Biomimetic Textiles - Engineered Biomimicry - page 251

consists of up to 10,000 β (1 → 4) linked d-glucose
units.
Wool is the hair of sheep. As such it is a protein,
a polypeptide composed of a specific set of amino
acids. Silk (from the silkworm) is the cocoon mate-
rial housing the pupae, and, like wool, silk is also
a polypeptide. The different amounts of the various
amino acids in wool vis-à-vis silk account for the
differences in the properties of the two fiber types.
In addition, owing to their different functions,
the physical structure of the fibers is different. Wool
fibers have scales (see Figure 10.1 ), whereas the
surface of silk fibers is relatively smooth. The pupae
are delicate and so need a smooth cradle in which
to grow. It is generally agreed [5] that the function
of scales on hair, which imparts a roughness to the
fiber, is to help keep the skin of the animal clean.
The protruding edge of the scale points away from
the skin, making more difficult the progression of
detritus from the tip of the hair to the skin. Also,
dead epidermal cells will be preferentially moved
from the surface of the skin toward the tip of the
fiber, i.e., away from the skin, as the hair grows.
It seems reasonable to presume that the natu-
ral world was the inspiration for the earliest
engineering forays into fiber manufacturing, or
fiber spinning. The process by which the natural
fibers, notably the silk materials but also hair, are
produced is essentially that of extrusion. Not all
fibrous materials in nature are formed through
extrusion, however. The growth of seed hairs
and plant stems does not follow an extrusion
model. Nor does the byssus thread of the mussel.
Nonetheless, the resulting fibers all have the
same microstructural characteristics. Of course,
natural extrusion is conducted under ambient
conditions, which is not the case for the synthet-
ics, but nonetheless the similarities are
remarkable.
Irrespective of whether the process is natural
or synthetic, the polymer, in a fluid state, is
pumped through a small orifice or, in the case of
synthetics, simultaneously through many small
orifices. The orifice is referred to as a spigot in
spider silk spinning. The plate containing a

and (silk worm) silk, and common synthetics
being polyesters, polyolefins, and polyamides.
Other commercial polymers used in synthetic
fibrous materials include the aromatic polyam-
ides (such as Kevlar®) and other high-perfor-
mance polymers. The common traits among these
materials derive from their being composed of
linear chains that have been oriented and locally
crystallized during fiber production, as discussed
later in this section. In contrast to fiber production
in natural systems, synthetic fiber production
generally requires the use of high temperatures
and pressures and/or unfriendly solvents. Con-
sequently, there has been much exploration into
the natural systems (silkworms and spiders being
the most common) in order to devise more sus-
tainable methods of fiber production.
Natural fibers are essentially exclusively com-
posed of either cellulose (e.g., cotton and flax) or
protein (e.g., wool and silk). As in the case of the
synthetic fibers, these natural polymers also
comprise (mostly) linear polymers with orienta-
tion and crystallinity. Because the cellulose mol-
ecule is consistent between plants, cotton, flax
(linen), ramie, and hemp all have the same basic
polymer structure. There are other constituents,
such as lignin, which vary in amount, however,
depending on the source. In addition, the degree
of crystallinity may also be different, which con-
tributes to the different properties of the fibers.
The cross-sectional shape of the fiber also con-
tributes to its properties. Cotton is bean-shaped
in cross-section, whereas ramie tends to be more
angular [5] ; hence, the flexural rigidity, being
strongly influenced by the moment of inertia, is
much different for each of these fibers. Cotton is
a seed hair, whereas the other fibers derive from
the plant itself, such as the stem. This difference
in function helps us understand why there are
marked differences in structure: Form follows
function. Common to all plant-derived fibers is
cellulose. An organic compound with the chemi-
cal formula (C 6 H 10 O 5 ) n , cellulose is, by this for-
mula, a polysaccharide. Unlike other saccharidic
materials, such as starch, the cellulose chain