Regenerated Cellulose Fibers

Keywords

Cellulose source • Wheat straw • Rice straw • bacterial cellulose • Acid tunic • Bamboo • Rayon

Apart from using conventional sources of cellulose such as wood, nonconventional resources such as sugarcane bagasse, bamboo, and bacteria have also been used to produce regenerated cellulose fibers. Viscose fibers with a linear density of 17 tex and length between 1.7 and 38 mm obtained from eucalyptus wood were modified with alkali solutions to improve the adhesive and surface properties [13Roj]. After treating with various concentrations of alkali, about 10-12 % decrease in % crystallinity was observed. Increase in diameter of fibers and decrease in contact angle after alkali treatment were also seen [13Roj].

Fabrics made from bamboo viscose were characterized for their structure and properties compared to traditional viscose rayon and cotton. Table 21.1 shows some of the comparative properties of the three types of fibers [12Mis]. Bamboo viscose had very similar properties compared to traditional viscose rayon. Compared to natural cellulose, viscose fibers and fabrics had considerably higher elongation. In addition, bamboo viscose is reported to have natural antibacterial property. Con­siderably lower numbers of bacterial colonies were observed on bamboo viscose compared to cotton and viscose as seen from Table 21.1.

Bacterial cellulose with a considerably high DP of 2700 was dissolved using NMMO and extruded into fibers on a laboratory scale. Fibers obtained had rela­tively low tenacity of 0.06-0.2 g/den and elongation of 3-8 % probably due to the poor drawing and orientation of the fibers during this process [11Gao]. Similar to using bacteria, bagasse obtained as the by-product after processing sugarcane was made into pulp and regenerated into fibers using NMMO/water as the solvent system. Pulp obtained was dissolved in NMMO/water at 120 °C for 2.5 h, and the cellulose solution was dry jet-wet spun into coagulation bath containing water, methanol, isopropanol, ethanol, or water/NMMO mixture (10 %) at 20 °C

[10Jal]. Fibers were hot drawn and also treated in an oven to orient the fibers. Some of the physical and tensile properties of the fibers obtained using various coagula­tion baths are reported in Table 21.2. Considerable variations in properties, espe­cially elongation, were seen with changing draw ratio and coagulation baths. Removal of NMMO from the fibers resulted in higher crystallinity and better orientation leading to higher strength and modulus. Stress-strain curves of fibers obtained from the different coagulation baths are shown in Fig. 21.1.

Wheat straw was steam exploded and grafted with various monomers to facili­tate dissolution and regeneration into fibers. Straw was steam exploded at 225 °C for 3 min and later treated with 2 % soda solution at 80 °C for 2 h and then bleached with 0.3 % sodium chlorite at 70 °C for 2 h. Acrylonitrile and methyl methacrylate were also grafted onto wheat straw. The treated or grafted straw was then added into dimethylacetamide at 150 °C for 30 min and later treated with lithium chloride solution (7 %) at 120 °C overnight until the samples were completely dissolved. Fibers were produced using both the dry and wet spinning system. Fibers obtained from grafted straw had considerably lower tensile properties especially at high draw ratios due to the difficulties in orienting the grafted polymers during extrusion and spinning [98Foc]. Table 21.3 provides some properties of fibers obtained from wheat straw and modified wheat straw.

Similar to wheat straw, rice straw has also been used to produce regenerated cellulose fibers. Rice straw chopped to about 10 cm in length was pretreated with 20 % NaOH to remove lignin and hemicellulose and then dissolved in NMMO at 100 °C. Fibers were extruded into a coagulation bath containing water and then drawn using take-up rollers. Untreated straw had a cellulose content of 44 %, 26 % hemicellulose, 15.8 % lignin, and 14 % ash compared to 60-94 % cellulose, 3-18 % hemicellulose, 1-5 % lignin, and 2-5.6 % ash after treatment depending on the extent of alkali used. Extruded fibers had diameters ranging from 10 to 25 ^m with circular cross section, and the typical fibrillated surface of regenerated cellulose fibers obtained by the NMMO process was observed. Fibers had a tenacity ranging from 1.9 to 3.1 g/den and modulus between 85 and 100 g/den similar to the fibers obtained from steam exploded and grafted wheat straw [01Lim].

Ascidic tunic, a by-product, was made into pulp and used to regenerate cellulose fibers. About 33 % of the tunic was obtained as pulp (DP 918) after bleaching and was dissolved (6 %) using NMMO/water (87/13) at 120 °C for 40 min. Cellulose solution was then extruded into a water bath at 30 °C. Dry and wet tensile properties of the fibers obtained and the crystallinity index are listed in Table 21.4. Properties of the fibers were found to be related to the concentration of the cellulose solution and the winding speed which governed the orientation of the cellulose crystals in the fibers. Increasing winding speed produced finer fibers with higher crystallinity and orientation leading to higher strength and modulus but lower elongation. Another distinguishing feature of the fibers was their high wet strength, more than 95 % of the dry strength due to the higher DP of the ascidic tunic pulp and higher crystallinity of the fibers [02Koo, 03Wan].

In another research, bagasse dissolved using NMMO system was extruded as fibers into a coagulation bath containing various concentrations of NMMO [11Yam]. Unique hollow fibers shown in Fig. 21.2 were obtained when the NMMO concentration in the coagulation bath was higher than 30 %. The size of the hollow center increased with increasing concentration of NMMO. When high solvent concentrations are present in the coagulation bath, the fibers do not precipi­tate completely but are rapidly coagulated in the washing step with water. During washing, the outer surface precipitates, but the inner core dissolves in water leading to the formation of the hollow center. Figure 21.2 shows the SEM picture of the hollow fibers obtained at three different concentrations of NMMO in the coagulation bath. Despite being hollow, the fibers obtained had tensile properties similar to that of the commercially produced lyocell fibers.

References

[98Foc] Focher, B., Marzetti, A., Marsano, E., Conio, G., Tealdi, A., Cosani, A., Terbojevich, M.: J. Appl. Polym. Sci. 67, 961 (1998)

[01Lim] Lim, S. K., Son, T. W., Lee, D. W., Park, B. K., Cho, K. M.: J. Appl. Polym. Sci. 82, 1705 (2001)

[02Koo] Koo, Y., Wang, Y., You, S., Kim, H.: J. Appl. Polym. Sci. 85, 1634 (2002)

[03Wan] Wang, Y., Koo, W., Kim, H.: Text. Res. J. 73(11), 998 (2003)

[10Jal] Jalaluddin, A., Yamamoto, A., Gotoh, Y., Nagura, M.: Text. Res. J. 80(17), 1846 (2010)

[11Gao] Gao, Q., Shen, X., Lu, X.: Carbohydr. Polym. 83, 1253 (2011)

[11Yam] Yamamoto, A., Uddin, A. J., Gotoh, Y., Nagura, M., Iwata, M.: J. Appl. Polym. Sci. 119, 3152 (2011)

[12Mis] Mishra, R., Behera, B. K., Pal, B. P.: J. Text. Inst. 103(3), 320 (2012)

[13Roj] Rojo, E., Alonso, M. V., Dominguez, J. C., Saz-Orozco, D., Oliet, M., Rodriquez, F.: J. Appl. Polym. Sci. 130, 2198 (2013)

Regenerated Protein Fibers

Keywords

Protein fiber • Regenerated protein fiber • Artificial silk • Azlon • Protein by-product • Spider silk • Recombinant protein

Natural silk exhibits extraordinary properties and is useful for various applications. However, silk is produced in limited quantities and is also not easy to be dissolved, modified, or manipulated for specific applications. With a goal to find an alternative to natural silk, attempts have been made to dissolve proteins and regenerate the proteins into fibers using various approaches. Regenerated protein fibers generally called “azlons” were commercially produced from the proteins in corn, soybean, peanuts, and milk and also poultry feathers during the early 1930s. The poor quality of the protein fibers produced, the use of toxic chemicals during fiber production, and the introduction of inexpensive regenerated cellulose and synthetic fibers led to the decline and eventual elimination of the azlons. Although currently there is very limited or no commercial-scale production of regenerated protein fibers, recent advances in biotechnology, increase in the availability of low-cost biofuel coproducts that contain proteins, environmental awareness on using nondegradable fibers, and distinct properties of protein fibers have renewed interests in regenerated protein fibers. Reproducing proteins through biotechnology, developing novel methods to dissolve proteins and improving the properties of fibers, and biomimicking are some of the approaches that are being considered to develop regenerated protein fibers. This chapter provides an overview of such approaches, properties of the fibers developed, and potential applications of the fibers.

Biothermoplastics from Renewable Resources

Keywords

Biothermoplastics • Polypropylene • Renewable resource • Polytrimethylene terephthalate • Polylactic acid

Biothermoplastics are considered to be those developed using polymers that are derived from renewable resources. Figure 63.1 lists some of the biopolymers obtained from bioresources, their structure and routes used to synthesize the biopolymers. In some cases such as poly(lactic acid), the entire polymer is derived from renewable resource whereas in the case of poly(trimethylene terephthalate), only one of the monomer is from an renewable resource [12Che]. As seen in Fig. 63.1, traditional synthetic polymers such as polypropylene (PP) have also been derived using biopolymers but have not been commercialized due to high cost and limitations in processing and properties. Properties of a few selected biopolymers are listed in Tables 63.1 and 63.2.

© Springer-Verlag Berlin Heidelberg 2015

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources, DOI 10.1007/978-3-662-45136-6_63

PHA polyhydroxyalkonates, PLA polylactic acid, PBS poly(butylenes succinate), PPC poly(propylene carbonate), PTT poly(trimethylene terephthalate), PE polyethylene, PP polypropylene, PET polyethylene terephthalate

PHA poly(hydroxyalkonates), PLA poly(lactic acid), PBS poly(butylenes succinate), PPC poly (propylene carbonate), PTT poly(trimethylene terephthalate), PE poly(ethylene), PP polypropyl­ene), PET poly(ethylene terephthalate)

Reference

[12Che] Chen, G., Patel, M. K.: Chem. Rev. 112, 2082 (2012)

Natural Cellulose Fibers from Renewable Resources

Keywords

Wheat straw • Rice straw • Fiber extraction • Lignin • Interlocked structure

Wheat is the fourth most popular crop in the world with a production of 675 million tons in 2012. About 1-1.2 tons of straw are generated per acre and wheat straw accounts for about 50 % by weight of the cereal produced. Straw is mainly used as animal fodder and bedding, for thatching, and for artistic works, and in many countries, wheat straw is burnt to prevent soilborne diseases. Extensive studies have been done to understand the potential of using wheat straw for pulp and paper production. However, wheat straw has a waxy covering on the surface and a unique morphological structure that makes it difficult for alkali to penetrate into the straw and separate fiber bundles with the length, fineness, and tensile properties required for textile and other high-value fibrous applications. As seen in Fig. 3.1, the individual cells or ultimate fibers in wheat straw have serrated edges that get interlocked with each other. It was found that a pretreatment with detergent and mechanical separation with steel balls were necessary before the alkaline treatment to obtain fiber bundles from wheat straw [07Red]. Fiber bundles obtained from wheat straw had tensile properties similar to kenaf as seen in Table 3.1. About 20 % fibers were obtained, but the fiber bundles obtained were considerably coarser than cotton and linen.

Similar to wheat, rice is also one of the most widely grown crops with a world production of about 720 million tons in 2012. Unlike other cereal straws, rice straw contains up to 15 % silica and 15 % lignin that makes it difficult to be processed into pulp and paper. Efforts to use rice straw as a source for fuel to power biomass plants have also not been economically attractive. In many countries, rice straw is mostly burnt and in developing countries used as animal feed and bedding and also for thatching. In addition to the straw, processing of rice removes the outer covering or the husk, equivalent to about 20 % by weight of the grain. The ultimate cells in rice straw are considerably smaller (0.6 mm in length and 8.1 pm in width) which makes the straw sensitive to fiber extraction conditions. As seen in Table 3.1, relatively

Table 3.1 Tensile properties and moisture regain of wheat and rice straw fibers compared with cotton, linen, and kenaf fibers

coarse rice straw fibers (240 tex) with length ranging from 2.5 to 8 cm were obtained after alkali and enzyme treatment [06Red]. Fibers obtained from rice straw had similar tensile properties compared to fibers obtained from wheat straw and jute. Rice straw fibers were blended with cotton (50:50 ratio) and processed on short staple cotton machinery into a 20s Ne yarn. The yarns were knitted into fabrics and dyed using reactive dyes.

References

[06Red] Reddy, N., Yang, Y.: J. Agric. Food Chem. 54, 8077 (2006)

[07Red] Reddy, N., Yang, Y.: J. Agric. Food Chem. 55, 8570 (2007)

Low-voltage actuators that operate based on changes in pH are used in biomedical devices and micromechanical systems [07Spi]. A dual mode actuation during pH switching was obtained using chitosan/polyaniline and carbon nanotube fibers. Chitosan fibers were co-spun with polyaniline to combine the reversible swelling ability in acidic/neutral solutions and the good solubility of polyaniline. Carbon nanotubes were added to increase the conductivity of the fiber [06Spi]. It was found that pH strains of about 2 % and electrochemical strains of 0.3 % were obtained, and the addition of carbon nanotubes was necessary to obtain electrochemical actuation, and the fibers were supposed to be useful for micro — and macro actuators [06Spi]. Chitosan fibers were used to self-assemble gold nanoparticles, and the composite fibers were intended for use as biosensors [06Wan]. Fibers (50 nm-5 pm in width) were formed via freeze-drying and partially cross-linked with glutaralde — hyde. These fibers were later dispersed into 0.025 mol/l aqueous tetrachloroauric acid and allowed to stay for 6 days. Nanoparticles formed experienced surface and interface interactions such as adsorption, formation of nuclei, crystal growth, dispersion, self-assembly, and agglomeration on the surface of the chitosan fibers. Gold flakes with thickness of 50 nm were formed, and the size of the flakes increased with increasing time. Spherical gold clusters (200 nm) containing gold nanoparticles of 5 nm diameter self-assembled on the fiber surface. Figure 26.11 shows gold nanoparticles beginning to self-assemble on the fibers (left) and fibers with a continuous gold coating (right) [06Wan]. To overcome the aggregation problems, single-walled carbon nanotubes were functionalized and incorporated into chitosan fibers to be used as actuators [08Oza]. Carbon nanotubes were treated with 70 % nitric acid to oxidize and functionalize the nanotubes by adding carbox­ylic groups. The addition of the nanotubes (0.4 wt%) increased the tenacity of the fibers to 180 MPa compared to 96 MPa for the non-functionalized nanotubes. The fibers also showed significant swelling and deswelling (up to 800 % volume change) with change in pH as seen in Fig. 26.12. The extent of change in the fibers was also dependent on the applied voltage and the concentration of the nanotubes and salt.

Chitosan fibers were considered to be versatile fibers for assembling proteins and provide distinct properties [08Shi]. Two His-tagged proteins (His-GFP) and His-protein G were used to biofunctionalize the fibers. Chitosan fibers were treated with nickel solution (0.01 M NiCl2 in 0.9 % NaOH, pH 5) for 1 h. Later, the fibers were treated with 5 % nonfat milk to block the nonspecific binding of proteins. The fibers were then treated with antibody-containing solutions, and the antigen binding was studied. It was found that the fibers were able to absorb antibodies, and the fibers would have potential protection and detection applications.

References

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[04Yua] Yuan, Y., Zhang, P., Yang, Y., Wang, X., Gu, X.: Biomaterials 25, 4273 (2004)

[05Fun] Funakoshi, T., Majima, T., Iwasaki, N., Yamane, S., Masuko, T., Minami, A., Harada, K., Tamura, H., Tokura, S., Nishimura, A.: J. Biomed. Mater. Res. 74A, 338 (2005) [06Qin] Qin, Y., Zhu, C., Chen, J., Chen, Y., Zhang, C.: J. Appl. Polym. Sci. 101, 766 (2006)

[06Spi] Spinks, G. M., Shin, S. R., Wallace, G. G., Whitten, P. G., Kim, S. I., Kim, S. J.: Sensor

Actuator B 115, 678 (2006)

[06Wan] Wang, R. H., Hu, Z. G., Liu, Y., Lu, H., Fei, B., Szeto, Y. S., Chan, W. L., Tao, X. M., Xin, J. H.: Biomacromolecules 7, 2719 (2006)

[07Qin] Qin, Y., Zhu, C., Chen, J., Zhong, J.: J. Appl. Polym. Sci. 104, 3622 (2007)

[07Spi] Spinks, G. M., Shin, S. R., Wallace, G. G., Whitten, P. G., Kim, I. Y., Kim, I. S., Kim, S. J.: Sensor Actuator B12, 616 (2007)

[07Wan] Wang, Q., Zhang, N., Hu, X., Yang, J., Du, Y.: Eur. J. Pharma. Biopharma. 66, 398 (2007)

[07Yan] Yang, Y. M., Hu, W., Wang, W. D., Gu, X. S.: J. Mater. Sci. Mater. Med. 18, 2117 (2007)

[08Hei] Heinemann, C., Heinemann, S., Lode, A., Bernhardt, A., Worch, H., Hanke, T.: Biomacromolecules 9(10), 2913 (2008)

[08Oza] Ozarkar, S., Jassal, M., Agarwal, A. K.: Smart Mater. Struct. 17, 1-8 (2008)

[08Shi] Shi, X., Wu, H., Tsao, C., Wang, K., Kobatake, E., Bentley, W. E., Payne, G. F.:

Biomacromolecules 9, 1417 (2008)

[08Wan] Wang, Q., Zhang, N., Hu, X., Yang, J., Du, J.: J. Biomed. Mater. Res. 85A, 881 (2008)

[09Hei] Heinemann, C., Heinemann, S., Lode, A., Bernhardt, A., Worch, H., Hanke, T.:

Biomacromolecules 10, 1305 (2009)

[09Lia] Lian, Q., Li, D., Jin, Z., Wang, J.: J. Bioact. Compat. Polym. 24, 113 (2009)

[09Yil] Yilgor, P., Tuzlakoglu, K., Reis, R. L., Hasirci, N., Hasirci, V.: Biomaterials 30, 3551 (2009)

[12Alb] Albanna, M. Z., Bou-Akl, T. H., Walters, H. L., Matthew, H. W.T.: J. Mech. Behav. Biomed. Mater. 5, 171 (2012)

[13Alb] Albanna, M. Z., Bou-akl, T. H., Blowytsky, O., Walters, H. L., Matthew, H. W.T.: J. Mech. Behav. Biomed. Mater. 20, 217 (2013)

[13Des] Desorme, M., Montembault, A., Lucas, J., Rochas, C., Bouet, T., David, L.: Carbohydr. Polym. 98, 50 (2013)

Electrospun Fibers from Biopolymers

Keywords

Electrospinning • Chitin • Solubility • Dissolution • Solvent • Deacetylation • Chitosan • Carbohydrate • Collagen • Blend • Alginate • Gelling • Polyethylene oxide • Polyvinyl alcohol • Core-sheath fiber • Biocompatibility • Hyaluronic acid • Formic acid • Cross-linking • Cell culture • Fibroblast • Cellulose • Ionic solvent • DMSO • Starch • Ethanol • Acetic acid • Polycaprolactone • Starch acetate • Drug loading • Tissue engineering

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

Bamboo • Renewable resource • Biofiber • Bast fiber • Alkali treatment

Considerable attention has been drawn towards the generally termed “bamboo fibers” in the last decade mainly because bamboo is a fast-growing (a meter or higher per day) biomass crop that needs minimum inputs and is renewable. How­ever, most reports or articles on bamboo fibers refer to regenerated cellulose fibers that are obtained using bamboo as a source and not the natural fibers extracted from bamboo stems/stalks. Nevertheless, natural fibers have been extracted from bam­boo, and some of the literature has been covered here despite bamboo not being a by-product and has to be grown independently. Companies are extracting natural cellulose fibers from bamboo stems and are selling them commercially. Litrax, a France-based company, is marketing “L1 natural bamboo bast fiber” that has been enzymatically extracted from bamboo stems. The extracted fibers have a linear density of about 5.2 denier and are supplied in various staple lengths up to 90 mm.

Bamboo is usually harvested after a period of 3 months and contains anywhere from 26 to 43 % cellulose, 21 to 31 % lignin, and 15 to 26 % hemicellulose [09Wai]. Fibers are extracted from bamboo using mechanical and chemical methods including enzymatic treatments [09Wai]. Bamboo chips were boiled with 4 % NaOH for 2 h under pressure to obtain fibers with lengths of 35 ± 5 mm and widths of 17 ± 3.4 ^m, but the tensile properties were not reported [10Kum]. Bamboo strips with widths from 1.5 to 1.75 cm and thickness between 0.65 and 0.75 mm were soaked with 0.1 NaOH for up to 72 h and later washed with water to obtain fiber bundles [00Des]. After the chemical separation, the treated bamboo was further subjected to mechanical separation using compression molding or roller mill techniques [00Des]. Properties of the fibers obtained using these two methods are given in Table 13.1. It should also be noted that the properties of fibers obtained from bamboo grown at different regions and age and even part of the bamboo and extraction conditions are considerably different. Bamboo fibers have

exceptional tensile properties mainly due to the unidirectional arrangement of fibrils [12Liu]. However, bamboo fibrils are arranged to the fiber axis at a low angle (2-10°) that leads to considerably low elongation of 1-2 %.

References

[00Des] Deshpande, A. P., Rao, M. B., Rao, C. L.: J. Appl. Polym. Sci. 76, 83 (2000) [09Wai] Waite, M.: J. Text. App. Text. Manag. 6(2), 1 (2009)

[10Kum] Kumar, S., Choudhary, V., Kumar, R.: J. Therm. Anal. Calorim. 102, 751 (2010) [12Liu] Liu, D., Song, J., Anderson, D. P., Chang, P. R., Hua, Y.: Cellulose 19, 1449 (2012)

Natural Protein Fibers

Keywords

Silk • Cocoon • Colored cocoon • Dyeing

Although silks are characterized by their bright color and luster, there are only limited colors in which cocoons are produced. Recently, attempts have been made to develop a new class of colored silks by feeding mulberry leaves mixed with fluorescent dyes to Bombyx mori insects. The dyes were predominantly taken up by the fibroin proteins, and the color was persistent even after degumming. These unique colored silks were found to have similar crystalline structure and tensile properties and also supported the attachment and growth of human colon fibroblasts [11Tan]. The presence of the fluorescent dye provided luminescent fibers (Fig. 37.1) that could enable the detection of cell attachment and spreading more easily. Such colored silk fibers would eliminate the need for dyeing and lead to substantial savings in energy, water, and other resources and also provide unique fibers for medical and other applications.

Control Rhodamine 101 Rhodamine 110 Rhodamine 8

a

b

c

d

Fig. 37.1 Digital images showing fluorescent B. mori cocoons and fibers obtained after feeding the insects with various fluorescent dyes [11Tan]

Reference

[11Tan] Tansil, N. C., Li, Y., Teng, C. P., Zhang, S., Win, K. Y., Chen, X., Liu, X. Y., Han, M.: Adv. Mater. 23, 1463 (2011)

Plant proteins such as zein, wheat gluten, and soyproteins that are obtained as coproducts during processing of cereal grains have been used for industrial applications including fiber production. Although wheat gluten and soyproteins are available in larger quantities and have lower cost than zein, zein has been more extensively studied for fiber production since zein dissolves in aqueous ethanol solutions and has excellent spinnability. Although zein has excellent solubility in aqueous ethanol, the influence of various solvents on the electrospinnability and properties of fibers obtained was studied by Selling et al. [07Sel]. Table 59.1 shows the solvent, conditions used, and the properties of the fibers obtained. Lower alcohol/water solutions produced ribbonlike fibers, whereas acetic acid produced fibers with round morphology and narrower diameter distribution.

The plant protein zein was electrospun into fibers with the addition of 10 % chitosan to improve antimicrobial activity [09Tor]. Zein and chitosan were dissolved separately, mixed in 2:1 proportion, and electrospun into fibers with diameters in the submicron range. Although preferable, addition of higher than

10 % chitosan resulted in the formation of excessive beads and it was not possible to obtain fibers. It was reported that addition of low amounts of chitosan provided substantial antimicrobial activity to the fibers. However, the stability of the fibers in aqueous media was not reported and no cross-linking was done. Ultrafine protein fibers with diameters between 150 and 600 nm were obtained from corn zein under the optimum conditions of 20 % protein, 70 % ethanol concentration, and voltage of 15 kV. Potential of the zein nanofiber mats to immobilize a plant polyphenol epigallocatechin gallate (EGCG) was investigated. Freshly spun fibers provided a relatively low immobilization power of 82 % compared to 98 % for fibers aged at 0 % relative humidity for 1 day. The electrospun fibers were considered to be suitable for encapsulation of biomolecules for food applications [09Li].

Although zein is easily electrospinnable, matrices developed from zein have poor aqueous stability and disintegrate upon immersion in aqueous media. To overcome this limitation, cross-linking of zein has been considered. In one such effort, zein was dissolved in acetic acid and glyoxal in various extents was added as the cross-linking agent. Optimizing conditions during electrospinning resulted in the production of fibers with diameters ranging from 0.3 to 67 pm [12Sel]. However, the stability of the matrices in various media was not studied. Cross-linking agents such as glyoxal and glutaraldehyde used to cross-link zein provide good water stability but are cytotoxic. To develop water stable and cytocompatible zein nanofibers, citric acid was used as the cross-linking agent. Both dry and wet cross-linking methods were developed to obtain electrospun matrices with desired properties. Uncross-linked matrices lost their morphology and became film-like when immersed in PBS (Fig. 59.2, left), whereas the cross-linked matrices were stable and retained their fibrous morphology even after 24 days (Fig. 59.2, middle). The cross-linked matrices were biocompatible and showed better potential for cell growth and proliferation (Fig. 59.2, right) than similar electrospun poly(lactic acid) matrices [10Jia, 12Jia].

To study the potential of using zein fibers for controlled release applications, three common (a, p, y) cyclodextrins were added into zein solution (in dimethyl formamide) to act as inclusion complexes which could attract and load biomolecules and electrospun into fibers with diameters between 100 and 400 nm

[12Kay]. Table 59.2 provides a comparison of the properties of the fibers obtained with various concentrations of zein in solution and different amounts of cyclodex­trin [12Kay]. Similarly curcumin, a natural antimicrobial agent, was added into zein and electrospun into fibers with average diameters of 310 nm. Addition of curcumin increased the fluorescence and in vitro degradation studies showed sustained release of curcumin and retention of free radical scavenging ability [12Bra].

In addition to zein, other plant proteins such as soyproteins and wheat gluten and gliadin have been made into regenerated films, fibers, and other materials. Since these non-prolamin proteins do not dissolve in electrospinnable solvents, it is difficult to produce electrospun fibers. However, some reports are available on electrospinning wheat gluten and soyproteins. For instance, ability of producing electrospun fibers from native and denatured wheat gluten was examined by Woerdeman [05Woe]. In another report, wheat gluten was mixed with poly(vinyl alcohol) (PVA), dithiothreitol (DTT), and thiolated poly vinyl alcohol (PVA) in water/propanol and electrospun into fibers [10Don].

In a unique approach, soyproteins were extracted using urea and reducing agents and the reduced soyproteins obtained were dissolved using aqueous buffers containing surfactants. The soyprotein solution could be electrospun into 3D fibrous scaffolds that supported the attachment, growth, proliferation, and differentiation of stem cells [13Cai]. Figure 59.3 shows an image of the 3D fibrous soyprotein scaffold developed using the novel approach.

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