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Biological Sciences and Bioengineering,
IIT Kanpur
Spider’s silk: Investigation of spinning process, web material and its properties
Rohit S. Gole 1 and Prateek Kumar 2
1, 2
Department of Biological Sciences and Bioengineering, Indian Institute of Technology Kanpur,
Kanpur-208016
Spider silk has outstanding mechanical & structural properties, despite its light and delicate appearance, like high
strength and exceptional toughness. Silk is twice as stretching nylon and eight times more strong than steel. Studying the
structural aspects, which bring out these properties, is of great interest to biopolymer research. It is tougher, stretchier,
and more waterproof than the silkworm's strands used today in fine garments. Among the many types of silk that have
been characterized till now, the most important varieties include cocoon silk fibroin from the silkworm, Bombyx mori &
the dragline silk from the spider Nephila clavipes. The building blocks of spider’s silk are primarily non essential amino
acids, Glycine and Alanine & the alignment of these amino acids in various ways is responsible for the unique
properties. Insects have found many ways of making a wide range of silk types, but web spiders are unique in
maintaining, in the same individual highly specialized glands producing different silks, often simultaneously. Spider spin
silks for its various purposes like, dragline silk used to stick the whole web to the wall, glue like sticky catching silk used
to catch its prey. Various spiders build up different web pattern, like all orb weaving spiders make suspended, sticky,
wheel-shaped orb webs which they place in openings between trees and shrubs where insects are likely to fly, others
build up ladder or spiral shaped web for catching their preys. Scientists are trying to find out: How spider spins silk in
its glands? Successful copying of the spider's internal processing and precise control over protein folding, combined
with knowledge of the gene sequences of its spinning threads, could permit industrial production of silk-based fibers with
unique properties.
Keywords: Spider, silk, silkworm, cocoon, glands, orb, spinning threads, web.
1. Introduction
Love them or hate them, we've got to admit
that spiders are some pretty impressive, wellequipped animals. They have a heavily
armored body and a highly effective venom
injection system. How many other creatures
can claim that? These remarkable adaptations
have made spiders some of the most
successful carnivores in history. In their 400
million years of existence, they've spread over
every part of the earth. Today, there are more
than 40,000 known spider species, and
potentially thousands more we haven't
discovered yet. The evolution of spider and its
correlation with the evolutionary success gives
a vivid and fascinating thought. The cooccurrence of the spider silk evolution with
spider speciation in the evolutionary history of
spiders draws our attention and motivates us
to study and find out the clever trick of the
nature. The two important evolutionary events
in spiders – the divergence of the advanced
spiders from the primitive spiders and the
divergence of modern orb weavers from the
primitive orb weavers coincide with the
evolution of two types of silk-producing
glands, the major ampullate gland and the
flagelliform gland. This suggests a link
between silk protein evolution and spider
evolution. Despite the process’s high ATP
requirements, spiders have evolved ways of
efficient cost management; they consume and
recycle their own silk.
Some spider Silk is the key to the
spider’s success, with behavior playing merely
a supporting role. Without their silk, spiders
would be weak-lings among their arthropod
peers (if they had survived at all). They are
soft-bodied and so are prone to physical
damage; they breathe through lungs, and are
in constant need of high humidity; and they
are wingless, and their hydraulically driven
legs buckle when overused. As a result,
spiders would have been no match for the
tough and virtually indestructible insects. But
silk gives spiders a distinct edge, and has
become the main weapon in an arms race with
their insect relatives, which are both their
main prey and major predators.
Interestingly, all the silk proteins,
including those of spiders show the
predominance of the three amino acids:
Alanine, Glycine and serine, the composition
of which, of course, varies with the silk type.
The variability lies in the sequence, length and
number of these repeats. Many spiders have
special adaptations that allow them to walk
easily on smooth and vertical surfaces. The
end of each leg is covered with thick brushes
of hair. All the tiny feet grip the small bumps
on whatever the spider is walking on, allowing
the spider to move easily over most terrain.
Here we describe the phylogeny of spider and
also give a brief description of its body
organs. Furthermore we describe the highly
efficient spinning glands and different
weaving patterns of web. And then we also
describe the structure and properties of silk.
Finally
we
look
at
the
different
biotechnological applications and further
research possible in this fascinating field.
2. Spider Basics
Spiders are look like insects and come in to
the category of Arthropoda, but they belong
to a completely different class of animals,
called Arachnida. Spiders make up the order
Araneae within this class, which also includes
mites, ticks and scorpions. While spiders vary
considerably in size, shape and behavior,
nearly all species share a basic set of
characteristics:
•
They have eight legs, made up of seven
segments each.
•
•
•
•
•
They feed primarily on insects.
They can inject venom into their prey.
They can produce silk.
They have a pair of small appendages on
the head, called pedipalps.
Their bodies are divided into two sections,
the cephalothorax and the abdomen,
joined by the thin pedicel.
The cephalothorax -- a fused head and
thorax -- distinguishes spiders from insects,
which have a separate head, thorax and
abdomen. The cephalothorax contains the
brain, stomach, eyes and mouth, and the
abdomen contains the heart, digestive system,
reproductive organs and lungs. Spiders have
two different types of respiratory systems -trachea and book lungs (most species have
both, but some have one or the other).
Compared to human beings, these respiratory
systems are very simple. Trachea is just long
tubes that run from a slit in the exoskeleton
through the body. Air flows in, oxygen
diffuses into the blood and carbon dioxide
diffuses into the air. The spider's normal
movement provides all of the necessary
energy to push air in and out. Book lungs are a
series of very thin, leaf-like structures. The
inside of each leaf is filled with blood, and the
outside is exposed to air. The spider's blood,
called hemolymph, circulates oxygen,
nutrients and hormones to the different organs
in the body. Unlike humans, spiders have an
open circulatory system. The spider's simple
heart -- a tube surrounded by a muscle, with a
one-way valve on each end -- pumps blood
into the body cavity, all around the spider's
organs. One of the most amazing things about
spiders is how much they can accomplish with
such a small brain. The spider's central
nervous system is made up of two relatively
simple ganglia, or nerve cell clusters,
connected to nerves leading to the spider's
various muscles and sensory systems. The
simple instructions encoded in these nerve
cells give spiders all the information they need
to undertake complex tasks, such as building
webs and attacking prey. Some species even
exhibit learning behavior. Spiders have an
exoskeleton -- a stiff support structure on the
outside of the body. Exoskeleton segments are
connected together with joints so the spider
can move them back and forth. Muscles
attached on the inside of the exoskeleton
contract to move the legs inward, but spiders
don't have any muscles to extend the legs back
out again. Instead, they have to force bodily
fluids (mainly blood) into the legs to push
them outward. If a spider loses too much body
water, it can't generate the necessary hydraulic
pressure to push its legs out. Spider can shed
its old cuticle exoskeleton (this is called
molting). Molting occurs frequently when a
spider is young and some of them can do it
throughout their life.
3. Silk’s structure
Spider silk is a biopolymer fiber. Its
composition is a mix of an amorphous
polymer (which makes the fiber elastic), and
chains of two of the simplest proteins (which
give it toughness). Out of 20 amino acids,
only Glycine and Alanine serve as a primary
constituent of silk. The Dragline silk of orbweb spider seems to be most studied in the
scientific research. The protein in dragline8
silk is fibroin (Mass of 200,000-300,000
Daltons) which is a combination of the
proteins spidroin 1 (Alanine-rich) and spidroin
2 (Glycine-rich), the exact composition of
these proteins depends on species. Fibroin
consists of approximately 40% Glycine and
25% Alanine as the major amino acids. The
remaining components are mostly glutamine,
serine, Leucine, Valine, Proline, tyrosine and
Arginine.
Spidroin6 contains polyalanine and
polyglycine rich (chains of Alanine and
Glycine respectively, these molecular chains
are linked together by hydrogen bonds)
regions where from 4 to 9 Alanine or Glycine
molecules are linked together in blocks. The
high elasticity of spider silk is due to Glycinerich regions where a sequence of multiple
(approximately 5 - dependant on silk type)
amino acids are continuously repeated. A 180°
turn (α-turn) occurs after each sequence,
resulting in α-spiral (or α-helix). Capture silk,
the most elastic kind, contains about 43
repeats on average and is able to extend 2-20
times (>200%) its original length whereas
most dragline silk for example will only repeat
about 9 times and is only able to extend about
30% of its original length, it is clear that the
repetitions and forming of the helixes (based
on the original amino acid sequence)
contributes considerably towards the silk's
resulting properties.
The fluid dope is a liquid crystalline
solution where the protein molecules can
move freely but some order is retained in that
the long axis of molecules lie parallel,
resulting in some crystalline properties. It is
thought that the spidroin molecules are coiled
in rod-shaped structures in solution and later
uncoil to form silk. During their passage
through the narrowing tubes to the spinneret
the protein molecules align and partial
crystallization occurs parallel to the fiber axis.
This occurs through self-assembly of the
molecules where the polyalanine regions are
linked together by hydrogen bonds to form
pleated β-sheets (these are well ordered
crystalline regions as shown in the figure 1).
These β-sheets act as crosslink between the
protein molecules and it is because of the
regular structure of these sheets that spider
silk has such a high tensile strength.
It is not purely coincidence that the
major amino acids in spider silk are Alanine
and Glycine. They are the simplest two amino
acids and do not contain bulky 'R' groups) so
are able to pack together tightly, resulting in
easier formation of the crystalline regions. The
crystalline (β-based) regions are hydrophobic
(anti-water) which aids the loss of water
during
the
polymerization
process
(solidification). This would also explain why
spider silk is insoluble as water cannot pass
through the densely spaced hydrogen bonds.
The Glycine-rich spiral regions of spidroin
aggregate to form amorphous areas and these
Figure 2: The above diagram shows how the highly
ordered β-sheets connect to the less crystalline regions
via extremely amorphous regions of Glycine-rich αhelixes.
3. Silk-spinning process
Figure 1: The above diagram shows how the crystalline
structure of the beta (β) sheets are formed by the polar
aligning of the protein molecules.
are the elastic regions of spider silk. Less
ordered Alanine-rich crystalline regions have
also been identified and these are thought to
connect the β-sheets to the amorphous
regions, it is these regions which give the silk
its elasticity. Overall, a generalized structure
of spider silk is considered to be crystalline
regions (Glycine-rich) in an amorphous matrix
(Alanine-rich).Kevlar has also a similar
structure.
The main thing that distinguishes spiders from
the rest of the animal kingdom is their ability
to spin silk, an extremely strong fiber. Most of
the spiders are able to sustain their lives on
these efficiently build webs, catching their
preys in them. Spiders have several spinneret
glands located at the spider's abdomen which
produce the silken thread. Each gland
produces a thread for a special purpose. Seven
different gland types have been identified till
now, although all the glands may not present
in the same species.
Scientists don't know exactly how
spiders form silk, but they do have a basic
idea of the spinning process. Spiders have
various specialized silk secreting glands that
secrete spinning dope (a solution containing
the protein molecule used to make the silk
fiber), which is dissolved in a water-based
solution inside the glands only. While in the
gland the secretion is a water soluble viscous
fluid, but upon being drawn through
spinnerets, its molecular arrangement changes
and becomes insoluble and ten times denser
than the fluid state. The spider pushes the
liquid solution through long ducts, leading to
microscopic spigots on the spider's spinnerets.
Spiders typically have two or three spinneret
pairs, located at the rear of the abdomen. Each
spigot has a valve that controls the thickness
and speed of the extruded material. As the
spigots pull the fibroin protein molecules out
of the ducts and extrude them into the air, the
molecules are stretched out and linked
together to form long strands. The spinnerets
wind these strands together to form the steady
silk fiber.
As an example we can discuss the
production of dragline silk by Nephila
clavipes, which is best understood among the
scientists. The major gland making this silk
consists of a long tail10 and a wider sac called
the Ampulla. The tail secretes the major part
of spinning dope, while the sac constitutes the
main storage place that leads, through a
funnel, to a tapering duct. The secretory part
of the gland has two distinct zones, the Azone occupying the tail and two third part of
the sac, and the B-zone comprises of the rest
part of the sac which run to the funnel. The
epithelium of the A-zone is composed of tall
columnar secretory cells of a single type,
packed with secretary granules. The A zone
secretes an aqueous and highly viscous, often
yellow solution of about 50% protein, which
will be mostly spidroin I and II (the main
proteins making up spider dragline silk). As
the A-zone secretion flows towards the funnel
it is coated by a colorless homogeneous
viscous liquid, possibly the glycoprotein
secreted in the B-zone. The glycoprotein may
help to plasticize the thread by maintaining a
high water content.
The silk material exits the gland
through a funnel that connects directly to the
long duct where actually protein orientations
into a fiber begin. The duct is thereby divided
into 3 limbs or sections which progressively
grow narrower towards the spinneret and
spigot. A valve is located just prior to the
spinneret. The duct itself has a thin cuticle,
which acts as a dialysis membrane and may
allow water and sodium ions out of the lumen,
and potassium ions, surfactants and lubricants
into the lumen to facilitate thread formation.
Within the glands and the first and second
loop of the spinning duct, the spider's dope is
liquid crystalline, with the main silk protein
constituent likely to be in a compact
conformation that allows it to be processed at
high concentrations. The duct's convergent, or
hyperbolic, geometry forces the dope flowing
along it to elongate at a constant rate which
also ensures that only low and uniform
stresses are generated which prevents
localized coagulation centers from forming
early before the dope have reached their
optimal
orientation.
Good
molecular
alignment contributes significantly to the
thread's toughness. The drawdown process is
mainly internal and starts in the third limb of
the duct. The high stress forces generated
during this stage of processing probably bring
the dope molecules into alignment, so that
they are able to join together with hydrogen
bonds to give the anti-parallel beta
conformation of the final thread. As the silk
protein molecules aggregate and crystallize,
they will become more hydrophobic, inducing
phase separation and hence the loss of water
from the surface of the solidifying thread.
From here finally, the thread is gripped by the
flexible and elastic lips of the spigot, through
which it passes to the outside world. The
spigot strips off the last of the aqueous layer
surrounding the thread, thus helping to retain
water in the spider, and also places the thread
under tension for the final air-drawing step.
Nephila clavipes has 3 pairs of
spinnerets (figure 3) called anterior laterals,
posterior laterals and posterior medians, and
can spin seven different types of silk made
from different silk glands. The dragline silk is
secreted by the major ampullate glands, exits
from spigots on the anterior lateral spinnerets.
Most spiders have six spinnerets, some have
four or two. They move independently and in
concert to build webs. Silk is not ejected under
pressure, but is drawn out by external means
by a force such as wind or gravity.
Most spiders have multiple silk glands,
which secrete different types of silk material
optimized for different purposes. By winding
different silk varieties together in varying
proportions, spiders can form a wide range of
fiber material. Spiders can also vary fiber
consistency by adjusting the spigots to form
smaller or larger strands. Spider might coat
the silk with various substances suited for
different purposes, for example a sticky
substance, or a water-proof material.
4. Weaving its web
Different spiders build up their webs in
different ways. The pattern & the structure of
the web vary among various species of spider.
We can take the example of Araneidae which
is a large family of spiders. All species in this
family, if they make webs, they make vertical
or horizontal orb webs. Some species
construct some sort of stabilizer in the center
of the web. Some members in this family do
not make web at all. Following is the list of
various orb web spiders:
•
•
•
•
•
•
Garden orb weaver spider (Figure 6)
Scorpion-tailed Spider (Figure 7)
Russian Tent Spider (Figure 8)
Banded Orb-weaving spider (Figure 9)
Tree-Stump Spider (Figure 10)
Pan spider (Figure 11)
The way spider builds up their webs is
extremely economical. The spider recycles
silk after use by eating it, so very little is
wasted. This way the spider regains its lost
energy in form of web by consuming the
proteins present in the web. The weight of silk
in the spider’s web is less than thousandth part
of weight of spider’s body.
This net-technology raises some
problems for the spider. The spider has to
make sure that the prey sticks to the web after
hurtling the web & do not tear off the web.
This problem can be solved by making the silk
very elastic but at the same time taking care of
the fact that the prey does not get recoiled
back. The web threads are capable of
stretching out to ten times their resting length
& they also recoil slowly enough not to
bounce the prey out of the web.
The next feature that the silk needs, in order to
keep the prey from escaping, is stickiness. The
substance that coats the silk in the reeling
operation is watery as well as sticky.
Spiders negotiate in a unique way from
sticking on to its own web. The legs spiders
are anointed with special oil which provides
some protection from the stickiness of the
web. Also spider while building up her web
leaves some of the threads i.e. the main spokes
that radiate out from the centre of the web,
non-sticky & she runs about on these main
spokes only, using the specially modified feet
(figure 4) ending in little claws to grip the fine
threads.
Figure 310. Image of the spinneret of the Cribellate orb web weaver utoborus showing
different types of spigots
Figure 411 : A scanning electron microscope
(SEM) micrograph of the foot of the jumping
spider E. arcuata. In addition to the tarsal
claws, a tuft of hair called a scopula is found
at the tip of the foot, which is what the spider
uses to attach itself to surfaces. The long
hairs which are distributed over the entire foot
are sensitive to touch. Magnification 200x.
(Image courtesy Institute Of Physics)
Figure 5 :These triangular setup tips stick to surfaces
by van der Waals force (20,000x magnification).
Photo: Institute of Physics
Eriophora transmarine
Figure 6
Cyrtophora moluccensis
Figure 8
Poltys illepidus
Figure 10
Arachnura higginsii
Figure 7
Argiope trifasciat
Figure 9
Pan Spider
Figure 11
The Garden Orb Web Spiders9 also called
Araneus Spiders, Garden Orb-weaver, and
Wheel weaving Spiders or Orb weaving
Spiders. They build vertical orb web in
garden and bushland. The way they construct
their web is nothing less than the job of a well
skilled engineer.
To build up their web they follow pattern
explained below with the help of figures 1222.
• Spider releases a single thread5 with at
its tip a tiny flattened silken sail or
kite. This is to let the wind to blow the
silk, until the silk connect with
something (Figure 12). This way the
spider sets up the bridge from where
on she will start building up her web
(Figure 13).
• Spider then came back and forwards
along the bridge thread, add more silk
on it to make it stronger. Then she will
start to set the centre of the web, &
give it a ‘V’ shape (Figure 14). The
two arms of ‘V’ are well placed to
make two of the major spokes of the
web.
• Spider fixes a new thread to the point
on the ‘V’ & reels herself down to
ground to find an anchor point &
fastens the vertical thread to the
surface (Figure 15).
• Then Spider put some frame threads
between the anchor points. This will be
the outside frame of the web (Figure
16).
• After putting the frame threads, Spider
starts to lay the radius threads. The silk
•
•
•
•
used to make the frames and radius is
not sticky. They serve for supporting
purpose. And Spider will walk on that
radius so she will not be entangled by
her own silk (Figure 17). Spider goes
back and ford from the web centre and
the frame to lay the radius (Figure 18).
Then Spider put the auxiliary spiral
silk, from centre out to the frame. This
auxiliary spiral silk is used by spider as
reference for laying the capture spiral,
the sticky silk. The auxiliary spiral silk
will be removed later (Figure 19).
When Spider reaches the outer most
frames, she returns and starts to lay
capture spiral, the sticky silk. She
spirals from outside towards the
centre. She uses the auxiliary spiral
silk as reference, and will remove it at
the mean time (Figure 20).
The web is finished when Spider laid
the stick silk close enough to the centre
(Figure 21).
Then she sits in the centre of the web,
with each leg on each silk sensing if
there is any prey get caught (Figure
22) .When the spider sit off the web
,she keeps in touch by a special signal
thread running from retreat to the
centre hub. This signal thread is under
tension & it instantly transmits the
signal to her about the prey being
caught in the web.
Figure 12
Figure 14
Figure 16
Figure 13
Figure 15
Figure 17
Figure 18
Figure 20
Figure 19
Figure 21
Figures 22
5. Biotechnological Applications of
Spider’s Silk
Current research in spider silk involves its
potential use as an incredibly strong and
versatile material. The interest in spider silk is
mainly due to a combination of its mechanical
properties and the non-polluting way in which
it is made. The production of modern manmade super-fibres such as Kevlar involves
petrochemical processing which contributes to
pollution.
Kevlar is also drawn from
concentrated sulphuric acid. In contrast, the
production of spider silk is completely
environmentally friendly. It is made by
spiders at ambient temperature and pressure
and is drawn from water. In addition, silk is
completely biodegradable. If the production
of spider silk ever becomes industrially viable,
it could replace Kevlar and be used to make a
diverse range of items such as:
•
•
•
•
•
•
•
Bullet-proof clothing
Wear-resistant lightweight clothing
Ropes, nets, seat belts, parachutes
Rust-free panels on motor vehicles or
boats
Biodegradable bottles
Bandages, surgical thread
Artificial tendons or ligaments,
supports for weak blood vessels.
Scientists hope to soon be able to spin spider
silk without the aid of spiders - achieving an
age-old human quest to harness one of nature's
most remarkable materials. Randy Lewis is a
professor of molecular biology at the
University of Wyoming in Laramie. His team
of researchers has successfully sequenced
genes related to spider-silk production—
uncovering the formula that spiders use to
make silk from proteins. The most common
method is introducing silk-spider genes into
other organisms so that they can produce silk
proteins that might later be used to create
artificial silk threads. Host organisms range
from simple bacteria to goats. There have
been attempts to generate transgenic tobacco7
and potato plants that express remarkable
amounts of recombinant Nephila clavipes
dragline proteins. In the process the team
acquired a better understanding of how the
silk's structure is related to its amazing
strength and elastic properties. By cracking
the genetic code of spider silk, scientists hope
not only to be able to duplicate the material
but perhaps even to improve on it. Over
hundreds of millions of years the 37,000
known species of spiders (and others
unknown) have evolved and diversified many
silks for their unique purposes. Best known
and studied is silk secreted by a spider's major
ampullate glands. The silk is also used to
create spiders' familiar "wagon wheel" webs.
Spider silk has incredible tensile strength and
is often touted as being several times stronger
than steel of the same thickness. What's even
more unique, however, is spider silk's
elasticity. "When we say spider silk is tougher
than things like Kevlar [a plastic used to make
body armor]. Kevlar has higher tensile
strength but it's not very stretchy," said Todd
Blackledge, an entomologist at the University
of Akron. These properties suggest a potential
for many applications for spider silk:
extremely thin sutures for eye or nerve
surgery, plasters and other wound covers,
artificial ligaments and tendons, textiles for
parachutes, protective clothing and body
armor, ropes, fishing nets, and so on. "One
that's initially surprising is air bags," Lewis
added. "Right now an air bag just sort of blasts
you back into a seat. But if it were made out
of this material it would actually be made to
absorb energy and really reduce impact."
Current research focuses around these
problems and a possible solution would be to
adapt the composition of silk proteins to alter
its properties. Research is still in its early
stages but unraveling the secrets of spider silk
is underway.
References:
1. Vollrath, F. & Knight, D.P. Liquid crystalline spinning of spider silk. Nature 410,
541–548(2001).
2. Vollrath, F. & Knight, D. P. Structure and function of the silk production pathway in the
spider Nephila edulis. Int. J. Biol. Macromol. 24, 243–249 (1998).
3. Lazaris, A., Arcidiacono, S., Huang, Y., Zhou, J.-F., Duguay, F., Chretien, N., Welsh, E.A.,
Soares, J.W. & Karatzas, C.N. Spider silk fibers spun from soluble recombinant silk
produced in mammalian cells. Science 295, 472–476 (2002).
4. Forbes, Peter. The Gecko’s foot, Bio-inspiration : Engineered from Nature. Chapter 3
“Nature’s Nylon”. Pp. 55-78
5. Dawkins, Richard. Climbing Mount Improbable.1996. Chapter 2 "Silken Fetters." Pp. 32-63
6. Seidel, A. et al. Regenerated spider silk: Processing, properties, and structure.
Macromolecules 33, 775–780 (2000).
7. Scheller, J., Guhrs, K.H., Grosse, F., Conrad, U. Production of spider silk proteins in
tobacco and potato. Nature Biotechnology 19, 573- 577(2001).
8. Van Beek, J. D., Hess, S., Vollrath, F. & Meier, B. H. The molecular structure of spider
dragline silk: Folding and orientation of the protein backbone. Proc. Natl Acad. Sci. USA 99,
10266–10271(2002).
9. http://www.geocities.com/brisbane_weavers/Garden_sp.htm
10. http://hubcap.clemson.edu/~ellisom/biomimeticmaterials/files/spiderbiology.htm
11. http://www.sciencedaily.com/releases/2004/04/040426054407.htm
12. http://www.wikipedia.org/