Page 54 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002J-63 May 18, 2001 14:16

Biomineralization and Biomimetic Materials 195

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cross-linked by cystines (sulfur–sulfur links). The high the stiffest synthetic fibers, such as Kevlar . The polymer
sulfur content is a defining characteristic of members of chains are wholly aligned with the fiber axis.
the keratin family and gives burning hair its characteristic Plant cells are hollow tubes with spirally wound fibrils.
smell. The fibrils confer great toughness by converting to Successive layers go in opposite senses, making a criss-
a more extended beta-sheet structure under stress. Feather cross pattern. The outer and inner layers may be wound at
keratin is a beta-sheet structure presumably for providing different angles to the rest. This is very like the winding of
higher stiffness. Amphibian skin is also a beta-sheet, again fibers on a composite pressure vessel, such as a pressurized
presumably to limit water swelling. The best-studied ex- gas tank. The wood composite structure is designed to
ample of keratin is wool, but there is also recent work on retain internal pressure or longitudinal compression with
hoof keratin. very high energy absorption on fracture as the windings
collapse inwards.
Plants again have the problem of how to manufacture
III. STRUCTURAL POLYSACCHARIDES the polymer without it enveloping and choking the pro-
duction site. It is laid down by a “track-laying” system,
Vincent (1980) discusses insect cuticle, which is a com- which seems to carry out the polymerization at the cell
posite of chitin fibers embedded in a cross-linked protein surface on a moving organelle and which draws glucose
matrix. Chitin is a polysaccharide, similar to cellulose but for polymerization through the membrane from inside.
with acetylamino substitution. Metabolically, polysaccha- In wood, hemicelluloses and lignin act as a matrix bond-
rides should be less expensive than protein since nitrogen ing the cellulose fibers into a composite structure. It is a
has a much lower abundance in the biosphere than carbon, puzzle that the stiffest, strongest, and cheapest of the bio-
hydrogen, and oxygen. It is not clear what improvement logical polymers is not used at all in animals.
in properties or processing led the insects to select chitin
in the place of cellulose as their main structural material.
The layered structures of cuticle, with various sequences IV. MINERALIZED TISSUES
of fiber orientation, do strongly resemble the layered
structures of carbon-fiber composite laminates. Given that In the synthetic world, some applications involve predom-
both insects and military aircraft are lightweight, roughly inantly tensile loading, the wall of a pressure vessel being
cylindrical systems, the resemblance cannot be acciden- one example. Much more commonly, parts will be loaded
tal. Gunderson and Schiavone (1995) have discussed how in compression or bending. While strong fibers in the form
insects adopt unbalanced or asymmetric layups that would of a rope can provide excellent resistance to tension, they
not be used in synthetic composites. Some of the patterns are of little use in compression. Large animals with an
also involve thick layers oriented along the major struc- external or internal skeleton will need stiff materials that
tural axes, with many fine layers forming the rotation from are more isotropic in their properties and so can withstand
one direction to the next, apparently to delocalize shear the varying stresses that come from moving around and
stresses that would cause delamination. colliding with other objects. Mineralized composites pro-
Many layered biological systems resemble cholesteric vide improved compressive properties over purely poly-
liquid crystals in the rotation of orientation from layer to mer structures. Compared to stiff polymers, minerals can
layer. This has raised recent interest in whether they actu- also be formed at a lower metabolic cost for a given level
ally are liquid crystals, in that the rotation forms sponta- of stiffness, but do increase the overall weight. A wide
neously as a result of interactions between fibers in suc- range of minerals is found in microbes, plants, and ani-
cessive layers . This would occur in a fluid state, which is mals but silica, calcium carbonate, and hydroxyapatite are
subsequently embedded in a hard matrix. The core ques- the most important.
tion is really whether the rotation pattern is directly con-
trolled by some form of oriented extrusion during the A. Silica
deposition process or is controlled through the surface
chemistry of fibrils deposited in successive layers. Silica occurs as spicules (short reinforcing rods) in
The mechanical properties of chitin are difficult to de- sponges. The spicules are typically 10 µm in diameter and
fine because large oriented samples are unavailable. In 100 µm long and may be simple rods or complex branched
cellulose, many plants contain bast fibers with very highly structures. They apparently form by aggregation of silica
aligned polymer, allowing us to calculate the stiffness of nanoparticles onto a thread of polysaccharide or protein
the polymer and so analyze the properties of wood and within a vesicle, a membrane-enclosed space inside the
other plant materials. The stiffness of cellulose in such organism. The material is amorphous, highly hydrated,
fibers, 40 GPa wet and 100 GPa dry, is comparable to and not fully dense. There is one example of a deep-sea