Biomedical Engineering Reference
In-Depth Information
transitions are introduced at a basic level. Readers
familiar with these basic polymer concepts may want
to consider skipping ahead to the next chapter.
words,
certain polymers
are more
rate
and
temperature sensitive than others.
As we shall see in subsequent sections and chap-
ters of this Handbook , when used for implants under
clinically relevant conditions, PEEK is relatively
insensitive to changes in rate and temperature.
Further explanation of general polymer concepts can
be found in the excellent textbook by Rodriguez [26] .
1.2 What Is a Polymer?
PEEK belongs to a class of materials known as
polymers or in lay terms more simply as plastics.
More specifically, PEEK is classified as a linear
homopolymer . Before proceeding to a definition of
PEEK, it is helpful to first understand the significance
of these italicized terms.
The definition of polymer has its origins in the
Greek, polumeres , meaning “having many parts.”
The repeating units, or monomer segments, of
a polymer can all be the same. In such a case, we
have a homopolymer ( Fig. 1.1 ).
When two or more different monomers are used,
the resulting material is classified as a copolymer .
However, PEEK is a homopolymer, and so
throughout this chapter we will focus our attention on
polymers having only a single monomer.
Polymers may be linear or branched ( Fig. 1.2 ).
The tendency for branching in a homopolymer
depends strongly on its synthesis conditions. The
distinguishing feature of a polymer d as compared
with a metal or ceramic d is its molecular size. In
a polymer such as PEEK, the molecule is a linear
chain of 100 monomer units with an average
molecular weight of 80,000 e 120,000 g/mol.
In general, the length and composition of the
molecular chain result in many unique attributes for
polymers, most notably the dependence of its
properties on the temperature and rate at which
deformations are applied. The rate and temperature
sensitivity of polymers are strongly dependent on
their chemical composition and structure. In other
1.3 What Is PEEK?
Commercialized for industry in the 1980s, PAEK
is a family of high-performance thermoplastic poly-
mers, consisting of an aromatic backbone molecular
chain, interconnected by ketone and ether functional
groups [27] . Thus, PEEK belongs to a larger family
of PAEK polymers, sometimes referred to as poly-
etherketones (PEKs) or more simply as “poly-
ketones.” The chemical formula of PEEK is shown in
Fig. 1.3 . Other members of the PAEK family that are
considered for implants include PEK and poly-
etherketoneketone (PEKK), with chemical structures
depicted in Fig. 1.4 . PEEK is the dominant member
of the PAEK polymer family for implant applications
and is consequently the main focus of this Handbook .
The chemical structure of PEEK, similar to its
PAEK cousins, confers stability at high temperatures
(exceeding 300 C), resistance to chemical and
radiation damage, compatibility with many rein-
forcing agents (such as glass and carbon fibers), and
greater strength (on a per mass basis) than many
metals, making it highly attractive in industrial
Figure 1.3 Chemical structure of PEEK.
Figure 1.1 Schematic representation of a homopolymer.
Figure 1.2 Schematic representation of
linear and
Figure 1.4 Chemical structure of
(A) PEK and (B)
branched homopolymers.
PEKK.
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