Biomedical Engineering Reference
In-Depth Information
spokes, the length, and density of sticky spiral material for catching bugs. The segments of the
photographed web are normally straight, but are seen curved in this figure due to the weight of
the accumulated droplets. The net is sufficiently strong to survive this increased load without
collapsing.
The spider generates the fiber while at the same time hanging on to it as it emerges cured and
flawless from its body. The web is generated at room temperature and at atmospheric pressure. The
spider has sufficient supply of raw materials for its silk to span great distances. It is common to see
webs spun in various shapes (including flat) between distant trees, and the web is amazingly large
compared to the size of the spider. Another interesting aspect of the spider web is the fact that it is a
sticky material intended to catch prey, but the spider itself is able to move freely on it without being
trapped.
The silk that is produced by a spider is far superior in toughness and elasticity to Kevlar
1
, which
is widely used as one of the leading materials in bullet proof vests, aerospace structures, and other
applications where there is a need for strong lightweight fibers. Though produced in water, at room
temperature and pressure, spider's silk is much stronger than steel. The tensile strength of the
radial threads of spider silk is 1154 MPa, while steel is 400 MPa (Vogel, 2003). Spiders eat flies and
digest them to produce the silk that comes out from their back ends, and spool the silk as it is
produced while preparing a web for trapping insects. This web is designed to catch insects that cross
the net and get stuck due to its stickiness and complexity. While the net is effective in catching
insects, the spider is able to maneuver on it without the risk of being caught in its own trap. Recent
progress in nano-technology reveals a promise for making fibers that are fine, continuous, and
enormously strong. For this purpose, an electrospinning technique was developed (Dzenis, 2004)
that allows producing 2-mm diameter fibers from polymer solutions and melts in high electric
fields. The resulting nano-fibers were found to be relatively uniform without requiring extensive
purification.
1.6.2 Honeybee as a Multiple Materials Producer
Another ''material manufacturing engineer'' found in nature is the honeybee. This insect can make
materials in volumes that far exceed the individual bee's size. Bees are well known for making
honey from the nectar that they collect from flowers. They also produce honeycomb from wax.
Historically, candles were made using this beeswax, but with the advent of the petroleum industry,
candles are now mostly made from paraffin wax. Another aspect of honeybee is that their bodies
produce a poison that causes great pain, which is injected, through a stinger, into the body of any
intruder who is perceived as a threat to the bee's colony.
1.6.3 Swallow as a Clay and Composite Materials Producer
The swallow makes its nest from mud and its own spit forming a composite structure that is strong.
The nest is shaped to fit the area onto which it is built. The swallow builds its nest under roofs and
other shelters that provide both protection and concealment. Figure 1.15 is a photograph of two
nests of swallow. A flock of swallows have gathered next to the nests. While the two nests are
different in shape they have similar characteristics and they both provided sufficient room for the
chicks to hatch and reach maturity. It is interesting to note that the birds in the photograph attach
themselves to the wall carrying their body weight on their claws, which secure them comfortably to
the stucco paint on the wall.
1.6.4 Fluorescence Materials in Fireflies and Road Signs
Fluorescence materials can be found in quite a few living species and these visible light-emitting
materials can be divided into two types:
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