Page 35 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

Biomaterials, Synthetic Synthesis, Fabrication, and Applications 175

and structure both play important roles in determining the between the molecules. These naturally increase with age
properties of the natural polymeric phase whether it is used leading to a less flexible material as someone gets older.
alone or in a composite form. Elastin is used in situations where a highly elastic fiber
is needed such as in ligaments and blood vessels (Table II).
The polypeptide chain of elastin contains high levels of
A. Natural Polymers
both glycine and alanine, is flexible and easily extended.
The key to the mechanical properties of fibrous materials Topreventinfiniteextensionthepeptidesequencecontains
(collagen, silks, chitin and cellulose are all natural exam- frequent lysine side chains that can be involved in cross-
ples and polyethylene and nylon are examples of synthetic links that allow the fibers to “snap-back” on removal of
polymers) lies in their structure, in particular the extent tension. Elastin is only ever found as fine fibers and is usu-
of their crystallinity. Crystallinity equates with material ally found in association with collagen and glycosamino-
rigidity, hardness and toughness, Table I. For crystalline glycans. The hydrophobic character of the protein (95%
polymers to form they must have great regularity of chain of the amino acids have nonpolar side chains) leads to wa-
structure and be linear in form. The presence of more ter becoming more ordered and the polymer less ordered
than one stereo-isomeric form of the polymer can cause when fibers of the material are stretched.
problems in structural alignment for synthetic polymers Carbohydrate-based natural polymers include gly-
but this is not a problem in natural polymers. Collagen or cosaminoglycans (polysaccharides containing amino
cellulose as only one stereo-isomer is utilized in the build- acids formerly known as mucopolysaccharides), proteo-
ing of these structures. When the level of crystallinity is glycans (covalently bonded complexes of protein and
less than 20%, the crystalline regions act as cross-links glycosaminoglycan in which the polysaccharide is the
in an amorphous polymer network with materials be- dominant feature) and protein–polysaccharide complexes
ing mechanically similar to cross-linked rubbers. Above where a complex is held together through noncovalent
40% crystallinity the materials become quite rigid and the linkages. Important examples are the chondroitin sulfates
mechanical properties of the polymers are largely time- and keratan sulfates of connective tissue, the dermatan
independent. The behavior of drawn fibers, such as those sulfates of skin and hyaluronic acid. All are polymers of
producedfromsyntheticpolymersmaybefurtherdifferent repeating disaccharide units in which one of the sugars
from the bulk material as the regions of amorphous char- is either N-acetylglucosamine or N-acetylgalactosamine.
acter are greatly extended and aligned perpendicular to Examples are given in Table II. A major function of this
the stress direction. In general, the Young’s modulus (the group of molecules is the formation of a matrix to support
stress–strain curve) of a bulk crystallized polymer is two and surround the protein components of skin and connec-
to three orders of magnitude greater than that of an amor- tive tissue. A schematic view of the proteoglycan complex
phouspolymersuchasrubberandthemodulusoforiented, in cartilage used to bind to collagen in a strong network
crystalline fibers is another order of magnitude greater is shown in Fig. 1. The filamentous structure contains at
still. its heart a single molecule of hyaluronic acid to which is
Collagen is a prime example of an important crystalline attached via noncovalent interactions proteins known as
polymer in mammals and is found in both rigid (e.g., “core” proteins. These in turn have chondroitin sulfate and
bone) and pliant materials (e.g., tendon, skin and cartilage)
(Table II). Each collagen chain is ca. 1000 amino acids
in length and is dominated by a 338 contiguous repeat-
ing triplet sequence in which every third amino acid is
glycine. Proline and hydroxyproline together account for
about 20% of the total amino acids. The structure of col-
lagen is based on the helical arrangement of three non-
coaxial, helical polypeptides, stabilized by inter-chain and
intra-chain hydrogen bonds with all the glycines facing the
centerofthetriplehelix.Individualcollagenmoleculesare
ca. 280 nm in length and 1.5 nm in width with adjacent
molecules being transposed in position relative to one an-
other by one quarter of their length in the axial direction.
Collagen is the protein used in tissues where strength or
toughness is required. In addition to the effect of its intrin-
sic chemical structure on the observed mechanical proper- FIGURE 1 A schematic view of the proteoglycan complex in
ties, part of its toughness arises from specific cross-links cartilage used to bind to collagen in a strong network.