Silkworm silk fibers

Silk fibers or monolayers of silk proteins have a number of potential biomedical applications. Biocompatibility tests have been carried out with scaffolds of fibers or solubilized silk proteins from the silkworm Bombyx mori (for review see Ref. [38]). Some biocompatibility problems have been reported, but this was probably due to contamination with residual sericin. More recent studies with well-defined silkworm silk fibers and films suggest that the core fibroin fibers show in vivo and in vivo biocompatibility that is comparable to other biomaterials, such as polyactic acid and collagen. Altmann et al. [39] showed that a silk-fiber matrix obtained from properly processed natural silkworm fibers is a suitable material for the attachment, expansion and differentiation of adult human progenitor bone marrow stromal cells. Also, the direct inflammatory potential of silkworm silk was studied using an in vitro system [40]. The authors claimed that their silk fibers were mostly immunologically inert in short and long term culture with murine macrophage cells. 

Silkworms spin composites of two silk fibers out of two converging silk glands. These fibers are surrounded by a glue-like seridn protein coating that holds the fibers and thus the cocoons together. The individual silkworm silk fibers (brin) are 10-12 xm in diameter with a triangular cross section. 

B.B. Mandal, S.C. Kundu, Biospinning by silkworms silk fiber matrices for tissue engineering applications, Acta Biomater. 6 (2) (2010) 360—371. 

Silkworm silk is another protein-based fiber produced naturally by the silkworm, B. Mori or other varieties of moth, and it is the only naturally and commercially produced continuous filament. Silkworm silk fiber is composed of a fibroin core and a sericin casing, and is relatively rigid because the sericin causes the filaments to adhere to one another. After the desericin treatment (with alkali treatment) silk has high tenacity, high luster, and good dimensional stability. A triangle cross section is its typical stracture, which results in the luster of silkworm silk (Fig. 2.10). 

Morphology of cells on natural silkworm silk fibers and nanofibers after 7 days in culture. 

Correspondingly, AFM and scanning electron microscope (SEM) results show that both spider and silkworm silk fibers are composed of numerous nano fibrils of diameter about 20-30nm, which are separated into crystalline and non-crystalline... 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

Fibers obtained from living organisms are known as animal fibers, e.g., wool, which is obtained from domestic sheep silk fiber, which is produced by the silkworm... 

The feeling of a spider web may be unsettling, but a similar natural material has been used for centuries to make silk fabric that is prized for its smooth texture. Silkworms produce the silk fibers used to make clothing. They feast on mulberry leaves and convert the molecules from these leaves into silk, from which they spin cocoons. 

The early work focused on a particular silkworm, Bombyx mori, that lives on mulberry bushes. There are other silkworms each with its own special properties, but in general most silk is still derived from the original strain of silkworm. Crystalline silk fiber is about four times stronger than steel on a weight basis. 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

It has been proposed, as for other silks, that the peptide chains in the stalks are aligned perpendicular to the long dimension of the fiber and are folded back on themselves many times to form a (3 sheet with only 8 residues between folds.3 The chains of silk fibroin, the major protein of silkworm silk, contain 50 repeats of the sequence 13... 

Silk is an animal fiber valued for its texture, strength, and luster. First prepared in ancient China, silk fabric was expensive, luxurious, and soft its popularity led to the development of a trade route known as the Silk Road leading from Asia to Europe. Early American entrepreneurs such as Benjamin Franklin promoted the silk industry in the colonies. Silkworms spin cocoons that are collected, steamed, and unwound to obtain the silk fiber by a process known as reeling. A number of fibers are twisted together to form a thread of raw silk. Threads are combined, cleaned, stretched, dyed, and woven into fine fabrics. Silk fibers have great tensile strength and are sometimes used in cordage. 

Although the amino acid sequence as well as the secondary structure of fibroin differs from those of spidroin, the fibers spun from these proteins, that is, silkworm silk and spider silk have comparable mechanical properties. These may be attributed to the structural characteristics, both at the molecular and filament level. The superior mechanical properties of silk-based materials, such as films, coatings, scaffolds, and fibers produced using reconstituted or recombinant silk proteins, are determined by their condensed structures. 

In view of the various level of structural organization, it is worthwhile to draw a comparison between natural silk fibers (silkworm silk and spider silk) and man-made silk-based materials. 

As to fibers, it was reported that the inferior mechanical properties of silk from cocoons compared to spider silk result from the silkworm spinning process. If silkworm silk is processed at a constant pulling speed rather than constant force pulling, it possesses excellent properties, approaching the spider dragline silk (Shao and Vollrath, 2002). This suggests that the silkworm silk has the potential to produce better fibers, and the regenerated fibroin, which is easy to harvest, has the possibility to be fabricated into a reconstituted super-fiber. 

Hudson et al. used Ca(NC>3)2—MeOH to dissolve silkworm silk and did not manage to obtain good fibers directly. They also casted film from such a solution and then dissolved the film in formic acid and trifluoroacetate acid... 

Zarkoob et al. (1998, 2004) were the first to report on the electrospinning of silkworm silk and Nephila clavipes dragline protein. They used an HFIP solution of protein as the spinning dope. The resulting fibers had a wide distribution in diameter and the continuity during spinning could be significantly improved. 

Zhou, L., Terry, A.E., Huang, Y.F., Shao, Z.Z., and Chen, X. "Metal element contents in silk gland and silk fiber of Bombyx mori silkworm". Acta Chimica Sinica 63(15), 1379-1382 (2005b). 

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