Biomedical Engineering Reference
In-Depth Information
natural selection, these transducers surpass any
current technological embodiment in terms of
sensitivity. Nonetheless, there are devices based
on micro electro mechanical systems (MEMS)
that make use of such things as piezoelectricity.
Most commercially available vibration and
acceleration sensors function by spring-mass
systems and electronic detectors. A cilia-mimetic
system utilizing conductive fibers has been sug-
gested
[29]
. Nanofibers of polypyrrole, tagged
with a magnetic material, were used as detectors
on giant magneto resistive multilayer sensors.
As the artificial cilia are deformed, the magnetic
tip interacts with the underlying sensor.
domains, imparting the elasticity needed for
specific function.
Engineering new customized protein fibers
that have designed mechanical properties pre-
sents a challenge. To achieve this goal, the
molecular architecture of the molecules com-
prising these fibers, as well as their assembly
process, needs to be fully understood in order to
control assembly in any manufacturing process.
Protein fibers are more accessible to needed
engineering analysis, and tremendous efforts
are being made in this field to understand the
structure/function relationship of protein poly-
mers. Because of the availability of technologies
allowing the manipulation of genes encoding
for these proteins, the perspective of designing
and manufacturing new protein-based poly-
mers is promising
[31]
.
10.3.3.4 Strong and Tough Fibers (Spiders,
Hagfish, and Mussels)
The basic components of animal fibers are
mainly structural fibrous proteins as found
in hair, tendon, cartilage, skin, arteries, and
muscles of mammals or in cuticles and silks
of arthropods, such as insects and arachnids.
The individual proteins making up these fibers
are of a specific amino-acid sequence, often
sharing specific amino-acid motifs from one
type of material to another. In addition, the
individual fibrous proteins have the ability to
assemble into a supramolecular network, such
as macroscopic fibers. The resulting network
structure, generally insoluble in water, is main-
tained by a combination of labile cross-linking,
hydrophobic interactions, hydrogen bonding,
and electrostatic interactions that are created
between distinct amino-acid molecules in the
protein
[30]
.
The molecular architecture (sequence and
composition) of the individual proteins form-
ing the network, together with the amount and
type of intermolecular interactions involved,
determine fiber mechanical and physical prop-
erties. Some of the fibers are composed of rub-
ber-like or elastomeric proteins such as elastin
or resilin that confer a high degree of elasticity
to the fiber while retaining relative strength.
Embedded within the matrix are crystalline
SPIDER SILK
Like insects, spiders also manufacture silk;
however, the silk gland location and associated
spinning systems are somewhat different in that
spiders do not use modified salivary glands as silk
glands. Spiders possess one or more silk glands
located in their abdomen that are each linked to
specialized external structures called
spinnerets
located on the ventral part of their abdomen. A
duct of a particular length and shape, depend-
ing on the spider and silk gland considered, links
each type of gland to a certain type of spinneret.
The soluble silk made by specialized cells in the
gland wall is collected in the lumen. The soluble
silk in a liquid crystalline phase
[32]
is slowly pro-
cessed into a fiber as the molecules move through
the long duct and are subjected to chemical and
physical stresses and modifications. The silk fiber
that the spider pulls out with its legs is insoluble
and exits through specialized hair-like structures
called
spigots
covering the spinnerets
[33]
.
The most characterized spider silks are the
ones produced by spiders belonging to the genera
Nephila
and
Araneus
. A picture of
Nephila calvipes
from my laboratory is shown in
Figure 10.6
.
These spiders, considered highly evolved spiders,
