Unit - Chemistry of Garments: Animal Fibres
Silk can be obtained from the cocoons of several types of
caterpillar or silkworm, but it is the Chinese silk moth
(mulberry silkworm Bombyx mori) reared in captivity that
is now mainly cultivated. See the Zanzibar Tribal Art web site for an
diagram of the life
cycle of the various silkworms as well as video clips of a Silk
factory in Beijing and
Thailand and the
Silk Worm Cycle (30+ videos)
Natural silk is one of the strongest textile fibres, and this can be
accounted for by the stretched-out molecular form. Silk (78%
protein) is much stiffer than wool in spite of both being
proteins made from amino acids chains. Silk fibres have fine
draping qualities and are naturally crease-resistant and bring
about a warm feel to the skin.
Of the 3-4000 metres of fibre in a cocoon, laid out as a figure of eight by the
movement of the head of the pupae, less that one third is
generally usable with much of the remainder being processed
Silk emitted by the silkworm consists of two main proteins,
sericin and fibroin, fibroin being the structural center of the
silk, and serecin being the sticky material surrounding it.
Fibroin is largely made up of the amino acids
Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets,
R = H, glycine; R = CH3
, alanine; R =
Hydrogen bonds form between chains, and side chains form above
and below the plane of the hydrogen bond network.
|fibroin protein in silk
The amino acid compositions of the silk proteins are shown below.
B. mori silk fibroin contains a high proportion of three
α-amino acids, glycine (G; Gly, 45%, R=H), alanine (A; Ala,
29%, R=CH3), and serine (S; Ser, 12%,
R=CH2OH), in the approximate molar ratio of 3:2:1,
respectively. Tyrosine, valine, aspartic acid, glutamic acid,
etc. make up the remaining 13%.
Reference: Fraser, R.B.D. and MacRae, T.P. Conformation of
Fibrous Proteins and Related Synthetic Polypeptides, Chapter 13.
Silks. Academic Press: New York, 1973; 293-343.
Silk protein composition, percentage of amino acids found
The high proportion (45%) of glycine, which is the smallest amino
acid, allows tight packing and the fibers are strong and
resistant to breaking. The tensile strength comes from the many
interseeded hydrogen bonds, and when stretched the force is
applied to these numerous bonds and they do not readily break.
Silk is one of the strongest natural fibers but loses up to 20%
of its strength when wet. It has a good moisture regain of 11%.
Its elasticity is moderate to poor: if elongated even a small
amount, it remains stretched. It can be weakened if exposed to
too much sunlight. It can be attacked by insects, especially
if left dirty.
Silk is a poor conductor of electricity and thus susceptible to
static cling. It is resistant to most mineral acids, except for
sulfuric acid, which dissolves it. It is yellowed by
To produce 1 kg of silk, 3000 silkworms must eat 104 kg of mulberry leaves.
It takes about 5000 silkworms to make a pure silk kimono.
Most of the pupa are killed in the process since it
is not practical to cut open the coccoon without damaging the
silk. A small proportion is kept for further breeding.
The following provides some basic information on how silk is
made. The commercial process of silk making is both complex and
- Sericulture - cultivation of the
- Hatching the Eggs - the female deposits 300
to 400 eggs at a time.
- The Feeding Period - for about six weeks the silkworm eats
chopped mulberry leaves almost continually.
- Spinning the Cocoon - this is done over a 3
to 8 day period. The fibroin is secreted by two salivary glands
and forced through openings called spinnerets. A second set of
glands secretes the sericin.
- Reeling the Filament - the cocoon is treated
with hot air, steam, or boiling water and the silk is then
unbound by softening the sericin and then delicately and
carefully unwinding or 'reeling' the filaments.
- Types of Silk - raw silk (still containing
the sericin) is twisted into a strand sufficiently strong for
weaving or knitting. This process of creating the silk yarn is
RNA deterines the sex of silk worms, The Scientist, May 2014.
The end goal is to find a way to produce all-male populations of the species.
Male silkworms weave cocoons that contain more silk of a higher quality than those produced by females.
Chem and Eng News, 12 Sept 2011, p 20.
Researchers at the Smithsonian Institution have developed a way
to estimate the age of ancient silk from just a piece of fluff
from priceless textiles. The new mass spectrometry-based
technique requires a silk sample size that is signficantly
smaller than what is needed for successful carbon-14 dating, the
only other scientific method that can date silk, explains Mehdi
Moini, a conservation chemist at the Smithsonian's Museum
Conservation Institute. Moini developed the technique with
Kathryn Klauenberg and Mary Ballard ( Anal. Chem.,
2011 DOI:10.1021/ac201746u). Silk is composed of intertwining
strands of protein extruded by a silkworm and has been used as a
textile for some 2,500 years in flags, tapestries, carpets, and
clothing. The Smithsonian team examined aspartic acid residues in
silk protein and found that, over time, aspartic acid racemizes,
changing from the L form to the D form. They discovered that by
measuring the L-to-D ratio, the age of the textile could be
Silkworms made to spin coloured silk
Since 2009 scientists from the
Singapore Agency for Science, Technology and Research,
have been working on a way to make silkworms produce coloured silk - a method
that bypasses the traditional silk-dyeing step and is also more environmentally-friendly.
In 2014 the use of azo dyes was reported from Mysore and Pune, India
In the last four days of the larva stage, the silkworms' diet is changed to
a mixture of mulberry powder and a fluorescent dye. Once digested, the silkworms start
producing coloured silk, forming cocoons that can even be made that glow in the dark.
An alternative method from Japan in 2013
involved genetically modifying the silkworms to produce the dyes.
Silkworms that eat carbon nanotubes and graphene spin tougher silk
Researchers reported in 2016 a clever way to make the gossamer threads even
stronger and tougher: by feeding silkworms graphene or single-walled carbon
(Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b03597).
The reinforced silk produced by the silkworms could find use in applications
such as durable protective fabrics, biodegradable medical implants, and
ecofriendly wearable electronics, they say.
Researchers have previously added dyes, antimicrobial agents, conductive polymers,
and nanoparticles to silk-either by treating spun silk with the additives or,
in some cases, by directly feeding the additives to silkworms.
In 2010 some US researchers
genetically engineered silkworms in which they incorporated specific DNAs taken from spiders.
When these transgenic silkworms spin their cocoons, the silk produced is not ordinary
silkworm silk, but, rather, a combination of silkworm silk and spider silk.
genetically engineered silk protein produced by the transgenic silkworms
has markedly improved elasticity and strength approaching that of native spider silk.
Golden Orb Silk Spider
was cultivated to produce some fabric for the 1900 Paris Exhibition, but
unfortunately that sample disappeared. In 2004 a textile designer, and an entrepreneur,
managed in three years work and using 1.2 million Golden silk orb-weavers
(collected in the wild and released some 30 minutes later after they produced
the silk) to produce a
shawl that was as exhibited at the Art Institute of Chicago.
By 2012 the two managed to produce a second, bigger
garment, a cape, that together with the shawl, were exhibited at the
Victoria and Albert Museum in London.
It has been shown possible to extract the spider silk gene and
use other organisms to produce the spider silk. In 2000, Canadian
biotechnology company Nexia successfully produced spider silk
protein in transgenic goats that carried the gene for it; the
milk produced by the goats contained significant quantities of
the protein, 1-2 grams of silk proteins per liter of milk
A Bullet Proof
Skin was recently produced from spider web obtained from
A comparison of Textile Fibres
Wool is the textile fiber obtained from sheep and certain other
animals, including cashmere from goats, mohair from goats, qiviut
from muskoxen, vicuña, alpaca, and camel from animals in
the camel family, and angora from rabbits.
Wool has several qualities that distinguish it from hair or fur:
it is crimped, it is elastic, and it grows in staples (clusters).
The term wool is usually restricted to describing the fibrous
protein derived from the specialized skin cells called follicles
Sheep grazing at Lyonville, Victoria, Australia
See the YouTube video clip on shearing merino
sheep in NSW, Australia
Wool and hair are fibres of protein (
keratin), that have a very
complicated structure consisting of dead cells, which emerge from
the hair follicles and are much overlaid with grease. The fleece
sheered during the warm season contains only about 43-50% of wool
by weight. The rest are the oils, fats, moisture and dirt.
The textile properties of wool have been appreciated for more
than 12,000 years. For example, among its many qualities are:
- Wool is insulating. It insulates against the cold as well as
heat, thanks to the quantity of air that is trapped in its
fibers. Wool keeps us warm during the winter and is pleasant to
wear during the summer.
- Wool is an excellent regulator of humidity : it can absorb up
to 30% of its weight in humidity without feeling damp or
breaking. This hydrophilic property of wool allows it to
- Wool has excellent elasticity and memory and of all the
natural fibres it is the most resistant to creasing. Thanks to
these properties, garments made from wool keep their shape and
- Wool has a natural affinity for dye : it is a textile fiber
that is easy to dye.
- Wool is naturally fire retardant.
- Additionally, wool is a natural resource : renewable and
Structure of wool
The outside of wool has a protective layer of scales called
Cuticle cells that overlap each other like roof tiles.
The interior of the wool fiber is called the Cortex and this
makes up about 90% of the fiber.
Cortical Cells have a complex interior structure that includes:
Twisted Molecular Chain and Helical Coil
These cells are protein chains that are coiled in a helical shape
like a spring. The chains are stiffened by hydrogen and disulfide
bonds, linking each coil of the helix, helping to prevent it
stretching. Though the helical coil is the smallest part of the
fibre this little spring gives wool its flexibility, elasticity
and resilience; helping wool fabric keep its shape and remain
These cells make up the units, lying inside the Matrix. The
microfibrils are like the steel that is embedded in concrete to
provide the strength and flexibility. The microfibrils contain
three right-handed helices wrapped around each other in a
left-handed coil where they are held together by more H-bonds and
sulfur bridges (protofibril). Nine of these protofibril coils
cluster around two more so that the microfibril contains a total
of eleven coils each consisting of three α-helices.
The matrix consists of high sulfur proteins. This makes the wool
absorbent because they attract water molecules. Wool can absorb
up to 30% of its weight in water and can also absorb and retain
large amounts of dye. The Matrix region is responsible for wool's
fire resistance and antistatic properties.
Inside the cortical cells are the macrofibrils that are made up
of bundles of hundreds of the even finer filaments (the
microfibrils). These are surrounded by the matrix region.
Fibrous keratin molecules supercoil to form a very stable,
left-handed superhelical motif to multimerise, forming filaments
consisting of multiple copies of the keratin monomer. See the
structure of proteins by Donald and Judiith Voet, and
Amino acid composition of keratin fibres
||Merino Sheep Wool
||g 100 g-1
||g 100 g-1
||g 100 g-1
N.H. Leon, J. Society of Cosmetic Chemists of Great Britain, 23,
The ability of the highly wound structures to unwind, even to the
extent of breaking the H-bonds, is what allows wool and hair to
stretch. Normally the shape is restored when the tension is
released and the H-bonds then reform. Note that a permanent
wave involves stretching and breaking the S-bridges so that
when the bridges reform they are in different positions.
Note that fingernails and claws are a form of α-keratin as well.
The Guinness World Book of Records for 2012 announced a winner for
the longest fingernails
and for the
where the record was over 16.8 m (55 feet).....
- In 2010/2011 75 million sheep were expected to be shorn in
Australia and production was estimated to be 350 million kg of
- Australia is the world's largest producer of wool, producing
21.5% of the world's greasy wool in 2008.
- While Australia produces more wool than any other country,
China has the largest sheep population.
Comparison between silk and wool.
One difference between silk (β-keratin) and wool
(α-keratin) is that in silk the amino acids, glycine,
alanine and serine are quite small with no bulky side-chains.
When combined together they do not form helices and instead lie
on top of each other to give pleated sheets of linked amno acids
with the glycine appearing on only one side of the sheets. The
sheets then stack on top of each other.
This planar structure is felt when you touch the smooth surface
of silk. Silk is less extensible than wool because its
polypeptide chains are all nearly fully extended already. The
limited flexibility comes from the weak interaction between the
sheets that allows one sheet to slide over another by breaking
the (loose) H-bonds.
How does the fibre structure of silk and wool influence the properties of
the final textile. (Include things like ability to maintain shape,
effect of moisture, etc.)
Much of the information in these course notes has been sourced
from Wikipedia under the Creative Commons License. Students
taking this course will be expected to contribute to Wikipedia as
a part of their course assignments.
Continue to Synthetic Fibres or
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Created August 2011. Links checked and/or last
modified 20th October 2016.