Page 39 - 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 179

SCHEME 1 Condensation polymerization.

toxic materials, the surrounding tissue dies. For nontoxic the polymer. If only one species of monomer is used
materials and those that dissolve, the surrounding tissue to build a macromolecule the product is termed an ho-
replaces the implant. For biologically nontoxic and inac- mopolymer, normally referred to as a polymer. If two
tive materials, a fibrous tissue capsule of variable thick- types of monomer unit are used the material is known
ness can form. For nontoxic and biologically active mate- as a copolymer and if three different monomers are used
rials, an interfacial bond forms. Materials used in medical then a terpolymer results.
procedures should be biocompatible with the host tissue. Polymers may be formed by condensation reac-
Biocompatibility of an implant material is deemed opti- tions between complimentary functional groups to
mal if it promotes the formation of normal tissue at its make poly(esters), poly(amides) and poly(urethanes)
surface, and in addition, if a contiguous interface capable (Scheme 1).
of supporting the loads that normally occur at the site of They may also be formed by free radical polymerisa-
implantation is established. tion of unsaturated compounds to give addition polymers
Hence the current goal is to produce materials that are (Scheme 2).
recognized and assimilated by the body. Examples of this class include poly(ethylene),
poly(vinyl chloride) and poly (methyl methacrylate).
A third route to polymers includes ring opening reac-
IV. MATERIALS OF CONSTRUCTION tions as in the formation of nylon-6 from ε-caprolactam
(Scheme 3).
All medical implants are composed of polymers, metals, Thestructure,chemicalfunctionalityandphysicalprop-
ceramics, or mixtures and composites of these materials. erties of the polymer phase and hence its potential in
Tissue replacement with synthetic materials requires se- medical devices depends on the monomers used and the
lection of a material or materials with physical properties method of synthesis. The presence of more than one func-
most similar to those of the natural tissue (Table I). tional monomer can lead to different arrangements of the
monomers in the polymer chain (Fig. 3). These structural
A. Synthetic Polymers variations, including effects due to chirality have an effect
on the way in which the polymer chains can pack together
These are the most widely used materials in health care and hence they have an effect on the physical properties
and are used in almost all phases of medical and/or dental
of the material itself.
treatment. Typical polymers in use are listed in Table II
Polymers are often compounded with a range of other
together with selected uses.
materials to lower the cost and improve the performance
of the proposed polymer device. Accelerators are used to
increase the extent and rate at which cross-linking between
1. Synthesis
the chains occurs. Antioxidants such as amines and hydro-
Polymers are large molecules made from many smaller quinones are added to minimize the cracking of a device
units called monomers that are chemically bonded to form when exposed to oxidants. Fillers such as carbon black,

SCHEME 2 Addition polymerization.