Biomedical Engineering Reference
In-Depth Information
Fig. 3.2.2-6 Copolymers used in medicine and their base monomers.
such as zinc and calcium are incorporated into the for-
mulation and interact with the carboxyl groups to pro-
duce a strong, hard material. The alginates, which are
polysaccharides derived from brown seaweed, also con-
tain anionic residues that will interact with cations and
water to form a gel. The alginates are used successfully to
dress deep wounds and are also being studied as tissue
engineering matrices.
appear at random on either side of the extended chain
backbone.
Atactic polymers usually cannot crystallize, and an
amorphous polymer results. Isotactic and syndiotactic
polymers may crystallize if conditions are favorable. PP is
an isotactic crystalline polymer used as sutures. Crystal-
line polymers, such as PE, also possess a higher level of
structure characterized by folded chain lamellar growth
that results in the formation of spherulites. These struc-
tures can be visualized in a polarized light microscope.
The solid state
Crystallinity
Tacticity
Polymers can be either amorphous or semicrystalline.
They can never be completely crystalline owing to lattice
defects that form disordered, amorphous regions. The
tendency of a polymer to crystallize is enhanced by the
small side groups and chain regularity. The presence of
crystallites in the polymer usually leads to enhanced
mechanical properties, unique thermal behavior, and in-
creased fatigue strength. These properties make semi-
crystalline polymers (often referred to simply as
crystalline polymers) desirable materials for biomedical
applications. Examples of crystalline polymers used as
biomaterials are PE, PP, PTFE, and PET.
Polymers are long-chain molecules and, as such, are
capable of assuming many conformations through rota-
tion of valence bonds. The extended chain or planar
zigzag conformation of PP is shown in
Fig. 3.2.2-8.
This
figure illustrates the concept of tacticity. Tacticity refers
to the arrangement of substituents (methyl groups in
the case of PP) around the extended polymer chain.
Chains in which all substituents are located on the same
side of the zigzag plane are isotactic, whereas syndiotac-
tic chains have substituents alternating from side to
side. In the atactic arrangement, the substituent groups
