Polymers - Orthopaedic Biomaterials in Research and Practice - page 134

However, some general relations are a useful guide in thinking about
choices between metals, polymers, and ceramics (including glasses, car-
bons, and other amorphous inorganic materials). These are summarized
in Table 6.1.
Other engineering properties that affect the selection of materials by
classes include wear rate, creep and stress relaxation rates, coefficient
of friction, endurance limit, nature and biologic effects of degradation
products, fabricability, appearance, availability, and cost.

General considerations concerning polymers

It is easy to picture a generic polymer: we think of polymers as “plas-
tics.” In this general view, polymers are relatively weak solid materials
that soften as temperature increases. They are all around us, more and
more so as new and improved polymers begin to displace metals (and
ceramics, especially glasses) from traditional applications. Thus, the
chrome steel automobile bumper is replaced by high-strength polymer
body components, and the glass quart milk bottle gave way to the plastic
gallon jug. Polymers also result in new products: compact audio discs,
clear food wrap, Frisbees, and so on.
Polymers and polymer-based composites represent one of the most
exciting areas of modern materials sciences. They combine moderate
strength, low cost, and easy raw material availability with the abil-
ity to regulate physical properties by design of composition, internal
structural arrangement, and processing. Furthermore, the ease with
which they may be colored and joined, by adhesive and other pro-
cesses, gives them an aesthetic flexibility lacking in more traditional
materials.
Nowhere has the impact of modern polymeric materials been greater
than in medicine, with the resulting wide use of polymeric disposable
supplies, dressings, and sutures and the incorporation of polymers into
medical devices and surgical instruments and implants. Many polymers
are used in orthopaedics, and the use of just two materials, ultrahigh-
molecular-weight poly(ethylene) (UHMWPE) and poly(methyl methac-
rylate) (PMMA), has radically transformed clinical practice by ushering
in the era of successful long-term joint replacement.

Structure Polymers are distinguished from metals and ceramics in that their struc-

tures (and therefore physical properties) are derived more directly from
primary molecular features than from interatomic bonds. This molecular
structure is characterized, in solid polymers, by high-molecular-weight
linear chains or networks formed of identical or similar smaller repeat-
ing units.
The repeating structural unit within a polymer is called a mer.
Polymers (literally “many mers”) result from the combination of mol-
ecules called monomers. A monomer is a molecule containing one or
more atoms that can each participate in two or more covalent bonds.
The most familiar such atom is carbon; having four electrons in its outer