Silk is a continuous protein fibre produced by the silk worm to form its cocoon (Cook, 1968). Very differently to materials such as wool, silk contains very small amounts of sulphar (Dhavalikar, 1962). There are two main types of the silk: ‘mulberry silk’ (produced by the Bombyx Mori), also called ‘cultivated silk’, and ‘wild silk’ of which Tussah silk is the most important representative. Mulberry silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori cultivated in provided habitats and fed with freshly picked mulberry leaves. Cultivated silk is different from Tussah silk as the Tussah variety is purely fed on oak leaves. Cultivated silks are fine, almost white (when degummed*) with soft filaments of lustre. Wild silks, on the other hand, are coarser, more irregular and brownish in appearance, and are never as white as the cultivated silk filament (Sandoz Colour Chronicle, 1990). Nearly 80-85% of the world’s silk production consists of cultivated silk.
* Raw silk is dull and slightly oily to the touch. It must be boiled off or degummed by which most of sericinet removed.
SILK FIBRE’S STRUCTURE
The spinning process of the silk worm has been described by a number of scientists such as Robson (Robson, 1985), Peters (Peters, 1963) and Mausersberger (Mausersberger, 1954) . The mature silk worm builds its cocoon by extracting a dense fluid from 2 structural glands. This substance is secreted through 2 ducts in the head of the silk worm and used in the form of a spinneret. The viscous part (known as fibroin) is then covered by another layer (known as sericin) which flows from the mentioned glands. As a result of this spinning process, the silk fibre is a mix of the aforementioned parts namely; sericin and fibroin. Sericin is also known as “silk gum” and is a minor element of the silk fibre (making up around 25% of the weight of raw silk) and holds a number of other components such as waxes, fats and pigments. Sericin is a yellow coloured, brittle and inelastic substance that has been proven to hold antibacterial abilities (Chang, 2005). It acts as an adhesive for the twin fibroin filaments and conceals the unique lustre of fibroin. Sericin is known as an amorphous structure and can be split from the fibroin layer by a process known as “hot soup solution”. Komatsu (Robson,1985; Gulrjani, 1992) claim that sericin may be split into further specialized sections such as Sericin I , Sericin II , Sericin III and Sericin IV by using their different solubilities in hot water and assessing the degree of solubility and density. The greatest sericin content is present in the outer layer of the cocoon whereas the least sericin proportion is present in the innermost layer of the cocoon.
Fibroin is the principal element of silk which is currently used and commercialized. The fibroin is a water “insoluble protein” (amounting to 75% of the weight of raw silk). Fibroin’s texture is highly affected by the crystalline structure that makes it durable, whilst still retaining a lot of the amino acids.
BENEFITS OF SILK :
Silk in its natural state consists of a single thread secreted by the silk worm and is made up of a double filament of protein material (fibroin) glued together with sericin, an allergenic and gummy substance that is normally extracted during the processing of the silk threads . Silk is comprised of perfectly smooth fibres that do not cause mechanical irritation of the skin. The structure of silk fibres is quite similar to that of human hair (97% proteins, 3% fat and waxy substances), thus allowing its use in surgery and also directly on scalded skin. Each silk thread is made up of many filaments more than 800m long which are highly resistant to mechanical and thermal forces. Silk helps to maintain the body temperature, by reducing the excessive sweating and moisture loss that can worsen xerosis*. Whereas silk allergy among workers in the silk industry is widely recognized, allergic reactions of consumers on a large scale have been only rarely described [Borelli .1999 , Caldon. 2001, ], as the final silk fabrics are nonallergenic [Wen.C , 1990].
* Xerosis is the medical term used to describe dry skin.
HEALING PROPERTIES OF SILK
A study performed by Sugihara et al.  in Japan examined the effects of a silk film on full-thickness skin wounds. They found that wounds dressed with the silk material healed 7 days faster than those covered with traditional dressing. The silk also enhanced collagen synthesis, reduced oedema and scarring due to inflammatory responses and promoted epithelialization*.
HYPO ALLERGENIC CAPABILITIES
Because silk is a natural bilayer structure , its natural protein structure, silk is the most hypoallergenic of all fabrics ( Wen. C , 1990)
Silk is highly absorbent: it can absorb up to 30% of its weight in moisture without feeling damp. Silk will absorb perspiration while letting your skin breathe.
Silk is made from the cocoons. Like nearly anything in nature, there are natural occurring substances in the cocoon of the silkworm that protect from various threats. Because the process of turning those cocoons into silk is a gentle one that does not strip away those natural substances, the benefits of them are still in the silk when you use it as part of your daily bedding.
SLEEPING TEMPERATURE REGULATION
Silk is a unique type of fabric due to its hollow formation that enables the texture to transpire the excess heat whilst still keeping in the needed amount of warmth . The fabric is comfortable and cool during the summer and cozy and warm in the winter. The temperature-regulating properties of silk give it the ability to warm and cool at the same time. When silk is applied to bedding products a study commissioned by the World Health Organization showed that the thermal effect is uncomparable to any other type of product ( such as cotton or artificial fibres) [Molloy et al ,1993]
The silk fibre is an extremely strong material. “A filament of silk is stronger than its equivalent in steel” (Crotch , 1956). Normally, the dry silk fibre has a tenacity varying from 2.4 to 5.1 grams per denier. The wet strength of the fibre is about 80-85% of the dry strength (Likitbanakorn,1991). The elongation at break of silk filaments is around 20-25% under normal conditions. The extension at break is 33% at 100% relative humidity (RH). Specific gravity of raw cultivated silk and raw tussah silk are 1.33 and 1.32g/cm-3 respectively. On the other hand, weighted silk has a specific gravity of more than 1.60 g/cm-3. Silk fibres heated at 140oC remain unaffected for a long period of time but decomposes very quickly at 175oC or more. Tsukada and Hirabayashi (1980) found that the strength and elongation of silk fibroin fibres were decreased when fibres were exposed to UV radiation. The degree of the crystal composite was not affected by the radiation treatment.
Silk is not dissolved by water at room temperature but silk may lose weight in boiling water at 100oC (Peters, 1963). Acids and alkalis cause hydrolysis of the polypeptide chains in the fibre. It has been claimed that pH values between 4 and 8 cause the least damage to the fibre (Peters, 1963). Acid hydrolysis tends to be more damaging to fibre than alkaline hydrolysis.
Acid hydrolysis occurs at nearly all the peptide linkages in the chain while alkali hydrolysis firstly attacks at the end of the peptide chains. Concentrated sulphuric acid and hydrochloric acid will dissolve the fibre while nitric acid colours silk yellow. Dilute acids do not attack the fibre under mild conditions.
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