Chitin, Chitosan, and Alginate Fibers

Keywords

Alginate fiber • Tensile properties • Cellulose • Nanocrystal • Montmorillonite • Carbon nanotube • Cross-linking

In view of the relatively low mechanical properties of alginate fibers, efforts have been made to improve the tensile properties and stability by adding various additives and also by cross-linking. In one such attempt, cellulose nanocrystals (CNC) were isolated and used as fillers to improve the properties of alginate fibers with the expectation that cellulose and alginate would have good compatibility and that the negatively charged sulfate groups on cellulose crystals would have electro­static interaction with the Ca2+ ions in the coagulation bath [10Ure]. Various levels of the nanocrystals (0-10 %) were added into the fibers, and the fibers were extruded at different jet-stretch ratios. It was found that increasing the level of CNC in the fibers decreased, whereas increasing the jet stretch increased the strength and elongation. The tenacity (0.1-0.22 g/den) of the fibers was very low even with the addition of CNC and extrusion at the highest jet speed possible [10Ure]. However, the fibers had about 38 % increase in tenacity and 123 % increase in modulus due to the addition of the filler and optimization of jet speed [10Ure]. Further investigation on the arrangements of the CNC in the alginate fibers showed that the degree or orientation decreased with increasing load of CNC. The interaction of the nanoparticles with the polymer introduced twists opposite to the direction of drawing, and at high concentrations, the crystallites oriented them­selves in a spiral manner in the alginate matrix, similar to the arrangement of fibrils in native cellulose [11Ure]. Such spiral arrangement decreased the strength and modulus of the fibers. However, it was reported that fibers with improved toughness could be obtained by controlling the processing conditions without sacrificing the strength of the fibers [11Ure]. Table 31.1 presents some of the properties of the fibers at two different jet-stretch ratios and different levels of nanoparticle loading.

Table 31.1 Properties of alginate fibers containing various levels of cellulose nanocrystals CNC load Spiral angle Width Tenacity Modulus Toughness (%) (deg) (deg) (x102 g/den) (g/den) (x102 g/den)

2.4 jet stretch 0 0 — 14.9 ± 0.4 3.1 ± 0.1 3.3 ± 0.2 2 3.6 ± 0.8 21 ± 5 15.2 ± 0.5 2.6 ± 0.1 3.9 ± 0.1 5 3.8 ± 0.5 15 ± 2 14.4 ± 0.3 2.8 ± 0.1 3.7 ± 0.1 10 — 16 ± 2 12.9 ± 0.3 2.5 ± 0.1 3.0 ± 0.1 20 3.6 ± 0.2 18 ± 0.3 17.8 ± 0.4 6.8 ± 0.2 2.4 ± 0.1

Max jet stretch 0 — — 14.9 ± 0.4 3.1 ± 0.1 3.3 ± 0.2 2 0 17 ± 3 18.0 ± 0.7 3.8 ± 0.2 3.6 ± 0.2 5 0 18 ± 1 18.7 ± 0.5 5.5 ± 0.1 2.4 ± 0.1 10 0 18 ± 0.6 20.6 ± 0.7 6.9 ± 0.2 2.5 ± 0.1 25 3.0 ± 0.2 17 ± 0.5 16.4 ± 0.6 6.2 ± 0.2 1.6 ± 0.1 20 3.2 ± 0.2 19 ± 0.4 16.4 ± 0.5 6.9 ± 0.2 1.8 ± 0.1 From Urena-Benavides et al. [10Ure]

Two types of nanoadditives p-tricalcium phosphate (TCP) (110 nm) or montmo — rillonite (MMT) (20-1,000 nm) were added (3 % on weight of the polymer) into calcium alginate fibers, and the influence of the additives on the fiber formation process and the properties of fibers were exhaustively investigated [10Bog]. The addition of the nanoadditives did not affect the viscosity and also did not alter the % crystallinity or crystal structure. Some of the properties of the fibers containing TCP or MMT and obtained at different draw ratios are compared in Table 31.2. The addition of the nanoparticles changed the water retention and the strength and elongation of the fibers. Increasing the draw ratio increased the strength, but the elongation showed varying trend probably because of the presence of the nanoadditives and also due to the varying calcium content in the fibers. Similarly, tricalcium phosphate (TCP) and silica in nanoform were added into zinc alginate fibers to improve the functional properties of the fibers. The addition of TCP was found to decrease the strength, whereas silica increased the strength of the fibers [09Mik, 10Mik]. Tensile strength of up to 3.3 g/den was obtained when silica was added compared to 2.6-2.7 g/den for fibers without the nanoadditive. The strength of the fibers with TCP was between 2.3 and 2.8 g/den compared to 2.8-3.3 g/den for fibers without TCP.

Silver nanoparticles were loaded onto alginate hydrogel fibers intended for wound healing [12Nei]. Alginate was spun into calcium chloride bath and formed into fibers. Later, the fibers were immersed in water-acetone mixture containing glutaraldehyde to cross-link the fibers. Silver nanoparticles were loaded onto the fibers via ion exchange by immersing in silver nitrate solution, and the excess of silver nitrate was removed. Then, the fibers were introduced into sodium borohy — dride solution to convert the silver ions into metallic silver. After this step, the fibers were washed three times in water and air-dried. The fibers obtained had

Table 31.2 Properties of alginate fibers containing various levels of tricalcium phosphate (TCP) and montmorillonite (MTT) (from Bogun et al. [10Bog] Fiber type Draw ratio Total pore volume (cm3/g) Water retention (%) Crystallinity (%) Tenacity (g/den) Elongation (%) Alginate + TCP 50 0.196 97.6 27 1.8 6.4 Alginate + TCP 70 0.078 93.0 27 2.8 10.4 Alginate + TCP 90 0.244 85.6 28 2.5 9.1 Alginate + TCP 110 0.214 86.3 26 2.2 7.4 Alginate + TCP 120 0.137 102.2 27 2.1 7.4 Alginate + MMT 50 0.127 107.5 26 2.5 9.9 Alginate + MMT 70 0.091 98.2 27 2.9 10.6 Alginate + MMT 90 0.064 101.3 26 2.8 10.0 Alginate + MMT 110 0.140 96.5 26 2.5 9.3 Alginate + MMT 120 0.172 118.7 29 2.2 10.4

superabsorbent properties and absorbed more than ten times their own weight of water because of the formation of ionizable COOH groups when carboxyl groups in alginate reacted with the catalyst HCl used during cross-linking. It was also reported that the chemically cross-linked fibers had a 20-fold swelling compared to about threefold for the ionically cross-linked fibers. The inclusion of silver nitrate did not affect the cytocompatibility of the fibers, and silver nanoparticles were also considered to be nontoxic within the range studied in this research. The presence of silver nanoparticles increased wound healing and also lowered inflammatory response. Figure 31.1 shows that the presence of silver nanoparticles led to higher epidermal thickness on wounds incised on mice suggesting better growth of fibroblasts.

A natural hydroxyapatite with average specific surface of 73.6 m2/g and particle size from 30 to 500 nm was used as an additive in calcium alginate fibers to improve tensile properties and porosity and make the fibers suitable for bone tissue engi­neering [09Bog]. To prepare the fibers, sodium alginate (7.4 %) was dissolved in distilled water with 3 % additive based on the weight of the alginate. After extrusion, the fibers were passed into a coagulation bath containing CaCl2 and HCl to substitute sodium with calcium and form calcium alginate fibers. Fibers were drawn to various extents in a two-step process. Some of the properties of the

Poitopttttive d«y 10 Fig. 31.1 Growth of epidermal tissue (after 10 days) for the alginate fibers containing various levels of silver nanoparticles from Neibert et al. [12Nei]. Reproduced with permission from Elsevier

fibers obtained with and without hydroxyapatite are listed in Table 31.3. Although the addition to hydroxyapatite did not show a major change in tensile properties, significant changes were observed in terms of % crystallinity and pore volume. The changes were thought to be due to the higher amounts of calcium when hydroxyap­atite was present. It was suggested that the fibers obtained were suitable for use in biocomposites intended for bone tissue regeneration [09Bog]. A unique aspect of the fibers was the relatively high moisture sorption (24-25 %) compared to the common cellulose and protein fibers.

Carbon nanotubes (23 %) were added into alginate fibers by electrostatic assem­bly, and the blend fibers were expected to be suitable for use in supercapacitors, artificial muscles, biomedical sensors, and other applications [11Sa]. Single-walled carbon nanotubes were coated with ionic surfactant sodium dodecyl sulfate (SDS) and then added into the sodium alginate solution. The solution was aged overnight and later wet spun using a syringe into an aqueous solution containing calcium chloride for the fibers to precipitate. A digital picture of the alginate-nanotube fiber formation is shown in Fig. 31.2. The addition of the nanotubes substantially increased the modulus and strength. Morphologically, fibers were found to fibrillate (Fig. 31.3) with increase in fibrillation as the concentration of the nanotubes increased [11Sa].

Nanosilica (25 nm diameter) was added into sodium alginate solution (5 %) and extruded into a calcium chloride bath. The addition of nanosilica increased the tenacity from 5.8 to about 8.2 g/den, and the elongation also increased from about 13 to 16 %. However, both the strength and elongation decreased at high levels of

Table 31.3 Influence of draw ratio on the properties of alginate and alginate hydroxyapatite blend fibers Fiber type Draw ratio Tenacity (g/den) Elongation (%) Crystallinity (%) Calcium content (%) Moisture sorption3 (%) Alginate + TCP 50 2.7 ±0.08 8.8 ±0.5 27 ±0.8 8.7 24.6 ±0.7 Alginate + TCP 70 3.2 ±0.08 10 ±0.4 31 ± 0.9 8.9 24.4 ±0.7 Alginate + TCP 90 2.7 ± 0.09 8.6 ±0.3 31 ± 0.9 9.0 24.5 ±0.7 Alginate + TCP 110 2.7 ±0.1 8.4 ±0.5 30 ±0.9 8.7 24.4 ±1.0 Alginate + TCP 120 2.5 ±0.1 7.7 ±0.4 32 ± 1.0 9.1 24.4 ±1.0 Alginate + hydroxyapatite 50 2.8 ±0.2 9.3 ±0.6 26 ±0.8 9.0 25.4 ±1.0 Alginate + hydroxyapatite 70 2.9 ±0.1 9.8 ±0.4 25 ±0.8 9.5 24.2 ± 0.7 Alginate + hydroxyapatite 90 2.7 ±0.1 9.4 ± 0.5 26 ±0.8 9.1 24.4 ±1.0 Alginate + hydroxyapatite 110 2.3 ±0.1 7.8 ±0.5 28 ±0.8 9.8 24.1 ±0.7 Alginate + hydroxyapatite 120 2.5 ±0.08 9.7 ±0.5 29 ±0.7 9.8 24.5 ±1.0 a65 % Humidity Reproduced from Bogun et al. [09Bog]

Fig. 31.2 Process of producing alginate-nanocomposite fibers. From Sa and Kornev [11Sa]. Reproduced with permission from Elsevier

Fig. 31.3 Confocal laser scanning image shows the striations formed in the alginate-nanotube composite fibers after addition of 23 % nanotubes (right) compared to fibers without any nanotubes (left). Reproduced with permission from Elsevier [11Sa]

silica (8 % or higher). Moisture absorption of the fibers did not change significantly, whereas water retention decreased from 72 to 54 % (9 % silica).

Porous calcium alginate fibers were loaded with TiO2 photocatalysts for mem­brane filtration process. It was expected that the high porosity in the alginate fiber could help to embed the catalysts inside the fiber and help to protect the catalysts from destabilization [12Pap]. Photodegradation of pollutants such as methyl orange was much higher in the alginate fibers containing TiO2 compared to the powder TiO2 due to the high surface area and excellent dispersion and stability of the catalysts in the fiber matrix [12Pap].

Although alginates have been widely used for medical applications, alginates lack specific cellular interactions that limit their use for regenerative applications [05Hou]. Chemical modifications have been used to make alginate more suitable for medical applications, but such modifications are often detrimental to cells and to sensitive biological agents. To avoid chemical modifications, a physical entrapment process was developed to incorporate bioactive molecules within alginate fibers. Fibers were first pre-swollen in sodium chloride/calcium chloride solutions and then immersed in a solution containing various concentrations of rhodamine-tagged polyethylene glycol. Later, the fibers were cross-linked by immersing in barium chloride (5 % w/v) for 15 min. A poly(L-lysine) (PLL) was coupled to GRCDS peptide and entrapped in the alginate fibers. Cell adhesion and proliferation studies on the fibers were done using mouse fibroblast (3T3) cells. It was reported that entrapping PLL-GRDS in the fibers promoted cell proliferation and growth.

Sodium alginate (3 %) was formed into alginic acid gel fibers and later dehydrated and cross-linked with glutaraldehyde to form superabsorbent filament fibers [00Kim]. Cross-linking was done at 50 °C for 4 h by immersing the fibers in various concentrations of glutaraldehyde and 0.1 % HCl. An absorbency of about 80 g/g of fibers was obtained for saline, whereas the absorbency was about 50 % for synthetic urine. Increasing the extent of cross-linking decreased the absorbency considerably. Uncross-linked fibers had tenacity of 1.2 g/den and elongation of

27.1 %. Cross-linking slightly decreased the breaking tenacity, but the elongation decreased by about 35 % at high levels of cross-linking.

References

[00Kim] Kim, Y., Yoon, K., Ko, S.: J. Appl. Polym. Sci. 78, 1797 (2000)

[05Hou] Hou, Q., Freeman, R., Buttery, L. D.K., Shakesheff, K. M.: Biomacromolecules 6, 734 (2005)

[09Bog] Bogun, M., Mikolajczyk, T., Rabiej, S.: J. Appl. Polym. Sci. 114, 70 (2009)

[09Mik] Mikolajczyk, T., Bogun, M., Kurzak, A., Szparaga, G.: Fibers Text East. Eur. 17(2), 12-18 (2009)

[10Bog] Bogun, M., Mikolajczyk, T., Rabiej, S.: Polym. Comp. 31, 1321 (2010)

[10Mik] Mikolajczyk, T., Bogun, M., Rabiej, S., Krol, P.: Fibers Text East. Eur. 18(6), 39-44

(2010)

[10Ure] Urena-Benavides, E. E., Brown, P. J., Kitchens, C. L.: Langmuir 26(17), 14263 (2010) [11Sa] Sa, V., Kornev, K. G.: Carbon 49, 1859 (2011)

[11Ure] Urena-Benavides, E. E., Kitchens, C. L.: Macromolecules 44, 3478 (2011)

[12Nei] Neibert, K., Gopishetty, V., Grigoryev, A., Tokarev, I., Al-Hajaj, N., Vorstenbosch, J., Philip, A., Minko, S., Maysinger, D.: Adv. Healthcare Mater. 1, 621 (2012) [12Pap] Papageorgiou, S. K., Katsaros, F. K., Favvas, E. P., Romanos, G. E., Athanasekou, C. P., Beltsios, K. G., Tzialla, O. I., Falaras, P.: Water Res. 46, 1858 (2012)