Regenerated Cellulose Fibers

Keywords

Cellulose • Dissolution • Room temperature • Ionic interaction • Solubility • Production properties

Room temperature ionic liquids are considered green solvents and typically have low vapor pressure and good dissolution power and are easily recyclable [10Mak]. Ionic liquids used to dissolve cellulose should have low melting points, should not decompose cellulose, and should be stable and easily recoverable and relatively inexpensive. Considerable research has been done on dissolving cellulose using ionic liquids. Some of the ionic liquids that have been used to dissolve cellulose with concentrations of 10 % or above are listed in Table 19.1 [10Mak]. Dissolution of cellulose by ionic liquids is mainly related to the combined properties of the cations and anions and the basicity of the hydrogen bonds. Smaller cations were found to be more efficient in dissolving cellulose, and cations containing hydroxyl end groups had lower solubility [05Zha]. This is because the hydroxyl groups in the cations react with acetate of chloride anions and compete with cellulose to form hydrogen bonds. Ionic liquids with high hydrogen bond basicity were also found to have better solubility for cellulose. Ionic liquids are considered to be non-derivatizing solvents for cellulose, and therefore changes to the structure of cellulose are not expected. However, depolymerization of cellulose has been observed at high temperature when ionic liquids such as 1-allyl-3- methylimidazolium chloride [AMIM][Cl] were used [05Zha], whereas [BMIM] [Cl] did not depolymerize cellulose.

Although there are no reports on commercial-scale production, several labora­tory and pilot-scale studies have been conducted on producing regenerated cellu­lose fibers using ionic liquids [08Kos]. Eucalyptus pre-hydrolysis sulfate pulp (DP 569) and cotton linter pulp (DP 454) were dissolved between 90 and 130 °C under pressure (700-705 mbar) in several ionic liquids and spun into fibers. Solubility of cellulose in the solvents ranged from 10 to 17 %. Some of the

properties of the fibers produced from cellulose dissolved in various ionic solvents are given in Table 19.2 [08Kos].

Fibers produced from ionic solvents have similar tensile strength but lower elongation and modulus compared to the properties of the fibers produced by the NMMO system. It was also seen that cellulose dissolved using the chloride anion produced fibers that had higher tenacity but lower elongation than those produced using acetate anions.

Two direct solvents 1-ethyl-3-methyl imidazolium acetate [EMIM][OAc] and 1-ethyl-3-methyl imidazolium diethyl phosphate ([EMIM][DEP]) which have melt­ing points below room temperature and have good solubility and recoverability were used to produce fibers from cotton linters (DP 759) and eucalyptus sulfite pulp (DP 592). The pulp was dissolved in the solvent at 85 °C for 2 h and later wet and dry spun into fibers. Both dry and wet spinning produced fibers with properties similar to that of the fibers obtained using the NMMO process except that the fibers

obtained from [EMIM][OAc] by dry spinning had low elongations as seen from Table 19.3. Fibers obtained from the ionic liquids had good strength and elongation retention when wet [13Ing].

Cotton pulp (5 %) with a DP of 514 was dissolved using [BMIM][Cl] by heating at 90 °C, and the solution was extruded through a spinneret with an orifice diameter of 0.15 mm. Various spinning speeds and draw ratios were chosen to study their influence of fiber properties. Properties of the fibers obtained at three different spinning speeds are compared in Table 19.4. Fibers obtained at different spinning speeds had considerably lower strength and elongation mainly due to the poor drawing. Increasing spinning speeds increased the crystallinity and crystal orienta­tion [12Jia]. Cai et al. dissolved wood pulp (8 %) with a DP of 722 using [BMIM] [Cl] and obtained fibers with a tenacity of 3.3 g/den and an elongation of 8 % [10Cai]. Fibers produced in both these studies had circular cross section similar to that of lyocell fibers, and the cellulose had diffraction patterns typical of cellulose II, suggesting that cellulose was transformed during the fiber formation. [BMIM] [Cl] was also used to dissolve wood pulp and bagasse pulp and produce fibers on a laboratory scale. Properties of the fibers are given in Table 19.5 along with the fibers produced from various other cellulose sources. Morphologically, regenerated cellulose fibers produced using ionic liquids as solvents had a circular cross section similar to the fibers obtained using the NMMO or NaOH/urea systems as seen from Fig. 19.1.

In addition to using traditional pulp, microcrystalline cellulose and bleached kraft pulp were converted into different degree of polymerization, dissolved using 1-ethyl-3-methyl imidazolium acetate [EMIMAc] at 70 °C for 12 h, and the relationships between DP and viscosity and mechanical properties were studied. A linear relationship was found between DP and tenacity in the dry and wet state.

Fiber tenacities ranged from 0.9 to 1.8 g/den when dry and between 0.1 and 0.5 g/ den when wet with the DP varying from 330 to 1,340. Elongation of the fibers varied between 5 and 12 % but did not show a strong correlation with DP [13Ols]. Kim and Jang used 1-ally-3-methylimidazolium chloride [AMIM]Cl to dissolve microcrystalline cellulose (DP 1740), filter paper (DP 2310) and cotton fabrics (DP 2730), and produced fibers with fineness between 10.7 and 12.0 tex. Fibers produced from cellulose with high DP resulted in fibers with higher strength and modulus but lower elongation [13Kim].

References

[00Hei] Heinze, T., Dicke, R., Koschella, A., Kull, A. H., Klohr, E.-A., Koch, W.: Macromol. Chem. Phys. 201, 627 (2000)

[02Swa] Swatloski, R. P., Spear, S. K.H., Rogers, R. D.: J. Am. Chem. Soc. 124, 4974 (2002) [05Hei] Heinze, T., Schwikal, K., Barthel, S.: Macromol. Biosci. 5, 520 (2005)

[05Zha] Zhang, H., Wu, J., Zhang, J., He, J.: Macromolecules 38, 8272 (2005)

[08DAn] D’Andolo, G., Szarvas, L., Massonne, K., Stegmann, V.: Ionic liquids for solubilizing polymers WO2008/043837.

[08Kos] Kosan, B., Michels, C., Meister, F. P.: Cellulose 15, 59 (2008)

[09Zha] Zhao, H., Jones, C. L., Baker, G. A., Zia, S., Olubajo, O., Person, V. N.: J. Biotechnol. 139, 47 (2009)

[10Cai] Cai, T., Zhang, H., Guo, Q., Shao, H., Hu, X.: J. Appl. Polym. Sci. 115, 1047 (2010) [10Mak] Maki-Arvela, P., Anugwom, I., Virtanen, P., Sjoholm, R., Mikkola, J. P.: Ind. Crop. Prod. 32, 175 (2010)

[11Jia] Jiang, W., Sun, L., Hao, A., Chen, J. Y.: Text. Res. J. 81(18), 1949 (2011)

[12Jia] Jiang, G., Yuan, Y., Wang, B., Yin, X., Mukuze, K. S., Huang, W., Zhang, Y., Wang,

H.: Cellulose 19, 1075 (2012)

[13Ing] Ingildeev, D., Effenberger, F., Bredereck, K., Hermanutz, F.: J. Appl. Polym. Sci. 128, 4141 (2013)

[13Kim] Kim, S., Jang, J.: Fibers Polym. 14(6), 909-914 (2013)

[13Ols] Olsson, C., Westman, G.: J. Appl. Polym. Sci. 127, 4542 (2013)

Natural Protein Fibers

Keywords

Poultry feather • Low density • Hollow structure • Honeycomb • Sound absorption

Poultry feathers are one of the most widely available, low-cost protein by-products. Unlike other protein sources, feathers have a unique hierarchical structure and low density that make them preferable for various applications. Figure 45.1 shows an image of the major parts of a feather. The central rachis or quill is a tough composite-like structure that extends throughout the length of the feather. Barbs are fibers that have lengths up to 4.5 cm in the case of chicken feathers. Barbules that have lengths of few mm are connected to the barbs similar to that of the barbs connecting to the quill as seen from the SEM image in Fig. 45.2 (left). A cross section of the feather quill and rachis reveals a unique honeycomb structure that is hollow as seen in Fig. 45.2 (right). This hollow structure is responsible for the lightweight and therefore low density (0.9 g/cm3) of feathers. In terms of physical structure, feather rachis and barbs were found to have typical diffraction pattern of a-keratin, but the orientation of the crystals in the rachis and barbs was found to be different [07Red]. Tensile properties of the chicken feather barbs are compared to those of turkey barbs and wool in Table 45.1. As seen from the table, the strength of the barbs is similar to that of wool but with lower elongation. It was reported that the chicken feather barbs could be hand twisted into yarns when blended with cotton fibers [07Red].

Fig. 45.1 Digital image of a chicken feather shows the main rachis or quill to which are connected the barbs that further branch out into barbules

Pe represents pennaceous and PI represents plumulaceous turkey feather barbs 1 g/den is approximately 115 MPa Reproduced from Reddy [07Red]

Reference

[07Red] Reddy, N.: J. Polym. Environ. 15, 81 (2007)

Fibers from Biotechnology

Keywords

Naturally colored cotton «Wax content • Moisture regain • Conventional cotton • Blend • Color fastness • Light fastness • Washing fastness • Colored cotton • Colored cotton production • Colored cotton processing • Scouring • Mordanting • K/S value • Ring spinning • Rotor spinning • Yarns • Fabrics • Genetic transfor­mation • Transgenic plant • Colored cotton • Restricted color • Lower yield • Tensile properties

62.1 Introduction

Production of natural fibers such as cotton requires substantial use of land, water, and other natural resources. Processing of the fibers into textiles also needs addi­tional water, energy, chemicals, and other resources. Textile processes such as dyeing are energy intensive and also release considerable amounts of wastewater containing dyes that cause environmental pollution. Although improvements in machinery and processes and increase in environmental awareness and regulations have made some textile processing environmentally friendly and sustainable, the majority of the textile processings, especially in the developing countries, are a cause for environmental concern. Considerable efforts have been made to reduce the waste generated during textile processing and/or to use sustainable and green materials. One such attempt has been to develop colored cottons that could elimi­nate the need for using dyestuffs, water, and energy required for dyeing.

Natural Cellulose Fibers from Renewable Resources

Keywords

Natural cellulose fiber • Cotton cultivation • Bast fiber • Alternative fiber

For centuries, mankind has been clothed using natural cellulose and protein fibers that have been almost entirely derived from dedicated sources. Cultivation of fiber crops and rearing of silkworms and sheep have been the traditional methods of obtaining cellulose and protein fibers, respectively. However, fiber crops were not just sources for clothing, but the by-products generated were major sources for food and means for substantial income. For instance, cotton seeds have been used as a source for oil and also as animal feed. Among the different types of fibers, natural cellulose fibers, mainly cotton, have been the most common source for fibers. Recently, the cultivation of cotton and other natural fibers has been declining due to the difficulties in growing cotton, better profits from biofuel crops such as corn and soybeans, and limited technological improvements in processing and using cotton-based textiles. Similarly, the supply of petroleum resources required for synthetic fibers at affordable prices could be questionable in the near future. At any given time, it can be expected that fuel needs would predominate the use of petroleum resources for textile fibers. In addition, increasing consumption, espe­cially in the developing countries, constraints on the natural resources required to produce fibers, and inability to increase the supply proportionate to the demand are expected to make most of the current fibers either too expensive or unavailable for commodity applications. This scenario is neither unrealistic nor unforeseeable. The production of natural fibers such as cotton is declining due to cotton farmers shifting to more profitable biofuel crops such as corn and soybeans. These biofuel crops are also less demanding in terms of resources required for cultivation, harvesting, and processing into final products. The decrease in cotton production could escalate further due to the demand for biofuels.

Attempts to find alternative sources for the natural cellulose fibers in current use have met with limited success due to cost and quality restrictions. Unlike cultivating exclusive fiber crops that require dedicated land, large amounts of water, energy, and labor, the residues such as stalks (stems), leaves, and husks left after harvesting cereal grains contain cellulose that can be extracted in the form of fibers. Similarly, coproducts obtained during the processing of cereal grains for food or fuel contain proteins that can be used to develop regenerated protein fibers. These by-products and coproducts of agricultural processing are available in abundance, are annually renewable, and are inexpensive. However, such agricul­tural by-products and coproducts have been relatively unexplored for fibrous applications. Attempts have been made to study the potential of using these by-products and coproducts to develop fibers, but there are no reports on commer­cial production of fibers or fibrous products from agricultural residues.

Corn, wheat, rice, and sorghum are the most common staple foods that are extensively grown across the world. Cultivation of these cereal crops inevitably generates substantial amounts of lignocellulosic by-products such as leaves, stems, husks, cobs, and straw, roughly equivalent to the amounts of grains harvested. In many instances, such as cotton, the amounts of by-products generated are typically much higher than the weight of the grain or fibers produced. These by-products are mostly been burned or buried on the ground, but traditional uses include animal feed and bedding, as fuel by burning, and to some extent as agricultural mulches. Increasing costs of agriculture and demand for sustainable raw materials have directed attention to understand the potential of using the by-products and coproducts generated during agricultural production and processing for various applications.

Chitin, Chitosan, and Alginate Fibers

Keywords

Controlled release • Antibacterial agent • Tissue engineering • Tissue engineer­ing scaffold • Biodegradation • 3D scaffold • Porous structure • Artificial nerve graft • Sensor • Actuator

26.1 Chitosan Fibers for Controlled Release Applications

Chitosan fibers were used to load silver particles by adding sodium hydrogen zirconium phosphate into the spinning solution [07Qin]. The silver particles were reported to be uniformly divided in the fibers and did not affect the color of the fibers. SEM image in Fig. 26.1 shows the presence of the nanoparticles on the surface of the fibers. Silver ions were released when the fibers were placed in water or aqueous protein solution. Incorporation of silver significantly increased the antibacterial activity of the fibers with greater than 98 % reduction for common bacteria. The ability of chitosan fibers to absorb silver and zinc ions that are delivered through wound dressings was investigated [06Qin]. Chitosan fibers were treated with silver nitrate and zinc chloride solutions, and the release of these ions in saline was studied. It was found that the silver-containing fibers had good antimicrobial properties, whereas the zinc-containing fibers could be used to deliver zinc ions for wound care applications. Figure 26.2 shows that the chitosan — and silver-containing fibers had a clear Escherichia coli inhibition compared to the viscose fiber-containing solution suggesting that the chitosan fibers inhibited bac­terial growth.

A blend of chitosan and polyethylene glycol (PEG) fibers was prepared, and the ability of the fibers to load and release salicylic acid as a model drug was studied [08Wan]. Chitosan and PEG were dissolved in acetic acid, and the two solutions were mixed to obtain 2 %, 5 %, 8 %, and 10 % PEG, and the solution was extruded into an ethanol coagulation bath containing 10 % tripolyphosphate. Two levels of salicylic acid were added onto the fibers, and the ability of the fibers to load and

release the drug was determined. The addition of PEG destroyed the close packing of chitosan molecules leading to a decrease in % crystallinity. Tensile strength and elongation of the fibers increased up to the addition of 6 % PEG and decreased at higher levels of PEG. The highest strength obtained was 1.5 g/den, and the breaking elongation obtained was 21 %. The presence of PEG was found to assist release of drug since PEG could dissolve in the release medium, but the amount of release was highly dependent on pH. In a similar approach, chitosan fibers were blended with starch (10 %, 30 %, 50 %, and 70 %) and loaded with salicylic acid. Tensile strength and elongation increased to 1.4 g/den and 24 %, respectively. With the addition of 30 % starch but decreased with higher levels of starch. Up to 100 % drug release was obtained within 1-7 days depending upon the concentration of starch in the fibers with higher ratio of starch leading to faster release [07Wan].

Fragrant chitosan fibers were prepared by suspending chitosan fibers in various types of aldehydes. Fibers produced had fineness ranging from 24 to 33 den, tenacity from 2.0 to 2.6 g/den, and elongation of 9.6-18.8 % [99Hir]. It was reported that a portion of the aldehydes was released into the air, whereas no release was observed when the fibers were closed in a glass vessel. The fragrant fibers were expected to be useful for air filters, cosmetics, textiles, and other applications.

Regenerated Protein Fibers

Keywords

Egg white • Amyloid protein • Dissolution • Hydrochloric acid • Cross-linking • Gellan gum • Polyion complexation

Amyloid proteins (lysozymes) found in egg white were regenerated into macro — and nanofibers using a wet-spinning approach [03Tsu, 11Mei]. Lysozyme was dissolved in 10 mM HCl and allowed to form nanofibers with diameters of 2.6 ± 0.7 nm and lengths in excess of 10 pm. To form macrofibers, the nanofibers were cross-linked with anionic polyelectrolyte gellan gum through interfibrillar interactions. Figure 56.1 shows images of the nanofibers and macrofibers obtained, and Table 56.1 provides a comparison of the properties of the fibers obtained after cross-linking to various extents. Cross-linking and increasing the concentration of the protein solution improved tensile properties as seen in Table 56.1. The tensile strength of the lysozyme fibers is considerably higher than that of the regenerated fibers produced from plant proteins but lower than that of natural Bombyx mori silk. When used for controlled release applications, a pH-triggered release of riboflavin molecules was obtained with 75 % of loaded drug released within 10 min at pH 7 compared to less than 5 % of the drug released at pH 2 suggesting that the fibers could be used for controlled drug release.

Protein Monomers

Fig. 56.1 SEM images of the nanofibers and macrofibers obtained from egg lysozyme (from [11Mei]). Reproduced with permission from the American Chemical Society

References

[03Tsu] Tsukada, M., Arai, T., Colonna, G. M., Boschi, A., Freddi, G.: J. Appl. Polym. Sci. 89, 638 (2003)

Meier, C., Welland, M. E.: Biomacromolecules 12, 3453 (2011)

Biocomposites from Renewable Resources

Keywords

Calcium alginate • Silane treatment • Blend with polyvinyl alcohol • Blend • Polyvinyl alcohol • Weight loss

Calcium alginate has also been studied as potential reinforcement for polypropyl­ene composites [10Kha]. Calcium alginate fibers with a diameter of 30 ± 10 pm were silane treated and added (10 %) into PP that was pre-pressed into sheets. Layers of alginate fibers were placed between PP sheets, and the sandwich structure was compression molded into composites at 180 °C. Some of the properties of the PP composites containing alginate fibers with and without the silane treatment are given in Table 72.1. As seen from the table, the addition of the alginate fibers had increased the strength, more than doubled the modulus and decreased the elonga­tion by several magnitudes. Bending properties also showed similar increase with the addition of the alginates. Weight loss ranging from 0.5 to 2.2 % was observed when the samples were buried in soil from 2 to 16 weeks. In a similar approach, calcium alginate fibers (120 MPa strength, 4.3 GPa modulus, and 75 % elongation) were mixed with poly(vinyl alcohol) and compression molded into composites [11Dey]. Some of the properties of the alginate reinforced PVA composites are given in Table 72.2. As seen from the table, incorporation of the alginate fibers leads to increase in strength from 10 to 16 MPa and also increase in modulus, but the elongation decreases drastically. Bending properties of the composites doubled. Degradation tests showed that the composites had lost about 50 % of their strength after being buried in soil for 2 months [11Dey].

References

[10Kha] Khan, A., Huq, T., Saha, M., Khan, R. A., Khan, M. A., Gafur, M. A.: J. Compos. Mater. 44(24), 2875 (2010)

Dey, K., Khan, R. A., Chowdhury, A. M.S.: Polym. Plast. Eng. Technol. 50, 698 (2011)

Part X

Natural Cellulose Fibers from Renewable Resources

Keywords

Switchgrass • High-yield • Cellulosic ethanol • Stem fiber • Leaf fiber

Switchgrass is a high-yielding, low-input biomass crop that is considered to be the most suitable crop for cellulosic ethanol production. Although not a by-product, switchgrass can be a source for fibers requiring fewer inputs to grow and could be economically more viable than traditional fiber crops such as jute and flax. In addition, about 25-30 % of switchgrass can be obtained as long fibers for high — value applications, and the remaining 20-25 % of short fibers and hemicellulose could still be used for ethanol production. Switchgrass consists of outer leaves (45 % of total plant weight) and inner sheath (stem) (55 % of total plant weight). Both the leaves and stems were used for fiber production [07Red]. As seen in Table 11.1, fibers obtained from switchgrass have very unique and distinct properties not seen in any other fiber obtained from lignocellulosic by-products. Fibers obtained from the leaves had high strength but low elongation similar to that of linen, whereas fibers from the stems of switchgrass had lower strength but high elongation, similar to that of cotton. A single plant producing two types of fibers with such distinct characteristics is unique. The relatively low fineness of fibers obtained from switchgrass leaves implies that the fibers could be processed on textile machinery. Low costs to grow, high fiber yield (20-25 %), and distinct fiber properties make switchgrass a crop with high potential for fiber production.

Reference

[07Red] Reddy, N., Yang, Y.: Biotechnol. Bioeng. 97(5), 1021 (2007)

Natural Protein Fibers

Keywords

Amino acid • Primary structure • a-helix • p-sheet • Nanofibril

Structurally, five types (coiled coil, p-strand, cross p-sheet, collagen triple helix, and polyglycine) of silk have been identified that vary in the amount of crystalline and amorphous regions and the arrangement (sequence and orientation) of the amino acids along the axis of the fiber. Each type of structure has a specific sequence of amino acids. For instance, the coiled-coil structure has seven amino acid residues, and the protein chains form a right-handed a-helix with 3.2 amino acids per turn. In the coiled-coil silks produced by some insect species such as honeybees and weaver ants, each fibroin contains 210 amino acid residues in the coiled-coil region with alanine-rich cores [07Sut]. A structural model for the coiled — coil silk is shown in Fig. 35.1. Coiled-coil silks were also found to contain unusually high levels of alanine and large hydrophobic residues. The high levels of alanine were required to stabilize the helices and facilitate coiled-coil formation [07Sut]. In a p-strand structure, alternating amino acid side chains form opposite faces of the sheet and in a cross-p sheet, the protein chains form p-strands of uniform length and alternating turns at which the direction of the protein chain reverses. In a collagen triple helix, three 32 helices intertwine and form a superhe­lix, and finally in a polyglycine structure, the protein chains form a right-handed helix with three amino acids per turn. Figure 35.2 illustrates the five different types of structures discussed here.

Morphologically, silks are composed of nanofibrils, similar to those seen in cotton. Atomic force microscopic images have shown that Bombyx mori and wild silk such as Antheraea pernyi are composed of nanofibrils or bundles of nanofibrils that are arranged in helices and in different layers. Mean width of the fibrils was found to be 90-170 nm, and the average angle was between 30 and 50°. No correlation was found between the fibril size and fiber properties, but the cross­angle influenced size of the fibers with larger fibers having higher angle.

Figure 35.3a shows an AFM image of the nanofibrils along the fiber axis, and Fig. 35.3b shows the layered structure in an A. pernyi fiber [00Put]. Similar AFM studies for Samia cynthia ricini fibroin have showed that fibroin molecules self — assemble to form highly ordered rodlike structures (0.4 nm long) and that the rods attach end to end due to the electrostatic interactions between the hydrophilic amino acid residues [03Ino]. Such an assembly leads to the formation of a textile like nanofabric as seen in Fig. 35.4. The nanofabric was 2 qm in width and 10 qm in length, and the height of the warp and weft was 0.4 nm which corresponds to the height of a single fibroin molecule. The distance between the adjacent threads was about 50-80 nm which confirms that the fabric was formed by an end-to-end assembly of the fibroin molecules. Fibroin molecules were also found to assemble in a comblike structure as seen in Fig. 35.5. In the case of Samia cynthia ricini, about 10 % of amino acids were found to be charged residues, and these residues are located in the non-helical part allowing the molecules to form aggregates through electrostatic interactions leading to an ordered structure [03Ino].

The structure and properties of silks have been extensively studied, and consid­erable variations in the secondary structures have been reported. Table 35.1 lists the P-sheet contents in various silks determined using different techniques. As seen in the table, there is considerable variation in the p-sheet contents that have been reported. This variation could be due to the inherent differences in the silks that were studied, instrumental differences, and interpretation of results. Particularly unique in terms of protein structure is the wild silkworm A. pernyi that is similar to the spidroins in spider silks than the common B. mori silk [11Fu]. To better understand the mechanism of fiber formation and its influence and structure and properties, silk fibers were forcibly extruded from A. pernyi silkworms, and the

structure and properties of the drawn fibers were studied. This approach avoided the formation of secondary structure during regeneration and was therefore direct evidence to the relationship between structure and properties. Unlike B. mori silk but similar to that of silk produced by major ampullates, the artificially produced A. pernyi silk fibers showed an obvious yield point during tensile testing. Immedi­ately after forced reeling, fibers had strength similar to that of forcibly reeled spider silk but a breaking strain between 0.3 and 0.6, better than that of spider silks. However, the reeled fibers did not show major contraction in water that happens when spider silk is immersed in water. Tensile properties of the fibers reeled from the silk before and water contraction are shown in Table 35.2.

2000 nm

1000 nm

Fig. 35.3 AFM images demonstrating the fibrillar structure along the fiber axis (left) and the layered structure (right) in A. pernyi silk fibers [00Put]. Reproduced with permission from Elsevier

Fig. 35.4 Tapping-mode image of S. c. ricini wild silk fibroin on a mica surface. The sample solution was cast over a mica surface 24 h after filtering. Scale bar: 500 nm [03Ino]. Reproduced with permission from the American Chemical Society

References

[00Put] Putthanarat, S., Stribeck, N., Fossey, S. A., Eby, R. K., Adams, W. W.: Polymer 41, 7735 (2000)

[03Ino] Inoue, S., Tsuda, H., Tanaka, T., Kobayashi, M., Magoshi, Y., Magoshi, J.: Nano Lett. 3 (10), 1329 (2003)

[07Sut] Sutherland, T. D., Weisman, S., Trueman, H. E., Sriskantha, A., Trueman, J. W.H., Haritos, V. S.: Mol. Biol. Evol. 24(11), 2424 (2007)

[10Sut] Sutherland, T. D., Young, J. H., Weisman, S., Hayashi, C. Y., Merritt, D. J.: Annu. Rev. Entomol. 55, 171 (2010)

[11Fu] Fu, C., Porter, D., Chen, X., Vollrath, F., Shao, Z.: Adv. Funct. Mater. 21, 729 (2011) [11Lin] Ling, S., Qi, Z., Knight, D. P., Shao, Z., Chen, X.: Biomacromolecules 12, 3344 (2011)

Electrospun Fibers from Biopolymers

Keywords

Silk fibroin • Electrospinning • Solvent • Recombinant protein • Lithium thiocyanate

59.1 Electrospun Fibers from Silk Fibroin

Due to the distinct advantage of silk for various applications, considerable attempts have been made to reproduce silk in the laboratory with specific properties for targeted applications. For instance, to exploit the advantages of protein-based biomaterials and nanostructures for medical applications, silk fibroin was electrospun into fibers [10Zha]. To form the fibers, silk (Bombyx mori) was first degummed to remove sericin. Later, the silk fibers were dissolved in 9.3 M lithium bromide solution at 60 °C, and the dissolved solution was dialyzed against a 2,000 molecular weight membrane to obtain a 3-7.2 % protein solution [10Zha]. In addition, lyophilized silk fibroin was also dissolved using HFIP at room tempera­ture. Silk solutions were blended with polyethylene oxide (PEO) to improve spinnability and enable fiber formation. Fibers with relatively larger diameters, between 700 and 880 nm, were obtained. Electrospun mats obtained were treated with methanol to induce crystallization in silk and transform the silk into p-sheet configuration. Methanol treatment removed PEO and increased the surface rough­ness of the fibers [02Jin]. In another study, silk fibroin has been electrospun and the potential of using the silk nanofibers for various applications has been studied [08Kaw, 05Kim]. Silk nanofibers with diameters from 8 to 2,500 nm have been produced and used for tissue engineering [09Zha].

Fibroin obtained from B. mori and Samia cynthia ricini and a recombinant protein containing sequences from both the silks were electrospun into fibers. To produce the fibers, B. mori silk fibroin was dissolved in 9 M lithium bromide at 40 °C and made into films. The silk films and ricini silk fibers were dissolved using hexafluoroacetone (HFA) solution in 2-10 % concentrations and electrospun into

fibers with diameters ranging from 100 to 1,000 nm. Proteins in the fibers assumed the p-sheet configuration for B. mori silk but not for S. ricini silk. Electrospun mats produced from B. mori had strength of 15 MPa and elongation of 40 % compared to strength of 20 MPa and elongation of 20 % for the S. ricini silk. Fibers with average diameters of 100 nm were obtained from the recombinant proteins, but no tensile properties were reported [03Ohg].

The biocompatibility and possibility of using electrospun fibroin membranes for bone regeneration were studied [05Kim]. Silk fibroin was dissolved in CaCl2/ CH3CH2OH/H2O (1:2:8 molar ratio) for 6 h at 70 °C to obtain sponges and the sponges were later dissolved in acetic acid to form the electrospun fibers. Cells grown on the scaffolds had ALPase activity and calcification similar to cells cultured on petri dishes. When implanted into a rabbit, the scaffolds supported cell attachment and growth and showed complete bone regeneration in 12 weeks as seen in Fig. 59.1 [05Kim].

Using the same approach of dissolving fibroin, Zhang et al. obtained fibroin fibers with widths between 234 and 1,016 nm. Fibers with grooves that facilitated cell attachment, growth, and spreading were obtained by coagulating the fibers in methanol [14Zha]. Treating with methanol promoted crystallization through con­formational transition of random coils to p-sheet structure [08Ale]. Similar improvement in degree of crystallinity was also observed. Murine fibroblasts (L929) showed good attachment and growth on the scaffold. Blends of silk fibroin and hydroxybutyl chitosan were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and trifluoroacetic acid (TFA) and electrospun into fibers with average diameters of 215 ± 84 nm for the pure fibroin and with a diameter of 315 ± 150 nm for the 50/50 fibroin-chitosan blend fibers.

Silk fibroin with and without PEO was electrospun into relatively coarse fibers with average diameter of 700 nm for potential use as tissue engineering scaffold. Fibroin was first degummed, later dissolved in 9.3 m lithium bromide (LiBr), and used for electrospinning [04Jin]. Inclusion of PEO improved the strength of the matrices obtained but inhibited cell growth. After removing the PEO, the fiber matrices were conducive to cell growth and extensive growth and proliferation of bone marrow stromal cells could be seen [04Jin]. Instead of using fibroin from B. mori, He et al. developed electrospun fibers from the fibroin from Antheraea mylitta. Proteins were dissolved in lithium thiocyanate and extruded into fibers with an average diameter of 422 nm. Structural analysis showed that the as-spun fibers had a complete a-helix/random coil configuration with no p-sheet content [13He]. However, after insolubilization, fibers were found to have both the a-helix and p-sheet configurations [13He]. Since in vivo biodegradation of the fibroin matrices is important for tissue engineering, it has been demonstrated that controlling recrystallization during fiber formation can provide matrices with desired degradation rates [12Kim]. By varying the ratio of ethanol/propanol, matrices that degraded between 14 weeks to one year were obtained.