Chitin, Chitosan, and Alginate Fibers

Keywords

Chitin • Chitosan • Deacetylation • Polysaccharide • Antimicrobial • Controlled release • Tissue engineering

Chitin is a polymer made from units of N-acetyl glucosamine as shown in Fig. 23.1. Chitin is the structural unit that provides strength to most invertebrates and is one of the most common biopolymers found in nature. Unlike most other polysaccharides, chitin contains about 6.9 % nitrogen which makes it useful as a chelating agent and also for various applications in the pharmaceutical, biomedical, paper, textile, photographic, and other applications. Chitin is also found in bacteria and fungi. In its native form, chitin is insoluble in common solvents and therefore has limited applications. Typically, chitin exists with an average molecular weight of 1.036 x 106 to 2.5 x 106 Da. Generally, chitin is deacetylated and obtained as chitosan which is soluble in aqueous acetic acid. Fibers have been obtained from chitin, chitosan, and several other chitin derivatives.

Regenerated Protein Fibers

Keywords

Hagfish slime • Protein • Dissolution • Formic acid • Mussel byssus • Mussel feet

Regenerated protein fibers were produced from the protein fibers (threads) found in hagfish slime [12Neg]. Proteins were solubilized in 98 % formic acid to obtain solutions (5, 7.5 %) that were spun into fibers and coagulated into an ethanol, methanol, or electrolyte buffer. However, fibers obtained were too weak and brittle. As an alternative approach, the protein solution was cast into films, and fibers were drawn from the films as shown in Fig. 49.1. Average fiber diameters obtained were between 46 and 137 ^m, and the length of the fibers was about 3 mm. Table 49.1 shows the tensile properties of the fibers obtained under various conditions. Tensile properties of the regenerated fibers were considerably lower compared to the properties of the natural slime threads but similar to that of the regenerated fibers produced from spider silks as seen in Table 49.2. Structural analysis using X-ray diffraction and Raman spectroscopy showed that the fibers were composed of about 67 % a-helix and 26 % p-sheet content. Drawing of the fibers was found to increase orientation but not the crystallinity of the fibers.

Proteins in mussel byssus have been extracted and regenerated into fibers [08Har, 09Har]. Proteins extracted from whole feet of mussel were dissolved in acetate buffer at various pHs (5.5-8). Fibers were hand drawn from the solution using a metal dissecting probe [08Har]. Figure 49.2 shows the formation of the fiber from the protein solution. It was found that the pH of the solution played a critical role in fiber formation. Fibers obtained had diameter in the range of 3-6 ^m, and TEM images showed alignment of filaments along the axis of the fiber. Table 49.3 provides a comparison of the tensile properties of the regenerated threads to the native distal and proximal byssal threads. As seen from the table, the regenerated protein fibers are considerably finer and have tensile strength and elongation comparable to that of the native threads. It was suggested that during drawing, the preCols align end-to-end and the histidine-rich termini or nearby preCols

Table 49.1 Mechanical properties of regenerated hagfish slime thread protein fibers produced using various conditions

Fig. 49.2 Formation of fibers by drawing between two metal probes [08Har]

(see Chap. 39) get cross-linked due to the interactions with the divalent metal ions [08Har].

In further continuation of their work, Harrington et al. extracted proteins from the four parts of the mussel feet to understand the role of the different proteins in the threads on the behavior of the threads [08Har]. Mussel feet were divided into four parts as proximal (PFP), distal (DFP), whole (WFP), and transition region. PreCols were separately purified from the PFP and DFP and the solution was used to draw

fibers. Table 49.4 provides a comparison of the properties of the fibers obtained from the PFP and DFP with the WFP protein fibers produced in an earlier research.

As seen from the table, the fibers produced from the DFP preCols have higher tensile strength but lower elongation than that of the fibers obtained from PFP preCols. The differences in the properties of the fibers obtained from DFP and PFP are mainly due to the varying contents of a — and p-sheets. It has been recognized that silks containing a high degree of antiparallel p-sheets have an extensibility of up to 50 %, whereas elastin-based materials such as spider silk can extend greater than 200 % [09Har]. Such mechanical gradient seen in byssal threads is considered to be very unique, and mimicking the byssal structure could lead to the develop­ment of new biomaterials.

References

[08Har] Harrington, M. J., Waite, J. H.: Biomacromolecules 9, 1480 (2008)

[09Har] Harrington, M. J., Waite, J. H.: Adv. Mater. 21, 440 (2009)

[12Neg] Negishi, A., Armstrong, C. L., Kreplak, L., Rheinstadter, M. C., Lim, L., Gillis, T. E., Fudge, D. S.: Biomacromolecules 13, 3475 (2012)

Biothermoplastics from Renewable Resources

Keywords

Bacterial polyester • Poly(3-hydroxybutyrate) • PLA • Core-sheath fiber • Knit­ted socks

Polyhydroxyalkonates are a diverse family of biopolyester produced by bacteria as energy and carbon storage materials. Poly(3-hydroxybutyrate) (PHB) is the most common type of PHA that is commercially used. PHB is an thermoplastic material with a melting temperature of about 180 °C and glass temperature that is below room temperature. Structure and properties of PHB are highly dependent on the conditions prevailing during fiber production. For instance, slow cooling from the melt produced large spherulites and rapid cooling results in amorphous state [01Yam]. It was suggested that PHB assumed orthorhombic or а-form or the P-zigzag form depending on the annealing conditions. PHB crystallized into ortho­rhombic form when annealed under high tension and into p-zigzag form when annealed under high tension [01Yam]. Based on X-ray diffraction patterns, it was found that the amorphous molecules transformed into orthorhombic crystal when annealed without tension and when annealed under tension, the amorphous regions were stretched and crystallized into the p-form [01Yam].

Polyhydroxybutyrate-valerate (PHBV) is a copolymer of PHB that is less stiffer but tougher than PHB. However, the low crystallization rate of PHBV makes it difficult to produce fibers. To overcome this limitation, PHBV was blended with PLA to produce core-sheath fibers. PHBV with a viscosity average molecular weight of 490 kDa was extruded to obtain pellets with lower molecular weight of 260 kDa and PLA was reduced to a molecular weight of about 90 kDa, and the two polymers were blended and extruded into fibers. Table 65.1 lists the conditions used and the properties of the fibers obtained. As seen in the table, it was not possible to obtain fibers with PHBV as the sheath and PLA as the core due to poor processabil­ity of PHBV. Tensile properties of the fibers were dependent on the draw ratio and to the amount of PLA in the blend. Biocomponent fibers with PLA as the sheath and

Table 65.1 Processing conditions and properties of PHBV-PLA blend fibers [12Huf] Core Sheath Temperature (°С) Draw ratio Strength (g/den) Strain {%) Modulus (GPa) Polymer Blend (wt%) Polymer Blend (wt%) Core Sheath PLA 100 — — 220 — 1.1 0.56 ±0.2 — 28.8 PLA 100 — — 220 — 4.5 3.1 ±0.2 29 ±2 47.2 PHBV 59 PLA 41 165 195 3 1.2 ±0.1 30±3 37.6 PHBV 62 PLA 38 165 195 3 1.4 ±0.2 26 ±9 42.4 PHBV 66 PLA 34 165 195 3 1.0 ±0.2 31 ± 10 38.4 PHBV 69 PLA 31 165 195 3 1.1 ± 0.2 29 ±7 36 PHBV 35 PLA 65 170 190 3 2.0 ±0.2 38 ± 9 44 PHBV 36 PLA 64 170 190 3.5 2.6 ±0.4 28 ±5 56.8 PHBV 29 PLA 71 170 190 3.5 2.3 ±0.2 34 ±6 46.4 PHBV 22 PLA 78 170 190 3.5 2.3 ±0.2 41 ± 6 51.2 PLA 49 PHBV 51 185 175 1.5 0.6 ±0.08 125 ±29 23.2 PLA 100 PHBV — 185 — 6 3.3 ±0.4 13 ± 3 52.8 PLA 100 PHBV — 185 — 6 3.3 ±0.5 17±4 48.0 PHBV 27 PLA 73 175 185 5.5 2.4 ±0.2 23 ±2 42.4 PHBV 29 PLA 71 175 185 5.5 2.7 ±0.2 23 ±2 48.0

PHBV as the core had tensile strength of 2.7 g/den and modulus of up to 56.8 g/den. In vitro biocompatibility studies did not show any toxicity and cells grew along the length of the fibers. A decrease in fiber strength by about 33 % was observed 4 weeks after incubation [12Huf]. Blends of PHBV and PLA were prepared and extruded into fibers between 210 and 235 °C. Blend fibers containing 5 and 10 % PHBV were knitted into socks [11Piv] shown in Fig. 65.1. Increasing take-up speed improved the tensile properties and addition of PHBV above 10 % led to a decrease

in tensile strength. SEM images (Fig. 65.2) of the fracture surface showed two distinct regions suggesting that the blends were incompatible even with a low PHBV content of 5 %.

References

[01Yam] Yamane, H., Terao, K., Hiki, S., Kumura, Y.: Polymer 42, 3241 (2001)

[11Piv] Pivsa-Art, S., Srisawat, N., O-Charoen, N., Pavasupree, S., Pivsa-Art, W.: Energy Procedia 9, 589 (2011)

[12Huf] Hufenus, R., Reifler, F. A., Maniura-Weber, K., Spierings, A., Zinn, M.: Macromol. Mater. Eng. 297, 75 (2012)

Natural Cellulose Fibers from Renewable Resources

Keywords

Cotton • By-product • Outer bark • Alkali treatment

The most prominent and oldest known natural cellulose fiber, cotton, has been grown and used for textiles since time immemorial. Cotton was grown in about 34.2 million hectares, and about 26 million tons of cotton was produced worldwide in 2012. In addition to the seed from which cotton fibers are harvested, cotton plants consist of stalks and leaves that are left as by-products, equivalent to 3-5 times the weight of the cotton fiber produced. Cotton stalks consist of an outer bark (20 % by weight of the stalk) and inner pith. The outer bark is fibrous and could be utilized as a source for fibers similar to the bast fibers produced from jute or flax plants. Treating the outer bark of cotton stalks with 2 N NaOH at boil for 1 h resulted in fibers with fineness of about 50 denier. These fibers had strength similar to cotton but lower elongation. When used as reinforcement for polypropylene composites, cotton stalk fibers provided similar tensile and flexural properties compared to jute fibers. Cotton stalks were treated at 150 °C in a mixture of 20 % sodium sulfide, 2 % anthraquinone, 2 % sodium silicate, and different concentrations of sodium hydrox­ide for 30 min. Concentration of sodium hydroxide considerably influenced the composition and properties of the fibers as seen in Table 5.1 [12Zho]. Substantially finer fibers (0.9 tex) have been produced by the high-temperature treatment reported by Zhou et al. compared to those produced by Reddy and Yang [09Red]. So far, no reports have been available on the processing of cotton stalk fibers into textiles or on the bleaching and dyeing of the cotton stalk fibers.

References

[09Red] Reddy, N., Yang, Y.: Bioresour. Technol. 100, 3563 (2009)

[12Zho] Zhou, L., Shao, J., Feng, X., Chen, J.: J. Appl. Polym. Sci. 125, 573 (2012)

Chitin, Chitosan, and Alginate Fibers

Keywords

Alginate • Seaweed • Alginate-chitosan • Gel properties • Wound healing

Alginates are polysaccharides found as the cell wall constituents in brown algae (Phaeophyceae) which is considered a seaweed. Although some quantities of alginate are found in most species of brown algae, certain species (Laminaria, Lessonia, Macrocystis, Sargassum) contain 30-45 % alginate by dry weight, and these species are used for extraction. In China, about 2 million tons of seaweeds are artificially cultured for alginate production. In 2009, about 27,000 tons of alginate with an estimated value of $318 million were reportedly produced. Alginates are extracted from raw seaweeds by treating with sodium hydroxide when the alginates in salt form are converted into the water-soluble sodium alginate [08Qin]. Chemically, alginates are linear polymers composed of 1,4,-P-d — mannuronic acid (M) and a-L-guluronic acid (G) residues. The amount of M and G residues and the proportion of the blocks of M and G residues vary between different species and are responsible for the variations in properties between the different alginates [08Qin]. Table 28.1 lists the percentage of M and G residues and the block structures found in commercially available alginates extracted from different types of Brown seaweeds.

Alginate is extensively used in the medical industry mainly as a wound dressing due to the excellent ability to sorb moisture and keep wounds dry. In addition, alginates are also used in food industries as stabilizer and as emulsifier in the textile industry for fiber production and other applications. The naturally occurring sodium alginate is water soluble but is made water insoluble by converting the sodium alginate into other forms. Some of the derivatives of sodium alginates that have been used for fibrous applications are calcium alginate, zinc alginate, copper alginate, barium alginate, aluminum alginate, beryllium alginate, and chromium alginate by changing the type of metal ion in the coagulation bath. Calcium chloride is the most common metal used since it is inexpensive, readily dissolves in water,

and is nontoxic, and therefore calcium alginate fibers are more extensively studied. In addition, alginates have been combined with other natural or synthetic polymers, and several additives have been included to improve the properties and make alginate fibers suitable for various applications.

Fibers have been made from alginates using various approaches and are reported to have unique ion-exchange, gel-forming, and medical properties. Alginate fibers have been widely used for wound dressing and other medical applications because alginates become gels by absorbing wound exudates which avoids the trauma/ discomfort when removing wound dressings [04Kni]. Gelling of alginate also keeps the wound moist and assists in better wound healing.

Sodium alginate fibers were formed by dissolving 5 % alginate in distilled water and aging the solution overnight and later extruding the solution into coagulation baths containing various non-solvents for alginate. Tensile properties of the fibers were dependent on the type of coagulation bath and the drawing speeds. Fibers with fineness of 0.7 den and tenacity of 1.1 g/den have been reported [95Kob].

Due to the relatively low tensile properties of sodium alginate fibers, several methods have been used to improve the properties of alginate fibers. Sodium alginate was mixed with graphene oxide in various ratios (0-8 %) and extruded into fibers by the wet spinning method at different draw ratios [12He]. The addition of graphene oxide substantially increased the tensile strength but decreased elon­gation as seen in Fig. 28.1. The increase in strength and decrease in elongation was also observed with increasing draw ratios from 0 to 100 %. Morphologically, outer surfaces of the fibers were considerably striated and rough as seen in Fig. 28.2 when higher concentrations of graphene oxide were used.

0.70

„ 0.65 —

0 60 — 0.50 — 0.40 —

— 12

0 25

Weight of Graphene Oxide W (%)

Fig. 28.1 Influence of addition of various levels of graphene oxide (GO) on the tensile properties of alginate fibers [12He]

Sodium alginate (1-6 %) was dissolved in water and extruded into coagulation bath containing hydrochloric acid to produce alginic acid fibers or into a calcium chloride (1-3 %) bath to produce calcium alginate fibers [04Kni]. Fibers produced were later coated with hydrolyzed and unhydrolyzed chitosans by passing through chitosan solutions (0-5 %) to enhance their suitability for medical applications. The addition of unhydrolyzed chitosan did not improve the strength but decreased the elongation considerably (Table 28.2) suggesting that chitosan acted as a coating and not as filler. Up to 25 % hydrolyzed chitosan could be added onto the fibers, and an increase in strength was also observed. The addition of chitosan, especially hydrolyzed chitosan, also provided better antibacterial activity [04Kni]. Fiber tenacities obtained were in the range of 1.4-2.8 g/den, and elongation was up to 30 %.

References

[95Kob] Kobayashi, Y., Kamishima, H., Fukuoka, S., Obika, H., Asaoka, T., Tenma, K.: US Patent 5474781 (1995)

[04Kni] Knill, C. J., Kennedy, J. F., Mistry, J., Miraftab, M., Smart, G., Groocock, M. R., Williams, H. J.: Carbohydr. Polymer. 55, 65 (2004)

[08Qin] Qin, Y.: Polym. Int. 57, 171 (2008)

[12He] He, Y., Zhang, N., Gong, Q., Qiu, H., Wang, W., Liu, Y., Gao, J.: Carbohydr. Polymer. 88, 1100 (2012)

Chitosan of different molecular weights and degree of deacetylation has been electrospun into fibers for medical, filtration, and other applications. Typically, chitosan with relatively low molecular weight is dissolved in acidic solution or using toxic solvents such as trifluoroacetic acid. Table 58.1 provides a comparison of the type of chitosan, solvents, and properties of fibers obtained. Although several

solvents and mixtures of acids and organic solvents were used to dissolve and produce electrospun fibers from pure chitosan, Ohkawa et al. claim that only trifluoroacetic acid was able to produce fibers. It was also found that addition of dichloromethane assisted in electrospinning and fibers with diameters of 380 nm were obtained [04Ohk]. Similarly, inclusion of surfactants either promoted or did not affect formation of chitosan-PEO nanofibers depending on the type of surfac­tant used. Nonionic surfactants with chitosan or ionic surfactants with neutral polymers such as polyethylene oxide (PEO) assisted in fiber formation whereas ionic surfactants and charged polymers led to formation of beads [09Kri].

Although chitosan can dissolve in dilute aqueous acetic acid, it was necessary to add other solvents to obtain electrospun fibers. Gong et al. were able to obtain electrospun chitosan fibers using acetic acid as solvent in concentrations from 10 to 100 %. Uniform fibers were obtained with increase in acetic acid concentration to 90 % and using chitosan with molecular weight of 106,000 mol/g and a solution concentration of 7 % [05Gen]. Other researchers have also showed that hydrolyzed chitosan with lower molecular weights could be electrospun into fibers using 70­90 % acetic acid [09Hom]. For instance, electrospun fibers with diameters of 140 nm were obtained using chitosan with molecular weight of 2.94 x 105 g/mol compared to fiber with diameters of 250-284 nm obtained when lower molecular weight chitosan was used [09Hom]. Similar results were also obtained by Vrieze et al. who produced chitosan fibers with diameters of 70 nm using 90 % acetic acid [07Vri].

Chitosan was dissolved in trifluoroacetic acid and methylene chloride and electrospun into oriented and non-oriented nanofibrous tubes with inner diameter of 1.2 mm and outer diameter of 2 mm with lengths of 15 mm [09Wan]. Images of the tubes containing oriented, unoriented, and a mixture of the two types of fibers are shown in Fig. 58.2a-c, respectively. The tubes were used to culture Schwann cells and also implanted into rats for nerve regeneration [09Wan]. Cells were found to align unidirectionally in the case of the oriented fiber mats, but such arrangement was not seen in the unoriented fiber mats as seen from Fig. 58.3. Scaffolds developed were considered to be suitable for autogenous nerve grafts.

A modified wet spinning approach was used to produce ultrafine fibers from chitosan. Chitosan (4 %) was dissolved in acetic acid and extruded through fine silicone rubber tubing into a coagulation bath consisting of either sodium tripolyphosphate/ethanol or 1 M NaOH/ethanol. Fibers obtained were washed

thoroughly with distilled water until the fibers were neutral in pH [11Pat]. Formation of the fibers in NaOH solution led to ionic cross-linking and fibers with good properties. In the dry state, the fibers had tensile strength in the range of 1-2.5 MPa but decreased to 100-300 kPa in the wet state. Chitosan fibers had a swelling of about 500 % in PBS compared to about 300 % for the chitosan- tripolyphosphate fibers.

Chitosan of various molecular weights was electrospun and then cross-linked with glutaraldehyde to improve water stability [07Sch]. Table 58.2 shows some of the properties of the chitosan fibers produced. As seen from the table, chitosan with medium molecular weights (190-310 kDa) had the highest viscosity and produced fibers with diameters of 172 nm. However, high molecular weight chitosan had better water stability. Cross-linking considerably decreased elongation and strength (from 1.4 to 1.2 MPa) but did not affect the modulus of the fibers. Cross-linked fibers were stable in acetic acid, water, and NaOH solution whereas the uncross — linked fibers disintegrated in water. A one-step cross-linking and electrospinning of chitosan fibers was done using glutaraldehyde as the cross-linking agent. The cross­linker (50 % water/50 % GA) solution was added to the spinning solution and the cross-linking occurred during electrospinning [07Sch]. Average diameter of the fibers obtained was about 128 nm, considerably lower than the average diameters of fibers (178 nm) obtained using a two-step cross-linking process. The fibers obtained were stable in acetic acid, water, and sodium hydroxide for up to 72 h.

A new set of cross-linkers were developed to improve the properties of electrospun chitosan mats. Genipin, hexamethylene-1,6-diaminocarboxysulfonate, and epichlorohydrin were added into the chitosan solution in various ratios and electrospun into matrices with fiber diameters of 267, 644, and 896 nm, respec­tively. Cross-linked mats showed good stability to dissolution at pHs 3, 7, and 12 after posttreatment with heat and alkali [12Aus]. In a similar approach, glycerol phosphate, tripolyphosphate, and tannic acid were identified and used as non-covalent cross-linkers for electrospun chitosan fibers. Glycerol phosphate and tannic acid cross-linked fibers had average diameters in the range of 145-334 and 145-554 nm, respectively, whereas tripolyphosphate cross-linking produced branched fibers with diameters between 117 and 462 nm. A two-step cross-linking was necessary for tannic acid to obtain fibers that were stable in 1 M acetic acid after immersion for 72 h [13Kie].

Quaternized chitosan was mixed with poly(vinylpyrrolidine) (PVP) and electrospun into fibers with antibacterial property. Fibers with diameters between

1.5 and 2.8 pm were obtained. However, the fibers were unstable and dissolved in water even after UV treatment. To improve stability, triethylene glycol diacrylate (TEGDA), 4,4-diazidostilbene-2,2-disulfonic acid disodium salt (DAS), and 2,2-dimethoxy-2-phenylacetophenone (DMPA) were used to improve cross-linking efficiency. After addition of these cross-linking enhancers, the fibers were found to be stable in water for up to 6 h. Fiber morphology, especially diameter, was found to decrease with increase in the chitosan content and was attributed to better solubility [07Ign].

Chitosan was PEGylated to improve solubility and enable fiber formation through electrospinning. Fibers with diameters between 40 and 360 nm were produced using tetrahydrofluoride and dimethylformamide (DMF) as a cosolvent system with the addition of triton X as surfactant [07Du]. PEGylated chitosan with a degree of substitution higher than 1.5 was completely soluble in electrospinnable solvents such as CHCl3DMF, DMSO, and THF.

Miscellaneous Applications of Biofibers from Renewable Resource

Keywords

Biofiber • Renewable resource • Electrode • Fiber • Chitosan • Nanocomposite • Supercapacitor

Chitosan fibers prepared through the wet-spinning approach were cross-linked with glutaraldehyde and later modified using polyalanine and multiwalled carbon nanotubes for potential use as electrode material for electrical double-layer capacitors [14Dor]. SEM images in Fig. 74.1 show the chitosan fibers modified using polyalanine and with MWCNT. The addition of polyalanine and CNTs onto chitosan fibers resulted in a porous structure shown in Fig. 74.1b. The conductivity of the chitosan/polyalanine/MWCNT fibers was 5.34 x 10-2 S cm-1 compared to

7.2 x 10-2 S cm-1 for the chitosan/polyalanine fibers. The nanocomposite fibers had a specific capacitance of 14.5 F cm-2 at a current density of 10 mA cm-2 suggesting that the fibers would be suitable as electrode materials.

In another study, wet spun chitosan fibers were in situ polymerized with aniline to form a biofiber hydrogel that had enhanced chemical and electrochemical actuation in response to pH and electrical stimulus [08Ism]. The presence of aniline was responsible for the electrochemical properties of the fibers. Aniline was found to be aggregated on the surface of the fibers, and the amount of aniline was lower at the center of the fibers. The electrical conductivity of the fibers at room temperature was 2.8 x 10-2 S cm-1, but the strain ratio and response time during electrochemi­cal actuation were dependent on the pH of the electrolyte. Similar approaches have been used to coat natural cellulose fibers with MnO2 and carbon nanotubes for potential use as substrates for supercapacitors [13Gui].

References

[08Ism] Ismail, Y. A., Shin, S. R., Shin, K. M., Yoon, S. G., Shon, K., Kim, S. I., Kim, S. J.: Sensor. Actuat. B 129(2), 834 (2008)

[13Gui] Gui, Z., Zhu, H., Gillette, E., Han, X., Rubloff, G. W., Liangbing, R., Lee, S. B.: ACS Nano 7(7), 6037 (2013)

[14Dor] Dorraji, S. M.S., Ahadzadeh, I., Rasoulifard, M. H.: Int. J. Hydrogen Energy 39, 9350 (2014)

Regenerated Cellulose Fibers

Keywords

Regenerated cellulose • Viscose • Rayon • Cellulose solvent

The production of regenerated cellulose fibers as early as the 1930s resulted in the generation of a new class of fibers. For several decades, the production of regenerated cellulose fibers such as viscose rayon and cuprammonium rayon was extensively done, and these fibers were considered to be ideal substitutes for the natural cellulose fibers. Traditionally, regenerated cellulose fibers were produced using wood as a source for cellulose. Regenerated cellulose fibers generally termed “rayon” were produced in various configurations and properties. Figure 15.1 depicts the cross section and Table 15.1 provides the properties of the different types of conventional regenerated cellulose fibers. As seen in the table, considerable variations in properties are observed depending on the cross section and the type (specifically degree of polymerization) of the cellulose used for fiber production. A rather distinguishing feature of the fibers which is also a major limitation of the regenerated cellulose fibers is their considerably lower wet strength compared to their dry strength, whereas the most common natural cellulose fiber cotton becomes stronger when wet. This unique behavior has been demonstrated to be mainly due to the poor crystallinity (30-35 %) of regular viscose fibers.

The advent of the relatively inexpensive synthetic fibers that also had good performance properties resulted in the gradual decline in the production of regenerated cellulose fibers. In addition, the production of regenerated cellulose fibers via the traditional xanthate process results in the generation of by-products that are harmful to the environment. Therefore, the production of regenerated cellulose fibers using the traditional approach is not being followed in developed countries. Although regenerated cellulose fibers are still being produced in consid­erable quantities, the raw materials used, the process of fiber production, and end uses have changed substantially. Modern methods of producing regenerated

Fig. 15.1 Type of cross-section of conventional regenerated cellulose fibers [95Woo, 01Sta]

cellulose are much more environmentally friendly and also use renewable raw materials. In this chapter, the new approaches of producing regenerated cellulose fibers, their advantages and limitations, and properties of the fibers produced are discussed in detail.

Ln UJ

References

[95Woo] Woodings, C. R.: Int. J. Biol. Macromol. 17(6), 305 (1995)

[01Sta] Stana-Kleinschek, K., Kreze, T., Ribitsch, V., Strnad, S.: Colloids Surf. A. 195, 275 (2001)

Natural Protein Fibers

Keywords

Weaver ant • Nanofiber web • Hollow fiber • Protein fiber • Biocompatibility • Electrospun fiber

Weaver ants (Fig. 39.1) belonging to the Oecophylla smaragdina family produce natural silks in the form of nanofibers that are connected together to form webs that resemble a piece of fabric as seen in Fig. 39.2. Fibers in the webs were hollow and had average diameters of 450 nm and had a unique architecture. As seen in Fig. 39.3, ants stick the fibers to form a web, and the connecting places were considerably stronger and resist alkali treatment even at boiling temperature. Although properties of individual fibers produced by the ant were not tested, webs produced by the ants were considerably stronger than electrospun protein nanofiber webs with substantially higher elongation (32 %) as seen in Table 39.1. It would be considerably challenging to produce nanofiber webs, especially with hollow nanofibers in the laboratory. Since ants are social insects unlike spiders, it would be possible to produce unique nanofibers webs by rearing the ants. It was found that the webs could be used as substrates for tissue engineering and could also load high amounts of drugs due to the presence of hollow fibers [11Red]. Other researchers have also reported that fibers in the weaver ant webs have diameters between 266 nm and 3 pm and that the proteins are mostly in the form of random coils and p-sheets [10Sir].

References

[10Sir] Siri, S., Maensiri, S.: Int. J. Biol. Macromol. 46, 529 (2010)

[11Red] Reddy, N., Xu, H., Yang, Y.: Biotechnol. Bioeng. 108(7), 1726 (2011)

Fibers from Biotechnology

Keywords

Bacterial cellulose • Fermentation • Bacteria source • Bacteria growth medium • Cellulose fibril • Wound healing • Bacterial cellulose • Bacterial cellulose production • Bacterial cellulose growth medium • Bacterial cellulose incubation • Bacterial cellulose yield • Bacterial cellulose properties • Static culture • Agitated culture • Agricultural waste • Feedstock • Switch grass • Wheat straw • Mechanical properties • Colored growth medium • Chitosan • Polyaniline • Alginate • Lithium hydroxide • Thiourea • Solvent • Supercapacitor • Antimicrobial membrane • Biocomposite • Conformability • Bacterial cellulose • Bacterial cellulose dissolution • Silk fabrics reinforcement • Silk fabrics • Bacterial cellulose chemical modification • Electrospinning • Novel biohybrid yarn

61.1 Introduction

The production of cellulose by Acetobacter xylinum was reported by A. J. Brown as early as 1886. From that time, bacterial cellulose (BC) has been used for biomedi­cal, environment, agriculture, electronic, food, and industrial applications [98Las, 14Moh]. Unlike most other sources of cellulose, BC does not contain lignin or hemicelluloses, making it ideally suited for various applications. In terms of structure, BC is composed of fibrils that have a width of about 1.5 nm and these fibrils are crystallized into microfibrils. BC has a relatively high level of crystallin­ity (60 %) and the degree of polymerization that can be as high as 16,000-20,000. Young’s modulus of a bacterial cellulose fibril has been reported to be in the range of 15-35 GPa and tensile strength between 200 and 300 MPa. However, other researchers have reported the modulus of a single bacterial cellulose fibril to be as high as 114 GPa, compared to a theoretical cellulose crystal modulus of 160 GPa. In addition to these features, BC has a water holding capacity of up to 100 times it weight and a linear thermal coefficient of expansion of only 0.1 x 10_6k_1. Typical
uses of bacterial cellulose have been as wound dressing. Bioprocess, Xcell, and Biofill are some of the products made from bacterial cellulose that are currently available on the market for wound healing [06Cza, 90Fon]. Other commercial scale applications of bacterial cellulose are in cosmetics, food, and electronics to some extent. The remarkably high wet tensile strength, biocompatibility, high porosity, and ability to be easily formed into various structures are considered to be some of the advantages of using bacterial cellulose for medical applications.