Anthony Chau
July 30, 2008
Cosmos Cluster 8
Spider Silk - the Material of the Future
Abstract:
Spider silk is well known for its strength yet elastic nature, and for this reason scientists
and engineers from a wide variety of fields have begun researching its structure and the
possibility and methods of synthesizing spider silk for industrial use. However, extraction of
silk from spiders is not cost effective, so most research is focused on synthesis of spider silk
either chemically or using DNA recombinant technology. Although research is still at an early
stage and ongoing, it is only a matter of time before spider silk can be successfully synthesized
for industrial use.
Results:
Nature is truly a master architect. With relatively few raw materials, it is able to create a
diverse array of biological life forms and biological materials essential for the survival of all
life forms on earth. Some of Nature’s materials continue to
amaze scientists and exceed the characteristics of artificial
materials. One such material is spider silk. Spiders rely on
their silk for a variety of functions, and their silk are
exceptionally light, tough, stiff, and extensible even when
compared to the strongest synthetic materials. Each family
of spider spins different types of silk, but silk from the
Figure 1: Nephila Clavipes
Nephila Clavipes (the golden orb-weaving spider) (see
figure 1) and Araneus diadematus (common garden spider) are the strongest among spiders,
and have been the focus of scientists’ research in recent years.[5]
Orb weaving spiders produce various types of silk from seven different glands. Dragline
and Viscid silk fibers are the strongest silk produced by orb weaving spiders, and are the most
closely studied. Dragline silk is used to make the radial threads in a web.[5] It is, pound for
pound, stronger than
steel, as stiff as nylon,
and can absorb ten
times more energy
than Kevlar. Dragline
silk can stretch to three
times its length before
snapping, whereas
Figure 2: Silk’s uses and tensile Strength
Kevlar snaps when stretched by three percent. Upon impact on dragline silk, most of the
energy is dissipated as heat.[7] Viscid silk, on the other hand, is sticky for prey capture and
stretches like rubber, but yet is still a third the strength of steel.[5] These desirable
characteristics have caught the attention of scientists and engineers from evolutionary biology
to mechanical engineering and material science.
Spider silks are fine fibers produced from fibrous-protein solutions.[5] Spider silk, like all
Figure 4: Beat Sheet Structure
other silk, is made up of protein polymers of
amino acids. The proteins MaSp-1 and
MaSp-2 make up dragline silk. The two
primary components of these proteins are
glycine (42%) and alanine (25%) while the
Figure 3: Beta Sheet
remainder of these proteins is made up of
bulkier amino acids such as tyrosine, glutamine, arginine, serine, and leucine. Dragline silk is
a semi-crystalline polymer, meaning it is part amorphous (random arrangement of chains) and
part crystalline (arranged in an orderly fashion).[4] Dragline silk differs from silkworm silk
because dragline silk consists of a larger percentage of bulky amino acid groups. There are
two crystalline alanine regions, one rigid crystal part and another less rigid crystal part.
Approximately 40% of the alanines
are highly oriented, and the other
60% are less so, yet still
crystalline.[3] The alanines are
mostly organized in polyalanine
sequences 5 to 10 residues in length, along with glycine-rich sequence containing the bulky
residues. These
alanine regions take up a beta sheet conformation (see figure 3 and 4), and make up the
crystalline regions.[8] Beta Sheets are regular secondary structures in proteins where hydrogen
bonding between fully extended polypeptide chains (beta strands) occur. Stacks of Beta sheets
are the predominant structure of the spider silk fibroin. The sheets are oriented along the axis
of the fiber, and are likely the cause of spider silk’s high tensile strength. (See figure 4) Beta
sheets are detected in both MaSp-1 and MaSp-2. In the non-polyalanine regions of MaSp-2,
proline occupies every fifth residue. A likely conformation for these regions is Beta coils.
MaSp-2 is predicted to have a structure similar to elastin, which suggests that MaSp-2 may be
responsible for the
elastic properties of
dragline silk.
Currently, models of
spider silk structure
suggest alternating
polyalanine
crystalline regions
and glycine and
proline-rich regions.
Figure 4: Spider Silk chemical structure
This likely explains
both the silk's strength and elasticity. The result of multiple hydrogen bonds in the beta sheets
that form the crystalline regions is a fiber that can resist separating. It is likely that glycine-rich
regions simply provide spacing for the crystalline region. The elasticity may then be a result of
the proline-rich regions that can stretch without breakdown of the beta sheet.[1] It is not
surprising that alanine and glycine, the two smallest amino acids, make up most of the
structure since they can pack together tightly and form crystal structures.[3] The combination
of hard beta sheet crystallites within in a soft rubbery glycine rich matrix is likely the main
reason for the exceptional mechanical strength and extensibility of dragline fibers[5]
As a result of its many desirable characteristics, Spider silk’s synthesis is currently a
major research focus for scientists and engineers from a wide variety of fields. There are three
possible approaches acquiring dragline silk or material with similar properties. The first
method is to extract it from a living source. However, raising spiders is not practical because
spiders tend to consume each other when placed closely together. The French naturalist
Rene-Antoine Ferchault de Reaumur was one of the first to used spider silk. She collected the
material from egg sacs, but eventually gave up due the large amount of spiders needed for
enough silk. Frenchman Bon de Saint-Hilaire in the eighteenth century tried to raise spiders
but failed too since spiders could not be raised in close quarters and require large separated
areas.[3]
Since raising spiders to extract their silk is not cost effective, one other option is to
synthesize the proteins chemically. At this point, researchers are still in the process of fully
understanding spider silk’s structure, so currently there is not enough information for the
synthesis of spider silk to be possible. Although both MaSp-1 and MaSp-2 are sequenced, the
difficulty of synthesizing spider silk lies in its change in solubility during its spinning process.
When spider spins silk, the silk is in a soluble liquid form in the body. However, when it
comes into contact with air, it hardens and becomes insoluble. The analysis of this process is
essential in understanding spider silk’s synthesis, but at the same time this process is difficult
to simulate in the lab. Furthermore, spider silk is a mixture of MaSp-1 and MaSp-2, so
studying these two proteins individually have been quite difficult. One major step in the spider
silk synthesis has been made, however. David A. Tirrel, a professor of polymer science and
engineering at the University of Massachusetts at Amherst successfully synthesized the
glycine and alanine rich sections of silk proteins that form the crystal portion.[3]
The most promising method is the use of recombinant DNA technology to coax another
organism to produce MaSp-1 and MaSp-2 in other organisms. Recombinant spider silk
proteins have been produced in bacteria and yeast, but with limited success. The problem with
this method lies in the highly repetitive structure and an unusual mRNA secondary structure
that makes translation inefficient, limiting the silk produced.[2] In addition some amino acids
are encoded by multiple codons. For example, alanine are encoded with the DNA codons
CGA, CGG, CGT, and CGC, while glycine can be encoded with DNA codons CCA, CCG,
CCT, and CCC. This leads to a problem when introduced into a different organism with
different codon preferences. Researchers at the US Army Natick Research Center in
Massachusetts, at DuPont Inc. in Wilmington, Delaware, and at several smaller companies and
academic institutions have tried to alter their gene species to produce spider silk proteins more
efficiently in bacteria with positive results.[3] However, recently transgenic plants may be a
feasible option for producing soluble recombinant spider silk, but more research is still
required.[2]
Conclusion:
Spider silk with its many desirable properties is truly a remarkable material that can be
useful in a wide variety of fields including medical, industry, and military. Although spider
silk research is still at an early stage, with all the research effort put in it is certain that
knowledge regarding spider silk will advance rapidly. DNA Recombinant technology is
especially promising in spider silk research and synthesis. With scientists and engineers from a
wide background all interested in using spider silk, it is only a matter of time before spider silk
can be used in society. When that time comes spider silk will truly be a revolutionary material.
Acknowledgements:
I would like to thank Dr. Mathew Peck as well as Professor Tim Patten for all the help and
giving me the inspiration for this research project.
Bibliography:
[1] Colgin, Mark A., and Randolph V. Lewis. "Spider silk: a biomaterial for the
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Pictures Sources:
Figure 1:
July 30, 2008. http://www.spiderzrule.com/spider806/IMG_3533_small.JPG
Figure 2:
Royal Society of Chemistry. July 30, 2008.
http://www.rsc.org/ej/SM/2006/b600098n/b600098n-f1.gif
Figure 3:
MCAT. July 30, 2008. http://www.mcat45.com/images/Beta-Sheets-MCAT.png
Figure 4:
Citizendium July 30, 2008. http://en.citizendium.org/images/2/29/BetaSheetByDEVolk.jpg
Figure 5:
July 30, 2008. http://www.scq.ubc.ca/wp-content/uploads/2006/07/silkstrand.gif