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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. 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
Silk
is produced by several insects, 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. Silk
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. Silk
Silk
production also occurs in Hymenoptera (bees , wasps , and ants ), silverfish , mayflies , thrips , leafhoppers , beetles , lacewings , fleas , flies , and midges . Other types of arthropod produce silk, most notably various arachnids such as spiders .

CONTENTS

* 1 Etymology

* 2 History

* 2.1 Wild silk * 2.2 China
China
* 2.3 India
India
* 2.4 Thailand
Thailand
* 2.5 Bangladesh * 2.6 Ancient Mediterranean * 2.7 Middle East
Middle East
* 2.8 Medieval and modern Europe
Europe
* 2.9 North America * 2.10 Malaysia
Malaysia
* 2.11 Vietnam
Vietnam

* 3 Production process

* 4 Properties

* 4.1 Physical properties * 4.2 Chemical properties

* 5 Uses

* 5.1 Applications as a biomaterial

* 5.1.1 Biocompatibility * 5.1.2 Biodegradability

* 6 Cultivation * 7 Animal rights * 8 See also * 9 References * 10 Cited sources * 11 Further reading * 12 External links

ETYMOLOGY

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_).

HISTORY

Main article: History of silk

WILD SILK

Woven silk textile from tomb no 1. at Mawangdui
Mawangdui
in Changsha , Hunan
Hunan
province, China
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
China
, South Asia
Asia
, and Europe
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. 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. A technique known as demineralizing allows the mineral layer around the cocoon of wild silk moths to be removed, 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.

CHINA

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

Silk
Silk
fabric was first developed in ancient China. The earliest example of silk fabric is from 3630 BC, and it was used as wrapping for the body of a child from a Yangshao culture site in Qingtaicun at Xingyang , Henan.

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
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
Asia
. Because of its texture and lustre, silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants. Silk
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
Jiangxi
province, dated to the Eastern Zhou Dynasty
Zhou Dynasty
roughly 2,500 years ago. 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
Han Dynasty
(202 BC-220 AD).

Silk
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. 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. The silk trade reached as far as the Indian subcontinent, the Middle East
Middle East
, Europe
Europe
, and North Africa
North Africa
. This trade was so extensive that the major set of trade routes between Europe
Europe
and Asia
Asia
came to be known as the Silk Road .

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

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

* Chinese silk making process

*

The silkworms and mulberry leaves are placed on trays. *

Twig frames for the silkworms are prepared. *

The cocoons are weighed. *

The cocoons are soaked and the silk is wound on spools. *

The silk is woven using a loom.

INDIA

Main article: Silk in the Indian subcontinent Silk
Silk
sari weaving at Kanchipuram
Kanchipuram

Silk
Silk
has a long history in India. It is known as _Resham_ in eastern and north India, and _Pattu_ in southern parts of India
India
. Recent archaeological discoveries in Harappa and Chanhu-daro suggest that sericulture , employing wild silk threads from native silkworm species, existed in South Asia
South Asia
during the time of the Indus Valley Civilization dating between 2450 BC and 2000 BC, while "hard and fast evidence" for silk production in China
China
dates back to around 2570 BC. Shelagh Vainker, a silk expert at the Ashmolean Museum in Oxford, who sees evidence for silk production in China
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."

India
India
is the second largest producer of silk in the world after China. About 97% of the raw silk comes from five Indian states, namely, Andhra Pradesh
Andhra Pradesh
, Karnataka
Karnataka
, Jammu and Kashmir
Jammu and Kashmir
, Tamil Nadu and West Bengal
West Bengal
. North Bangalore, the upcoming site of a $20 million " Silk
Silk
City" Ramanagara and Mysore
Mysore
, contribute to a majority of silk production in Karnataka. _ Antheraea assamensis_, the endemic species in the state of Assam, India
India
A traditional Banarasi sari with gold brocade

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

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

THAILAND

Main article: Thai silk

Silk
Silk
is produced year-round in Thailand
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
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.

BANGLADESH

Main article: Rajshahi silk

The Rajshahi Division
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
Bengal
was the leading exporter of silk between the 16th and 19th centuries.

ANCIENT MEDITERRANEAN

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

In the _ Odyssey
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) which could refer to the lustrous quality of silk fabric. Aristotle
Aristotle
wrote of _Coa vestis _, a wild silk textile from Kos
Kos
. Sea silk from certain large sea shells was also valued. The Roman Empire
Roman Empire
knew of and traded in silk, and Chinese silk was the most highly priced luxury good imported by them. During the reign of emperor Tiberius
Tiberius
, sumptuary laws were passed that forbade men from wearing silk garments, but these proved ineffectual. Despite the popularity of silk, the secret of silk-making only reached Europe
Europe
around AD 550, via the Byzantine Empire . Legend has it that monks working for the emperor Justinian I smuggled silkworm eggs to Constantinople
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

In the Torah
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
Bible
into Arabic
Arabic
, Rabbi Saadia Gaon
Saadia Gaon
, translates this phrase explicitly as "crimson silk" – חריר קרמז حرير قرمز.

In Islamic
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. 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
Islamic
world because of its permissibility for women, and due to the presence of non-Muslim communities. The Muslim Moors
Moors
brought silk with them to Spain during their conquest of the Iberian Peninsula
Iberian Peninsula
.

MEDIEVAL AND MODERN EUROPE

Silk
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
Calabria
. The silk of Catanzaro supplied almost all of Europe
Europe
and was sold in a large market fair in the port of Reggio Calabria
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.

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
Genoa
, Venice
Venice
and Florence
Florence
.

The Silk
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.

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

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. "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
Congleton
. 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.

* Medieval and modern Europe

*

Dress
Dress
made from silk. *

Bed covered with silk *

A hundred year old pattern of silk called "Almgrensrosen" *

The necktie originates from the cravat , a neckband made from silk

Yếm
Yếm
– the traditional silken bra in Vietnam
Vietnam

NORTH AMERICA

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
World War II
interrupted the silk trade from Asia, and silk prices increased dramatically. 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

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

VIETNAM

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

PRODUCTION PROCESS

The process of silk production is known as sericulture . The entire production process of silk can be divided into several steps which are typically handled by different entities. 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.

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 . :104 The major silk producers are China
China
(54%) and India (14%). Other statistics:

TOP TEN COCOONS (REELABLE) PRODUCERS — 2005

COUNTRY PRODUCTION (INT $1000) FOOTNOTE PRODUCTION (1000 KG) FOOTNOTE

People\'s Republic of China
China
978,013 C 290,003 F

India
India
259,679 C 77,000 F

Uzbekistan
Uzbekistan
57,332 C 17,000 F

Brazil
Brazil
37,097 C 11,000 F

Iran
Iran
20,235 C 6,088 F

Thailand
Thailand
16,862 C 5,000 F

Vietnam
Vietnam
10,117 C 3,000 F

Democratic People\'s Republic of Korea
Korea
5,059 C 1,500 F

Romania
Romania
3,372 C 1,000 F

Japan
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.

PROPERTIES

Models in silk dresses at the MoMo Falana fashion show

PHYSICAL PROPERTIES

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

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

Silk
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.'

Silk
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.

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 LINEAR DENSITY (DTEX) DIAMETER (μM) COEFF. VARIATION

Moth
Moth
: _ Bombyx mori _ 1.17 12.9 24.8%

Spider
Spider
: _Argiope aurentia _ 0.14 3.57 14.8%

CHEMICAL PROPERTIES

Silk
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
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

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

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 . New uses and manufacturing techniques have been found for silk for making everything from disposable cups to drug delivery systems and holograms.

APPLICATIONS AS A BIOMATERIAL

Silk
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. Silks from _Bombyx mori_, a kind of cultivated silkworm, are the most widely investigated silks.

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 ) linked by a single disulfide bond 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, 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 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

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 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. In addition, silk fibroin exhibits an inflammatory response similar to that of tissue culture plastic in vitro when assessed with human mesenchymal stem cell s (hMSCs) or lower than collagen and PLA when implant rat MSCs with silk fibroin films in vivo. Thus, appropriate degumming and sterilization will assure the biocompatibility of silk fibroin, which is further validated by in vivo experiments on rats and pigs. 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 and diseases have been triggered by the degraded products of silk fibroin.

Biodegradability

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. showed the in vivo degradation of silk via aqueous 3-D scaffolds implanted into Lewis rats. Enzyme
Enzyme
s 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

Thai man spools silk Cocoon

Silk
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.

ANIMAL RIGHTS

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. 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. Since silk cultivation kills silkworms, possibly painfully, People for the Ethical Treatment of Animals (PETA) urges people not to buy silk items.

SEE ALSO

* Art silk * Mommes * Rayon
Rayon
* Sea silk * Silk waste * Spider
Spider
silk * International Year of Natural Fibres

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CITED SOURCES

* Hill, John E. 2004. _The Peoples of the West from the Weilüe_ 魏略 _by Yu Huan_ 魚豢_: A Third Century Chinese Account Composed between 239 and 265 AD._ Draft annotated English translation. Appendix E.

FURTHER READING

* Callandine, Anthony (1993). "Lombe's Mill: An Exercise in reconstruction". _Industrial Archaeology Review_. Maney Publishing. XVI (1). ISSN 0309-0728 . * Feltwell, John. 1990. _The Story of Silk_. Alan Sutton Publishing ISBN 0-86299-611-2 * Good, Irene. 1995. "On the question of silk in pre-Han Eurasia" _Antiquity_ Vol. 69, Number 266, December 1995, pp. 959–968 * Kuhn, Dieter. 1995. " Silk
Silk
Weaving
Weaving
in Ancient China: From Geometric Figures to Patterns of Pictorial Likeness." _Chinese Science_ 12 (1995): pp. 77–114. * Liu, Xinru (1996). _ Silk
Silk
and Religion: An Exploration of Material Life and the Thought of People, AD 600–1200_. Oxford University Press. * Liu, Xinru (2010). _The Silk Road in World History_. Oxford University Press. ISBN 978-0-19-516174-8 ; ISBN 978-0-19-533810-2 (pbk). * Rayner, Hollins (1903). _ Silk
Silk
throwing and waste silk spinning_. Scott, Greenwood, Van Nostrand. * Sung, Ying-Hsing. 1637. _Chinese Technology in the Seventeenth Century – T'ien-kung K'ai-wu_. Translated and annotated by E-tu Zen Sun and Shiou-chuan Sun. Pennsylvania State University Press, 1966. Reprint: Dover, 1997. Chap. 2. Clothing materials. * Kadolph, Sara J. Textiles. 10th ed. Upper Saddle River: Pearson Prentice Hall, 2007. 76–81. * Ricci, G, et al. "Clinical Effectiveness of a Silk
Silk
Fabric in the Treatment of Atopic Dermatitis", _British Journal of Dermatology_ (2004) Issue 150. Pages 127 – 131

EXTERNAL LINKS

* References to silk by Roman and Byzantine writers * A series of maps depicting the global trade in silk * History of traditional silk in martial arts uniforms * Raising silkworms in classrooms for educational purposes (with photos) * New thread in fabric of insect

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