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Four of the most important domesticated silk moths. Top to bottom:
"Bombyx mori, "Hyalophora cecropia, "Antheraea pernyi, "Samia cynthia.
From "Meyers Konversations-Lexikon (1885–1892)
Silk
""Silk (Chinese characters).svg
"Silk" in "seal script (top), Traditional (middle), and Simplified (bottom) Chinese
"Traditional Chinese
"Simplified Chinese
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A silk-producing "raspy cricket

Silk is a natural "protein "fiber, some forms of which can be "woven into "textiles. The protein fiber of silk is composed mainly of "fibroin and is produced by certain insect larvae to form cocoons.[1] The best-known silk is obtained from the "cocoons of the "larvae of the "mulberry "silkworm "Bombyx mori reared in captivity ("sericulture). The shimmering appearance of silk is due to the triangular "prism-like structure of the silk fibre, which allows silk cloth to refract incoming light at different "angles, thus producing different colors.

Silk is produced by several insects, like silk worms but generally only the silk of moth caterpillars has been used for textile manufacturing. There has been some research into other types of silk, which differ at the molecular level.[2] Silk is mainly produced by the "larvae of insects undergoing "complete metamorphosis, but some insects such as "webspinners and "raspy crickets produce silk throughout their lives.[3] Silk production also occurs in "Hymenoptera ("bees, "wasps, and "ants), "silverfish, "mayflies, "thrips, "leafhoppers, "beetles, "lacewings, "fleas, "flies, and "midges.[2] Other types of "arthropod produce silk, most notably various "arachnids such as "spiders.

Contents

Etymology[edit]

The word silk comes from "Old English sioloc, from "Greek σηρικός serikos, "silken", ultimately from an Asian source (cf. Chinese si "silk", Manchurian sirghe, Mongolian sirkek).[4]

History[edit]

Wild silk[edit]

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Woven silk textile from tomb no 1. at "Mawangdui in "Changsha, "Hunan province, "China, from the "Western Han Dynasty, 2nd century BC

Several kinds of "wild silk, which are produced by "caterpillars other than the "mulberry silkworm, have been known and used in "China, "South Asia, and "Europe since ancient times. However, the scale of production was always far smaller than for cultivated silks. There are several reasons for this: first, they differ from the domesticated varieties in colour and texture and are therefore less uniform; second, cocoons gathered in the wild have usually had the pupa emerge from them before being discovered so the silk thread that makes up the cocoon has been torn into shorter lengths; and third, many wild cocoons are covered in a mineral layer that prevents attempts to reel from them long strands of silk.[5] Thus, the only way to obtain silk suitable for spinning into textiles in areas where commercial silks are not cultivated was by tedious and labor-intensive "carding.

Commercial silks originate from reared silkworm pupae, which are bred to produce a white-colored silk thread with no mineral on the surface. The pupae are killed by either dipping them in boiling water before the adult moths emerge or by piercing them with a needle. These factors all contribute to the ability of the whole cocoon to be unravelled as one continuous thread, permitting a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.[6][7] A technique known as "demineralizing allows the mineral layer around the cocoon of wild silk moths to be removed,[8] leaving only variability in color as a barrier to creating a commercial silk industry based on wild silks in the parts of the world where wild silk moths thrive, such as in Africa and South America.

"Genetic modification of domesticated silkworms is used to facilitate the production of more useful types of silk.[9]

China[edit]

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A painting depicting women inspecting silk, early 12th century, ink and color on silk, by "Emperor Huizong of Song.
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Portrait of a silk merchant in Guangzhou, "Qing dynasty, from "Peabody Essex Museum

Silk was first developed in ancient China.[10][11]

The earliest example of silk has been found in tombs at the "neolithic site "Jiahu in "Henan, and dates back 8,500 years.[12][13] Silk fabric from 3630 BC was used as wrapping for the body of a child from a "Yangshao culture site in Qingtaicun at "Xingyang, Henan.[10][14]

Legend gives credit for developing silk to a Chinese empress, "Leizu (Hsi-Ling-Shih, Lei-Tzu). Silks were originally reserved for the Emperors of China for their own use and gifts to others, but spread gradually through "Chinese culture and trade both geographically and socially, and then to many regions of "Asia. Because of its texture and lustre, silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants. Silk was in great demand, and became a staple of pre-"industrial international "trade. In July 2007, archaeologists discovered intricately woven and dyed silk "textiles in a tomb in "Jiangxi province, dated to the Eastern "Zhou Dynasty roughly 2,500 years ago.[15] Although historians have suspected a long history of a formative textile industry in ancient China, this find of silk textiles employing "complicated techniques" of weaving and dyeing provides direct evidence for silks dating before the "Mawangdui-discovery and other silks dating to the "Han Dynasty (202 BC-220 AD).[15]

Silk is described in a chapter on mulberry planting by Si Shengzhi of the Western Han (206 BC – 9 AD). There is a surviving calendar for silk production in an Eastern Han (25–220 AD) document. The two other known works on silk from the Han period are lost.[10] The first evidence of the long distance silk trade is the finding of silk in the hair of an "Egyptian "mummy of the 21st dynasty, c.1070 BC.[16] The silk trade reached as far as the "Indian subcontinent, the "Middle East, "Europe, and "North Africa. This trade was so extensive that the major set of trade routes between Europe and Asia came to be known as the "Silk Road.

The "Emperors of China strove to keep knowledge of "sericulture secret to maintain the Chinese "monopoly. Nonetheless sericulture reached "Korea with technological aid from China around 200 BC[17] the ancient "Kingdom of Khotan by AD 50,[18] and India by AD 140.[19]

In the ancient era, silk from China was the most lucrative and sought-after luxury item traded across the Eurasian continent,[20] and many civilizations, such as the ancient Persians, benefited economically from trade.[20]

India[edit]

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Silk "sari weaving at Kanchipuram

Silk has a long history in India. It is known as Resham in eastern and north India, and Pattu in southern parts of "India. Recent archaeological discoveries in "Harappa and "Chanhu-daro suggest that "sericulture, employing "wild silk threads from native "silkworm species, existed in "South Asia during the time of the "Indus Valley Civilization (now in "Pakistan) dating between 2450 BC and 2000 BC, while "hard and fast evidence" for silk production in China dates back to around 2570 BC.[21][22] Shelagh Vainker, a silk expert at the "Ashmolean Museum in Oxford, who sees evidence for silk production in China "significantly earlier" than 2500–2000 BC, suggests, "people of the Indus civilization either harvested silkworm cocoons or traded with people who did, and that they knew a considerable amount about silk."[21]

India is the second largest producer of silk in the world after China. About 97% of the raw mulberry silk comes from five Indian states, namely, "Andhra Pradesh, "Karnataka, "Jammu and Kashmir, "Tamil Nadu and "West Bengal.[23] North Bangalore, the upcoming site of a $20 million "Silk City" "Ramanagara and "Mysore, contribute to a majority of silk production in Karnataka.[24]

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Antheraea assamensis, the endemic species in the state of Assam, India
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A traditional "Banarasi sari with gold "brocade

In "Tamil Nadu, mulberry cultivation is concentrated in the "Coimbatore, "Erode, "Tiruppur, "Salem and "Dharmapuri districts. "Hyderabad, "Andhra Pradesh, and "Gobichettipalayam, "Tamil Nadu, were the first locations to have automated silk reeling units in India.[25]

India is also the largest consumer of silk in the world. The tradition of wearing silk sarees for marriages and other auspicious ceremonies is a custom in Assam and southern parts of India. Silk is considered to be a symbol of royalty, and, historically, silk was used primarily by the upper classes. Silk garments and "sarees produced in "Kanchipuram, "Pochampally, "Dharmavaram, "Mysore, "Arani in the south, "Banaras in the north, "Bhagalpur and "Murshidabad in the east are well recognized. In the northeastern state of "Assam, three different types of silk are produced, collectively called "Assam silk: Muga, Eri and Pat silk. Muga, the golden silk, and Eri are produced by silkworms that are native only to Assam.

Thailand[edit]

Silk is produced year-round in Thailand by two types of silkworms, the cultured Bombycidae and wild Saturniidae. Most production is after the rice harvest in the southern and northeastern parts of the country. Women traditionally weave silk on hand looms and pass the skill on to their daughters, as weaving is considered to be a sign of maturity and eligibility for marriage. Thai silk textiles often use complicated patterns in various colours and styles. Most regions of Thailand have their own typical silks. A single thread "filament is too thin to use on its own so women combine many threads to produce a thicker, usable fiber. They do this by hand-reeling the threads onto a wooden spindle to produce a uniform strand of raw silk. The process takes around 40 hours to produce a half kilogram of silk. Many local operations use a reeling machine for this task, but some silk threads are still hand-reeled. The difference is that hand-reeled threads produce three grades of silk: two fine grades that are ideal for lightweight fabrics, and a thick grade for heavier material.

The silk fabric is soaked in extremely cold water and bleached before dyeing to remove the natural yellow coloring of Thai silk yarn. To do this, skeins of silk thread are immersed in large tubs of "hydrogen peroxide. Once washed and dried, the silk is woven on a traditional hand-operated loom.[26]

Bangladesh[edit]

The "Rajshahi Division of northern Bangladesh is the hub of the country's silk industry. There are three types of silk produced in the region: mulberry, endi and tassar. "Bengali silk was a major item of international trade for centuries. It was known as Ganges silk in medieval Europe. Bengal was the leading exporter of silk between the 16th and 19th centuries.[27]

Ancient Mediterranean[edit]

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The "Gunthertuch, an 11th-century silk celebrating a "Byzantine emperor's triumph

In the "Odyssey, 19.233, when Odysseus, while pretending to be someone else, is questioned by Penelope about her husband's clothing, he says that he wore a shirt "gleaming like the skin of a dried onion" (varies with translations, literal translation here)[28] which could refer to the lustrous quality of silk fabric. "Aristotle wrote of "Coa vestis, a wild silk textile from "Kos. "Sea silk from certain large sea shells was also valued. The "Roman Empire knew of and traded in silk, and Chinese silk was the most highly priced luxury good imported by them.[20] During the reign of emperor "Tiberius, "sumptuary laws were passed that forbade men from wearing silk garments, but these proved ineffectual.[29] The "Historia Augusta mentions that the 3rd Century AD emperor "Elagabalus was the first Roman to wear garments of pure silk, whereas it had been customary to wear fabrics of silk/cotton or silk/linen blends.[30] Despite the popularity of silk, the secret of silk-making only reached Europe around AD 550, via the "Byzantine Empire. Legend has it that monks working for the emperor "Justinian I "smuggled silkworm eggs to "Constantinople in hollow canes from China. All top-quality looms and weavers were located inside the "Great Palace complex in Constantinople, and the cloth produced was used in imperial robes or in diplomacy, as gifts to foreign dignitaries. The remainder was sold at very high prices.

Middle East[edit]

In the "Torah, a scarlet cloth item called in Hebrew "sheni tola'at" שני תולעת – literally "crimson of the worm" – is described as being used in purification ceremonies, such as those following a leprosy outbreak (Leviticus 14), alongside "cedar wood and "hyssop ("za'atar). Eminent scholar and leading medieval translator of "Jewish sources and books of the "Bible into "Arabic, Rabbi "Saadia Gaon, translates this phrase explicitly as "crimson silk" – חריר קרמז حرير قرمز.

In "Islamic teachings, Muslim men are forbidden to wear silk. Many religious jurists believe the reasoning behind the prohibition lies in avoiding clothing for men that can be considered feminine or extravagant.[31] There are disputes regarding the amount of silk a fabric can consist of (e.g., whether a small decorative silk piece on a cotton caftan is permissible or not) for it to be lawful for men to wear, but the dominant opinion of most Muslim scholars is that the wearing of silk by men is forbidden. Modern attire has raised a number of issues, including, for instance, the permissibility of wearing silk "neckties, which are masculine articles of clothing.

Despite injunctions against silk for men, silk has retained its popularity in the "Islamic world because of its permissibility for women, and due to the presence of non-Muslim communities. The Muslim "Moors brought silk with them to Spain during their conquest of the "Iberian Peninsula.

Medieval and modern Europe[edit]

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Silk satin leaf, wood sticks and guards, c. 1890

Italy was the most important producer of silk during the Medieval age. The first center to introduce silk production to Italy was the city of "Catanzaro during the 11th century in the region of "Calabria. The silk of Catanzaro supplied almost all of Europe and was sold in a large market fair in the port of "Reggio Calabria, to Spanish, Venetian, Genovese and Dutch merchants. Catanzaro became the lace capital of the world with a large silkworm breeding facility that produced all the laces and linens used in the Vatican. The city was world-famous for its fine fabrication of silks, velvets, damasks and brocades.[32]

Another notable center was the Italian "city-state of "Lucca which largely financed itself through silk-production and silk-trading, beginning in the 12th century. Other Italian cities involved in silk production were "Genoa, "Venice and "Florence.

The "Silk Exchange in Valencia from the 15th century—where previously in 1348 also perxal ("percale) was traded as some kind of silk—illustrates the power and wealth of one of the great Mediterranean mercantile cities.[33][34]

Silk was produced in and exported from the province of "Granada, Spain, especially the "Alpujarras region, until the "Moriscos, whose industry it was, were expelled from Granada in 1571.[35][36]

Since the 15th century, silk production in France has been centered around the city of "Lyon where many mechanic tools for mass production were first introduced in the 17th century.

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"La charmante rencontre", rare 18th century embroidery in silk of Lyon (private collection)

"James I attempted to establish silk production in England, purchasing and planting 100,000 mulberry trees, some on land adjacent to "Hampton Court Palace, but they were of a species unsuited to the silk worms, and the attempt failed. In 1732 John Guardivaglio set up a silk throwing enterprise at "Logwood mill in Stockport; in 1744, Burton Mill was erected in "Macclesfield; and in 1753 Old Mill was built in "Congleton.[37] These three towns remained the centre of the English silk throwing industry until silk throwing was replaced by "silk waste spinning. British enterprise also established silk filature in "Cyprus in 1928. In England in the mid-20th century, raw silk was produced at "Lullingstone Castle in Kent. Silkworms were raised and reeled under the direction of Zoe Lady Hart Dyke, later moving to "Ayot St Lawrence in Hertfordshire in 1956.[38]

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"Yếm – the traditional silken bra in Vietnam

North America[edit]

King "James I introduced silk-growing to the American colonies around 1619, ostensibly to discourage "tobacco planting. The "Shakers in Kentucky adopted the practice. In the 19th century a new attempt at a silk industry began with European-born workers in "Paterson, New Jersey, and the city became a silk center in the United States. "Manchester, Connecticut emerged as center of the silk industry in America from the late 19th through the mid-20th century. The Cheney Brothers Historic District showcases mills refurbished as apartments and includes nearby museums.

"World War II interrupted the silk trade from Asia, and silk prices increased dramatically.[42] U.S. industry began to look for substitutes, which led to the use of "synthetics such as "nylon. Synthetic silks have also been made from "lyocell, a type of "cellulose fiber, and are often difficult to distinguish from real silk (see "spider silk for more on synthetic silks).

Malaysia[edit]

In "Terengganu, which is now part of "Malaysia, a second generation of silkworm was being imported as early as 1764 for the country's silk textile industry, especially "songket.[43] However, since the 1980s, Malaysia is no longer engaged in sericulture but does plant mulberry trees.

Vietnam[edit]

In Vietnamese legend, silk appeared in the sixth dynasty of "Hùng Vương.

Production process[edit]

The process of silk production is known as "sericulture.[44] The entire production process of silk can be divided into several steps which are typically handled by different entities["clarification needed]. Extracting raw silk starts by cultivating the silkworms on mulberry leaves. Once the worms start pupating in their cocoons, these are dissolved in boiling water in order for individual long fibres to be extracted and fed into the spinning reel.[45]

To produce 1 kg of silk, 104 kg of mulberry leaves must be eaten by 3000 silkworms. It takes about 5000 silkworms to make a pure silk "kimono.[46]:104 The major silk producers are "China (54%) and "India (14%).[47] Other statistics:[48]

Top Ten Cocoons (Reelable) Producers — 2005
Country Production (Int $1000) Footnote Production (1000 kg) Footnote
 "People's Republic of China 978,013 C 290,003 F
 "India 259,679 C 77,000 F
 "Uzbekistan 57,332 C 17,000 F
 "Brazil 37,097 C 11,000 F
 "Iran 20,235 C 6,088 F
 "Thailand 16,862 C 5,000 F
 "Vietnam 10,117 C 3,000 F
 "Democratic People's Republic of Korea 5,059 C 1,500 F
 "Romania 3,372 C 1,000 F
 "Japan 2,023 C 600 F
No symbol = official figure, F = FAO estimate,*= Unofficial figure, C = Calculated figure;

Production in Int $1000 have been calculated based on 1999–2001 international prices
Source: Food And Agricultural Organization of United Nations: Economic And Social Department: The Statistical Division

The environmental impact of silk production is potentially large when compared with other natural fibers. A life cycle assessment of Indian silk production shows that the production process has a large carbon and water footprint, mainly due to the fact that it is an animal-derived fiber and more inputs such as fertilizer and water are needed per unit of fiber produced.[49]

Properties[edit]

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Models in silk dresses at the MoMo Falana fashion show

Physical properties[edit]

Silk fibers from the Bombyx mori silkworm have a "triangular "cross section with rounded corners, 5–10 "μm wide. The fibroin-heavy chain is composed mostly of "beta-sheets, due to a 59-mer amino acid repeat sequence with some variations.[50] The flat surfaces of the fibrils reflect "light at many angles, giving silk a natural sheen. The cross-section from other silkworms can vary in shape and diameter: crescent-like for Anaphe and elongated wedge for tussah. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins that act like "glue, to form a bave. Bave diameters for tussah silk can reach 65 μm. See cited reference for cross-sectional SEM photographs.[51]

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Raw silk of domesticated silk worms, showing its natural shine.

Silk has a smooth, soft texture that is not slippery, unlike many "synthetic fibers.

Silk is one of the strongest natural fibers, but it loses up to 20% of its strength when wet. It has a good "moisture regain of 11%. Its "elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.

One example of the durable nature of silk over other fabrics is demonstrated by the recovery in 1840 of silk garments from a "wreck of 1782: 'The most durable article found has been silk; for besides pieces of cloaks and lace, a pair of black satin breeches, and a large satin waistcoat with flaps, were got up, of which the silk was perfect, but the lining entirely gone ... from the thread giving way ... No articles of dress of woollen cloth have yet been found.'[52]

Silk is a poor conductor of "electricity and thus susceptible to "static cling.

Unwashed silk chiffon may "shrink up to 8% due to a relaxation of the fiber macrostructure, so silk should either be washed prior to garment construction, or "dry cleaned. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage due to molecular-level deformation.

Natural and synthetic silk is known to manifest "piezoelectric properties in proteins, probably due to its molecular structure.[53]

Silkworm silk was used as the standard for the "denier, a measurement of "linear density in fibers. Silkworm silk therefore has a linear density of approximately 1 den, or 1.1 "dtex.

Comparison of silk fibers[54] Linear density (dtex) Diameter (μm) Coeff. variation
"Moth: "Bombyx mori 1.17 12.9 24.8%
"Spider: Argiope aurentia 0.14 3.57 14.8%

Chemical properties[edit]

Silk emitted by the silkworm consists of two main proteins, "sericin and "fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it. Fibroin is made up of the "amino acids "Gly-"Ser-Gly-"Ala-Gly-Ala and forms "beta pleated sheets. "Hydrogen bonds form between chains, and side chains form above and below the plane of the hydrogen bond network.

The high proportion (50%) of glycine allows tight packing. This is because glycine's R group is only a hydrogen and so is not as sterically constrained. The addition of alanine and serine makes the fibres strong and resistant to breaking. This tensile strength is due to the many interceded hydrogen bonds, and when stretched the force is applied to these numerous bonds and they do not break.

Silk is resistant to most "mineral acids, except for "sulfuric acid, which dissolves it. It is yellowed by perspiration. Chlorine bleach will also destroy silk fabrics.

Uses[edit]

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Silk filaments being unravelled from silk cocoons, "Cappadocia, "Turkey, 2007.

Silk's "absorbency makes it comfortable to wear in warm weather and while active. Its low conductivity keeps warm air close to the skin during cold weather. It is often used for clothing such as "shirts, "ties, "blouses, formal "dresses, high fashion clothes, "lining, "lingerie, "pajamas, "robes, dress "suits, sun dresses and Eastern "folk costumes. For practical use, silk is excellent as clothing that protects from many biting "insects that would ordinarily pierce clothing, such as "mosquitoes and "horseflies. Silk's attractive lustre and drape makes it suitable for many "furnishing applications. It is used for "upholstery, wall coverings, window treatments (if blended with another fiber), "rugs, "bedding and wall hangings.["citation needed] While on the decline now, due to artificial fibers, silk has had many industrial and commercial uses, such as in "parachutes, bicycle "tires, "comforter filling and "artillery "gunpowder bags.[55]

Fabrics that are often made from silk include "charmeuse, "habutai, "chiffon, "taffeta, "crepe de chine, "dupioni, "noil, "tussah, and "shantung, among others.

A special manufacturing process removes the outer "sericin coating of the silk, which makes it suitable as non-absorbable "surgical sutures. This process has also recently led to the introduction of specialist silk underclothing, which has been used for skin conditions including "eczema.[56][57] New uses and manufacturing techniques have been found for silk for making everything from disposable cups to drug delivery systems and holograms.[58]

Applications as a biomaterial[edit]

Silk has been considered as a luxurious textile since 3630 BC. However, it started to serve also as a biomedical material for suture in surgeries decades ago. In the past 30 years, it has been widely studied and used as a biomaterial, which refers to materials used for medical applications in organisms, due to its excellent properties, including remarkable mechanical properties, comparative biocompatibility, tunable degradation rates in vitro and in vivo, the ease to load cellular growth factors (for example, BMP-2), and the ability to be processed into several other formats such as films, gels, particles, and scaffolds.[59] Silks from Bombyx mori, a kind of cultivated silkworm, are the most widely investigated silks.[60]

Silks derived from Bombyx mori are generally made of two parts: the "silk fibroin fiber which contains a light chain of 25kDa and a heavy chain of 350kDa (or 390kDa[61]) linked by a single disulfide bond[62] and a glue-like protein, "sericin, comprising 25 to 30 percentage by weight. Silk fibroin contains hydrophobic "Beta sheet blocks, interrupted by small hydrophilic groups. And the beta-sheets contribute much to the high mechanical strength of silk fibers, which achieves 740 MPa, tens of times that of "poly(lactic acid) and hundreds of times that of "collagen. This impressive mechanical strength has made silk fibroin very competitive for applications in biomaterials. Indeed, silk fibers have found their way into tendon tissue engineering,[63] where mechanical properties matter greatly. In addition, mechanical properties of silks from various kinds of silkworms vary widely, which provides more choices for their use in tissue engineering.

Most products fabricated from regenerated silk are weak and brittle, with only ~1–2% of the mechanical strength of native silk fibers due to the absence of appropriate secondary and hierarchical structure,

Source Organisms[64] Tensile strength

(g/den)

Tensile modulus

(g/den)

Breaking

strain (%)

Bombyx mori 4.3–5.2 84–121 10.0–23.4
Antheraea mylitta 2.5–4.5 66–70 26–39
Philosamia cynthia ricini 1.9–3.5 29–31 28.0–24.0
Coscinocera hercules 5 ± 1 87 ± 17 12 ± 5
Hyalophora euryalus 2.7 ± 0.9 59 ± 18 11 ± 6
Rothschildia hesperis 3.3 ± 0.8 71 ± 16 10 ± 4
Eupackardia calleta 2.8 ± 0.7 58 ± 18 12 ± 6
Rothschildia lebeau 3.1 ± 0.8 54 ± 14 16 ± 7
Antheraea oculea 3.1 ± 0.8 57 ± 15 15 ± 7
Hyalophora gloveri 2.8 ± 0.4 48 ± 13 19 ± 7
Copaxa multifenestrata 0.9 ± 0.2 39 ± 6 4 ± 3

Biocompatibility[edit]

Biocompatibility, i.e., the ability to what level the silk will cause an immune response, is definitely a critical issue for biomaterials. The biocompatibility of silk arose during its increasing clinical use. Indeed, wax or silicone is usually used as a coating to avoid fraying and potential immune responses[59] when silk fibers serve as suture materials. Although the lack of detailed characterization of silk fibers, such as the extent of the removal of sericin, the surface chemical properties of coating material, and the process used, make it difficult to determine the real immune response of silk fibers in literature, it is generally believed that sericin is the major cause of immune response. Thus, the removal of sericin is an essential step to assure biocompatibility in biomaterial applications of silk. However, further research fails to prove clearly the contribution of sericin to inflammatory responses based on isolated sericin and sericin based biomaterials.[65] In addition, silk fibroin exhibits an inflammatory response similar to that of tissue culture plastic in vitro[66][67] when assessed with human mesenchymal "stem cells (hMSCs) or lower than collagen and PLA when implant rat MSCs with silk fibroin films in vivo.[67] Thus, appropriate degumming and sterilization will assure the biocompatibility of silk fibroin, which is further validated by in vivo experiments on rats and pigs.[68] There are still concerns about the long-term safety of silk-based biomaterials in the human body in contrast to these promising results. Even though silk sutures serve well, they exist and interact within a limited period depending on the recovery of wounds (several weeks), much shorter than that in tissue engineering. Another concern arises from biodegradation because the biocompatibility of silk fibroin does not necessarily assure the biocompatibility of the decomposed products. In fact, different levels of immune responses[69][70] and diseases[71] have been triggered by the degraded products of silk fibroin.

Biodegradability[edit]

Biodegradability (also known as "biodegradation)--the ability to be disintegrated by biological approaches, including bacteria, fungi, and cells—is another significant property of biomaterials today. Biodegradable materials can minimize the pain of patients from surgeries, especially in tissue engineering, there is no need of surgery in order to remove the scaffold implanted. Wang et al.[72] showed the in vivo degradation of silk via aqueous 3-D scaffolds implanted into Lewis rats. "Enzymes are the means used to achieve degradation of silk in vitro. Protease XIV from Streptomyces griseus and α-chymotrypsin from bovine pancreases are the two popular enzymes for silk degradation. In addition, "gamma-radiation, as well as "cell metabolism, can also regulate the degradation of silk.

Compared with synthetic biomaterials such as "polyglycolides and "polylactides, silk is obviously advantageous in some aspects in biodegradation. The acidic degraded products of polyglycolides and polylactides will decrease the pH of the ambient environment and thus adversely influence the metabolism of cells, which is not an issue for silk. In addition, silk materials can retain strength over a desired period from weeks to months as needed by mediating the content of beta sheets.

Cultivation[edit]

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Thai man spools silk
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"Cocoon

Silk "moths lay eggs on specially prepared paper. The eggs hatch and the caterpillars (silkworms) are fed fresh "mulberry leaves. After about 35 days and 4 moltings, the caterpillars are 10,000 times heavier than when hatched and are ready to begin spinning a cocoon. A straw frame is placed over the tray of caterpillars, and each caterpillar begins spinning a cocoon by moving its head in a pattern. Two glands produce liquid silk and force it through openings in the head called spinnerets. Liquid silk is coated in sericin, a water-soluble protective gum, and solidifies on contact with the air. Within 2–3 days, the caterpillar spins about 1 mile of filament and is completely encased in a cocoon. The silk farmers then heat the cocoons to kill them, leaving some to "metamorphose into moths to breed the next generation of caterpillars. Harvested cocoons are then soaked in boiling water to soften the sericin holding the silk fibers together in a cocoon shape. The fibers are then unwound to produce a continuous thread. Since a single thread is too fine and fragile for commercial use, anywhere from three to ten strands are spun together to form a single thread of silk.[73]

Animal rights[edit]

As the process of harvesting the silk from the cocoon kills the larvae by boiling them, sericulture has been criticized by animal welfare and rights activists.[74] "Mohandas Gandhi was critical of silk production based on the "Ahimsa philosophy which led to promotion of cotton and Ahimsa silk, a type of "wild silk made from the cocoons of wild and semi-wild silk moths.[75] Since silk cultivation kills silkworms, possibly painfully,[76]["better source needed] "People for the Ethical Treatment of Animals (PETA) urges people not to buy silk items.[77]

See also[edit]

References[edit]

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Cited sources[edit]

Further reading[edit]

External links[edit]

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