Silk, artificial structure

The structural differences between natural and man-made silk materials determine the property differences. Even minor changes in the chemical structures give rise to a wide range of variability in mechanical properties (Porter et al., 2005). How this happens is both a challenge to understand and to control when fabricating silk artificially. 

The overall performance difference between the artificial fibroin silk and natural silk is induced by many factors. Composition of the spinning dope is critical but not the only factor. Important to understand is that the spinning process which determines the condensed structure of silk is crucial. It suggest that knowing the spinning process details it should be feasible to produce high-performance silk artificially and "design" silk. 

Fu, C., Shao, Z., Fritz, V., 2009. Animal silks their structures, properties and artificial production. Chemical Communications (Cambridge, England) 43, 6515—6529. 

The structures of some natural protein-based materials, such as silk and wool, result in strong, tough fibers. Spiders and silkworms use proteins as a structural material of remarkable strength (Fig. 19.22). Chemists are duplicating nature by making artificial spider silk (Fig. 19.23), which is one of the strongest fibers known. 

Preparation of Sections of the Fibres.—Identification of animal or vegetable fibres is based mainly on the physical characters, the fibres being mostly so transparent that their form and structure, and particularly the thickness of the walls and the form of the internal canal or lumen, are easily observed. In some cases, however, owing to the close resemblance between certain fibres, recognition is doubtful. This is the case, for instance, with the poorer qualities of flax and hemp, and with certain types of artificial silk. In these instances the transverse sections of the fibres are studied, these permitting of observation of the thickness of the walls, the strata... 

First, a review of the properties and the structure of silk is given followed by a discussion on the relationship between molecular composition, assembled protein structure, and mechanical properties. Second, artificial spinning of silk proteins and their bioapplications are emphasized. Finally, the potential role of silk proteins in biomineralization is introduced and discussed. 

Substantially more work is done on the elucidation of properties of regenerated silk, compared to film materials in view of the difficulty to fabricate uniform films. From Table 3, it can be concluded that the mechanical properties of man-made silk materials are inferior to the natural ones. This can be attributed to the fact that the final properties are greatly affected by the structural hierarchies. Typically, the "artificial" materials do not contain the controllable microstructure and supramolecular structure that natural ones possess. 

The wet spinning of regenerated spidroin was reported in the early 1990s by Jelinski et al. They dissolved spider silk in hexafluoroisopro-panol (HFIP) at a concentration of 2.5 wt% to produce an artificial fiber using water, methanol, isopropanol, and acetone as coagulation bath. The reconstituted silk could only be shaped in acetone but the structure... 

Pioneering work in fibroin wet spinning can be traced back to 1930s. After that, little work has been done until the late 1980s, when more research was done to investigate the spinning dope systems, and structure and properties of the artificial fibroin silk. The composition of the dope is very important to the properties of the final fiber. Several kinds of solvents, such as LiBr—EtOH, Ca(NOo,)2—MeOH, formic acid, HFIP, hexafluoro acetone (HFA), and so on, are used to prepare the spinning dope (Table 4). Very recently, an ionic liquid was used as dope solvent (Phillips et al., 2005). 

Zhao, C.H., Yao, J.M., Masuda, H., Kishore, R., and Asakura, T. "Structural characterization and artificial fiber formation of Bombyx mori silk fibroin in hexafluoro-iso-propanol solvent system". Biopolymers 69(2), 253-259 (2003). 

The result was one of the most extraordinary sets of papers in 20th-century science. Seven appeared together, dominating the May 1951 issue of the Proceedings of the National Academy of Sciences. There was a detailed description of the pleated sheet for silk. There was a new model for the protein in feathers, and new ideas about the structure of artificial proteins, globular proteins, and muscle. 

Clarifying the structure and functions of protein materials in the solid state provides an index with respect to the design of artificial biomaterials. Solid-state NMR has been used as a powerful means for elucidating structure and dynamics in addition to the X-ray diffraction method [le]. The structure and dynamics of some fibrous proteins, such as wool, silk, collagen, tropomyosin, etc., have been characterized using characteristic solid-state NMR chemical shifts as stated above, and much more new information obtained in addition to the results provided by X-ray diffraction. And the individual advantages of these two methods are complementary with each other. Details of appli-... 

Commercial and Artificial Processing. Commercially, silkworm cocoons are extracted in hot soapy water to remove the sticky sericin protein. The remaining fibroin or structural silk is reeled onto spools, yielding approximately 300—1200 m of usable thread per cocoon. These threads can be dyed or modified for textile applications. Production levels of silk textiles in 1992 were 67,000 metric tons worldwide. The highest levels were in China, at 30,000 t, followed by japan, at 17,000 t, and other Asian and Oceanian countries, at 14,000 t (24). Less than 3000 metric tons are produced annually in each of eastern Europe, western Europe, and Latin America almost no production exists in North America, the Middle East, or Africa. 1993 projections were for a continued worldwide increase in silk textile production to 75,000 metric tons by 1997 and 90,000 metric tons by 2002 (24). 

In the mid twenties several circumstances permitted a revised orientation of both content and style of areas of research at the Central Research Laboratory. In 1925 the Technical Committee (TEA) of I.G. Farben discussed the possibilities for producing artificial fibres. At this time, I.G. Farben was the second largest producer of artificial fibres in Germany. Therefore polymer chemistry became more important for the company at the same time as dyestuffs chemistry lost its former position. However, the science of synthetic, semi-synthetic and natural polymers was not yet established in the same way as structural chemistry was for organic dyestuffs, pharmaceuticals, and intermediates. Colloid chemists regarded substances such as cellulose, silk, and wool as... 

Most artificial protein polymers expressed to date have been inspired by sequences found in natural proteins. Structural proteins such as elastin, collagen, and silk exhibit usefid material properties and many researchers have attempted to capture these properties in well-defined engineered protein polymers. There is a wealth of knowledge related to these systems that will be discussed in depth in individual chapters of this volume. We will highlight only seleaed aspects of these protein polymers here. 

A direct application of these results to the behavior of high polymeric substances is not, however, feasible, because we seldom meet with rigid rods in the latter, but usually have to assume the existence of more or less mobile thread or worm-like structures. For this reason, Eirich and Sverak have extended their experiments to flexible model materials, which could be realized in the form of swollen fine filaments of artificial silk, or as thin flexible filament crystals (cholesterol, azo dyes etc.). Such preparations prove to be very uniform in regard to length and axial ratio of the separate particles, but they offer an irregular aspect under the microscope because of their flexibility. 

Commercial and Artificial Processing. Commercially, silkworm cocoons are extracted in hot soapy water to remove the sticky sericin protein. The remaining fibroin or structural silk is reeled onto spools, yielding approximately 300-1200 m of usable thread per cocoon. These threads can be dyed or modified... 

PU is a strong, hard-wearing, tear-resistant, flexible, oil-resistant, and blood-compatible polymer. The functional properties of natural macromolecules can be merged with those of synthetic polymers having controllable structures and properties for the production of polymer/protein hybrids. In tissue engineering, silk fibroin/PU blend film can be used as scaffold material for artificial blood vessels [466] (Figure 2.62). Bacterial synthesized cellulose, which was designed... 

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