et al., 2016). Clay imparts antimicrobial property to sericin-based nanofibrous and acts as afiller to reinforcethe mats. As the concentration of clay in the mats increased, the fiber diameters increased, while the pore size decreased. This would enhance the filtration capacity of the prepared mats.
Series of researches have been exploredtodevelop and utilize of SS/PVA nanofibrous mat in medical applications. Electro-spun SS/chitosan/PVA nanofibers decorated with in-situ synthesized AgNPs with superior bactericidal property, was successfully fabricated (Goudarzi et al., 2014). In another approach, the induction of the epithelial-mesenchymal transition of A549 cell by the electro-spun sericin/PVA nanofiber has been reported (Yan et al., 2017).
Gilotra succeeded to electro-spun PVA/SS with a wide range of porous structures which made it suitable for wound dressing with enhanced binding capacity than pure PVA mats (Gilotra et al., 2018). Chao and coworkers assessed the antimicrobial properties of SS/PVA nanofibers and found that the addition of an antibacterial agent (e.g., tigecycline) to the nano-fibers would enlarge the inhibition zones in presence of Gram -ve bacteria (Chao et al., 2018). Similarly, PVA/SS/chitosan (CS)/ tetracycline (TCN) porous nanofibers were fabricated for the sake of in-vitro and in-vivo possible applications as wound healing (Bakhsheshi-Rad et al., 2020).
Nowadays, for the first time, a novel multifunctional core -shell nanofiber (CSF) with hydrophilic core and hydrophobic shell has been designed (Tuancharoensri et al., 2022). The hydrophilic core was represented by PVA/SS part, while poly(lactide-co-glycolide), PLGA, had the role of the hydrophobic shell. They were fabricated via coaxial electro-spinning. Whereas, the core layer acts as a carrier for the bioactive agents as well as other materials (which are soluble in water), the shell layer acts as a shield to protect the early liberated water-soluble materials from the core component.
These CSFs have the ability impart bactericidal activity, as well as cell adhesion and growth. These smart multi-functional electro-spun nanofibers showed a high potential for usage as a novel system in the biomedical field that can (i) incorporate more bioactive materials and growth factors embedded into the core (e.g.,SS/PVA nanofibers); (ii) contain antibacterial agents within the shell (as AgNPs or cinnamon essential oil); and (iii) function as an ECM of the extra space present within the 3D structure of CSFs scaffolds to enable cells to migrate and grow (Tuancharoensri et al., 2022).

Sericin/PCL nanofibers and their utilization
Li and coworkers produced electro-spun three-dimensional PCL/sericin porous nanofibrous scaffold with high hydrophilic capability resulted from sericin, which promote human primary skin fibroblast cells adhesion and growth on the scaffolds (Li et al. 2011). Adopting emulsion electro-spinning technique, SS (middle silk gland)/PCL blends were successfully electro-spun, and the fabricated nanofibrous mats were eventually utilized in-vitro and in-vivo trials .
Currently, SS-based nanofibrous mats for diabetic foot ulcer were developed by Anand et al. The nanofibrous mats were prepared by electro-spinning SS mixed with, each separately, PCL and cellulose acetate (CA), containing ferulic acid (FA). This blending was carried to in order to enhance the efficiency of the prepared SS-based nanofibers in wound-healing and to improve their mechanical properties. Ferulic acid has anti-inflammatory, anticancer, antimicrobial, and antidiabetic properties with extensive therapeutic activities (Anand et al., 2022).

Sericin/Aliphatic polyester-based nanofibers and their utilization
Electro-spinning technique was adopted to fabricate a mat for wound healing made from nitrofurazone (NFZ)-loaded PLA/SS nanofibers and NFZ-loaded PLA nanofibers. The NFZ-loaded PLA/ sericin nanofibers act as the first layer and then the NFZ-loaded PLA nanofibers are the second layer to prepare the dual-layer fiber dressings . The incorporating of the antibiotic drug nitrofurazone enhanced the antibacterial activities of the dual layer nanofiber blends. In a current study, SS-PLGA scaffolds were electro-spun with ketoprofen for periodontal diseases (Chachlioutaki et al. 2022). These SS-PLGA scaffolds displayed enhanced hydrophilicity and mechanical strength with favorable attachment and proliferation of human gingival fibroblasts and a sustained anti-inflammatory action in the treatment of periodontal diseases.

Sericin/Natural polymer nanofibers and their utilization
SS-loaded electro-spun nanofibrous composite scaffold was fabricated via blending a cationic gelatin/ hyaluronan (HU)/chondroitin sulfate (ChS), and utilized in dermal tissue engineering (Bhowmick, Scharnweber, and Koul 2016). Sericin, HU and ChS were used as bioactive components to mimic the extracellular micro environment, and cationic gelatin as a base polymer to prepare electro-spun scaffolds, which can be used as skin wound care system. These findings were supported by the results of in-vitro trials. Recently, a benign one-pot method for synthesis of new SS-encapsulated silver nanoclusters (SS-AgNCs) was conducted (Mehdi et al., 2021). The bactericidal efficiency of the prepared materials was assessed by its incorporation within ultrafine electro-spun cellulose acetate (CA) fibers. Mehdi et al. proposed an acceptable mechanism for the formation of SS/AgNCs.

Smart textile application of sericin-based nanofibers
In the recent years, development environmentally responsive or stimuli-responsive smart materials showed great interest. In light of this, the multi-function sericin/poly(N-isopropylacrylamide), PNIPAM,/PEO nanofibers was fabricated and then grafted onto the surfaces of cotton textiles to endow the cotton textiles with outstanding stimuli-responsive functionalities (Li et al., 2021). The sericin/PNIPAM/PEO spinning solutions were electro-spun on the cleaned cotton fabric followed by glutaraldehyde vapor cross-linking. The multi-function modified cotton fabric can be used as a sensor by virtue of its ability to response to temperature and pH alterations. The treated cotton fabrics had superior antimicrobial activity.

Future outlook
Electro-spinning is an easily applicable and versatile technology, however this process faces some technical challenges, such as the low rate of production caused by the use of needles or nozzles as spinnerets. The multi-needle spinnerets, which were introduced recently to the market, would be appropriate for relatively high rate of nano-fiber production. However, the needle clogging will remain unsolved issue. Needleless electro-spinning (NLES) is an ideal system to promote the rate of production of nanofiber production away from any complication which may arose from the use of needles and nozzles in the electro-spinning operation. A major challenge which would face the NLES is to control the different process parameters which determine the quality and homogeneity of the electrospun nanofibers. On the other hand, an environmental concern arose from the volatile organic solvents which are usually used for dissolution of polymers before electro-spinning which may result in some health and safety issues.
With the growing demand on the production of nano-fibrous materials in various applications, and under the pressure of environmental legislation in most of the world, research works will be obliged to direct toward the following topics: • Interdisciplinary research projects should be directed toward proper utilization of natural resources as well as their wastes; viz. wool and natural silk. The specialists in agriculture, animal production, nutrition, plant and animal biotechnology, nanotechnology, physics, and textile chemistry and technology are invited to share their ideas together to achieve this goal. • More effort should be directed toward finding new applications for keratinous and proteinic wastes. This would have positive impact from the economic and environmental points of views. • Blending keratin and sericin with other natural as well as synthetic polymers would result in electro-spinnable materials with new properties for new applications. • Under the expected water crisis in some parts of the world within the next few years, it is mandatory to optimize the characteristics of electro-spun nanofibers to enhance their capacity for water filtration and purification. This would include the fiber diameter, pore size, surface area, specific volume, chemical composition, etc. • In the upcoming few years, and in the presence of huge numbers of pathogens surviving around us causing pandemic, it will be of prime importance to exert more effort toward the protection and safety of human beings against the current and expected attacking microorganisms. Keratin or sericin-based electro-spun nanofibers would play a vital role in production of protective masks. • Recyclability/reuse of nano-fibrous materials would be a significant anxiety in various fields. An interesting topic which deserves great work in the future is "The use of recycled materials" taking into consideration the economic, ecologic and social-related aspects.

Conclusion
The proteinic biopolymers keratin and sericin are appropriate for production of nanofibrous materials if properly blended with some synthetic as well as natural polymeric materials. The keratin and/or sericin-containing composites can be electro-spun into nano-fibers using various electro-spinning techniques. Due to their huge specific area and presence of polar functional groups, keratin and/or sericin-based nanofibers are and will be utilized in various applications in the medical, ecological and industrial fields. However, some technical and ecological concerns restrict the commercialization of production of nanofibers to a limit which is far beyond the importance of nanofibers. These include the slow rate of production and the ecologically unacceptable solvents which are used to dissolve the polymers before electro-spinning. Further investigations should be directed toward finding the proper solution to these problems in our route to large-scale production and utilization of keratin and sericin-based nanofibers.

Disclosure statement
No potential conflict of interest was reported by the authors.