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 %

Type of silkworm	Fineness (denier)	Tenacity (g/den)	Elongation (%)	Modulus (g/den)
Bagworm	0.9 ± 0.1	3.2 ± 1.0	15.3 ± 6.2	45 ± 12
Tasar	1.6-2.9	3.9-4.5	26-39	67-70
Eri	1.3-2.7	1.9-3.5	24-27	29-31
Muga	1.9-3.2	4.6-4.9	26-41	66-74

Table 36.1 Comparison of some of the tensile properties of fibers obtained from various silkworms [00Raj, 10Red1]

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

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

Fig. 36.4 SEM images depicting the hairlike protrusions seen in Anaphe panda cocoon hair (a) and the spinelike structure seen in Gonometa postica spine (b). From Teshome et al. [12Tes]

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,

Fig. 36.5 Digital picture of a bagworm cocoon with plant material attached on the surface (left). The right image shows the inside of the cocoon that has appearance similar to a woven bag

Fig. 36.6 SEM images of the fibers extracted from bagworm cocoons [10Red1]

Table 36.2 Comparison of the properties of natural silk fibers obtained from bag worms with B. mori and two common wild silk fibers [10Red1] Fiber Bag worm B. mori A. mylitta P. ricini Fineness (denier) 0.9 ± 0.1 0.4-1.1 4.7-10.7 2.3-3.6 Breaking tenacity (g/den) 3.2 ± 1.0 4.3-5.2 2.5-4.5 1.9-3.5 Breaking elongation (%) 15.3 ± 6.2 10.0-23.4 26-39 24-27 Young’s modulus (g/den) 45 ± 12 84-121 66-70 29-31

Fig. 36.7 Digital images of the unique perforated and shiny cocoons produced by Argema mittrei (left) and Argema mimosae (right)

Fig. 36.8 SEM image of the surface of fibers obtained from Argema mittrei

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

Table 36.3 Properties of silk fibers produced by uncommon silkworms Insect Fiber fineness (denier) Tenacity (g/den) Breaking elongation (%) Young’s modulus (g/den) References Cecropia 2.0 3.8 ± 0.6 14.8 ± 6.8 68 ± 9.9 [10Red2] A. atlas 2.0 4.3 ± 0.8 18.7 ± 9.3 48 ± 18 [13Red1] A. mittrei 12.1 1.6 ± 0.4 10.7 ± 0.9 46 ± 7.1 [12Red2] Bagworms 0.9 3.2 ± 1.0 15.3 ± 6.2 45 ± 12 [10Red1] E. calleta 2.0 2.8 ± 0.7 11.8 ± 5.5 58 ± 18 [13Red2] R. lebeau 2.7 3.3 ± 1.2 12.3 ± 4.4 64 ± 17 [13Red3]

Table 36.4 Comparison of the tensile properties of silk fibers produced by various Saturniidae insects [10Red3] Silk-producing insect Fineness (Denier) Tensile strength (g/den) Breaking elongation (%) Young’s modulus (g/den) C. Hercules 1.5 5.0 ± 1.2 12.1 ± 5.1 87 ± 17 H. euryalus 1.7 2.7 ± 0.9 11.1 ± 5.8 59 ± 18 R. hesperis 1.7 3.3 ± 0.8 9.5 ± 4.4 71 ± 16 E. calleta 2.0 2.8 ± 0.7 11.8 ± 5.5 58 ± 18 R. lebeau 2.2 3.1 ± 0.8 15.5 ± 6.7 54 ± 14 A. oculae 2.9 3.1 ± 0.8 14.5 ± 6.6 57 ± 15 H. gloveri 4.0 2.8 ± 0.4 19.3 ± 6.9 48 ± 13 C. multifenestrata 7.8 0.9 ± 0.2 4.1 ± 2.7 39 ± 6 B. mori 0.4—1.1 4.3-5.2 10.0-23.4 84-121 A. mylitta 4.7-10.7 2.5-4.5 26-39 66-70 P. ricini 1.3-2.7 1.9-3.5 24-28 29-31

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

Fig. 36.9 Digital picture revealing the outer (a), middle (b), and inside layers (c) of cecropia cocoons

Table 36.5 Properties of silk fibers obtained from the three layers in cecropia cocoons compared with B. mori silk and common wild silks [10Red2] Fiber Cecropia B. mori A. mylitta P. ricini Outer Intermediate Inner Fineness (denier) 1.7 2.0 1.7 0.4—1.1 4.7—10.7 2.3—3.6 Strength (g/den) 4.3 ± 0.7 3.8 ± 0.6 4.3 ± 1.1 4.3-5.2 2.5—4.5 1.9—3.5 Elongation (%) 12.6 ± 6.5 14.8 ± 6.8 12.6 ± 5.9 10.0— 23.4 26—39 24—27 Modulus (g/den) 92 ± 15 68 ± 9.9 82 ± 19 84—121 66—70 29—31 Moisture regain (%) 13.4 12.6 10.5 8.5 10.5 10.0

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.

Fig. 36.10 SEM image of the protein fibers produced from Aposthonia gurneyi. Scale bar is 1 pm. Reproduced from Okada et al. [08Oka] with permission from Elsevier

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