Page 173 - Handbook of Properties of Textile and Technical Fibres

150 Handbook of Properties of Textile and Technical Fibres

such as polyacrylamide, poly(vinyl alcohol) and fabricated into the form of films,
hydrogels, and tissue-engineering scaffolds (Zhang, 2002; Yooyod et al., 2016).

5.2.1.2 Technology and silk production
The elements, which go into the making of silk, by whatever animal, need to be stored
in a gland. It then needs to be secreted before being spun into filaments. This method of
production of silks is common to all types of animals producing silk; nevertheless,
some important differences can be highlighted such as the number and characteristics
of glands producing the polymers and the way these polymers are spun. Furthermore,
the period of life when the organism is able to produce silk differs depending on the
creature.
The life cycle of the B. mori involves two phases. First, the larval phase starts when
caterpillars hatch from eggs laid by the moths. Their growth goes through five larval
steps separated by metamorphosis. The caterpillar produces its silk to build a cocoon as
protection for its metamorphosis into a pupa. Finally, the butterfly phase is necessary
for reproduction permitting the production of eggs and completion of the cycle. The
B. mori is therefore only able to produce silk during a short part of its life, which is
the fifth period in the larval stage. The silkworms feed until they have stored up enough
energy to enter the cocoon stage. While they are growing they have to be protected
from loud noises, drafts, strong smells, chemicals traces, etc.
The pair of B. mori glands grows very fast with the increase of the size being due to
the growth of each cell and not an increase of the number of cells. As represented in
Fig. 5.6, the glands have three parts: the main central part with a Z shape, diameter
w3e4 mm in the middle and length w60 mm; two very fine tubes, the excretor (diam-
eter w0.05e0.3 mm, length 35 mm), which terminates in a spinneret located in the
head of the larva; and a broader one (w0.4e0.8 mm, w100 mm) where fibroin is
produced by the rear gland cells. The sericin coating is made in the anterior region
of the central Z part and envelops the fibroin core, which lead to the sericin-coated
fibroin fiber after the passage through the double spinneret. Recent Raman and IR
in vivo analysis (Percot et al., 2014) confirms that the silk precursor contains initially
a lot of water (Dinh et al., 2008) that then decreases and the material is ready to be spun
when the pH goes from 6.5 to 4 and the Ca 2þ concentration increases. The importance
of ion transfer, notably copper (Zhou et al., 2003), calcium, and magnesium, can be
noticed by stabilizing the polymer in solution and certainly controls its solidification
(Pérez-Rigueiro et al., 2007). These ions are incorporated in the fibroin in the medium
gland leading to the formation of “salt bridges” between the carboxyl groups of the
macromolecules. The sericin, with its high content in hydrophilic amino acids, absorbs
some of the water allowing the fibroin core to contain less water, helping to increase
the fibroin viscosity and hence forming the silk bave (Percot et al., 2014; Li et al.,
2003).
When it is time to build their cocoons, the worms extract the stocked substance,
which hardens when it comes into contact with air. Silkworms spend 1e3 days spin-
ning a cocoon around them until they look like puffy, white balls. After 8 or 9 days in a
warm, dry place the cocoons are ready to be unwound. First they are steamed or baked