Regenerated Cellulose Fibers

Keywords

Biomass • Lignin • Cellulose • Cellulose dissolution • Fiber • Fiber properties

Conventional approach of producing regenerated cellulose through ionic liquids is based on using pulp (>90 % cellulose) obtained from various sources. Recently, attempts have been made to directly use the biomass containing cellulose, hemicel — lulose, and lignin and produce composite fibers. Such an approach would avoid the need to produce pulp and substantially reduce the cost of the fiber and the use of chemicals. Biomass from oak, bagasse, and pine was used with and without pretreatment to produce fibers using 1-ethyl-3-methylimidazolium acetate as the solvent. Type and composition of biomass, conditions used for pretreatment, and dissolution and composition and properties of the fibers obtained and compared in Table 20.1. Fiber production conditions varied depending on the type of pulp, and it was found that fibers containing higher amounts of cellulose had higher strength and elongation [11Sun]. Also, pretreatment resulted in higher cellulose content and therefore better properties. Bagasse could be dissolved and made into fibers using low — or high-temperature dissolution, whereas wood cellulose required the use of high temperatures but shorter time. The ability to directly dissolve lignocellulosic sources and produce fibers could lead to novel fibers and also significant cost reductions. However, the viability of commercial-scale production of this process and the properties of the fibers that can be obtained is not known. In addition, the presence of lignin and hemicellulose could affect further processing (dyeing, etc.) of the fibers, and the properties of the fibers could be adversely affected.

Ionic liquids have been studied as greener alternatives to produce viscose from biomasses [13Cas]. 1-Allyl-3-methylimidazolium chloride (Amimcl) was used to dissolve wood (4 % w/w) obtained from pine (Pinus radiata) and eucalyptus (E. globulus) in a microwave at 110 °C for 10 min and later at 120 °C for 20 min. The dissolved wood cellulose was added into dimethyl sulfoxide (11/3)

Table 20.1 Sources and dissolution conditions of biomass and composition and mechanical properties of the resulting fibers Biomass composition Tensile properties Fiber composition Biomass Cellulose [%] Lignin [%] Dissolution condition Stress [g/den] Strain [%] Cellulose [%] Lignin [%] Oak 79 23.8 175 °С, 30 min 0.8 4 — — Oak 79 23.8 175 °С, 30 min 0.3 2 — — Bagasse 57.8 22.7 185 °С, 10 min 1.0 6 63.6 25.3 Bagasse 57.8 22.7 110 °С, 16 h 0.7 8 57.3 22.3 Pulp DP 1056 100 0 90 °С, 30 min 1.7 10 — — Mixture 44.2 31.8 90 °С, 30 min 1.0 1 — — Pine 44.2 31.8 175 °С, 30 min 0.4 2 55.9 32.4 Pretreated pine 56.9 30.3 175 °С, 30 min 1.7 13 63.2 30.6 Reproduced from [11 Sun]

to help in the filtration of the dissolved substances. Later, the dissolved cellulose was added into dry methanol and stirred at 300 rpm and 40 °C for 10 min to regenerate the cellulose. About 16 % cellulose from pine and 11 % of cellulose were reported to be regenerated from pine and eucalyptus, respectively [13Cas].

References

[11Sun] Sun, N., Li, W., Stoner, B., Jiang, X., Lu, X., Rogers, R. D.: Green Chem. 13, 1159 (2011)

[13Cas] Casas, A., Alonso, M. V., Oliet, M., Santos, T. M., Rodriguez, F.: Carbohydr. Polym. 92, 1946 (2013)

Natural Protein Fibers

Keywords

Animal hair • Protein fiber • Diameter • Scale • Rabbit

Although most animals contain hairs on their skin, limited studies have been conducted on understanding the structure and properties of animal hairs except for wool from different types of sheep. Zhang et al. had studied the structural characteristics of rabbit hair and found that the hair fibers had average diameters which varied between 10 and 20 ^m and the fibers had scales similar to those seen on wool and the cross section of the fibers revealed a hollow center similar to that seen in feathers [11Zha]. Since there is limited literature available on animal hair fibers and an innumerable number of animals with hair exist, this topic has not been reviewed here.

Reference

[11Zha] Zhang, Y., Zheng, Q., Wang, X., Liu, H.: Adv. Mater. Res. 332-334, 1073 (2011)

Researchers have attempted to develop and promote colored cotton in the previous 15-20 years [01Pri]. Small-scale production of colored cellulose fibers in light tan, cinnamon, green, pink, black, and red has been done and attempts have been made

to process the colored cottons on textile machinery and develop textiles. In one study, colored cotton fibers (light and dark cinnamon, champagne, and green) were studied for their properties and the potential of converting the processed fibers into textiles was investigated on full-scale ring and rotor spinning machinery [01Pri]. Tables 62.1 and 62.2 provide selected properties of the naturally colored cottons used in the study. As seen in the tables, colored cotton contains consider­ably higher amounts of hemicelluloses and lignin and also waxes on the surface that imparts hydrophobicity to the fibers. The presence of wax and natural pigments provides the colored fibers resistance to growth against Aspergillus niger. However, the naturally colored cottons were found to severely degrade when buried in the soil along with substantial loss in fiber strength, but the rate of degradation was much lower than that of white cotton [10Che]. The fineness of the ring spun yarns produced was considerably low and the yarns showed considerable variations in properties as seen in Table 62.3. Similar results were also observed for the rotor spun yarns as seen from Table 62.4.

Morphologically, the white cottons and colored cottons had similar features as seen in Fig. 62.1. Microfibrils with lengths between 85 and 225 nm and diameters between 13 and 22 ^m are seen on all four fibers.

In another study, two naturally colored cottons (camel brown and olive green) were blended with conventional J-34 white cotton and spun into 8 s count ring spun yarns. Properties of the blended yarns were studied and the yarns were used as weft in various ratios and the fabrics obtained were scoured and bleached using various chemicals [02Par]. Tables 62.5 and 62.6 provide a comparison of the changes in K/S values for the cotton fabrics containing various extents of colored cotton fibers after scouring and exposure to light [02Par]. Substantial changes in the K/S values

are observed especially for blends containing higher amounts of colored cottons. Light fastness rating also showed that the blends containing higher amounts of colored cotton had lower color fastness, but the fastness improves after treatment with the various chemicals as seen in Table 62.7 [02Par].

Structural behavior and influence of different chemicals on the properties of two types (brown and green) of cotton were studied by Ishtiaque et al. [00Ish]. Increase in the hardness of water increased color intensity with K/S values increasing from 0 to 50 for the green cotton and from 50 to 80 for the brown cotton when the water

hardness was increased from 0 to 400 ppm. The increase in shade depth with increasing water hardness was considered to be due to the interaction of the cotton with metallic salts like calcium and magnesium present in the water. Similar effect was also seen when the pH of water (90 °C) was increased from 7 to 11. However, the effect of increase in shade depth was more pronounced for the brown cotton compared to the green cotton. Bleaching of the fibers with hydrogen peroxide resulted in near complete removal of color for both the cottons. To obtain cotton with other colors, the green and brown cotton fibers were treated with various mordants and the changes in K/S values were observed. Table 62.8 provides the K/S values for the brown and green cottons after treating with various mordants. It was

also found that the colored cottons had higher flame resistance and better thermal degradation [00Ish]. Other researchers have also reported that colored cottons have better thermal resistance than white cottons. Degradation of colored cottons was observed at about 390 °C compared to 370 °C for the white cottons [01Par] which was attributed to the higher amounts of metals. Similarly, the colored cottons had higher flame resistance as seen from the higher limiting oxygen index (LOI) values in Table 62.9.

In addition to the limited colors available, the low moisture absorption of colored cottons is a major limitation. Colored cottons have moisture regain of about 3.9 % compared to 8.6 % for regular white cotton. The presence of fat and pectin on the surface was considered to be the major reason for the low moisture absorption of colored cottons. Gu has reported that colored cottons have a fat, lignin, and pectin content of 4.3, 9.3, and 0.5 %, respectively, compared to 0.6, 0, and 1.2 % for regular cotton [05Gu]. To increase the moisture absorption of colored cotton, the fibers were treated with hot water and various concentrations of sodium hydroxide. Table 62.10 shows that the moisture regain of the fibers increases substantially after treating with sodium hydroxide but without affecting the tensile properties. In a similar study, the effect of scouring and enzyme treatment on the moisture regain of buffalo brown and coyote brown cottons was investigated [09Kan]. A general trend of higher moisture regain was observed after the treatment [09Kan]. Figure 62.2a, b shows the extent of increase in moisture regain after various treatments. As seen in the charts, lipase provided the lowest increase in moisture regain. Figure 62.2c summarizes the changes in moisture regain after the various treatments [09Kan]. Other studies have shown that the color of the fibers becomes darker and deeper after scouring [08Kan]. It was also observed that the fiber pigment moved toward the outer portion of the fiber from the center during alkali treatment. Some pigments were also released from the fibers into the scouring bath. SEM images showed that the fibers became round and circular and, longitudinally, the fibers become flat as opposed to their natural twisted conformation [09Kan].

Table 62.9 Comparison of the flammability of white and colored cottons before and after treating with ferrous sulfate, aluminum sulfate, and copper sulfate [01 Par, 06Par] Cotton Fabric weight (g/nr) Thickness (mm) LOI (%) Untreated Fe treated A1 treated Cu treated Warp Weft Warp Weft Warp Weft Warp Weft White 270.65 0.87 18.9 19.0 19.2 20.1 19.9 20.2 20.1 22.9 Brown 268.21 0.83 22.4 22.6 23.1 26.5 22.8 25.9 23.2 26.0 Green 280.10 0.86 22.5 25.4 21.2 23.6 20.8 23.2 21.1 23.3

Table 62.10 Increase in the moisture regain (average ± CV %) of colored cotton fibers after treating under various concentrations of alkali [05Gu] 0 % Alkali 3 % Alkali 5 % Alkali Property 40 °С, 30 min 60 °С, 45 min 60 °С, 80 min 60 °С, 30 min 80 °С, 45 min 40 °С, 60 min 80 °С, 30 min 40 °С, 45 min 60 °С, 60 min Moisture regain (%) 8.15 ±15 8.4 ±11 7.5 ± 16 8.6 ± 13 8.5 ± 12 8.7 ±14 8.6 ±11 8.8 ±11 8.7 ±10 Tensile strength (cN) 1.8 ±17 1.6 ± 12 1.9 ±15 1.8 ± 14 1.7 ±13 1.6± 11 1.8 ±16 1.8 ± 14 2.0 ±14

WTMM

A

enzyme treated

Scoured

Scoured

Scoured

(СаСОЗ) and single

Ire dted

ТгмДталІ oonson

Fig. 62.2 Changes in the moisture regain of the colored cottons after various treatments

Recent studies have also reported that colored cottons contain up to 2.5 times higher wax content than white cottons [10Pan]. In the case of brown cotton, it was reported that the greater the color of the fibers, the higher was the wax content. Consequently, the colored fibers had lower cellulose content, particularly, the green cellulose fibers. Fibers containing higher levels of cellulose were found to have better fiber length, fiber strength, fineness, lint index, boll weight, and other fiber properties. An acidophic layer was reported on the secondary cellular wall of the green fibers but not seen on the other fibers [10Pan]. Further investigations by staining with osmium tetroxide have revealed the presence of a series of concentric rings in the secondary cell wall that formed a lamella pattern characteristic of a substance called suberin [99Ric]. Suberin was suggested to form a network of polymer molecules with the assistance of glycerol and therefore the colored fibers had higher hydrophobicity. Colored cottons were found to have cellulose I structure and similar unit cell dimensions compared to white cotton. However, crystallite dimensions for the colored cottons were different and varied with the treatment for

cottons. As seen in Tables 62.11 and 62.12, the crystallites in green cotton become larger after extraction with ethanol. The degree of crystallinity also showed an increase. In another study on the microcrystalline size of naturally colored cottons, it was reported that the crystallite sizes based on 101 and 002 reflections of white and colored cotton were similar whereas the 10I crystallite was smaller [00Che]. The lattice parameters and crystallite sizes for the cottons studied are given in Tables 62.13 and 62.14 for comparison.

a, b, c are the dimensions of the unit cell in three dimensions. у is the interfacial angle, V is the volume of the unit cell, and p is the calculated density of the cellulose in the crystalline regions of the fiber.

Instead of using alkali, Demir et al. have used atmospheric plasma treatment to remove the wax on the surface and increase the hydrophylicity of the fibers [11Dem] and corresponding changes in the properties of the fibers were investigated. Unlike the alkali treatments where considerable changes in K/S values were observed, plasma treatment did not cause any change in the K/S values probably because the plasma could not penetrate inside the fiber and reach the pigment located in the middle and around the lumen of the fibers [11Dem]. It was suggested that plasma treatment would be an environmentally friendly approach to treat colored cottons and make them processable for textile applications.

An in-depth investigation was conducted to determine the possibility of devel­oping specialty textile products from colored cottons. Brown, green, and white cottons were characterized for their structure and properties and then made into needle punched fabrics [02Kim]. Table 62.15 provides a comparison of the properties of the cottons used in this study. The brown and white cotton were better thermal insulators than the green cotton and therefore the green cotton burned quicker than the other two colored cottons. The fibers could be processed on small-scale spinning equipment and made into yarns. Similarly, the fibers were also made into non-woven webs. It was suggested that blending the colored cottons with synthetic fibers such as lyocell was necessary to obtain products with good properties.

Naturally colored cottons were hydrolyzed using acid and the nanofibers obtained were studied as potential sources for developing various products. The naturally colored cottons were able to retain their color in a nanofiber suspension as shown in Fig. 62.3. It was suggested that the solutions from the colored cottons could be used to develop colored plastics without the need for additional dyes [10Tei].

62.3 Genetic Transformations of Colored Cotton

Studies have been done to genetically transform colored cotton and introduce the colored cotton into other plants. An Agrobacterium-mediated transformation of green-colored cotton was done to induce callus formation from hypocotyl explants on Murashige and Skoog medium containing 2,4-dichlorophenoxyacetic acid and kinetin. Among four different genotypes studied, embryogenic calli and plant regeneration was only observed in G9803 with 32 individual regenerants resistant to kanamycin being generated within 7 months. The transformation frequency was about 17.8 % and was confirmed using southern blot analysis and RT-PCR. Figure 62.4 shows the digital pictures of the generation of the transgenic plants [06Wei].

62.4 Limitations of Colored Cottons

In addition to the limited colors possible, there are several other restrictions of naturally colored cottons that have limited their commercial applications. Colored cottons have considerably lower yields than white cottons. In a study by Hua et al.,

Table 62.15 Properties of white and three-colored cotton used to develop non-woven fabrics determined using high volume instruments (HVI), advanced fiber information system (AFIS), and image analysis (IA) Mean length (in.) Fineness (mtex) Micronaire Maturity Fiber HVI AFIS AFIS FMT IA HVI FMT IA AFIS FMT IA White 0.94 0.98 164.7 183.6 166.8 4.1 4.08 3.78 0.46 0.47 0.36 Brown 14 0.83 0.91 166.6 179.2 149.9 4.2 4.20 3.52 0.47 0.49 0.40 Brown 15 0.60 0.75 163.7 152.1 170.9 3.0 3.17 2.76 0.43 0.42 0.27 Green 14 0.71 0.85 154.6 189.9 150.5 2.9 3.17 2.37 0.43 0.33 0.25

it was reported that brown cotton fiber and green cotton fiber had about 33.6 and

41.9 % lower yields than white cottons [09Hua]. About 17.4 and 11 % reduction in fiber lengths was also observed (Table 62.16). Vigorous vegetative growth was considered to be one of the major reason for the low yield and quality of the colored cottons. Table 62.17 provides a comparison of the properties of the white and colored cottons. As seen in the table, the colored cottons have lower boll numbers, boll mass, and considerably lower lint yield. Tensile properties of the fibers showed that the colored fibers had lower strength but higher elongation. Colored fibers also had substantially lower micronaire compared to the white cottons [09Hua].

References

[99Ric] Richards, A. F., Rowe, T., Stankovic, U., Elesini, U. S.: J. Text. Inst. 90(4), 493 (1999)

[00Che] Chen, H., Yokochi, A.: J. Appl. Polym. Sci. 76, 1466 (2000)

[00Ish] Ishtiaque, S. M., Parmar, M. S., Chakraborty, M.: Colourage 47(9), 18 (2000)

[01Par] Parmar, M. S., Chakraborty, M.: Text. Res. J. 71(12), 1099 (2001)

[01Pri] Price, J. B., Cui, X., Calamari, T. A., McDainel, R. G.: Text. Res. J. 71, 993 (2001) [02Kim] Kimmel, L.: AATCC Rev. 5, 25 (2002)

[02Par] Parmar, M. S., Sharma, R. P.: Ind. J. Fibre Text. Res. 27, 397 (2002)

[05Gu] Gu, H.: J. Text. Inst. 96(4), 247 (2005)

[06Par] Parmar, M. S., Giri, C. C., Singh, M., Chabbra, J.: Colourage 53(7), 57 (2006)

[06Wei] Weizhul, S., Gao, P., Sun, J., Wang, H., Luo, X., Jiao, M., Wang, Z., Xia, G.: In Vitro Cell Dev. Biol. Plant 12, 439 (2006)

[08Kan] Kang, S. Y.: AATCC Rev. 8(7), 38 (2008)

[09Hua] Hua, S., Yuan, S., Shamsi, I. H., Zhao, X., Zhang, X., Liu, Y., Wen, G., Wang, X., Zhang, H. A.: Crop Sci. 49, 983 (2009)

[09Kan] Kang, S. Y., Epps, H. H.: J. Text. Inst. 100(7), 598 (2009)

[10Che] Chen, H., Cluver, K.: Text. Res. J. 80(20), 2188 (2010)

[10Pan] Pan, Z., Sun, D., Sun, J., Zhou, Z., Jia, Y., Pang, B., Ma, Z., Du, X.: Europhytica 173,

141 (2010)

[10Tei] Teixeira, E. M., Correa, A. C., Manzoli, A., Leite, F. L., Oliveria, C. R., Mattoso, L. H. C.: Cellulose 17, 595 (2010)

[11Dem] Demir, A., Ozdogan, E., Ozdil, N., Gurel, A.: J. Appl. Polym. Sci. 119, 1410 (2011)

Natural Cellulose Fibers from Renewable Resources

Keywords

Corn • Husk • Stover • Natural cellulose fiber • Fiber extraction • Lignocel — lulosics • Fiber properties

Corn or maize is the second largest agricultural crop grown in the world, second only to sugarcane with 875 million tons produced in the world in 2012. Cultivation of corn generates stover (stalk, leaves, and husk) as by-product that has been considered for a variety of uses. In developed countries such as the United States, the recent efforts on producing cellulosic biofuels from biomass have led to the use of corn stover as feedstock for cellulosic ethanol. However, substantial quantities of corn stover are still left unused and are available for industrial use at low cost. Currently, a ton of corn stover baled and ready to be shipped is estimated to cost about $40-$50, making stover one of the cheapest lignocellulosic sources. Corn stover typically consists of about 50 % stalk, 23 % leaves, 15 % cobs, and 14 % husk. The stalks consist of an inner pith and outer rind which is the source for fibers. Cornhusks (ears, shucks) are fibrous structures that can be up to 20 cm in length and have been traditionally used for decoration, food wrapping, and other applications.

Due to the large availability and low cost, the potential of obtaining fibers from cornhusks and cornstalks had been explored. Fibers have been produced from cornhusks and cornstalks for textile and composite applications. To extract fibers from husks or stalks, the raw materials are treated in alkali solutions at high temperatures (85-90 °C) for a desired time [05Red1, 05Red2, 05Red3]. Stalks require more severe chemical and/or physical treatment conditions and produce relatively inferior quality of fibers compared to the fibers obtained from husks. After treatment, the fibers are washed to remove the dissolved substances and short fibers. An additional enzyme treatment may be done to remove hemicellulose and lignin and obtain finer fibers. Typical yield of fibers from husks or stalks varies from 10 to 30 % depending on the severity of the treatment and quality of fibers desired. The long length of cornhusks provides a unique opportunity to obtain fibers with

Table 2.1 Properties of fibers obtained from comhusks and cornstalks

lengths suitable for processing on both the short and long staple spinning systems and the ability to blend cornhusk fibers with cotton, linen, wool, or other fibers. As seen in Table 2.1, cornhusk fibers with lengths of up to 20 cm were obtained [05Red3]. Longer cornhusk fibers had lower strength but higher elongations, and fibers obtained from cornstalks had similar strength but substantially lower elongation than cornhusk fibers. Interestingly, the cornhusk fibers have high elongations similar to the fibers obtained from coconut and Borassus husks, whereas the cornstalk fibers have elongations typical to bast fibers (1-3 %). These differences in elongation are mainly due to the amount of cellulose and arrangement of cellulose to the fiber axis. Rather than using husks from dried stover, green husks were collected and used for fiber extraction at various treatment conditions. Considerable variations in fiber composition and properties were observed with stronger conditions providing fibers with higher cellulose content and strength [13Yil].

Cornhusk fibers were also bleached and dyed and processed on spinning machines to produce yarns. Bleached cornhusk fibers had a CIE whiteness index (WI) of 74 compared to a CIE WI of 80 for cotton [07Sal]. Digital images of cornhusk fibers before and after bleaching are shown in Fig. 2.1. Similarly, cornhusk fibers were found to have higher dye pickup than cotton fibers under similar dyeing conditions [11Red]. Fibers obtained from cornhusks were blended

Fig. 2.2 Sweater developed from a 50/50 blend of cornhusk fibers and cotton

with cotton and polyester and processed on both the ring and rotor spinning machineries [06Red]. It was found that blending cornhusks with cotton provided yarns with good strength and elongation retention as seen in Table 2.2. Cornhusk fiber-blended polyester yarns had higher strength and elongation retention than corresponding polyester/cotton blends of the same count and proportion [06Red]. The cotton/cornhusk-blended (65/35) ring-spun yarns were knitted into a garment (Fig. 2.2) and dyed with reactive dyes.

References

[05Red1] Reddy, N., Yang, Y.: Green Chem. 7(4), 190 (2005)

[05Red2] Reddy, N., Yang, Y.: AATCC Rev. 5(7), 24 (2005)

[05Red3] Reddy, N., Yang, Y: Polymer. 46, 5494 (2005)

[06Red] Reddy, N., Yang, Y., McAlister III, D. D.: Indian J. Fibre Text. Res. 31(4), 537 (2006) [07Sal] Salam, A., Reddy, N., Yang, Y.: Ind. Eng. Chem. Res. 46, 1452 (2007)

[11Red] Reddy, N., Thillainayagam, V. A., Yang, Y.: Ind. Eng. Chem. Res. 50, 5642 (2011) [13Yil] Yilmaz, N. D.: Indian J. Fibre Text. Res. 38, 29 (2013)

Chitosan fibers with improved biological and mechanical properties and intended for tissue engineering applications were prepared by varying the concentrations of acetic acid, ammonia, and the cross-linking agent heparin and by varying the annealing temperature [13Alb]. Optimizing the concentration of acetic acid to 2 % improved fiber strength and stiffness by twofold, and using 25 % ammonia solution during coagulation decreased fiber diameters and increased strength by 200 %. Increase in the strength was attributed to increase in % crystallinity. Annealing also increased strength but resulted in discoloration of the fibers as seen from Fig. 26.3. Cross-linking with heparin decreased fiber strength. Fibers with very low tensile strength of about 0.05 g/den (5 MPa) and breaking strain of 0.2 % were obtained using the optimized fiber production conditions. The physical and chemical treatments did not affect biocompatibility, whereas cross-linking with heparin improved the attachment and proliferation of porcine valvular interstitial cells.

Chitosan fiber scaffolds intended for tissue engineering were coated with colla­gen type I and used to study biocompatibility using human bone marrow stromal cells (hBMSCs) and with murine osteoblast cells [08Hei, 09Hei]. Good adhesion, proliferation, and osteogenic differentiation were observed, and the fibers were considered as excellent scaffolds for hBMSCs. As seen in Fig. 26.4, chitosan fibers coated with collagen (b) showed extensive growth of actin compared to the uncoated fibers (a) after 14 days of culture. The collagen coating is seen in red on the right panel. Similar results were obtained for chitosan fibers cultured with

osteoblasts, but collagen coating did not improve the viability or proliferation [08Hei].

Chitosan fibers (167 den; tenacity 2.1 g/den and elongation of 10.3 %) were N-acetylated, and the influence of acetylation on the in vitro and in vivo biodegra­dation of the fibers was studied [07Yan]. Acetylation was done by immersing the fibers in acetic anhydride at 25 °C. The degree of acetylation was controlled to

7.7 %, 21.6 %, 40.9 %, 61.2 %, 82.5 %, and 93.4 % by varying the reaction time from 10 to 120 min. Degradation of the fibers was studied in pH 7.4 buffer containing 4 mg/ml lysozyme, and it was found that chitosan fibers did not biodegrade, whereas acetylated fibers degraded to different extents depending on the degree of acetylation. Up to 100 % biodegradation was obtained for the fibers acetylated to 93.4 % after immersion in PBS for 9 days. Fibers acetylated to three levels were embedded into mice for 6 months. After 6 months, the mice were

sacrificed, and it was found that the fibers with higher degree of acetylation (93 %) had degraded in the body. As seen in Fig. 26.5, fibers with lower degree of acetylation (7.7 %) were dense and had degraded to a lower extent than the fibers with 93 % acetylation that had completely assimilated in the surrounding tissue.

Obtaining scaffolds with porosity and mechanical properties required for bone tissue engineering has been a challenge [09Lia]. A rapid prototyping and rapid tool technique were used to reinforce calcium phosphate cement composites with chitosan fibers, and the biocompatibility of the scaffold was studied. It was found that attachment and proliferation of mesenchymal stem cells were higher on the scaffolds containing chitosan fibers, particularly at the interface of the chitosan fibers and calcium phosphate cement. As seen in Fig. 26.6, scaffolds containing chitosan fibers showed extensive spreading of the actin filaments indicating better compatibility compared to the fibers without the chitosan fibers.

Porous heart valve scaffolds made from chitosan were reinforced with chitosan fibers (2-10 %) and found to improve the mechanical properties of the scaffolds. Table 26.1 provides a comparison of the properties of the chitosan scaffolds with and without the fibers as reinforcement. To further improve the properties of the scaffolds, cross-linking with heparin was done, but no significant increase in fiber strength was seen but the strength of the scaffold was higher [12Alb]. Properties of

the chitosan and chitosan fiber-reinforced heart valve scaffold are compared with that of human pulmonary and aorta valves in Table 26.2.

In another study, heart valves with desired properties could be obtained by controlling the amount, length, and tensile properties of the fibers. Core-shell fibers with chitosan as core and calcium phosphate as shell and intended for bone tissue engineering with different properties were developed by varying the coagulation bath conditions. Analysis of the fibers showed that the Ca and P atoms were distributed mostly on the surface of the fibers. Tensile properties of the fibers were found to increase with increasing concentration of chitosan. No porosity or cell culture studies were done [04Mat].

3D fibrous chitosan scaffolds containing poly(lactic acid-co-glycolic acid) nanocapsules loaded with the bone morphogenetic growth factor BMP-2 and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanoparticles loaded with BMP-7 were used to culture rat bone marrow mesenchymal stem cells (MSCs) for bone tissue engineering [09Yil]. Fibers were prepared using two concentrations (4 and 6 %) of chitosan, and chitosan was also blended with polyethylene oxide (PEO). The addition of PEO resulted in a rough fiber surface but improved stability of the fibers. MSCs showed higher initial cell proliferation and increased ALP activity for the chitosan fibers containing PEO fibers. Figure 26.7 shows SEM images of the MSCs growing on the chitosan scaffolds (top) and chitosan-PEO scaffolds (bottom) after 21 days of incubation clearly demonstrating that the inclusion of PEO promoted cell growth. In addition to adding PEO, growth factor containing nanocapsules made from PLGA and PHBV were inserted onto the fibers via mixing with the spinning solution or seeded onto the fibers after fiber produc­tion. Higher release rate was obtained by loading the nanocapsules on the surface after fiber formation. PLGA nanocapsules were found to have better release com­pared to PHBV nanocapsules after 21 days of testing.

3D fibrous mesh scaffolds were also prepared from chitosan for tissue engineer­ing applications, and the biocompatibility and cytotoxicity were evaluated using mouse fibroblasts and human osteoblasts [04Tuz]. Fibers obtained had strength of about 205 MPa and elongation of 8.5 % and swelled rapidly to about 110 % within 30 min. Immersion of the fibers in simulated body fluid for 30 days led to the formation of films composed of calcium phosphate as shown in Fig. 26.8 indicating the bioactivity of the fibers. 3D fiber meshes had swelling of up to 160 % within 50 min of immersion in NaCl solution but preserved their integrity. Extensive attachment and growth of cells and formation of bridges between cells were observed suggesting that the fibers were not cytotoxic.

Table 26.2 Comparison of the tensile properties of chitosan-based scaffolds with the human pulmonary and aorta valves (reproduced from Albanna et al. [12Alb]) Pulmonary valve Aorta valve Property Chitosan Reinforced Radial Circumferential Radial Circumferential Tensile strength (kPa) 58 ±28 220 ±17 290 ±60 2,780 ±1,050 320 ±40 1,740 ±290 Strain {%) 90 ±30 55 ±10 30 ±4 19.4 ±3.91 24 ±4 18.4 ±7.6 Modulus of elasticity (kPa) 70 ±10 400 ±140 1,320 ±930 16,050 ± 2,020 1,950 ±150 15,340 ±3,480

Chitosan fibers were proposed to serve as bioartificial nerve grafts to treat peripheral nerve injuries and to evaluate this possibility, Schwann cells were cultured on chitosan fibers and compared to chitosan films for cell attachment, growth, and proliferation. Schwann cells were found to have a spherical and long shape, grew contact extensions, and migrated faster on the fibers compared to films [04Yua]. After 14 days of culture, cells formed chains on the chitosan fibers as seen in Fig. 26.9.

Collagen fibers were coated with hyaluronic acid (HA) and used for ligament tissue engineering. It was reported that incorporating HA increased mechanical properties of the fibers and also promoted the attachment and proliferation of fibroblasts [05Fun].

Alcogels were developed by coagulating chitosan fibers in hydroalcoholic solutions than the conventional approach of using alkali baths or ammonia vapors. Chitosan was dissolved using acetic acid into which an alcohol such as 1,2, propanediol was added, and fibers shown in Fig. 26.10 were extruded by hot-air drying. It was proposed that the alcohol system increased the entanglement density and chitosan chain interactions that led to improved mechanical properties [13Des]. Fibers produced using the hydroalcohol approach had a large proportion of anhydrous crystals. However, information on the properties of the hydroalcohol fibers is not available.

Electrospun Fibers from Biopolymers

Keywords

Electrospinning • Microfiber • Nanofiber • High surface area • Porosity • Medical application • Biotechnology • Energy

Electrospinning is a process where polymeric solutions are extruded through a charged electrical field consisting of + vely and — vely charged source/collector. Fibers in the nano — to micrometer scale are produced by controlling the distance between needle and collector and voltage and other parameters during electrospinning. Electrospun fibers are preferred for a variety of applications due to their high surface area, ability to develop fibrous matrices with desired porosity and pore size, and comparatively easy biodegradability. Due to these advantages, electrospun fibers have been considered suitable as tissue engineering scaffolds and other medical applications, reinforcement for composites, filters for biotechnologi­cal applications, protective clothing and smart textiles, and in energy and electronic applications such as batteries/cell and capacitors, sensors, and catalysts [14Bra]. Due to their wide acceptability and unique properties, attempts have been made to develop electrospun fibers from almost every possible raw material. Reports are available on producing electrospun fibers from polysaccharides such as cellulose and chitosan, proteins such as silk fibroin and gelatin, synthetic polymers such as polypropylene and poly(lactic acid), and even from metals such as TiO2 [08For]. This part provides an overview of the biopolymers including polysaccharides, proteins, and synthetic polymers that have been used to develop electrospun fibers. Since there is a massive amount of literature in developing electrospun fibers, especially from synthetic polymers, our focus in this part is to only cover electrospun fibers produced from polysaccharides and proteins and synthetic biopolymers such as poly(lactic acid) and poly(ethylene glycol) that are derived from renewable resources.

References

[08For] Formo, E., Lee, E., Campbell, D., Xia, Y.: Nano Lett. 8(2), 668 (2008)

[14Bra] Braghirolli, D., Steffens, D., Pranke, P.: Drug Discov. Today 19(6), 743 (2014)

Keywords

Biofiber • Renewable resource • Supercapacitor • Membrane filtration

In addition to the textile, medical, and composite and other applications discussed in the previous parts, researchers have also attempted to use biofibers for some unique and novel end uses. This part provides an overview of the use of biofibers in some unconventional applications.

Natural Cellulose Fibers from Renewable Resources

Keywords

Cannabis • Hemp fiber • Outer bark • Reinforcement

Belonging to the same family (Cannabaceae) and genus Cannabis as hemp, hop (Humulus lupulus L.) is a plant grown for its flower, an ingredient used in most beer. After harvesting the flower, the hop plants are cut and considered as waste. Hop stems contain an outer bark and inner pith, typical of any bast fiber plant. The fibrous outer bark has been used to produce long-length fibers (10-15 cm) with tensile properties comparable to that of hemp [09Red]. Hop stem fibers also had cellulose crystal structure similar to that of hemp as seen in Table 12.1. Untreated hop stems and fibers obtained from hop stems have been used as reinforcement for composites [10Zou].

Table 12.1 Properties of fibers obtained from hop stems [09Red]

References

[09Red] Reddy, N., Yang, Y.: Carbohydr. Polym. 77(4), 898 (2009)

[10Zou] Zou, Y., Reddy, N., Yang, Y.: J. Appl. Polym. Sci. 116, 2366 (2010)

Natural Protein Fibers

Keywords

Wild silk • Life cycle • Degumming • Calcium oxalate • Saturniidae • Biospinning • Tissue engineering • Matrix • Biocompatibility

Wild or non-mulberry silks are produced from various species of insects. Most popular non-mulberry silks that are commercially available are tasar (Antheraea mylitta), eri (Samia cynthia ricini), and muga (Antheraea assamensis). Properties of these three common types of wild silks are compared to Bombyx mori silk in Table 36.1. A typical life cycle of a wild silkworm (Antheraea mylitta) is shown in Fig. 36.1 [12Kun]. During production of the wild silk fibers, in addition to the cocoons, some sericin proteins are extruded external to the cocoons and are called peduncles. These peduncles (Fig. 36.2) act as reservoir for sericin and are seen only in the non-mulberry silks. Silk produced in these peduncles was found to be similar to the sericin in the cocoons [06Das] with proteins having molecular weight of 200 kDa and mainly composed of glycine and serine with 36.7 % p-sheets, 52.7 % random coils, and 10.6 % turns with no helices. Other researchers have suggested that Antheraea mylitta contains polyalanine repeat sequences, and fibroin extracted from the silk gland of Antheraea mylitta had a molecular mass of 395 kDa with monomers of approximately 197 kDa [09Ach]. To determine the structural differences using nuclear magnetic resonance (NMR), 13C and 15N labeling, select amino acids were orally fed to the fifth instar larvae. Silk obtained contained 75 % alanine and 65 % glycine residues, the alanine content being much greater than that found in B. mori silk [99Asa, 04Asa]. In the solid state, the glycine-rich regions stretched up to 10 times indicating that p-sheets were predominant.

The effect of organic solvents on the tensile properties of commonly known wild silkworm Antheraea assamensis was studied by Talukdar et al. [11Tal]. It was found that methanol — and phenol-treated fibers had higher strength than those treated with DMSO, formaldehyde, toluene, benzene, and DMF. Elongation of the fibers decreased by about 15 % after treating with benzene, whereas a 20 %

non-mulberry silkworm.

Antheraea mylitta

Pupae taken out

I rum cocoons

Fig. 36.1 Typical life cycle/different stages of non-mulberry Indian tropical tasar silkworm, Antheraea mylitta, as an example. Reproduced from Kundu et al. [12Kun] with permission from Wiley

Fig. 36.2 Image of an A. mylitta cocoon with the peduncle that contains silk similar to that of sericin in the cocoons

increase in modulus was observed after treating with many solvents. Similar to Antheraea mylitta silk, considerable differences in structure and properties of Samia cynthia ricini silk have also been reported [99Asa].

The structure and physical properties of cocoons produced by 25 different types of silkworms were analyzed by Chen et al. Images of some of the cocoons studied and their morphological features are shown in Fig. 36.3. As seen in the figure, the structure, shape, and size of the cocoons and the surface of the fibers differed considerably between cocoons [12Che]. Although no correlation was found between cocoon structure and fiber properties, four different types of cocoons were classified based on their structures: weak cocoons with maximum stress

і e

VVv

Sotumio txivomo

Fig. 36.3 Images of the cocoons and their surfaces produced from different insects from Chen et al. [12Che]

experienced at elongation of 15-20 %, lattice cocoons that have a loose structure, brittle cocoons that have maximum strain at about 15-25 %, and tough cocoons that have multiple yield points and show failure after extending to 40-60 % [12Che]. Tensile strength of the fibers in these cocoons ranged from 2.3 to 5.2 g/den, and elongation ranged from 7 to 30 %. In a similar study, the microstructure of African wild silk cocoon shells and fibers was studied by Teshome et. al. [12Tes]. Cocoons and fibers from the different silkworms revealed unique and distinct features. For instance, hairlike protrusions (Fig. 36.4) were found on Anaphe panda cocoons, and spines were seen on Gonometa postica cocoons. A common feature with several wild silk cocoons was the presence of calcium oxalate crystals that requires the use of harsh chemicals and severe conditions for degumming. The weight of the cocoons ranged from 63 to 140 mg with the heaviest cocoons being produced by G. postica and A. panda.

Cocoons produced by the insect (Thyridopteryx ephemeraeformis) commonly called as bagworm consist of ultrafine fibers that are tightly constructed in the form of a bag [10Red1]. Figure 36.5 (left) shows an image of the bagworm cocoon with the plant material attached. When the plant material is removed, the outer and inner layers (Fig. 36.5, left and right, respectively) of the cocoons resemble that of a woven bag. Degumming of the cocoons results in the formation of fibers with average diameter of 2.9 qm and fineness of 0.9 den [10Red1]. Figure 36.6 shows an SEM image of the fibers obtained from bagworm cocoons. Tensile properties of the fibers obtained from the bagworm cocoons are compared with other wild silks and with B. mori silk in Table 36.2. As seen in the table, bagworm fibers had similar tenacity but lower elongation as the common wild silkworms Antheraea mylitta and P. ricini [10Red1]. Unlike the B. mori and common wild silks, bagworm silk had considerably low levels (1.4 %) of glycine and alanine (5.2 %) but considerably high levels of leucine, glutamic acid, and lysine.

Fibers were also obtained from the cocoons of the wild silkworm Actias lunas that produced cocoons with an average weight of 200 mg. Unlike B. mori silks, about 66 % of amino acids of Actias lunas cocoons were composed of alanine,

glycine, serine, and tyrosine compared to 90 % in B. mori silks [12Red1]. Tensile properties of the fibers were similar to that of B. mori with breaking tenacity of 4.3 g/den and elongation of 11 % as seen in Table 36.2.

Unlike most other silks, insects belonging to the Argema family produce unique cocoons that have perforations and a shiny appearance as seen in Fig. 36.7. It has been reported that the perforations are present to drain the water that accumulates in the cocoons in the tropical environment of the insects. Fibers were obtained from Argema mimosae and Argema mittrei that had properties considerably different than the common silks. Morphologically, fibers extracted from these cocoons had a rectangular cross section (Fig. 36.8) and were flat and ribbonlike compared to the classic triangular cross section of mulberry silks. Argema mittrei had considerably low strength of 1.5 g/den and elongation of 11 % [12Red2]. Argema mimosae cocoons had average weight of about 1 g and produced fibers with an average strength of 2 g/den and elongation of 13 %, similar to that of A. mittrei but

considerably lower than that of the common silks. Another distinguishing feature of the A. mittrei and also A. mimosae silk fibers is their considerably larger diameter compared to other common silks as seen in Table 36.3.

It is perceivable that considerable variations occur in the properties of silk fibers obtained from different insect species. However, insects belonging to the same species also produce fibers with highly distinct and unique properties. Reddy et al. have studied the properties of silk fibers produced from various uncommon Saturniidae and found that the fibers from different insects belonging to the same family had considerably different properties [10Red3]. As seen in Table 36.4, insects from the same species produced fibers with tenacity ranging from 0.9 to 5 g/den and elongation ranging from 4 to 20 %. Differences in the fiber properties were attributed to the variations in the type and amount of amino acids, physical structure, and environmental habitats of the insects [10Red3].

As with the variations of fiber properties between different species and within the same species, fibers in different layers of a single cocoon could also exhibit varying properties. However, the outer, middle, and inner layers of cecropia cocoons (Fig. 36.9) were found to have similar composition and tensile properties [10Red2]. As seen in Table 36.5, the tensile properties of the fibers were better than those of the wild silk fibers and similar to that of B. mori silk. It was suggested that

cecropia were easier to rear than the B. mori silks due to fewer diet restrictions, produced larger cocoons, and could therefore be a better alternative to the tradi­tional silk.

An Australian web spinner (Aposthonia gurneyi) is considered to produce the finest known silk fibers with an average diameter of 65 nm [08Oka] as seen in Fig. 36.10. The silk was predominantly composed of p-sheet structure with exten­sive glycine-serine repeat units (GSGSGS) similar to the GAGAGS repeats found in silkworm fibroin. However, the tensile properties and other structure of the fibers were not reported.

References

[99Asa] Asakura, T., Ito, T., Okudaira, M., Kameda, T.: Macromolecules 32, 4940 (1999) [00Raj] Rajkhowa, R., Gupta, V. B., Kothari, V. K.: J. Appl. Polym. Sci. 77, 2418 (2000)

[04Asa] Asakura, T., Tanaka, C., Yang, M., Yao, J., Kurokawa, M.: Biomaterials 25,

617 (2004)

[06Das] Dash, R., Mukherjee, S., Kundu, S. C.: Int. J. Biol. Macromol. 38, 255 (2006) [08Oka] Okada, S., Weisman, S., Trueman, H. E., Mudie, S. T., Haritos, V. S., Sutherland, T. D.: Int. J. Biol. Macromol. 43, 271 (2008)

[09Ach] Acharya, C., Ghosh, S. K., Kundu, S. C.: Acta Biomater. 09, 429 (2009)

[10Red1] Reddy, N., Yang, Y.: J. Mater. Sci. 45, 6617 (2010)

[10Red2] Reddy, N., Yang, Y.: J. Mater. Sci. 45, 4414 (2010)

[10Red3] Reddy, N., Yang, Y.: Int. J. Biol. Macromol. 46(4), 419 (2010)

[11Tal] Talukdar, B., Saikia, M., Handique, P. J., Devi, D.: Int. J. Pure Appl. Sci. Tech. 7(1), 81 (2011)

[12Che] Chen, F., Porter, D., Vollrath, F.: J. R. Soc. Interface 9, 2299 (2012)

[12Kun] Kundu, S. C., Kundu, B., Talukdar, S., Bano, S., Nayak, S., Kundu, J., Mandal, B. B.,

Bhardwaj, N., Botlagunta, M., Dash, B. C., Acharya, C., Ghosh, A. K.: Biopolymers 97, 455 (2012)

[12Red1] Reddy, N., Yang, Y.: J. Polym. Environ. 20(3), 659 (2012)

[12Red2] Reddy, N., Jiang, Q., Yang, Y.: J. Biobased Mater. Bioenergy. Res. 6(5), 558 (2012)

[12Tes] Teshome, A., Vollrath, F., Raina, S. K., Kabaru, J. M., Onyari, J.: Int. J. Biol.

Macromol. 50, 63 (2012)

[13Red1] Reddy, N., Zhao, Y., Yang, Y.: J. Polym. Environ. 21(1), 16-23 (2013)

[13Red2] Reddy, N., Jiang, Q., Yang, Y.: J. Biomater. Sci. Polym. Ed. 24(4), 460 (2013)

[13Red3] Reddy, N., Jiang, Q., Yang, Y.: J. Biomater. Sci. Polym. Ed. 24(7), 820 (2013)

Proteins in the innermost portion of egg shell were dissolved using acetic acid and 3-mercaptopropionic acid or in 0.2 % aqueous NaOH and the solution was electrospun into fibers. Due to the low viscosity of the egg proteins, PEO was added in ratios of 3.5-5 wt % to improve spinnability. Fibers obtained were treated with methanol or 1,3-dicyclohexylcarbodiimide (DCC) to improve stability in water [04Yi]. Electrospinning and amount of PEO substantially affected fiber diameters that ranged between 300 nm and 20 pm. As-spun fibers immediately disappeared when immersed in water, but treating the fibers with methanol or DCC provided water stable fibers.