1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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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