DUET, Gazipur-1700, Bangladesh

A review on mulberry silk and spider silk fiber, and their mechanical

behavior.

Engr. Md. Mizanur Rahman 1

M.Sc. in Textile Engineering

Department of Textile Engineering,

Dhaka University of Engineering and Technology (DUET) Gazipur - 1700, Dhaka,

Bangladesh

Abstract

Silk is a natural protein filament, spun by the caterpillars of various butter flies. Its filament

density is 1.34 g/cm which make it a medium weight fiber. Very light weight silk textile

materials may be manufacturing from silk filaments. The increasing demand for greener and

biodegradable materials leading to the satisfaction of society requires a compelling towards the

Natural fibers. This review presents the natural fiber i.e. mulberry silk and spider silk

fiber/polymer provides ecological and environmental advantages. Both plant- and animal-based

natural fibers have been studied for their mechanical properties and potential contributions to

renewable, completely biodegradable and also given more applications in many fields. In recent

years, the reported exceptional nature of silk lead to increased interest in silk for biomedical

applications. A comparison of the structural differences between spider and silkworm silks will

follow. Some of the different types of silk produced by orb weaving spiders, their main functions

and structural features will be described. Silk is one of the strongest natural fibers, it is originally

from Bombyx mori and can be available industrially at a low cost. Spider silk is one outstanding

fibrous biomaterial which consists almost entirely of large proteins. It has tensile strengths

comparable to steel and some silks are nearly as elastic as rubber on a weight to weight basis. In

combining these two properties, silks reveal a toughness that is two to three times that of

synthetic fibers like Nylon or Kevlar. Spider silk is also antimicrobial, hypoallergenic and

completely biodegradable. This review that an artificially produced silk or silk-like material

formed to possess specific desired properties will allow the overcoming of present limitations. ©

2006 Elsevier Ltd.

Keywords: A review on mulberry silk and spider silk fiber, and their mechanical behavior in

different application area.

DHAKA UNIVERSITY OF ENGINEERING AND TECHNOLOGY

1.1 Introduction

The term silk normally refers to a wide range of continuous filaments spun by the several species

of Lepidoptera and Arthropoda. Natural silk which are commercially known and produced in the

world. Among them mulberry silk is the most important and contributes as much as 90 per cent

of world production, therefore, the term "silk" in general refers to the silk of the mulberry

silkworm. Cultivation of the silkworm is known as sericulture. Although many insects produce

silk, only the filament produced by Bombyx mori, the mulberry silk moth and a few others in the

same genus, is used by the commercial silk industry Three other commercially important types

fall into the category of non-mulberry silks namely: Eri silk; Tasar silk; and Muga silk. There

are also other types of non-mulberry silk, which are mostly wild and exploited in Africa and

Asia, are Anaphe silk, Fagara silk, Coan silk, Mussel silk and Spider silk.

Silk is highly valued because it possesses many excellent properties. Not only does it look

lustrous and feel luxurious, but it is also lightweight, resilient, and extremely strong—one

filament of silk is stronger than a comparable filament of steel! Although fabric manufacturers

have created less costly alternatives to silk, such as nylon and polyester, silk is still in a class by

itself.

The major silk producing countries in the world are; China, India, Uzbekistan, Brazil, Japan,

Republic of Korea, Thailand, Vietnam, Iran, etc. Few other countries are also engaged in the

production of cocoons and raw silk in negligible quantities; i.e. Kenya, Botswana, Nigeria,

Zambia, Zimbabwe, Bangladesh, Colombia, Egypt, Nepal, Bulgaria, Turkey, Uganda, Malaysia,

Romania, Bolivia, etc.

The major silk consumers of the world are; USA, Italy, Japan, India, France, China, United

Kingdom, Switzerland, Germany, UAE, Korea, Viet Nam, etc.

Spider silk-another non-insect variety – is soft and fine, but also strong and elastic. The

commercial production of this silk comes from certain Madagascan species, including Nephila

madagascarensis, Miranda aurentia and Epeira. As the spinning tubes (spinne-rules) are in the

fourth and fifth abdominal segments, about a dozen individuals are confined by their abdominal

part to a frame from which the accumulated fibre is reeled out four or five times a

month. Because of the high cost of production, spider silk is not used in the textile industry;

however, durability and resistance to extreme temperature and humidity make it indispensable

for cross hairs in optical instruments. Silk fiber-reinforced composites have the advantage of

being relatively light; they have impact resistance, a high specific strength, high specific stiffness

and are extremely elastic and resilient. In this chapter, we will discuss the use of silk fiber as

reinforcement in polymer composites as well as their different properties.

1.2 Chemical composition and physical/mechanical properties of mulberry

silk and spider silk

1.2.1 Chemical composition and physical/mechanical properties of spider silk

Spider silk is a natural polypeptide, polymeric protein and is in the scleroprotein group which

also encompasses collagen (in ligaments) and keratin (nails and hair). These are all proteins

which provide structure. The protein in dragline silk is fibroin (Mr 200,000-300,000) which is a

combination of the proteins spidroin 1 and spidroin 2. The exact composition of these proteins

depends on factors including species and diet. Fibroin consists of approximately 42% glycine

and 25% alanine as the major amino acids. The remaining components are mostly glutamine,

serine, leucine, valine, proline, tyrosine and arginine. Spidroin 1 and spidroin 2 differ mainly in

their content of proline and tyrosine.

Alanine Glycine

Length of the silk polymer is about 140nm which is slightly longer than wool polymer, and about

0.9 nm thick. The important chemical groupings of the silk polymer are the peptide groups which

give rise to hydrogen bonds, and the carboxyl and amine groups which give rise to the salt

linkages. Polymer system of silk is composed of layers of folded linear polymers. This results in

65-70% crystalline polymer system.

The Nephila Clavipe spider silk fibers were subjected to transverse cyclic loading at a

compressive speed of 0.3 cm/s. under ambient and wet conditions, the compressive modulus of

the fiber tested in ambient condition was 0.58 Gpa. And the fiber experienced a high degree of

permanent deformation (~20%). As shown in Figure 4, the ability of spider silk to resist

transverse compression is lower than all the other textile fibers, indicating a high level of

anisotopy.

1.3 The origin of strength and toughness of spider silk

The duct’s convergent, or hyperbolic, geometry forces the dope flowing along it to elongate at a

practically constant rate [37]. As a result, the spherical droplets in the dope extend to form the

long, thin, and axially orientated canaliculi, which are thought to contribute to the thread’s

toughness [38]. The constant nature of the elongation also ensures that only low and uniform

stresses are generated. This prevents localized coagulation centers from forming prematurely

[39] before the silk protein molecules in the dope have reached their optimal orientation. As in

any other spinning, good molecular alignment contributes significantly to the thread’s toughness

[40–43]. A much higher stress is generated during the rapid extension when the forming thread

suddenly stretches, narrows, and pulls away from the walls of the third limb of the duct. These

high forces bring the dope molecules into alignment so that they are able to join together with

hydrogen bonds to give anti-parallel beta conformation of the final thread. The spider’s

simultaneous and internal drawdown and material processing using phase-separation differs from

industrial spinning, where the solvent escapes to the surface at the die’s external opening. In case

of spider silk, the inside drawing process offers the obvious advantage: most of the water from

the dope can be recycled by absorption from the duct. More importantly – the duct acts as a

combined internal die and treatment bath [44].

The absolute size of the molecules and their size distribution are also important parameters

affecting the toughness of the final thread – the spider silk containing predominantly a single

large protein, with little variability of molecular weight [45].

Fig. spider silk production cycle

1.4.1 The Liquid Crystalline Phase

The processing of water soluble high molecular weight silk proteins into water insoluble fibers in

both spiders and silkworms involves many factors like disulfide bond formation, cat-ion

interactions, glycosylation, and other chemical or physical steps. Physical shear generated during

spinning the soluble silk appears in a large part responsible for conversion to the insoluble silk

fiber in the natural spinning process [14]. Compared to silkworm, the major ampullate gland in

the spider is smaller and there is no sericin contribution in the middle region of the gland. The

process leads to the formation of a lyotropic liquid crystalline phase prior to spinning in the

spider and is responsible for the different silks [15].

As observed by Knight and Vollrath [16], the molecules seem to be in a nematic phase – a

unique phase that flows as a liquid but maintains the orientational order characteristic of a

crystal. Liquid crystallinity allows the viscous silk protein solution to flow slowly through the

storage sac and duct while the molecules form complex alignment patterns. In the spider’s gland,

the long axes of the rod-shaped silk protein molecules or molecular aggregates appear [17] to be

oriented perpendicular to the secreting epithelium walls when close to them, but gradually bend

over with increasing distance until, along the midline of the glandular sac, they lie parallel to the

long axis.

This arrangement is commonly seen when ‘nematic discotic’ liquid crystals are confined in

narrow tubes. This type of liquid crystal forms bilayered disks in which the rod-shaped

molecules are arranged perpendicular to the plane of the disk. It seems likely that the

perpendicular arrangement of the silk protein molecules at the secreting epithelium walls and the

subsequent nematic escape into an arrangement that is parallel to the long axis of the silk gland.

The spinning duct prevents the liquid crystalline dope from breaking up into numerous small

domains. This in turn suppresses the formation of disclinations, a form of defect somewhat

analogous to dislocations in solid crystals, which diminish the tensile strength of the spun thread

[1920].

1.4 Elasticity of spider silk

The extreme elasticity of this natural fiber comes from long spirals in the protein’s configuration,

[46] propose researchers from the University of Wyoming in Laramie. The helices present in the

protein molecules act as molecular springs and make it elastic. A strand of spider silk, of normal

size, stretched 5 times and 20 times its original length showing its extensibility.

It has been found that capture silk protein which has a chain of thousands of amino acids having

regions in which a sequence of five amino acids is repeated over and over, as many as 63 times.

The researchers suggest that the segments of the protein with the repeating blocks form long,

spring like shapes. At the end of each five-amino-acid block, the protein kinks back on itself in a

180 turn. The series of turns eventually forms a spiral that ‘looks exactly like a molecular spring’

1.5 Life cycle/production cycle of mulberry silk

The secret to silk production is the tiny creature known as the silkworm, which is the caterpillar

of the silk moth Bombyx mori. It feeds solely on the leaves of mulberry trees. Only one other

species of moth, the Antheraea mylitta, also produces silk fiber.

The life cycle of the Bombyx mori begins with eggs laid by the adult moth. The larvae emerge

from the eggs and feed on mulberry leaves. In the larval stage, the Bombyx is the caterpillar

known as the silkworm. The silkworm spins a protective cocoon around itself so it can safely

transform into a chrysalis. In nature, the chrysalis breaks through the cocoon and emerges as a

moth. The moths mate and the female lays 300 to 400 eggs. A few days after emerging from the

cocoon, the moths die and the life cycle continues.

.

Life cycle of mulberry silk Manual spinning system of mulberry silk

The cultivation of silkworms for the purpose of producing silk is called sericulture. Over the

centuries, sericulture has been developed and refined to a precise science. Sericulture involves

raising healthy eggs through the chrysalis stage when the worm is encased in its silky cocoon.

The chrysalis inside is destroyed before it can break out of the cocoon so that the precious silk

filament remains intact.

Generally, one cocoon produces between 1,000 and 2,000 feet of silk filament, made essentially

of two elements. The fiber, called fibroin, makes up between 75 and 90%, and sericin, the gum

secreted by the caterpillar to glue the fiber into a cocoon, comprises about 10-25% of silk. Other

elements include fats, salts, and wax. To make one yard of silk material, about 3,000 cocoons are

used.

But the percentage is variable in respect of silkworm strain, seasons and ecological conditions

(like temperature, humidity etc.) at different geographical locations [14]. N. E. region of India is

a treasure-house of silkworms producing all the economically important varieties of natural silk

viz. muga, pat, oak tasar and eri. Muga (Antheraea assamensis), oak tasar (Antheraea pernei), eri

(Philosomia ricini) are categorized as nonmulberry silk while the pat (Bombyx mori) silk is

known as mulberry silk [3]. The semi-domesticated silkworm variety muga silkworm, Antheraea

assamensis.

Insects rearing preparing a cocoon Collecting yarn/filament from cocoon

1.5 Thermal Properties

Spider dragline silk is thermally stable to 230 C. Two thermal transitions have been observed,

one at 75 C, presumed to represent localized mobility in the non-crystalline regions of the silk

fiber, and one at 210 C attributed to a glass transition (T) [69].g

2.2 Mechanical properties/behavior of mulberry silk

2.2.1 Tensile properties

All of our cocoons have a similar general form to their tensile stress–strain deformation profile in

the plane of the cocoon wall. The stress rises with strain to a maximum value and the gradient of

this curve can change once or twice through apparent yield points until stress falls relatively

rapidly after the maximum. Looking at all the stress–strain profiles in figure 3, we see that these

yield points are quite consistent in strain 12–18% across almost all the cocoons, but their

combinations and permutations in stress create an interesting diversity.

3.1 Applications

The excellent properties have prompted extensive interest in silks in general for a wide range of

material applications in the textile and biomedical fields. Chemically modified, coextruded, and

grafted silkworm fibers have been prepared to expand textile materials applications for these

protein fibers [86]. Cosmetics and consumer products containing silkworm have also been

marketed [87]. Membranes, which possess a high permeation coefficient for oxygen and good

optical properties, have been evaluated for contact lens material and in biosensor systems [88]. It

is also predicted that spider silk would be replacing some now traditional man-made fibers;

techno-silks might find a use in novel applications. It is likely that in the foreseeable future silk

proteins would be designed from scratch and thus make fibers to order, assuming we fully

understand the form–function relationship. A first experiment on natural dragline silk that has

been modified suggest that its desirable mechanical properties can indeed be maintained while at

the same time adding totally new properties.

Special silk-fiber composites might be used in microelectronics and fiber optics or as ‘smart’

structural fabrics with anti-static properties. Electrostatic properties may also lead to a first

market for the more complex mini-machine silks of the capture thread type, be they of the

droplet or woolly kind, and they might find employment in active filters. For medical

applications, surgical threads, biomaterial membranes, and scaffolds, cell-growth supporting

substrates and controlled release matrices are envisioned due to the low inflammatory potential

of the silk proteins, the antithrombic nature of the material, and the opportunity to generate a

wide range of mechanical properties by bioengineering the primary sequences contained in the

silk. Most current applications for silks involve silkworm fibroin due to the limited availability

of spider silks.

3.2 Result and discussion

Silk from the embiopteran species Antipaluria urichi and Aposthonia ceylonica were studied

using SEM, TEM, FT-IR, WAXD and NMR spectroscopy to characterize the molecular-level

protein structure as well as a hydrophobic surface coating rich in long-chain lipids and

alkanes. Fig. 1 shows both optical and SEM images of insects and silk produced from An. urichi.

The insects produce silk out of their tarsal organs, or forelimbs, creating very thin sheets of silk

protecting the colonies. An example of a silk in a natural, arboreal setting can be seen in Fig. 1A.

Fiber diameters from An. urichi were determined using SEM and TEM microscopy. Previous

work by Collin et al.5,6

reported fiber diameters in the range of 500–800 nm. However, the

authors used polarized light microscopy techniques and thus could not resolve fibers below the

optical resolution limit.Fig. 1C shows how one could easily be fooled; it is likely that the authors

were observing bundles of webspinner silks and were unable to resolve fine detail. Fig. 2 shows

a histogram of An. urichi fiber diameter measurements made from 68 isolated fibers over

multiple SEM images and 82 fibers from TEM images. The fibers for SEM imaging had been

coated with a layer of gold approximately 15 nm thick, thus 30 nm was subtracted from each

edge to edge measurement. The average size was 93 ± 15 nm (one standard deviation), which is

more consistent with the 65 nm fibers reported for a different webspinner species.4 For TEM

imaging, fiber bundles were stained by submerging in an aqueous solution containing 0.5%

uranyl acetate for 30 minutes prior to resin embedment. The average fiber diameter over 82

measurements from 3 separate TEM images was 100 ± 15 nm. This is slightly larger than the 93

nm average result from SEM images, but we note that these fibers were soaked in an aqueous-

based uranyl acetate stain prior to resin embedment and thus we are potentially observing a slight

swelling of the fibers due to water absorption. This observation brings doubt into the validity of

previous mechanical testing results on embiid silks. Webspinner silks show elasticity similar to

spider silks (15–40% extensibility), but silk strengths were reported at only about 150 MPa.5,6

If

correct, this is many times weaker than spider dragline fibers and silkworm silk. As a silk used

primarily for structural and protective purposes and not for absorbing impact, it would be

surprising if webspinner fibers possess similar gigapascal-level strengths as spider dragline

fibers. Nevertheless, embiopteran silk galleries must be strong enough to both deter predators,

which often walk on top of the silk predators, which often walk on top of the silk.

Fig. 2

Fiber diameter distribution for silk obtained from adult female Antipaluria urichi using electron

microscopy. 82 and 68 separate measurements were combined from multiple TEM (A) and SEM

(B) images, respectively. The results indicate that An. urichi silk ...

To the best of our knowledge, the only available mechanical data obtained on silk produced by

webspinner insects is unreliable due to improper fiber diameter measurements. Therefore in an

attempt to better estimate embiid fiber tensile properties, we collected tensile stress–strain curves

on silk bundles prepared for An. urichi. Samples were prepared by carefully brushing an E-

shaped cardboard card across the tarsus of adult female insects of An. urichi. Fibers were

superglued to each of the three anchor points on the E. Stress–strain curves were obtained by

stretching one side of the E-shaped card at a rate of 1% per second, while the other unstretched

side was analyzed using SEM to approximately obtain the number of fibers present. Additional

experimental details and results are included as ESI.† Results suggest that webspinner silks are

significantly stronger than previously thought; we observed an average of 500 MPa mean

ultimate stress and about 30% extensibility over 14 measurements. Due to the small fiber

diameters and extreme difficulty in obtaining consistent samples, this result should only be

interpreted as a rough estimate.

3.4 Conclusions and scope of future work

Sericulture is an ancient science, and the modern age has not brought great changes to silk

manufacture. Rather, man-made fibers such as polyester, nylon, and acetate have replaced silk in

many instances. But many of the qualities of silk cannot be reproduced. For example, silk is

stronger than an equivalent strand of steel. Some recent research has focused on the molecular

structure of silk as it emerges from the silkworm, in order to better understand how new, stronger

artificial fibers might be constructed. Silk spun by the silkworm starts out as a liquid secretion.

The liquid passes through a brief interim state with a semi-ordered molecular structure known as

nematic liquid crystal, before it solidifies into a fiber. Materials scientists have been able to

manufacture durable fibers using liquid crystal source material, but only at high temperatures or

under extreme pressure. Researchers are continuing to study the silkworm to determine how

liquid crystal is transformed into fiber at ordinary temperatures and pressures.

Spider silks are semicrystalline biopolymers with extraordinary mechanical properties, which

have evolved into a wide range of forms and functions, spun through the process of liquid crystal

spinning. This process of spinning as modified by the spider has several advantages: (1) It is

virtually free of uncontrolled re-orientation of molecules after emergence from the die; (2) the

force required to spin is minimal; and (3) prealignment of molecules in the unspun dope

minimize defects. An important lesson to learn from the spider is how it stores protein dope

molecules in a highly concentrated liquid crystalline state and then extends these in the spinning

duct to form a supremely tough thread.

The combination of high strength and super toughness is likely to push dragline- silks into

impact and tear-proof textiles or other structural fabrics where strong, flexible materials are

desirable. Techno-silks might benefit from the fact that environmental concerns are growing and

that the market is already primed and waiting for artificial spider silks.

Silk has set the standard in luxury fabrics for several millennia. The origins of silk date back to

Ancient China. Legend has it that a Chinese princess was sipping tea in her garden when a

cocoon fell into her cup, and the hot tea loosened the long strand of silk. Ancient literature,

however, attributes the popularization of silk to the Chinese Empress Si-Ling, to around

2600 B.C. Called the Goddess of the Silkworm, Si-Ling apparently raised silkworms and

designed a loom for making silk fabrics. Finished silk products were about half of the world's

total at about $3 billion.

In the future of biomaterial Spider silk production would be done from bacteria. silk molecules

are stretched by a mechanical actuator to increase fiber strength. These mechanical

improvements produce uniform spider silk and remove human error from the spinning process.

As a result, the synthetic silk is much closer to the natural fibers produced by the female black

widow spider than what was previously possible, and the procedure provides a scalable ground

work to utilize spider silk in material manufacturing.

Due to their mechanical properties, synthetic spider silks have numerous manufacturing and

industrial applications. Of particular interest is the high tensile strength of black widow silk,

which is comparable to Kevlar in strength, but is lighter and of a lower density. If scientists

could reproduce the mechanical properties of spider spun silk in the laboratory, the material

could be used to replace Kevlar, carbon fiber and steel. Increased production of this

new biomaterial will have an impact on a wide variety of products where spider silk's properties

are valuable, ranging from bulletproof vests and aircraft bodies to bridge cables and medical

sutures.

While scientists have been able to produce spider silk with the same biochemical integrity of the

natural fibers for some time, it has remained difficult to mimic a spider's "post-spin" techniques.

The natural post-spin process stretches the fiber in order to align the fiber molecules, and

increases the fiber's tensile strength. To solve this problem, Dr. Craig Vierra from the University

of the Pacific developed a technique that removes human variability by using a mechanical

actuator. Built by Dr. Vierra and his laboratory group, the mechanical actuator can reliably

stretch fibers to a specified length, mimicking the spider's natural post-spin.

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