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Cocoon Silk: A Natural Architecture

Adrian Attard Trevisan*†, Melanie Hart* , Vicente Yuste*,  Anna Hjoellund*, Daniel Galle*, Oskar Bergman*

*R&D Department ,Sense of Nature Ltd , †London Metropolitan University

Abstract

The paper discusses both the benefits and the effects of silk fibres in humans with the added advantages which counter a number of health conditions due to the silk’s chemical structure. The study describes and analyses both the processes taking place to refine silk as well as the long term benefits derived from its natural properties. The work includes a meticulous literature review of a number of studies which have been instrumental to this investigative area of study.

Introduction

Silk is a continuous protein fibre produced by the silkworm so as to form its cocoon (Cook, 1968). Silk fibres are also produced by some spiders belonging to the Arachnida family (Cook, 1968; Robson, 1985). Unlike the silkworms’ fibre, the spiders’ fibre cannot be commercially produced, and therefore, the silk fibre referred to within this study refers to silkworm produced silk.

The full silk production process

Figure 1 illustrates the full silk production (from worm to silk)

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 produced by silkworm larvae 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.

Mulberry Silkworms

Mulberry silkworms are divided into 3 different (yet related) groups (Venugapal, 1991). The Univoltine breed is usually associated with Europe. As a reaction to the cold climate, the eggs are dormant in winter and hatched and cross fertilize only by spring. Because of this, they generate silkworms only once a year. The second type of breed is called Bivoltine and is normally found in regions such as Japan, China and Korea. The breeding process varies from the first group and is done twice a year. This is because of the warmer climate that makes it possible for two lifecycles to take place. The Polyvoltine breed is only found in tropical climates. After eggs are laid by female butterflies, the eggs are hatched within nine to twelve days, making it possible to have up to 8 separate lifecycles throughout a single year. The major groups of silkworms fall under the univoltine and bivoltine categories.

Silk Fibre’s Structure

The spinning process of the silkworm has been described by a number of scientists such as Robson (Robson, 1985), Peters (Peters, 1963) and Mausersberger (Mausersberger, 1954). The mature silkworm builds its cocoon by extracting a dense fluid from 2 structural glands. This substance is secreted through 2 ducts in the head of the silkworm 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.

Production of Silk

One of the first operations in silk production is a reeling process (Das,1992). Before being reeled, the cocoons must be sorted carefully by size, quality and defects. The cocoons are immersed into a hot water bath which softens the sericin holding the filaments together. The pupae in fresh cocoons are killed at this stage and the filament ends can be found on the surface of cocoons, unwound and twisted from several cocoons in order to make a raw silk, multi-filament, yarn between 20 and 25 denier (the equivalent weight in grams of 9000m of silk). Moreover, this process also gets rid of a certain small amount of sericin in the fibre.

Frequently, the filaments are accumulated further by a process referred to as “throwing”, the process of producing twisted silk yarn from reeled silk (Dixit, 1990). Hence it produces “thrown silk” which is a twisted yarn suitable for producing a specific fabric through knitting or weaving.

Degumming of Silk

 

the degumming process

Figure 2 illustrates the degumming process

Degumming is at the heart of the wet processing of raw silk due to the fact that the raw silk contains the two components fibroin and sericin which covers the filament. Sericin contains some impurities, such as, waxes, fats, mineral salts and pigments. Sericin has the same amino acid residues as fibroin but the proportions contained in both components are quite different. As a result of this, the degumming process on silk must be carefully carried out in the appropriate conditions otherwise the fibroin may be damaged.  The main purposes of the degumming process are; 1) to remove the sericin from the fibre, 2) to remove impurities (eg. waxes, fats and mineral salts) affecting both the dyeing and printing processes, 3) to make the fibre highly absorbent for dyes and chemicals, and 4) to reveal the lustre of fibroin while improving the appearance of the fibre (Saligram, 1993). The sericin has to be removed from the fibre but the fibroin must not be damaged in the process.
The degumming processes are classified into 5 separate methods:

  1. Degumming with water under pressure at 115ᵒC
  2. Degumming with soap ( ca. 98ᵒC)
  3. Degumming with synthetic detergent ((ca . 98ᵒC)
  4. Degumming with Acid
  5. Degumming with Enzymes

Enzymes

Known as “bio-catalysts”, enzymes are proteins which catalyse a specific chemical reaction. Generally, appearing in living cells, enzymes work at atmosphering pressure and in mild conditions (for example at 40ᵒC , and a pH of 80). Different enzymes may cause hydrolysis, reduction, oxidation, coagulation and decomposition reactions. Hydrolytic enzymes are commonly used in the textile industry: for example with cellulose (Ashkenazi, 1992), trypsin and papain (Gulrajani, 1992).

the enzymes composition in silk fibres

Figure 3 illustrates the Enzymes composition in silk fibres

Nalankilli (1992) mentions the following properties associated with enzymes:

  1. Enzymes are complex and higher in molecular weight proteins,
  2. Enzymes are susceptible to high temperatures and pH values outside their optimum ranges because they can be denatured,
  3. Enzymes react at specific parts of the substrate molecule depending on the active sites and types,
  4. Enzyme reactions are reversible reactions.

Material Resistance

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.

Strong hot caustic alkalis will readily dissolve the fibre. Weak alkalis such as soap, borax and ammonia normally dissolve sericin but they also attack fibroin when the treatment time at boiling point is prolonged. Commonly used dry cleaning solvents do not dissolve the fibre.

Controlling Degummed Silk Quality

There are several methods for assessing the efficiency of the degumming process, such as;

  1. The Gravimetric Method ( Sandoz Colour Chronicle , 1991 ; Shukla et al , 1992),
  2. Staining methods with some dyes which distinguish between fibroin and sericin, for example, C.I Direct Red 2, C.I Direct Blue 22, C.I Direct Red 61, C.I Direct Green 9 ( Bianchi and Colonna, 1992),
  3. The degree of fibre damage may be assessed by determining the viscosity of the degumming solution, or the tensile properties and the primary amino group content of the fibre,
  4. The efficiency of degumming can be assessed using the scanning electron microscope (SEM).

The gravimetric method is commonly used in the assessment of the efficiency of the degumming process (Gulrajani, 1993; Tendulkar, 1990). The gravimetric method is the most simple method, measuring the weight loss of the sample. However, it may not reveal any residual sericin and it does not assess damage to the fibroin. Tensile strength gives more information about damage to the treated sample. SEM examination (Sharma, 1993 ; Gulrajani,1993; Greaves, 1990; Brzezinski, 1989) may be more suitable for revealing fibre damage and the residual sericin.

Unlike the gravimetric method, the staining method is a method to confirm if any sericin is left on the sample but it cannot assess this quantitatively. The determination of the viscosity of the degumming solution and determination of the primary amino group contents of the fibre are more difficult to interpret and there are a few investigators using these methods.

The tensile strength testing method was not considered to be suitable for the present experiments since the silk samples were uneven because of the domestic reeling process.  After careful considerations, the gravimetric method was chosen to assess the degumming in the present experiments. Some of the degummed fibres were examined in the SEM.

Conclusion

The numerous features and complex composite of the silk architecture makes it extremely difficult to emulate. Over these last centuries a number of different cultures and scientists looked into this extraordinary material from a number of different angles, focusing specifically on fibroin, which is only half of the silk fibre. Unfortunately, not a lot of scientific literature looks at cocoon silk and sericin from an applied point of view. A number of opportunities may be created to modify and put to use the sericin part of the silk fibre in a commercial way.

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